Elemental Selenium in the Synthesis of Selenaheterocycles

Abstract

An overview of the known methods of introducing selenium under the action of elemental selenium into the structures of various saturated, unsaturated, and heteroaromatic selenacycles containing C–Se, N–Se, B–Se, Ge–Se and P–Se bonds is presented. These methods include metal, iodine, bromine or chlorine exchange for selenium and the direct cyclization of 1-(2-bromoaryl)benzimidazoles, polyunsaturated hydrocarbons, acetylenes, propargylic amines, 3-halogenaryl amides, aryl amides, diazo-compounds, 2-aminoacetophenone, and the annulation of ethynyl arenes. Three- and four-component reactions utilizing elemental selenium as one of the components and leading to selenium-containing heterocycles are presented as well.

1. Introduction

The synthesis of organoselenium compounds, and especially selenium-containing heterocycles, continues to be a very active research area since the 1980s, when the results for the synthetic ebselen, an organoselenium compound 2-phenyl-1,2-benzoselenazol-3-one, revealed promising antioxidant properties of this heterocycle [1,2]. Nowadays, a variety of these compounds are known, which demonstrate antimicrobial, biocidal, anti-inflammatory, antioxidant and free radical scavenging activities [3,4,5,6,7,8,9,10,11]. Among them, a number of organoselenium compounds have been found suitable for the treatment of the most common ailments—cardiovascular, cancer, viral diseases and AIDS [2,12,13,14,15,16,17]. Their practical application in medicine for the treatment of tumors and cancers is a subject of current intense interest [18,19,20,21,22,23,24]. In material science, the utilization of selenium-containing heterocycles in developing organic conductors, semiconductors, electroconducting materials, paramagnetics and optoelectronics is another area of current interest [25,26,27,28,29,30,31].

The selenorganic heterocycles are usually prepared by the direct introduction of selenium into the organic scaffold or by exchange with another atom. Over the last half century, a number of selenylating agents have been introduced into the practice, including nucleophilic H₂Se, NaHSe, Na₂Se, Li₂Se, KSeCN, (Me₃Si)₂Se, electrophilic organylselenyl halides, selenium di- and tetrahalides [32,33,34,35,36,37,38]. But the most direct synthetic way to obtain selenaheterocycles consists of introducing elemental selenium into the parent organic molecule. Elemental selenium lacks the drawbacks of other selenylating agents such as toxicity, difficulty of preparation and handling as well as instability. At the same time, since this approach leads to the synthesis of selenium heterocycles by excluding additional manipulations with selenium such as, for instance, the generation of sodium or potassium selenides or diselenides Na₂Se, K₂Se, Na₂Se₂, K₂Se₂ or selenium di- or tetrahalides SeX₂, SeX₄ it seems quite prospective.

In 2019, a small review [39] was published in which some examples of elemental selenium introduction into the molecules of different heterocycles were presented. Quite a number of reviews depicting the syntheses of various selenium-containing compounds also included examples of the selenacyles formation due to the use of elemental selenium [40,41,42,43,44,45,46,47,48,49,50,51]. A fairly extensive overview of the use of elemental selenium for the syntheses of different classes of organoselenium compounds, including various heterocycles, were presented by Ma et al. in 2021 [52] and Guo et al. in 2022 [53]. However, the use of elemental selenium for the synthesis of heterocycles has neither been considered in the literature as a separate subject nor in more detail. This review presents the currently known data related to the synthesis of selenium-containing heterocycles using elemental selenium as a selenylating reagent, which will allow us to establish the limits of the use of elemental selenium in the construction of various heterocyclic systems.

2. Synthesis of Selenium-Containing Heterocycles by Metal–Selenium Exchange in Cyclometallated Derivatives of Olefins, Allenes, Acetylenes and Aromatics

The exchange of metal for selenium in metallacyclopentanes 1, -cyclopent-2-enes 2 and –cyclopenta-2,4-dienes 3, generated in situ by Dzhemilev reaction (M = Al [54,55,56,57] and Mg [57,58,59]) through the catalytic cycloalumination or cyclomagnesation of alkenes and alkynes in the presence of catalytic amounts of Ti and Zr complexes, can be regarded as one of the earliest methods for the introduction of elemental selenium to heterocyclic compounds. Reactions result in a formation of selenium-containing five-membered saturated tetrahydroselenophenes 4, and unsaturated dihydroselenophenes 5 and selenophenes 6 [54,56,59,60] (Scheme 1).

Based on this methodology, Dyakonov et al. [60] developed a one-pot synthesis of fused five-membered selenium heterocycles via the cyclometallation of methylenecyclobutane 7 and allenes 8a,b using alkyl derivatives of Al and Mg. The reaction of the resulting alumina- and magnesacarbocycles with elemental selenium afforded various spiro-, bi- and tricyclotetrahydroselenophenes 9a,b,c and bi- and tricycloselenophenes 10a,b in high yields (Scheme 2).

The cycloalumination of cyclotetradeca-1,8-diyne 11 in the presence of Zr catalyst (Cp₂ZrCl₂) involved both triple bonds of the diyne to present the isomeric tricyclic bisaluminacyclopentenes 12 and 13 in a 1:1 ratio in a 91% yield. The reaction of the latter with an excess of elemental selenium in boiling benzene afforded a mixture of the regioisomeric 8,20-diselenatricyclo [15.3.01,17.07,11]eicosa-1(17),7(11)-diene 14 and 8,18-diselenatricyclo [15.3.01,17.07,11]eicosa-1(17),7(11)-diene 15 in a 1: 1 ratio and a 69% total yield [61] (Scheme 3).

The exchange of mercury for selenium in a mercury derivative of biphenyl 18 prepared by the treatment of diiodobiphenyl 16 with lithium and mercury chloride (II) at 200 °C led to dibenzoselenophene 6a [62] (Scheme 4). Reaction proceeds through the intermediate formation of a dilithium derivative of biphenyl 17.

3. Synthesis of Selenium-Containing Heterocycles via Lithium–Selenium Exchange in Lithium Derivatives of Organic Compounds

The most developed method of introducing selenium into a heterocycle molecule is lithium–selenium exchange, which sometimes presents results that are difficult to achieve by other methods.

For instance, if to treat the intermediate dilithium biphenyl 17 mentioned above (Scheme 4) with elemental selenium in air, instead of mercury chloride, another product, dibenzo[1,2]diselenine 19, is formed at the expense of direct lithium–selenium exchange, followed by aerial oxidation [62] (Scheme 5).

The reaction of the highly crowded trisilylmethyllithium compound (PhMe₂Si)₃CLi with elemental Se resulted in a variety of products, which include the triselane [(PhMe₂Si)₃CSe]₂Se, the unexpected diselane [(PhMe₂Si)₂HCSe]₂ and the novel heterocycle s-tetraselenane [(PhMe₂Si)₂CSeSe]₂ 20 [63] (Scheme 6). The structure of the latter could be elucidated from the NMR spectroscopic data and was confirmed by the crystal structure, which displays the SeSeCSeSeC cycle in twist form.

The 1,2-diselenine-containing-fused π-conjugated compounds 24 were synthesized, starting from bis(o-haloaryl)diacetylenes 21 via a one-pot intramolecular triple cyclization reaction [64]. Further deselenation of 1,2-diselenine under the action of Cu afforded five-membered heteroacenes 25 [64] (Scheme 7).

The formation of heterole-1,2-diselenin-heterole tricyclic structures 24 in this work was explained by the three-stage process, including the dilithiation of 21 with t-BuLi in THF, followed by trapping with elemental selenium to produce the dianionic species 22. In the second step, the anionic centers attack the inner carbon atoms of the diacetylene moiety, generating a new dianionic species which is trapped with the remaining elemental selenium to afford the doubly cyclized dianionic intermediate 23. The fused 1,2-diselenines 24 were obtained in the final step by the oxidation of the latter with potassium ferricyanide (III) in a 1 M NaOH aqueous solution (Scheme 7).

Similarly, the treatment of triphenylene derivative 26 with n-BuLi, followed by elemental selenium, afforded a 70% yield of heteroacene 27, which was quantitatively transformed to the triselenasumanene derivative 28 by a solid-state deselenation over copper powder [65] (Scheme 8).

The dilithiation of 1,4-dibromo-2,5-bis(phenylethynyl)benzene 30a with t-BuLi followed by treatment with selenium powder resulted in a synthesis of the corresponding 2,6-diphenybenzo [1,2-b:4,5-b′]diselenophene 31a (DPh-BDS) [66]. The parent 30a was prepared in this work by the iodation of 1,4-dibromobenzene 29 and subsequent Sonogashira coupling with phenylacetylene [66] (Scheme 9). Other BDS derivatives 31b–d with biphenyl-, p-hexylphenyl- and trimethylsilyl subsistents can also be synthesized by the same method using the corresponding acetylenes [66,67,68,69,70] (Scheme 9).

The treatment of disubstituted acetylene, 2,2’-dibromodiphenylacetylene 32a, with tert-butyllithium followed by elemental selenium insertion in the Li derivative of acetylene 32a resulted in a intramolecular ring closure to afford [1]benzoseleno [3,2-b][1]benzoselenophene 33a [71] (Scheme 10). According to this procedure, [1]benzothieno [3,2-b][1]benzothiophene 33c and [1]benzotelluro [3,2-b][1]benzotellurophene 33c were also obtained.

Dinaphtho [1,2-b:2′,1′-d]selenophene 35 was prepared by the exchange of lithium for selenium in the lithium derivative of sulfonamide generated by the treatment of sulfonamide 34 with n-BuLi in tetramethylethylenediamine (TMEDA). Furthermore, the diselenide 36 was formed in this reaction [72] (Scheme 11).

An ebselen analog 39 was synthesized by treatment with selenium in THF of the lithium derivative of bisdihydrooxazole 37, produced by the interaction of the latter with LDA in the presence of TMEDA [73,74]. It is assumed that the product 39 is formed due to the spontaneous disproportionation of the intermediate diselenide 38 (Scheme 12).

A general approach to ebselen and its derivatives 42 involving the use of elemental selenium was described. It includes the ortholithiation of benzanilides 40, the subsequent insertion of elemental selenium into benzanilide-derived dianion 41 and the cyclization of selenium-containing dianion to ebselen derivatives 42 in yields up to 14% [75,76] (Scheme 13).

Selenazoloindoles 44 were prepared from the readily available N-alkynylindoles 43 via annulation through the introduction of n-BuLi and the sequential exchange of lithium for selenium under the action of elemental selenium. The simultaneous lithiation of triple bond to generate intermediate lithium selenolate results in the formation of the corresponding 3-alkylselanyl derivatives 44 on interaction with alkyl bromides [77] (Scheme 14).

Treatment of 4,5,11,12-tetrabromo-N,N′-di-n-butyl-2,7,9,14-tetrakis(trimethylsilyl) tetraphenyleno [1,16-bcd:8,9-b′c′d′]dipyrrole 45 with excess of BuLi at −78 °C, and then with elemental selenium in THF at room temperature afforded diazadiseleno [8]circulene 46 thanks to the exchange of lithium for selenium in the lithium derivatives formed in the first stage [78] (Scheme 15).

Kobayashi et al. developed a synthetic approach to benzoselenazole-2(3H)-thiones 50, 2-(alkylsulfanyl)benzoselenazoles 51 and S-(benzoselenazol-2-yl)thiocarboxylates 52 [79] (Scheme 16). Treatment of 1-bromo-2-isothiocyanatobenzene 47 with BuLi to produce 2-lithiophenyl isothiocyanates 48, which were further reacted with selenium powder, yielded lithium benzo-1,3-selenazole-2-thiolate 49. The quenching of this anion with the corresponding nucleophiles afforded the above benzoselenazoles 50–52.

4. Synthesis of Selenium-Containing Heterocycles by Exchange of I or Br for Se

The exchange of halogens for selenium is another powerful method for the preparation of selenium-containing heterocycles.

Thus, diarylselenophenes 6 were obtained in 49–90% yields by the base-catalyzed Se–I exchange reaction of diaryliodonium salts 53 in DMSO at 80 °C [80] (Scheme 17). Diaryliodonium salts with both electron-rich and electron-deficient substituents can be used in this reaction.

The annulation of ortho-alkenyl aryliodides 55 under the action of elemental selenium in the presence of CuI results in substituted benzoselenophenes 56 [81,82] (Scheme 18). The starting aryliodides 55 have been prepared here by the addition of arylzinc reagents 54 to alkynes in the presence of the cobalt–Xantphos complex to form o-alkenyl arylzinc intermediates and the subsequent substitution of the zinc substituent for iodine under the action of I₂ [82].

Similarly, the Cu-catalyzed reaction of 2-(2-iodophenyl)-1H-indoles 57 and Se powder in DMSO at 110 °C affords benzoselenopheno [3,2-b]indole derivatives 58 [83] (Scheme 19).

2-(2-Iodophenyl)imidazo [1,2-a]pyridine derivatives 59a in similar conditions, under the action of elemental selenium, result in novel benzo[b]selenophene-fused imidazo [1,2-a]pyridines 60 [84] (Scheme 20). Both intramolecular cyclizations involve the Ullmann-type Se-arylation and C(sp²)–H selenation reactions. However, reaction with imidazopyridine derivatives 59a in contrast to reaction with indoles 57 proceeds under aerobic conditions. Products were prepared here in moderate-to-high yields.

An alternative method for the synthesis of benzo[b]selenophene-fused imidazo [1,2-a]pyridines 60 through ligand- and base-free CuI-catalyzed cyclization of 2-(2-bromophenyl) imidazo [1,2-a]pyridine derivatives 59b under the action of elemental selenium in air was also described [85] (Scheme 21).

A copper-catalyzed reaction between 2-bromobenzothioamides 2-Br-RC₆H₃C(S)NHR¹ (R = H, 5-Me, 5-Cl, 3-Me, etc.; R¹ = Ph, pyridin-2-yl, 9H-fluoren-2-yl, etc.) 61 and Se involves sulfur rearrangement and enables access to benzothiaselenoles 62 in the presence of Cs₂CO₃. In the absence of Se, the reaction affords dibenzodithiocines 63 (R = H, 3-OMe, 2-Me, etc.) via two consecutive C(sp²)-S Ullmann couplings [86] (Scheme 22).

A novel and efficient procedure for one-pot regio- and stereospecific synthesis of benzo [1,4,2]thiaselenazine 1,1-dioxides 66 via [Cu]-catalyzed ring closure reaction between N-alkynyl-2-iodobenzene sulfonamides 64 and elemental Se in N-methyl-2-pyrrolidone (NMP) at 90 °C for 20 h has been developed [87]. Its generality was illustrated by extension to the synthesis of seven-membered benzothiaselenazepines 67 from N-(3-phenylprop-2-yn-1-yl)-2-iodobenzene sulfonamides 65 (Scheme 23). The involvement of water in the reaction is demonstrated by the incorporation of 2D at the olefinic site by using D₂O in place of water.

A similar procedure was used for the synthesis of 2,3-dihydro-1,4-benzoxaselenines 69 from 2-iodoaryl propargyl ethers 68 [88] (Scheme 24).

A CuBr₂-catalyzed annulation of 2-bromo-N-arylbenzimidamide 70 with selenium powder was shown to be a general convenient method for the preparation of benzo[d]isoselenazoles 71 in good yields [89] (Scheme 25). This synthetic strategy demonstrates good functional group tolerance. Furthermore, the corresponding products could be converted into N-aryl indoles 72 in the reactions with diarylacetylenes 32c via rhodium^III-catalyzed ortho-C–H activation of the N-phenyl ring, providing an efficient approach for axial aromatic molecules.

5. Synthesis of Benzoselenazoles by Cyclization of 1-(2-Bromoaryl)benzimidazoles under Action of Selenium

The ring-closure reaction of 1-(2-bromoaryl)benzimidazoles 73 with Se powder was promoted by Cs₂CO₃ in DMF at 150 °C and afforded novel tetracyclic heterocycles—benzimidazo [2,1-b]benzoselenoazoles 74 [90] (Scheme 26). As compared to the above methodology, the proposed mechanism of this reaction involves the deprotonation of the imidazole ring at the 2-position and C(Het)–Se bond formation. Consequent ring closure via the S_NAr reaction by attack of the selenide anion on the phenyl group containing bromine generates the target tetracyclic molecule (Scheme 26). Single-crystal X-ray analysis of the parent benzimidazo [2,1-b]benzoselenoazole 74a (R = R¹ = H) revealed that the tetracyclic ring is almost planar.

6. Cyclization of Polyunsaturated Hydrocarbons under the Action of Elemental Selenium

Another method that can be regarded as one of the earliest for the synthesis of selenium-containing heterocycles is the cyclization of polyunsaturated hydrocarbons under the action of elemental selenium.

The selenation of tetraarylbutatrienes R₂C=C=C=CR₂ 75 (R = 4-R¹C₆H₄; R¹ = Me, H, Cl) in DMF in the presence of DBU afforded 1,2,5-triselenepanes 76, while sulfurization resulted in 1,2,3,4,5-pentathiepanes 77. The further degradation of 1,2,5-triselenepanes 76 resulted in the formation of benzoselenophene derivatives 78 [91] (Scheme 27).

[4+1]-Cycloaddition of elemental selenium to trifluoromethyl derivatives of 1,3-diene 79 in an autoclave without a solvent and in the presence of anhydrous trifluoroacetic acid anhydride (TFAA) as catalyst presented 2,4-substituted 2,5-dihydroselenophenes 80 [92] (Scheme 28).

The reaction of diaryldiynes 81 with elemental selenium, in contrast to 1,3-dienes, afforded—on heating to 170–230 °C—various diselenolodiselenole derivatives 82 [93] (Scheme 29).

The direction of the reaction with diynes changes on the addition of hydrazine monohydrate and KOH to elemental selenium. Thus, the treatment of diphenyl diacetylene 81a with Se/N₂H₄^.H₂O/KOH system afforded 2,5-diphenylselenophene 6b due to the generation of K₂Se in the reaction mixture [94]. Similarly, 1,3-butadiyne-bridged carbazole dimer 83 afforded selenophene-bridged carbazole dimer—isophlorin 84, which, upon oxidation with MnO₂ in CH₂Cl₂, led to selenaporphyrin 85 [95] (Scheme 30).

7. Selenophenes via Cyclization of Acetylenes under Action of Elemental Selenium

One of the best known methods for producing selenophenes is the interaction of terminal and disubstituted acetylenes with elemental selenium in benzene at elevated pressure. A reaction of 3-butyn-2-one 32b with Se at 205–215 °C in C₆H₆ in stainless autoclave resulted in 2,4- and 2,5-diacetylselenophenes 6c and 6d, while tetraphenylselenophene 6e was prepared in a similar method to that of PhC≡CPh 32c [96] (Scheme 31). The proposed mechanism for the formation of selenophenes involves the intermediate generation of diselenins 86, which, under reaction conditions, are subjected to deselenation.

8. A Carbonylative Cyclization of Propargylic Amines with Elemental Selenium

Various 1,3-selenazolidin-2-ones 88 were prepared via a carbonylative cyclization of propargylic amines 87 with elemental selenium [97] (Scheme 32). In this process, as a safe and convenient solid CO source, benzene-1,3,5-triyl triformate (TFBen) was employed using t-BuOK as the promoter. A broad class of substrates was effectively transformed into the desired products under mild conditions.

When in the same process with TFBen as a CO source where DBU was used as a reaction promoter while n-C₄F₉I was used as the iodide source, a carbonylative cyclization of propargylic amines 87 with elemental selenium afforded (E)-5-(iodomethylene)-1,3-selenazolidin-2-ones 89 in up to 95% yields [98] (Scheme 33).

An alternative method for the preparation of 5-alkylidene-1,3-selenazolin-2-ones 88 was described by Fujiwara et al. [99], who suggested the 5-exo-dig cyclization of selenolate intermediate 90 generated by the reaction of propargylic amines 87 with elemental selenium and CO using DBU as a promoter (Scheme 34).

A similar CuI-catalyzed cyclocarbonylation of homopropargylamine 91 under the action of CO in the presence of elemental Se afforded the corresponding selenazinan-2-one derivative 92 [99] (Scheme 35).

9. Synthesis of 1,2,3,5,6,7-Hexaselenacyclooctane via Se–Cl Exchange in 1-Chloro-2,2-bis(diethylamino)ethene under Action of Elemental Selenium

The possibility of Se–Cl exchange in chloroethenes was demonstrated by the synthesis of 4,8-bis[bis(diethylamino)methylene]-1,2,3,5,6,7-hexaselenacyclooctane 94 [100,101]. Treatment of 1-chloro-2,2-bis(diethylamino)ethene 93 with elemental Se in refluxing benzene resulted in compound 94 in 60% yield (Scheme 36). Its structure was determined by XRD analysis. The compound 94 was shown to behave as 2,2-bis(diethylamino)-2-ethylium-1-diselenocarboxylate 95 toward a range of reagents. Thus, with di(methyl) acetylenedicarboxylate 32c, it reacted to provide 1,3-diselenole 96 in high yield (Scheme 36). Evidence for the dissociation of 94 into 95 in solutions was provided by IR, UV/visible and ¹H-, ¹³C- and ⁷⁷Se-NMR spectra.

10. Synthesis of Ebselen via Cyclization of 3-Halogenaryl Amides and Aryl Amides under Action of Selenium

A general method was developed for the synthesis of the aforementioned biologically important ebselen and related analogs 42 containing a Se–N bond. It involves an efficient copper-catalyzed selenium–nitrogen coupling reaction between various 2-chloro, 2-bromo, 2-iodo-arylamides 97 and selenium powder [102,103] (Scheme 37). This copper-catalyzed reaction tolerates functional groups such as amides, hydroxyls, ethers, nitro, fluorides and chlorides. The best results have been obtained by using a combination of potassium carbonate as a base, or iodo-/bromo-arylamide substrates and copper iodide catalyst.

In order to prepare the ebselen analogs bearing an 8-quinolyl moiety, the arylamides 97 without halogen substituents at aryl moieties were also used. This efficient Ni-catalyzed selenation reaction was carried out in DMF at 120 °C in air and afforded for 24 h the corresponding benzoselenazole derivatives 42 in good yields [104] (Scheme 38).

11. Annulation of Ethynyl Arenes under Action of Selenium

Action of elemental Se on ortho-monoalkynyl-substituted perylene diimide (PDI) 98 in dimethylacetamide (DMA) at 140 °C resulted in highly regioselective heteroannulation to form selenophene-fused polycyclic product 99 [105] (Scheme 39).

12. Formation of Heterocycles via Cyclization of Diazo-Compounds under Action of Selenium

The reaction of bis(diazo)octamethyldecane 100 with elemental selenium in DBU at 130 °C yielded 1,2-di-tert-butyl-3,3,6,6-tetramethylcyclohexene 101 as the major product along with trans-3,8-di-tert-butyl-4,4,7,7-tetramethyl-1,2-diselenocane 102, while the analogous reaction of the reagent 100 with elemental sulfur in DBU resulted in trans-3,8-di-tert-butyl-4,4,7,7-tetramethyl-1,2-dithiocane 103 as the only product [106] (Scheme 40). The reaction of 3,9-bis(diazo)-2,2,4,4,8,8,10,10-octamethylundecane 104 with elemental selenium in DBU at 80 °C resulted in the formation of cyclic triselenide, cis-4,10-di-tert-butyl-5,5,9,9-tetramethyl-1,2,3-triselenecane 105 as the only identifiable product [106] (Scheme 40). The structures of the heterocycles 103 and 105 were confirmed by X-ray crystallography.

13. Formation of Selenazolines via Action of Selenium on N-Acyl-2-oxazolidinones

An efficient method for the preparation of chiral selenazolines (4,5-dihydro-1,3-selenazoles) 107 from N-acyl-2-oxazolidinones 106 and elemental Se in the presence of amine and hydrochlorosilane was suggested by Shibahara et al. [107] (Scheme 41). Suggested reaction mechanisms include the selenative rearrangement of N-acyl-2-oxazolidinones 106 and the elimination of O=C=Se species. A similar selenative rearrangement was observed in the reaction of free oxazolidinone 108 carried out under the same selenation condition and affording selenazolidinone 109 in moderate yield [107] (Scheme 42).

14. Carbonylative Cyclization of 2-Aminoacetophenone under Action of Se and CO

2H-3,1-Benzoselenazin-2-one 111 was prepared in 58% yield by carbonylation of o-aminoacetophenone 110 with Se and CO (under 30 atm pressure) in the presence of N-methylpyrrolidone at 100 °C for 20 h [108] (Scheme 43). The reducing agent, H₂Se, was presumably formed by the reaction of selenium, carbon monoxide and water.

15. Synthesis of 1,3-Oxaselenoles from Carbonyl-Stabilized Sulfonium Ylides under Action of Selenium

Carbonyl-stabilized sulfonium ylides 112a,b readily react with elemental selenium to afford 1,3-oxaselenole derivatives 113a,b in good yields, thus providing a simple method for constructing these ring systems, which use easily accessible compounds as starting materials [109] (Scheme 44).

16. Synthesis of 1,3-Diselenole-2-selones by Interaction of Terminal Acetylenes with Se, CSe₂ and BuLi

4-Methylthio-5-(2-methoxycarbonylethylthio)-1,3-diselenole-2-selone 116a have been prepared in high yields from methylsulfanyl acetylene 32e or 1-methylsylfanyl-1,2-dichloroethylene 114 by their lithyation with 1 or 2 equivalents of BuLi to result in lithium acetylenide, followed by consequent reaction with elemental selenium and carbon diselenide, and finally with methyl 3-thiocyanateproponate 115 [110] (Scheme 45).

4,5-Alkylenedichalcogeno-substituted 1,3-diselenole-2-selones 116 have been prepared in a similar way by a one-pot synthetic method, including the successive treatment of trimethylsilylacetylene 32f with BuLi, Se, CSe₂ and finally, α,ω-bis(chalcogenocyanato)alkanes NCZ(CH₂)_nZCN (Z = S, Se; n = 1–3) 117 [111] (Scheme 46).

17. Introduction of Selenium into Organic Molecule via Carbanion

The treatment of the mesyloxymethyl-substituted β-lactams 118a–c with elemental selenium and t-BuOK in THF/DMF led to the synthesis of the cis-configurated biologically active isodethiaselenapenam 119 as well as isodethiaselenacephems 120a,b [112] (Scheme 47). The key step of this synthetic approach involved the addition of Se to the corresponding carbanions, followed by internal alkylation.

18. Formation of Heterocycles with Se–El Bonds (El = B, Ge, P)

18.1. Formation of Heterocycles with Se–B Bonds

The treatment of annulated 1,4,2,5-diazadiborinine 121 with elemental selenium resulted in the oxidative addition of selenium, which proceeded regioselectively at the boron centers of diborine 121 to present a bicyclo [2.2.2] molecule 122 with a B–Se–Se–B unit, which can be deemed a heavier analog of diboraperoxide [113] (Scheme 48).

The reaction of diborene 123 with elemental selenium is shown to afford diboraselenirane 124 [114] (Scheme 49). This reaction is reminiscent of the sequestration of subvalent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. Although carbon is too electronegative to affect the reduction in elements with a lower relative electronegativity, the highly reducing nature of the B–B double bond enables reactions with Se⁰.

The reductive insertion of elemental selenium into the B–B triple bond of the first stable diboryne 125 [115] under ultrasonic agitation has led to the synthesis of an Se-bridged cyclic compound containing boron stabilized by N-heterocyclic carbene (NHC) 126. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled a six-electron reduction reaction, resulting in a [2.2.1]-bicyclic system wherein bridgehead B atoms are spanned by three selenium bridges [116] (Scheme 50). Unfortunately, no yields have been reported for both compounds 124 and 126.

Reaction of silaborene, R₂Si=B(tmp) (R = SiMeBu-t₂, tmp = 2,2,6,6-tetramethylpiperidine) 127 with elemental selenium in THF afforded the novel three-membered ring product, selenasilaborirane 128. In contrast, the oxidation of 127 under an O₂ atmosphere produced the four-membered ring, 1,3,2,4-dioxasilaboretane 129 [117] (Scheme 51). Compounds 128 and 129 were studied using XRD analysis.

The treatment of the stable 1-phospha-2-boraacenaphthene 130 with elemental selenium afforded the unique heterocycle, 2-selena-1-phospha-3-boraphenalene 131 through the insertion of the selenium atom into a P–B bond of acenaphtene 130. Further selenation of phenalene 131 led to 2-selena-1-phospha-3-boraphenalene-1-selenide 132 [118]. The unique dynamic behavior of phosphine selenide 132 in solution was explained by facile selenium exchange in the molecule (Scheme 52).

The carborane-fused heterocycles 134a–c were prepared in good isolated yield via the reaction of carborane-fused zirconacyclopentane 133 with elemental selenium as well as with sulfur and tellurium [119] (Scheme 53). This approach represents a promising route to obtain functionalized carboranes that are difficult to access through conventional methods.

18.2. Formation of Heterocycles with Se–Ge Bonds

The reaction of stable hafnocene-based bicyclo [2.1.1]hexene germylene 135 with elemental selenium provides access to 1,3-diselena-2,4-digermetane 137a formed as a 87:13 mixture of cis- and trans-isomers (Scheme 54). This strongly colored four-membered germanium heterocycle, which is the formal dimer of a heavy ketone 136, was characterized via NMR and UV spectroscopy as well as the results of an XRD analysis [120]. The same reactions were realized in the case of elemental sulfur and tellurium [120] (Scheme 54).

Alternatively, 2,2,4,4-tetrakis [2-(dimethylamino)phenyl]-1,3-diselena-2,4-digermetane 137d was prepared by the reaction of {tris [2-(dimethylamino)phenyl]germyl}lithium (R₃GeLi) 138 with elemental selenium [121] (Scheme 55). The crystal structure of this heterocyclic compound has been determined by XRD analysis. The authors explained the formation of 1,3-diselena-2,4-digermetane either by an intermediate generation of germanselone and its intramolecular formal head-to-tail [2+2] cycloaddition or an intermolecular nucleophilic attack of the selenide ion at the germanium atom of another molecule.

The treatment of stable cyclic digermenes, 1,2-digermacyclobutene 139 and antiaromatic 1,2-digermacyclobutadiene derivatives 140, with elemental selenium yielded novel [Ge₂Se₂/₃C_x] heterocycles, 5,6-diselena-1,4-digermabicyclo [2.1.1]hexane 141, 5,6,7-triselena-1,4-digermabicyclo [2.2.1]hept-2-ene (1,2,4,3,5-triselenadigermolane) 142 and 5,6-diselena-1,4-digermabicyclo [2.1.1]hex-2-ene 143 (Scheme 56), which should be a convenient procedure for the preparation of the cyclic tetrel selenides [122,123]. These Ge-containing polyselenide products were isolated and characterized using X-ray crystallography.

The formation of selenagermanium heterocycle via the selenation of C=Ge bond was exemplified by the reaction of germabenzene 144a and 2-germanaphtalene 144b bearing a Tbt group (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) with elemental selenium in THF, which resulted in the cyclization and formation of only cyclic triselenides, 1,2,3,4-triselenagermolanes 145a and 145b [124] (Scheme 57). These new cyclic triselenides containing a germanium atom were characterized via NMR spectroscopy and elemental analysis.

18.3. Formation of Se–P Heterocycles via Insertion of Se into P=P and P–P Bonds

The reactions of P=P systems kinetically stabilized by 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) or 2,6-bis[bis(trimethylsilyl)methyl]- 4-[tris(trimethylsilyl)methyl]phenyl (Bbt) groups—TbtP=PTbt 146a, TbtP=PFc (Fc = ferrocenyl) 146b [125] or BbtP=PBbt 146c [126]—with elemental selenium in the presence of triethylamine, which resulted in the formation of the corresponding selenadiphosphiranes 147 (Scheme 58). The molecular structures of these three-membered heterocyclic compounds were confirmed by spectroscopic analysis.

The oxidative addition of elemental selenium to the homocyclic pentamer (PhP)₅ in refluxing toluene afforded various five-membered P–Se heterocycles. By varying the molar ratio of (PhP)₅ to selenium, the different selenaphospholanes 148–150 as well as the red crystalline solid 2,4-diphenyl-1,3,2,4-diselenadiphosphetan-2,4-diselenide, or the so-called Woollins reagent (WR) 151, were prepared by this method [127,128,129] (Scheme 59). Karaghiosoff and co-workers extended this oxidative route to other (RP)₅ homocyclic pentamers, R=Me, Et, 4-Me₂NC₆H₄ and 4-MeOC₆H₄ [44,130,131].

The oxidative addition of elemental selenium to tetraphospholanes (PhP)₄CR₂ 152a,b (R = H (a), Me (b)), prepared from (PPh)₅ by a reduction with potassium to form the phosphorus chain species, K₂P₄Ph₄, and further by its cyclization with dichloromethane and 2,2-dichloropropane, afforded 4-, 5-, and 6-membered heterocycles—2,3-diselena-1,4-diphospholane 153, selenadiphosphetanes 154a and 154b, tetraselenadiphosphinane 155, and WR 151 [132] (Scheme 60). The main products were five-membered diselenadiphospholane 153 and four-membered 2-selena-1,3-diphosphetane 154b prepared from the compound 152 in 94% and 68% yields, correspondingly. Six-membered tetraselenadiphosphinane 155, WR 151 and four-membered 2-selena-1,3-diphosphetane 154a were formed only in small quantities. Crystallographic analysis revealed a trans-configuration of exocyclic Ph in the formed heterocycles 153 and 154b.

18.4. Formation of Se–P Heterocycles through Interaction of Se with Methylenephosphorane

The reaction of tert-butylarylmethylenetriphenylphosphoranes R-p-C₆H₄C(t-Bu)=PPh₃ (R = OMe, OPh) 156 with elemental selenium afforded the corresponding five-membered 1,2,4-triselenolanes 157 as trans-isomers, four-membered 1,3-diselenetanes 158 and Ph₃P=Se [133] (Scheme 61). Triselenolanes 157 were shown to in fact be formed from the selenation of the 1,3-diselenetanes 158, which were the dimerization products of initially generated selenoketones.

19. Formation of Heterocycles through Three- and Four-Component Reactions Involving Elemental Selenium

Reactions involving the interaction of elemental selenium with organic substrates can also be found among multicomponent reactions with selenium, since selenium in them directly interacts with an organic intermediate.

2-Aryl-1,3-benzoselenazoles 161 have been formed in a selenium-mediated decarboxylative cyclization of 2-chloronitrobenzenes and chloronitropyridines 159, and aryl- and hetaryl (pyridine and thiophene) acetic acids 160 under metal-free conditions using N-methylpiperidine (NMP) as a base (Scheme 62). The reactions proceeded in moderate-to-good yields with good functional tolerance [134].

Another approach to preparation of 2-substituted 1,3-benzoselenazole derivatives 161 consisted in (1) the three-component one-pot reactions of readily available 2-iodoanilines 162, arylacetic acids 160 or arylmethyl chlorides 163, as well as selenium powder in the presence of CuBr in DMSO at 120 °C [135], or (2) in the three-component reactions of 2-iodoanilines 162, aromatic and heteroaromatic aldehydes 164, as well as elemental Se in DMSO at 120 °C in the presence of a Cu powder catalyst [136]. Substituted 1,3-benzoselenazoles 161 were prepared by these methods in moderate-to-high yields (Scheme 63).

The three-component assembly of 1-substituted indoles 165, aromatic ketones 166 and selenium powder were enabled by the IBr-promoted highly selective double C–H selenylation/annulations. This protocol provided a novel access to a diverse variety of selenopheno [2,3-b]indoles 167 with good efficacy and a broad functional group compatibility [137] (Scheme 64). However, with 2-aryl- and hetaryl-substituted indoles 165, the same three-component assembly afforded indolyl-substituted benzoselenophenes 56 via the selective formation of one C–C and two C–Se bonds (Scheme 65). Acetophenones with both EWG and EDG were converted to the corresponding products [138]. The reaction mechanism of these two reactions is based on the generation of a 3-vinylindole intermediate and oxidative dual CH selenylation. Annulation in the case of 2-unsubstituted indoles proceeds at the indole substituent, and in the case of 2-arylsubstituted indoles, at the aryl substituent of intermediate 3-vinylindole.

The base-promoted three-component cascade reaction of ortho-functionalized isocyanides 168, secondary amines 169, and elemental Se in 1,2-dichloroethane (DCE) at room temperature under metal-free conditions afforded 2-amino-3,1-benzoselenazines 170 in high yields [139] (Scheme 66).

Alternatively, the three-component mixture of isocyanides 168, arylamidine hydrochlorides 171 and elemental Se successfully reacted in the presence of N,N-diisopropylethylamine (DIPEA) as an efficient base to present a series of 1,2,4-selenadiazol-5-amine derivatives 172 [140] (Scheme 67).

Another three-component reaction of isocyanides 168, alk-2-yn-1-ols 173 and elemental selenium afforded, in the presence of DBU, 2-imino-4-alkylidene-1,3-oxaselenolanes 175 in high yields via the intramolecular addition of selenolate moieties of the generated in situ oxyimidoyl selenoates 174 to the carbon–carbon triple bond (Scheme 68) [141].

2-Substituted naphtho [2,1-d][1,3]selenazoles 178 and naphtho [1,2-d][1,3]selenazoles 179 were prepared in generally high yields for 178 and modest yields for 179 via efficient molecular iodine-catalyzed three-component cascade reactions from naphthalen-2-amine 176 in a case of [1,3]selenazoles 178, and naphthalen-1-amine 177 in a case of [1,3]selenazoles 179, aldehydes 164 and selenium powder [142] (Scheme 69). This approach has the advantages of metal-free conditions, simple operation and available raw materials. The possible reaction mechanism involves the formation of imine intermediates and consequent radical process triggered by iodine radicals.

The four-component reaction of (2-benzimidazolyl) acetonitrile 180, CS₂, isothiocyanate 181 and elemental selenium led to a zwitterionic azaselenadithiapentalene 182 [143] (Scheme 70). The structure of the product has been established by XRD analysis. The proposed reaction mechanism comprises the addition of the anion of 180 to CS₂ to yield intermediate A, which then reacts with selenium to result in intermediate B. The ring closure of the latter to C, the addition of 4-bromophenyl isothiocyanate to present D and cyclization lead to the final product 182, via tautomerization and protonation. It is worthwhile to mention that the same reaction with elemental sulfur results in tetracyclic [1,3]thiazolo [4′,5′:4,5]pyrimido [1,6-a]benzimidazol-2(3H)-thione (Figure 1) [143].

20. Conclusions

In conclusion, metal-to-selenium exchange is the most widely used method for introducing selenium into the heterocycle molecules, with lithium being mainly used as the metal, which is due to the ease of the starting organic compounds lithiation and the ease of further lithium-to-selenium exchange. The synthesis of 1,3-diselenol-2-selones by the interaction of terminal acetylenes with butyllithium, selenium and carbon diselenide can be considered as a variant of this method.

For the construction of five-, six- and seven-membered unsaturated and saturated selenacycles, the exchange of iodine, bromine or chlorine for selenium in a presence of CuI or CuO is also often used. In particular, this method is utilized in the synthesis of ebselen and its analogs from 2-halogenarylamides and selenium.

For the synthesis of a selenium element-containing heterocycles, the introduction or addition of elemental selenium at El–El or C–El bond is mainly used, which distinguishes this methodology from the preparation of C–selenium containing heterocycles, where the introduction of selenium with carbon–carbon bond rupture, or the direct addition of selenium at carbon–carbon bond is impossible.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figure and Schemes

Scheme 1. Metal–selenium exchange in metallacyclopentanes 1, -cyclopent-2-enes 2 and -cyclopenta-2,4-dienes 3.

Scheme 2. Cyclometallation of methylenecyclobutane 7 and allenes 8a,b.

Scheme 3. Cycloalumination of cyclotetradeca-1,8-diyne 11 with consequent Al–Se exchange.

Scheme 4. Hg–Se exchange in a mercury derivative of biphenyl 18.

Scheme 5. Li–Se exchange in dilithium biphenyl 17.

Scheme 6. Formation of s-tetraselane 20 by Li–Se exchange in (PhMe2Si)3CLi.

Scheme 7. Triple cyclization of bis(o-haloaryl)diacetylenes 21 by action of BuLi/Se.

Scheme 8. Synthesis of heteroacene 27 by action of BuLi/Se on triphenylene derivative 26.

Scheme 9. Synthesis of BDSs 31 by action of t-BuLi/Se on 1,4-dibromo-2,5-bis(organylethynyl)benzenes 30.

Scheme 10. Annulation of 2,2′-dibromodiphenylacetylene 32a under action of t-BuLi/Se as well as t-BuLi/Te, t-BuLi/S.

Scheme 11. Synthesis of dinaphtho [1,2-b:2′,1′-d]selenophene 35 by Li–Se exchange in lithium derivative of sulfonamide 34.

Scheme 12. Synthesis of ebselen analog 39 by Li–Se exchange in lithium derivative of bisdihydrooxazole 37.

Scheme 13. Synthesis of ebselen and its derivatives 42 by Li–Se exchange in lithium derivatives of benzanilides 40.

Scheme 14. Synthesis of selenazoloindoles 44 by annulation of N-alkynylindoles 43 under action of BuLi/Se.

Scheme 15. Synthesis of diazadiseleno [8]circulene 46 from 4,5,11,12-tetrabromo-N,N′-di-n-butyl-2,7,9,14-tetrakis(trimethylsilyl) tetraphenyleno [1,16-bcd:8,9-b′c′d′]dipyrrole 45 via Li–Se exchange.

Scheme 16. Synthesis of lithium benzo-1,3-selenazole-2-thiolate 49 from 1-bromo-2-isothiocyanatobenzene 47 by action of BuLi/Se and consequent transformation of thiolate 49 into thiones 50, sulfides 51 and thiocarboxylates 52.

Scheme 17. Synthesis of dibenzoselenophenes 6 by Se–I exchange in diaryliodonium salts 53.

Scheme 18. Synthesis of benzoselenophenes 56 by annulation of ortho-alkenyl aryliodides 55 via Se–I exchange.

Scheme 19. Synthesis of benzoselenopheno [3,2-b]indoles 58 by annulation of 2-(2-iodophenyl)-1H-indoles 57 via Se–I exchange.

Scheme 20. Synthesis of benzo[b]selenophene-fused imidazo [1,2-a]pyridines 60 by annulation of 2-(2-iodophenyl)imidazo [1,2-a]pyridine derivatives 59a via Se–I exchange.

Scheme 21. Synthesis of benzo[b]selenophene-fused imidazo [1,2-a]pyridines 60 by annulation of 2-(2-bromophenyl) imidazo [1,2-a]pyridine derivatives 59b via Se–Br exchange.

Scheme 22. Synthesis of benzothiaselenoles 62 by Se–Br exchange in 2-bromobenzothioamides 61 and consequent sulfur rearrangement.

Scheme 23. Synthesis of benzo [1,4,2]thiaselenazine 1,1-dioxides 66 and benzothiaselenazepines 67 by Se–I exchange with ring closure in N-alkynyl-2-iodobenzene sulfonamides 64 and N-(3-phenylprop-2-yn-1-yl)-2-iodobenzene sulfonamides 65.

Scheme 24. Synthesis of 2,3-dihydro-1,4-benzoxaselenines 69 by Se–I exchange and ring closure in 2-iodoaryl propargyl ethers 68.

Scheme 25. Synthesis of benzo[d]isoselenazoles 71 by CuBr2-catalyzed annulation of 2-bromo-N-arylbenzimidamide 70 with selenium and conversion of 71 into N-aryl indoles 72.

Scheme 26. Ring closure in 1-(2-bromoaryl)benzimidazoles 73 with Se powder.

Scheme 27. Selenation of tetraarylbutatrienes 75 with elemental Se to afford 1,2,5-triselenepanes 76 as compared to sulfurization.

Scheme 28. Synthesis of 2,5-dihydroselenophenes 80 by cyclization of 1,3-diene 79 under action of elemental Se.

Scheme 29. Synthesis of diselenolodiselenole derivatives 82 by action of elemental Se on diaryldiynes 81.

Scheme 30. Formation of selenophene moieties in compounds 6b and 84 by action of Se/N2H4.H2O/KOH on diynes 81a and 83.

Scheme 31. Formation of selenophenes 6c, 6d and 6e by action of elemental Se on acetylenes 32b and 32c.

Scheme 32. Synthesis of 1,3-selenazolidin-2-ones 88 by cyclization of propargylic amines 87 with elemental Se in the presence of benzene-1,3,5-triyl triformate (TFBen).

Scheme 33. Synthesis of (E)-5-(iodomethylene)-1,3-selenazolidin-2-ones 89.

Scheme 34. Alternative method for synthesis of 1,3-selenazolin-2-ones 88 by cyclization of propargylic amines 87 with elemental Se and CO.

Scheme 35. Synthesis of selenazinan-2-one 92 by cyclization of homopropargylamine 91 under action of elemental Se/CO.

Scheme 36. Synthesis of 1,2,3,5,6,7-hexaselenacyclooctane 94 by Se–Cl exchange in 1-chloro-2,2-bis(diethylamino)ethene 93 under action of elemental Se.

Scheme 37. Synthesis of ebselen and its analogs 42 by cyclization of 2-halogen arylamides 97 under action of elemental Se.

Scheme 38. Synthesis of ebselen and its analogs 42 by Ni-catalyzed cyclization of arylamides 97 with elemental Se.

Scheme 39. Annulation of ortho-monoalkynyl-substituted perylene diimide (PDI) 98 with elemental Se.

Scheme 40. Cyclization of bis(diazo)octamethyldecane 100 and bis(diazo)octamethylundecane 104 under action of elemental Se.

Scheme 41. Synthesis of chiral selenazolines 107 under action of elemental Se on N-acyl-2-oxazolidinones 106.

Scheme 42. Synthesis of selenazolidinone 109 under action of elemental Se on oxazolidinone 108.

Scheme 43. Synthesis of 2H-3,1-benzoselenazin-2-one 111 by carbonylation of o-aminoacetophenone 110 with elemental Se/CO.

Scheme 44. Cyclization of sulfonium ylides 112a,b under action of elemental Se.

Scheme 45. Synthesis of 4-methylthio-5-(2-methoxycarbonylethylthio)-1,3-diselenole-2-selone 116a by action of elemental Se/CSe2 on lithum acetylenide generated from methylsulfanyl acetylene 32e or 1-methylsylfanyl-1,2-dichloroethylene 114.

Scheme 46. Synthesis of 4,5-alkylenedichalcogeno-substituted 1,3-diselenole-2-selones 116 from trimethylsilylacetylene 32f under action of BuLi/Se/CSe2.

Scheme 47. Cyclization of mesyloxymethyl-substituted β-lactams 118a-c with elemental Se/t-BuOK.

Scheme 48. Formation of bicyclo [2.2.2] system 122 by treatment of 1,4,2,5-diazadiborinine 121 with elemental Se.

Scheme 49. Synthesis of diboraselenirane 124 by action of elemental Se on diborene 123.

Scheme 50. Insertion of elemental Se into B≡B bond of diboryne 125.

Scheme 51. Synthesis of selenasilaborirane 128 by insertion of elemental Se into Si=B bond of silaborene 127 as compared to oxidation leading to 1,3,2,4-dioxasilaboretane 129.

Scheme 52. Insertion of elemental Se into P–B bond of 1-phospha-2-boraacenaphthene 130.

Scheme 53. Zr–Se(S,Te) exchange in carborane-fused zirconacyclopentane 133.

Scheme 54. Synthesis of 1,3-diselena-2,4-digermetane 137a as well as 1,3-ditellura-2,4-digermetane 137b and 1,3-dithia-2,4-digermetane 137c by action of elemental Se (Te, S) on hafnocene-based bicyclo [2.1.1]hexene germylene 135.

Scheme 55. Synthesis of 1,3-diselena-2,4-digermetane 137d by Se–Li exchange in triphenylgermyl lithium 138.

Scheme 56. Synthesis of bicycle Ge-containing polyselenides 141–143 by insertion of elemental Se into Ge=Ge bonds of 1,2-digermacyclobutene 139 and 1,2-digermacyclobutadiene derivatives 140.

Scheme 57. Synthesis of 1,2,3,4-triselenagermolanes 145a,b by insertion of elemental Se into C=Ge bond.

Scheme 58. Synthesis of selenadiphosphiranes 147 by insertion of elemental Se into P=P bond.

Scheme 59. Synthesis of selenaphospholanes 148–150 and Woollins reagent 151 by insertion of elemental Se into P–P bond of homocyclic pentamer (PhP)5.

Scheme 60. Synthesis of 4-, 5-, and 6-membered P, Se-containing heterocycles by oxidative addition of elemental Se to tetraphospholanes (PhP)4CR2 152a,b.

Scheme 61. Formation of 1,2,4-triselenolanes 157 and 1,3-diselenetanes 158 by interaction of elemental Se with methylenetriphenylphosphoranes 156.

Scheme 62. Synthesis of 2-aryl-1,3-benzoselenazoles 161 by decarboxylative cyclization of 2-chloronitrobenzenes and chloronitropyridines 159, and aryl- and hetaryl (pyridine and thiophene) acetic acids 160 in the presence of elemental Se.

Scheme 63. Synthesis of 2-substituted-1,3-benzoselenazoles 161 by three-component reactions of 2-iodoanilines 162, arylacetic acids 160, or arylmethyl chlorides 163, or aldehydes 164 and selenium powder.

Scheme 64. Synthesis of selenopheno [2,3-b]indoles 167 by three-component reaction of 1-substituted indoles 165, aromatic ketones 166 and elemental Se.

Scheme 65. Synthesis of indolyl-substituted benzoselenophenes 56 by three-component reaction of 2-aryl- and hetaryl-substituted indoles 165, aromatic ketones 166 and elemental Se.

Scheme 66. Synthesis of 2-amino-3,1-benzoselenazines 170 by three-component cascade reaction of isocyanides 168, secondary amines 169 and elemental Se.

Scheme 67. Synthesis of 1,2,4-selenadiazol-5-amine derivatives 172 by three-component reaction of isocyanides 168, arylamidine hydrochlorides 171 and elemental Se.

Scheme 68. Synthesis of 2-imino-4-alkylidene-1,3-oxaselenolanes 175 by three-component reaction of isocyanides 168, alk-2-yn-1-ols 173 and elemental Se.

Scheme 69. Syntheses of naphtho [2,1-d][1,3]selenazoles 178 and naphtho [1,2-d][1,3]selenazoles 179 by molecular iodine-catalyzed three-component cascade reactions from naphthalen-2(1)-amines 176 or 177, aldehydes 164 and elemental Se.

Scheme 70. Formation of zwitterionic azaselenadithiapentalene 182 by four-component reaction of (2-benzimidazolyl) acetonitrile 180, isothiocyanate 181, CS2 and elemental Se.

Figure 1. [1,3]thiazolo [4′,5′:4,5]pyrimido [1,6-a]benzimidazol-2(3H)-thione.

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