Triton-X-100 as an Organic Catalyst for One-Pot Synthesis of Arylmethyl-H-phosphinic Acids from Red Phosphorus and Arylmethyl Halides in the KOH/H2O/Toluene Multiphase Superbase System

Abstract

Triton-X-100, a polyethylene glycol 4-(tert-octyl)phenyl ether, has been found to be an active micellar organic catalyst for the one-pot selective synthesis of arylmethyl-H-phosphinic acids in up to 65% yields by the direct phosphinylation of arylmethyl halides with red phosphorus in the KOH/H₂O/toluene multiphase superbase system. The catalyst demonstrates a good recyclability. As a result, an expeditious method for the chemoselective synthesis of arylmethyl-H-phosphinic acids—versatile sought-after organophosphorus compounds—has been developed. The synthesis is implemented via direct alkylation/oxidation of red phosphorus with arylmethyl halides, promoted by superbase hydroxide anions using Triton-X-100 (a commercial off-the-shelf organic recyclable micellar catalyst). The reaction comprises the hydroxide anions-assisted disassembly of P_red 3D polymer molecules triggered by the separation from the potassium cation in ordinary crown-like micelles to produce polyphosphide anions in aqueous phase. Further, polyphosphide anions are alkylated with arylmethyl halides in organic phase in the presence of the catalytic Triton-X-100 reverse micelles and alkylated polyphosphide species undergo the double hydroxylation. The advantages of the strategy developed include chemoselectivity, benign and accessible starting reagents, catalyst recyclability, and facile one-pot implementation.

1. Introduction

Organic-H-phosphinic acids represent a sought-after class of organophosphorus compounds, which find application as pharmaceutical intermediates [1,2,3], rare-earth and actinoid elements extraction and separation [4,5,6,7], reagents for hydrometallurgy [8,9], ligands for metal complex catalysts [10], flame retardants [11], and nanoparticle stabilizers [12].

Since benzyl-H-phosphinic acids are more active building blocks compared to alkyl-H-phosphinic acids, this family of intermediates is used in fine and industrial organophosphorus chemistry. They are employed as starting materials for the preparation of valuable phosphonic, diorganylphosphinic, and aminophosphinic acids, or their derivatives [13,14,15,16] and for other important organophosphorus compounds [17,18] including optical active ones [19] for medicine [20,21,22,23,24], in particular, for the design of pro-drugs of NAALADase inhibitors and agents to treat prostate cancer [25,26], as well as MRI [27] or specific targeting of tumors with radioactive isotopes [28]. Benzyl-H-phosphinic acids and their functionally substituted derivatives are exploited as catalysts in polycondensation reactions [29], to create GABA_B agonist [22] and aminopeptidaze inhibitors [30], to prevent the serious complications of heparin therapy (HIT, HITT) [31].

The existing multistep methods for their synthesis include the hydrolysis of toxic and irritative arylmethyl dichlorophosphines [32], oxidations of flammable primary phosphines [33], reaction (EtO)₂PCl with Grignard reagents [34], reactions of hypophosphorous acid (H₃PO₂), hypophosphites [35] and alkyl (or TMS) phosphinates or phosphonates with benzylic alcohols [36] or alkyl halides [14,22,30,37,38,39,40,41], mainly in the presence of Pd catalysts [36,40,41], under microwave activation [40] or flammable and moisture- and air sensitive organometallic reagents (BuLi or Grignard reagents) [34,37] (Scheme 1).

The application of elemental phosphorus (especially its benign and accessible red analogue) as an alternative phosphorylating agent to form the C-P bond is now emerging as an environmentally and commercially appealing strategy for the preparation of organophosphorus compounds [42,43,44,45,46]. The key stage of this strategy is the auto-redox reaction of elemental phosphorus promoted by activated (superbasic) hydroxide anions (Scheme 2):

On a pro forma basis, the auto redox process is the reduction of one P⁰ atom with another P atom, which is consequently oxidized to P³⁺.

Superbasic hydroxide ions are generated in the systems consisting of MOH (M = Na, K) and strongly polar non-hydroxylic solvents (DMSO, HMPA, NMP, DMF etc.), capable of forming complexes with alkali metal cations thereby releasing ⁻OH anions. A similar activation of the alkali metal hydroxides takes place under phase-transfer conditions (PTC), when poorly solvated ⁻OH ions are transferred by bulky ammonium cations from the aqueous to organic phase. In all these cases, the pKa values of these systems lie within 20–30 logarithmic units that correspond to the superbasicity level [47]. During the above integrated redox process (Scheme 2), along with two ultimately reduced and oxidized phosphorus-containing species (P³⁻, P(OH)₃), other anions with phosphorus atoms in a lower reduced state such as P²⁻OH, P¹⁻(OH)₂ are formed. These anionic intermediates are taken up by different electrophiles (organic halides, alkenes or alkynes or protons) to deliver organic phosphines [45,48,49], phosphine oxides [45,50], or phosphinic acids [45,51,52,53]. The chemoselectivity of these reactions is determined by the electrophile structure and depends on the process conditions (the nature of the base, activity of the hydroxide ion, catalyst character, reaction duration, temperature, and the order of the reactants feeding).

Recently, a simple and straightforward synthesis of long-chain alkyl-H-phosphinic acids (Alk = C₄–C₁₈) based on the alkylation/oxidation of red phosphorus (P_red) in the two-phase aq.KOH (50%)/toluene system with various phase-transfer catalysts (TEBAC, TBAB, DB18C6) [53,54] or micellar catalysts (CTAB, alkyl-PEGs) [54,55] has been reported (Scheme 3). However, as yet, the less accessible class of organophosphorus compounds from those obtained by the above elemental phosphorus-based approach remains alkyl-H-phosphinic and -phosphonic acids (21–90% yields) [54,55], while, unfortunately, arylalkyl-H-phosphinic acids—which are much more practically valuable—have been synthesized by this method usually in complex mixtures of products with low yields (PTC method), or have not been synthesized at all (micellar method). As follows from the above, the chemoselectivity issue remains a challenge for further advancement toward the synthesis of benzyl-H-phosphinic acids.

As to other approaches to the elemental phosphorus-based synthesis of organophosphorus acids, it is worthwhile to mention that the reaction of red phosphorus with benzaldehyde in conc. 57% HI (reflux, 6 h) gave benzylphosphonic in 25% yield; the possible formation of phosphinic acid being not observed [42] Scheme 4.

In the present work, we disclose a novel catalytic approach to address the above challenge. The implementation of this approach was exemplified by the synthesis of arylmethyl-H-phosphinic acids using, for the first time, the micellar catalysis with alkylaryl ether of polyethylene glycols for the alkylation/oxidation of P_red in arylmethyl halides/aq.KOH (50–55%)/toluene multiphase system.

2. Results

2.1. Optimization of the Reaction Conditions

The synthesis of arylmethyl-H-phosphinic acids by the straightforward phosphinylation of benzyl halides 1a–f with red phosphorus (Scheme 5) has been implemented, as mentioned above, in multiphase systems comprising saturated (~50%) aq.KOH and toluene (two liquid phases), powdered red phosphorus (P_red), and micellar aggregates (solid phases) and a phase-transfer catalyst (cationic, anionic, and nonionic surfactants, Figure 1) at 85–90 °C for 4 h under an argon blanket (until the complete conversion of BnCl).

First, we studied the effect of the catalyst nature on the yield of benzyl-H-phosphinic acid using benzyl chloride as the reference electrophile (Scheme 5, Table 1). The constant reaction parameters were the loading of elemental phosphorus (0.1 g-atom, 3.1 g) and KOH·0.5H₂O (20 g), as well as volumes of water and toluene (13 and 60 mL, correspondingly). The selective representative results of the experiments are collected in Table 1.

The preliminary experiments showed that the yield of benzyl-H-phosphinic acid most strongly depended on the catalyst nature. In Table 1, we focused on this very reaction parameter, while other reaction conditions were placed in the footnotes. As seen from Table 1, the yield of benzyl-H-phosphinic acid progressively increased upon the changing nature of the catalyst from ionic (Table 1, entries 1–8) to nonionic (Table 1, entries 9–15). Screening of catalysts showed that the phosphinylation under the action of 5 mol% TBAB and TEBAC ensured only yields of 4–7% (entries 2 and 3), while Bu₃N, [Ph₄P]Br, and even some micellar catalysts (DPB, SDS, Stearate Na), were not effective at all (Table 1, entries 1, 4, 5, 7, 8). The most active ionic catalyst of the phosphinylation studied proved to be cetyltrimethylammonium bromide (CTAB), i.e., a typical micelle-forming agent (Table 1, entry 6). This is likely due to the competitive delivery of hydroxide anions to organic phase via the normal phase-transfer mechanism that results in the additional side transformation of BnCl to secondary and tertiary phosphine oxides according to [45], and also to BnOH and dibenzyl ether.

Surprisingly, neither dibenzo-18-crown-6 (DB18C6) nor polyethylene glycols (PEG_600,1000) and its alkyl esters (Hex₂PEG₆₀₀, DodecMPEG₅₅₀)—which turned out to be very active in the reaction phosphinylation of long chain alkyl bromides as micellar catalysts (Scheme 3) [54,55]—had any significant effect on the yield of acid 2a compared with classic PTC surfactants (cf. in Table 1, entries 10–12 and 2, 3, 9).

However, when we used oligoethoxylated alkylphenols as catalysts, the acid yields were already 50% (for Triton-X-100) and 16% (for Nonoxynol-12). Probably, such a different result can be explained by the nature of micelles formed in the organic phase (vide infra, Section 3).

Subsequently, a brief optimization of the reaction conditions with CTAB and Triton-X-100, as the best catalysts (Table 2), showed that the yield and selectivity of benzyl-H-phosphinic acid most strongly depended on temperature and basicity (here the KOH/H₂O ratio).

As follows from Table 2, Triton-X-100 is the best catalyst for the phosphinylation of benzyl chloride, and the reaction should be conducted at ≥95 °C and reactants molar ratio: BnCl:P:KOH:H₂O as ~1:5:15:36 and the Triton-X-100 concentration should be no less 2.5 mol%. During our previous research in this area, toluene was shown to be the organic phase of choice for similar phosphinylation reactions.

2.2. Study on Substrate Scope

Further, to verify the generality of the phosphinylation in question, we have extended the above elaborated optimal conditions for the synthesis of phosphinic acid 2a (Table 2, entry 12) over other arylmethyl halides. The results of the experiments are summarized in Scheme 6. In all the cases, the conversion of benzyl halides 1a–g (~100%) exceeded noticeably the yields of H-phosphinic acids (Table 1 and Table 2), resulting from both the side reactions of arylmethyl halides with hydroxide ions and their participation in double or triple alkylation of the polyphosphide anions to afford minor amounts of the corresponding diorganyl- or triorganylphosphine oxides. The latter processes were previously employed for the synthesis of di- and triarylmethylphosphine oxides from red phosphorus in aqueous KOH under phase-transfer conditions [45]. For instance, the above side processes were typically expressed in the case of 4-methoxybenzyl chloride. The expected 4-methoxybenzyl-H-phosphinic acid (2f) was formed in about 10% yield, while the major products, tris(4-methoxybenzyl)phosphine oxides were formed in 26% yield and small amounts of the corresponding primary 4-benzylphosphine (δ_P = −122 ppm, ¹J_PH = 194 Hz) and dibenzylphosphine oxide (δ_P = 36.7 ppm, ¹J_PH = 469 Hz), were also detected. Such a low yield of 4-methoxybenzyl-H-phosphinic acid is explained by the high reactivity of 4-methoxybenzyl chloride, which is prone to polycondensation to form oligomers [56].

The substituents Me (in methylene moiety), halogens (Br, F) in phenyl ring, and naphthyl fragment of arylmethyl halogenides 1b–f diminished the yield of H-phosphinic acids 2b–f. The replacement of chlorine by bromine at the methylene fragment of arylmethyl halogenide considerably reduced the yield of acid 2a (7%) because of the greater reactivity of the latter, which resulted in the predominant phosphinylation up to tribenzylphosphine oxide. As for arylmethyl iodides, it is inappropriate to use them in this process, because they are expensive, toxic, and undergo side processes associated with their easier solvolysis. The branched diphenylmethyl chloride, under the above conditions, did not participate in the phosphinylation studied at all, thus indicating the significant steric effect in this reaction (Scheme 7).

Overall, the strongly basic conditions, which generate superbasic hydroxide anions, do not allow the substrate scope to be extended over the substituents sensitive to alkalis. For instance, when we tried the reaction with 3-acylbenzylchloride, only a polymer product containing no phosphorus was isolated, probably due to aldol/crotone condensation of the ketone function accompanied by solvolysis of the chloromethyl group [57]. In an attempt to extend the catalytic process studied over heterocyclic analogues of benzyl chlorides, we tried in this synthesis 2-chloromethylthiophene, which under optimum conditions gave 2-thienylmethyl-H-phosphinic acid in 6% yield only, while the major product was tris(2-thienylmethyl)phosphine oxide (40% yield). This indicates that the chloromethyl group in this case is more electrophilic compared to that in benzyl chloride, hence, the nucleophilic substitution of the chlorine atom with P-centred anions becomes preferable. Thus, it is obvious that to extend the synthesis over the heterocyclic representatives, a special systematic exploration is required.

3. Discussion

The absence of regularity in the catalyst structure/product yield relationship implies a sophisticated interplay between the micelles of difference structures and, hence, their catalytic activity. The results of the previous paper [55] (vide supra, Scheme 3) allow us to infer that the catalysts of choice for the process studied are dialkylated PEGs with longer alkyls and a molecular mass within 600–1000. These Alkyl_-PEGs were formed in situ by the phosphinylation of n-AlkBr with PEGs. Therefore, we assumed that the benzyl ethers of PEGs that should be formed in situ under the conditions of this reaction could also be active catalysts. However, Table 1 shows that this is not the case.

The fact that the steric factors of catalysts actually matter in this reaction follows from the small yields (~10%) of the corresponding H-phosphinic acid (2a) obtained when various alkyl-(or benzyl)PEGs were used (see Table 1). If the PEG contained either n-alkyl or phenyl groups as a tail, then the acid yield usually did not exceed 15% (Table 1, entries 10–14). Only when PEG had the brunched alkylphenyl tail (t-OctC₆H₄), the acid yield achieved 50% (Table 1, entry 15). Apparently, the branched bulky substituent (t-OctC₆H₄) in the catalyst prevented the penetration of the second molecule of arylmethyl chloride 1 into the micelle sphere, which once again shows the importance of steric effects for reaching the target chemoselectivity of the process.

As far as the mechanism is concerned, the synthesis of H-phosphinic acids includes the two main reactions: (i) the splitting of the P-P bond of P_red 3D polymer by the activated hydroxide anion to produce polyphosphide phosphorus-centered anions, and (ii) the benzylation of polyphosphide anions with arylmethyl halides. The first process (Scheme 8) occurs in aqueous KOH (50–55%) and is catalyzed by micelles A composed of a hydrophobic alkylphenyl core (inside of the micelles) and an outer hydrophilic PEG corona with K⁺ cations, alike to the crown-ether case.

Consequently, hydroxide ions, which normally accompany potassium cations, are departed from the cations and, hence, become more active. These more basic hydroxide anions cleave the P-P bond which allows for the disassembly of P_red polymer to oligomeric polyphosphide anions B, which are transferred to the toluene phase as the reverse micelle architecture C (having the hydrophilic PEG core and hydrophobic alkylphenyl corona). This anion transfer to organic phase likely passes through the liomesophase aggregation.

In organic phase, alkylation/oxidation of the polyphosphide anions proceeds, promoted by the separation from K⁺ cation in reverse micelles C. Fundamentally, this phosphinylation is the nucleophilic substitution of the halogen atom in arylmethyl halides 1a–g by activated polyphosphide anions (Scheme 9), which successfully compete with hydroxide anions as more nucleophilic (super-nucleophiles) due to α-positioned phosphorus atoms (so-called “α-effect”). The benzylated polyphosphide species D undergo the further P-P bond cleavage by the activated hydroxide anions, which regioselectively attack the phosphorus atom bound with benzyl radical as it is more positively charged due to the higher electronegativity of the adjacent carbon atom. Moreover, this phosphorus atom is not so much sterically screened as the adjacent phosphorus atom surrounded by the remaining 3D polymer fragments. The incompletely oxidized intermediates E thus generated are then subjected to the P-P bond cleavage with hydroxide anions, which again regioselectively attack the more positively charged oxidized P atom to liberate the anions of H-phosphinic acids 2a–f (with K⁺ cation); the latter being taken up by aqueous phase. The remaining phosphorus-centered anions F and G undergo benzylation with benzyl halides to deliver the target benzyl-H-phosphinic acids.

As is clear from the preceding analysis, such a type of micellar catalysis considerably differs from traditional phase-transfer processes implemented in our previous works [45]. The mechanism of this catalysis merges both normal and reverse micelles in a one-pot procedure. The normal micelles act in aqueous phase, promoting the P-P bond cleavage, while the reverse micelles are active in organic phase, catalyzing the nucleophilic substitution of the halogen atom in arylmethyl halogenides by polyphosphide anions, which are likely delivered there and inverted via mesa-phase. Furthermore, a beneficial property of this multi-function micellar catalyst (Triton-X-100, unlike ionic PTC-catalysts, Table 1) is that it can be recyclable, because it is mainly kept in organic phase and can be used for further phosphinylation. In the preliminary recyclability testing, we reached 34% and 20% yield of benzyl-H-phosphinic acid on the second and third cycles, respectively. The recyclability indices were improved when we skipped the aqueous work-up in the first step of the isolation procedure: the yields of benzyl-H-phosphinic acid became 47, 25 and 30% on the second, third, and fourth cycles, correspondingly. This shows that recyclability indices are open for further increasing. The observed yield dropping is likely due to the increasing content of the side secondary and tertiary phosphine oxides and the products of the benzyl halides solvolysis. Note, all the syntheses have been performed on the gram-scale, implying a good scaling-up of the method developed.

4. Materials and Methods

4.1. General Considerations

¹H, ¹³C and ³¹P NMR spectra were recorded on a 400.13, 100.61, and 161.98 MHz instrument, respectively, equipped with an inverse gradient 5 mm probe in CDCl₃ or DMSO. The ¹H NMR chemical shifts are expressed with respect to residual protonated CDCl₃ (7.27 ppm), which served as an internal standard. The ¹³C NMR shifts are expressed with respect to the CDCl₃ (77.0 ppm). An amount of 85% H₃PO₄/D₂O was used as the external standard for ³¹P NMR. The assignment of signals in the ¹H NMR spectrum was made using 2D COSY and NOESY experiments. Resonance signals of carbon atoms were assigned based on 2D ¹H-¹³C HSQC and ¹H-¹³C HMBC experiments. Coupling constants (J) were measured from one-dimensional spectra, and multiplicities were abbreviated as follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), and m (multiplet). IR spectra were recorded on a two-beam Bruker Vertex 70 spectrometer, in a microlayer from chloroform. The C, H microanalyses were performed on a Flash EA 1112 SHNS-O/MAS analyzer. Melting points were established using a Kofler micro hot-stage apparatus (Wagner & Munz GmbH, München, Germany). Commercially available red phosphorus (KSAN Sia, Riga, Latvia) was purified by consecutive washing with aq.NaOH (1–2%), H₂O, EtOH, and Et₂O to remove all acidic impurities, dried in vacuum at 25–30 °C to constant weight, and stored under inert atmosphere (N₂). Usual commercial toluene, KOH∙0.5H₂O (~15% water), catalysts, and arylmethyl halogenides were used without further purification. All reagents were commercially available.

4.2. General Procedure for the Synthesis of H-Phosphinic Acids 2

To a well-stirred (700–800 rpm) mixture of red phosphorus (0.10 g-atom), KOH∙0.5H₂O (0.308 mol), H₂O (0.25–0.60 mol), Triton-X-100 (2.5–10 mol%), toluene (40–50 mL), and arylmethyl halogenide (1) (5–20 mmol) in toluene (10 mL) was added within 1 min at the appropriate temperature (see Supplementary Materials), and the suspension was stirred for 3 h under Ar. Following this, the reaction mixture was cooled to r.t., and diluted in 2x with distilled water. The reaction mixture was filtered to remove unreacted red phosphorus and the aqueous alkaline layer was thoroughly separated, washed with DCM (3 × 10 mL) to remove the residual catalyst, acidified with hydrochloric acid to pH 2–3, and extracted with chloroform (3 × 30 mL). The chloroform extract was dried with Na₂SO₄, the solvent was removed under reduced pressure and the pure acid was dried under vacuum.

5. Conclusions

A scrutinized screening of fifteen ionic and nonionic compounds (quaternary ammonium salts, salts of higher fatty acids, ethers of oligoethylene glycols) for the catalysis of red phosphorus arylmethylation by arylmethyl halides in the concentrated aqueous KOH with toluene organic phase has revealed that 4-(tert-octyl)phenyl ether of oligoethylene glycol (Triton-X-100) is a catalyst of choice ensuring, in case of benzyl chloride, chemoselective synthesis of the target benzyl-H-phosphinic acid in 65% yield, completely avoiding the formation the side benzylphosphonic and dibenzylphosphinic acids. The reaction tolerates various arylmethyl chlorides and bromides having substituents at both methyl and phenyl groups as well as naphthylmethyl chloride, securing 10–65% yields of the corresponding H-phosphinic acids.

The phosphinylation found represents a complex multi-phase micellar catalytic process, which is a combination of the disassembling of the P_red 3D polymer by the activated hydroxide anion to shorter polyphosphide species in aqueous phase, and nucleophilic substitution of halides (in arylmethyl halides) by polyphosphide anions followed by further reactions with hydroxide anions that take place in organic phase.

Thus, a new expedient chemoselective approach to the synthesis of the sought-after benzyl type H-phosphinic acids using a commercially available micellar catalyst (Triton-X-100) has been developed, thereby paving a shorter, more economic, and environmentally safer way (compared to the existing synthesis of similar compounds) to the valuable organophosphorus reagents for biomedical application.

Author Contributions

V.A.K. and S.F.M. designed the experiments of the project; N.A.B. and R.I.F. performed the experiments; V.A.K. drafted this manuscript; A.I.A. made assignments of NMR signals and established the structure; V.A.K. and B.A.T. proofread the manuscript and supervised the studies. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the Baikal Analytical Centre of Collective Use of the SB RAS for the equipment.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figure, Schemes and Tables

Scheme 1. Selective examples of known methods of benzyl-H-phosphinic acids syntheses.

Scheme 2. Auto-redox reaction of elemental phosphorus under superbasic conditions.

Scheme 3. Phosphinylation of long-chain alkyl bromides with red phosphorus.

Scheme 4. Reaction of red phosphorus with benzaldehyde in acidic conditions.

Figure 1. Representative surfactants.

Scheme 5. Phosphinylation of benzyl chloride with red phosphorus in superbasic multiphase systems.

Scheme 6. Scope of Triton-X-100-catalyzed phosphinylation of arylmethyl chlorides with red phosphorus in superbasic multiphase systems. Reaction conditions are as follows: 1 aq.KOH (~55%); 2 aq.KOH (~70%); 3 aq.KOH (~50%); 4 2.5 mol% of the catalyst used; 5 5 mol% of the catalyst used.

Scheme 7. Attempt of phosphinylation of diphenylmethyl chloride with red phosphorus.

Scheme 8. Triton-X-100-catalyzed generation of polyphosphide anions by Pred disassembling by the activated hydroxide anions.

Scheme 9. Reverse micelle-catalyzed benzylation/oxidation of polyphosphide anions in the organic phase.

Table 1

Catalyst screening in the phosphinylation of benzyl chloride 1a with red phosphorus ¹.

Entry	Catalyst	Conversion of Pred, %	Content of Acids in the Crude Product
			2a	3	4
1	Bu3N	99	0	0	0
2	TBAB	40	7	1	11
3 2	TEBAC	59	4	1	5
4	[Ph4P]Br	98	0	0	0
5	DPB	97	0	0	0
6	CTAB	70	31	2	6
7 3	Stearate Na	77	0	0	2
8	SDS	89	0	0	0
9	DB18C6	74	10	0	6
10	PEG1000	70	15	2	2
11	Hex2PEG600	73	11	2	3
12	DodecMPEG550	96	27	trace	0
13	Bn2PEG600	87	11	trace	0
14	Nonoxynol-12	97	16	0	3
15	Triton-X-100	73	50	0	4

¹ Reaction conditions are as follows: (i) 0.1 g-atom (3.10 g) of P_red, 0.02 mol (2.54 g) of BnCl (1a) (feeding time 1.5–2 h), 0.31 mol (20.00 g) of KOH·0.5H₂O, 13 mL of H₂O, 60 mL of toluene, 5 mol% of a catalyst, argon atmosphere, 85–90 °C, 4 h; (ii) The aqueous layer of the reaction mixture was separated and treated by 35–40 mL aq.HCl (35%). ² Duration of the reaction was 5 h. ³ Duration of the reaction was 6 h.

Table 2

Optimization of the reaction parameters (phosphinylation of benzylchloride 1a with red phosphorus).

Entry	Catalyst(mol%)	Temp., °C	Feeding Time of BnCl	Reaction Time	Concentration of KOH, %	Conversion of Pred, %	31P NMR Yield of Acids
							2a	3	4
1	CTAB (5)	85–90	2 h	6	50	99	31	2	6
2	CTAB (5)	90–95	1.25 h	6	50	98	16	0	0
3	CTAB (5)	90–95	0.7 h	1	50	47	8	3	9
4	CTAB (5)	90–95	1.5 h	3	50	48	32	0	0
5	CTAB (5)	95–97	1 h	3	50	61	34	0	0
6	Triton-X-100 (10)	80	1 min	3	55	76	40	0	0
7	Triton-X-100 (5)	85–90	2 h	4.5	55	71	41	2	4
8	Triton-X-100 (2.5)	80–85	2 h	5	55	83	3	1	2
9	Triton-X-100 (1)	80–85	2 h	5	50	92	2	1	3
10	Triton-X-100 (5)	85–90	1.5 h	4	50	73	50	0	4
11	Triton-X-100 (2.5)	85–90	2 h	6	55	92	63	0	2
12	Triton-X-100 (2.5)	95–97	1 min	3	55	97	65	0	0

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040720/s1: experimental procedures, spectral and analytical data, copies of NMR spectra [58,59,60].

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