A C-H bond activation-based catalytic approach to tetrasubstituted chiral allenes

Abstract

Enantioselective synthesis of fully substituted allenes has been a challenge due to the non-rigid nature of the axial chirality, which spreads over three carbon atoms. Here we show the commercially available simple Rh complex may catalyse the CMD (concerted metalation/deprotonation)-based reaction of the readily available arenes with sterically congested tertiary propargylic carbonates at ambient temperature affording fully substituted allenes. It is confirmed that the excellent designed regioselectivity for the C-C triple bond insertion is induced by the coordination of the carbonyl group in the directing carbonate group as well as the steric effect of the tertiary O-linked carbon atom. When an optically active carbonate was used, surprisingly high efficiency of chirality transfer was realized, affording fully substituted allenes in excellent enantiomeric excess (ee).

ARTICLE

Received 7 May 2015 | Accepted 25 Jun 2015 | Published 6 Aug 2015

DOI: 10.1038/ncomms8946 OPEN

A CH bond activation-based catalytic approach to tetrasubstituted chiral allenes

Shangze Wu1, Xin Huang1, Wangteng Wu1, Pengbin Li1, Chunling Fu1 & Shengming Ma1

Enantioselective synthesis of fully substituted allenes has been a challenge due to the non-rigid nature of the axial chirality, which spreads over three carbon atoms. Here we show the commercially available simple Rh complex may catalyse the CMD (concerted metalation/deprotonation)-based reaction of the readily available arenes with sterically congested tertiary propargylic carbonates at ambient temperature affording fully substituted allenes. It is conrmed that the excellent designed regioselectivity for the CC triple bond insertion is induced by the coordination of the carbonyl group in the directing carbonate group as well as the steric effect of the tertiary O-linked carbon atom. When an optically active carbonate was used, surprisingly high efciency of chirality transfer was realized, affording fully substituted allenes in excellent enantiomeric excess (ee).

1 Laboratory of Molecular Recognition and Synthesis, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China. Correspondence and requests for materials should be addressed to S.M. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 1

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946

Allenes have been becoming more and more important due to their presence in nature and the recent demonstration of their great potentials in modern organic chemistry

with very nice reactivities, which led to their versatile applications in the efcient total syntheses of some natural products and drugs14. Thus, the highly selective synthesis of allenes is of great importance510. On the basis of recent advancements, the enantioselective synthesis of 1,3-disubstituted or 1,1,3-trisubstituted allenes has been at least partially solved6, however, enantioselective synthesis of tetrasubstituted is still a challenge610. Here we propose that the well-established CMD (concerted metalation/deprotonation) process of arenes1116 would generate organometallic intermediates in situ, which would, in principle, react with 2-alkynylic derivatives with an appropriate leaving group to afford allenes via regioselectivity-reversed insertion and thermodynamically non-favoured b-elimination (Fig. 1b). However, the challenge is the regioselectivity of the carbometalation of the CC triple bond with the in situ generated aryl metallic species1722 since the reported insertion reaction with the CC triple bond in propargylic alcohols affording the [4 2]-products with the

undesired regioselectivity18. To achieve our goal, we proposed to reverse the regioselectivity by installing a directing entity in the propargylic leaving group, thus avoiding the formation of undesired B-type insertion intermediate18. In addition, it should be noted that stereospecic b-elimination of the targeted intermediate A providing highly optically active allenes is challenging as the Rh-catalysed reaction of stoichiometric amount each of aryl boronic acids and optically active propargylic acetates with forming allenes in an extremely low efciency of chirality transfer due to the mixed syn- and anti-non-stereospecic b-elimination2325. In such a well-designed process, the oxidation state of the metal in the catalyst would remain the same, thus, making it catalytic in [M] and free of any oxidants for the regeneration of catalyst in many of CH functionalization reactions1116.

Herein, we wish to report the realization of such a concept for the synthesis of tetrasubstituted allenes via the commercially available [Cp*RhCl2]2 catalysed coupling between readily available essential chemicals, that is, arenes and 2-alkynylic carbonates with a surprisingly high efciency for point-to-axial chirality transfer. After careful studies, it is believed that the directing group and the steric effect of the tertiary 2-alkynylic carbonates are responsible for the reversed regioselectivity for the insertion of the CC triple bond.

ResultsIdentication of the leaving group. With the purpose of synthesizing the most challenging tetra-subtituted allenes, we rst tried to identify a proper leaving group for the tertiary propargylic alcohol (Fig. 2): When N-methoxybenzamide 1a was reacted with tertiary propargylic methyl ether 2a1 in a mixed solvent of MeOH and water (20/1) under the catalysis of [Cp*RhCl2]2 at room temperature, no allene was formed with 1a being recovered; when we introduced a phosphate as leaving group (2a2), interestingly the expected tetrasubstituted allene 3aa was afforded in 8% NMR yield!18 When propargylic acetate 2a3 was used, the yield was improved to 46%; with these results in hand, we envisioned to have a methoxy group to replace the methyl group in the acetyl unit to increase the electron density of the carbonyl oxygen, that is, carbonate: to our delight, the carbonate 2a was indeed the best and the yield was further improved to 76%. After further optimization of the parameters of the reaction, the reaction of 1.2 equiv. 1a and 2a in a mixed solvent of MeOH and water (20/1) under the catalysis of [Cp*RhCl2]2 with 30 mol% NaOAc as base was chosen as the optimized reaction conditions for further study (for details of optimization, see Supplementary Table 1).

Substrate scope. The reaction proceeded well on a gram scale, affording 3aa in 82% yield (Fig. 3). Having the optimized reaction conditions in hand, the generality of the reaction was then investigated. Electron-donating groups such as t-butyl and methoxy as well as electron-withdrawing groups such as CF3 and

F in the aryl unit were all tolerated in the amides giving the corresponding fully substituted allenes in good to excellent yields (Fig. 3; 3aa, 3ba, 3ca, 3da, 3ea, 3bf and 3cf). It was noteworthy that when a meta-substituted 1d was used, the less hindered CH bond was exclusively functionalized and an o-uorine atom did not hamper the reaction affording 3ea in 79% yield (Fig. 3, 3da and 3ea). The R1, R2, and R3 groups in the 2-alkynylic carbonate could be alkyl or aryl groups (Fig. 3; 3ac, 3ad, 3ae, 3af, 3ag, 3ah, 3bf and 3cf). Substrates with sterically hindered i-Bu or o-tolyl groups also worked (Fig. 3, 3ac and 3ag).

Compatibility of functional groups. To show the robustness of our newly developed catalytic allene synthesis strategy, substrates with synthetically useful yet sensitive functional groups were explored without any protection: to our delight, Cl, Br, CO2Me,

CN and even unprotected free alcohol were all well tolerated,

a

CMD process of arenes and the subsequent reaction with a CC triple bond

RH

Cat. [M]

R1

R2

R2

R1 R1

R2 R2

RH

R[M]

R[M]

+

R M M R

b

This work: RM formation via CMD-regiospecific insertion affording A-stereospecific -elimination

catalytic in [M]

R1

R3

X

R1

R2

Cat. [M]

R2

DG Y

DG

R3

R

R3 R3

R2

R1

R1

R M

X

M R

X

DG

A

B

Figure 1 | Carbometallations of alkynes. (a) CMD-based reaction of RH with alkynes. (b) This work: new concept and challenge for allene synthesis.

2 NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946 ARTICLE

O

O

OMe

OMe

N H

N H

+ Bu

2 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, room temperature

Bu

LG

1a 2

3aa

Bu

Bu

Bu

Bu

OMe

O O

O

P

O

O

OEt

O

OEt

OMe

2a1 2a1 2a3 2a

Yields were determined by 1H NMR using CH2Br2 as internal standard.

Figure 2 | The effect of leaving groups. A carbonate was found to be the most efcient.

+ R R

R

no reaction

8% 46% 76%

O

O

OMe

OMe

R

N H

R

R

R

OCO Me

2 mol% [Cp*RhCl2]2a

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C

1 2

R

R

3

O

OMe

3aa, R = H, 80% (82%)b

3ca, R = 4-OMe, 74%c

3ba, R = 4-t-Bu, 73%

3ea, R = 2-F, 79%

3da, R = 3-CF3, 83%

O

Bu

OMe 3ac, R1= Bu, R2 = i-Bu, 68%

3ae, R1= Ph, R2 = Me, 80%g3af, R1= Ph, R2 = Et, 79%g3ag, R1= o-MeC6H4, R2 = Me, 77%g

3ad, R1= n-C5H11, R2= Ph, 56%d,e,f

R

NH N

H

R

O

O

O

OMe

OMe

OMe

N H

NH N

H

N H

Bu

t-Bu

Ph

MeO

Ph

Et

Et

Et

Et

3ah, 77%

3bf, 73%h 3cf, 74%i

Figure 3 | Substrate scope. Electron-withdrawing and electron-donating as well as bulky groups were all tolerated. aThe reaction was conducted with 1 (1.2 mmol), 2 (1 mmol), [Cp*RhCl2]2 (0.02 mmol), NaOAc (0.3 mmol), MeOH (6 ml), and H2O (0.3 ml) and monitored by TLC. bReaction was conducted on 7 mmol scale. cReaction was conducted at 10 C, 1a (1.3 mmol) and NaOAc (1 mmol) was used. dReaction was conducted at room

temperature. e1a (2 mmol), [Cp*RhCl2]2 (0.05 mmol) was used. fThe acetate was used instead because of the unstability of the carbonate. g1a (1.5 mmol), [Cp*RhCl2]2 (0.04 mmol) was used. h1b (1.5 mmol), [Cp*RhCl2]2 (0.04 mmol) was used. i1c (1.5 mmol), [Cp*RhCl2]2 (0.04 mmol) was used.

showing the broad synthetically attractive functional group compatibility (Fig. 4; 3fa, 3ff, 3ga, 3hi, 3ij and 3al). The structure of 3fa was further conrmed by X-ray diffraction study. (For details of the X-ray diffraction study of 3fa, see Supplementary Dataset 1 and pages 9697 in the Supplementary File.) Interestingly, even g-(1-alkynyl)-g-lactone 2m may be used to afford tetrasubstituted g-allenoic acid 3am directly in an atom-economic way (Fig. 4; 3am).

Regioselectivity. The regioselectivity is an important issue when 41 CH bonds could be activated. When N-methoxy-2-naphthylcarboxylic amide 1j was used to react with 2a, the less hindered CH bond was exclusively functionalized affording 3ja (Fig. 5, equation (a)). A heteroaryl N-methoxyamide 1k was also a suitable substrate and CH bond in the thiene moiety was

exclusively functionalized giving 3ka in 81% yield (Fig. 5, equation (b); also see: Fig. 3; 3da).

Other directing groups. With this protocol in hand, we reasoned that such a concept should be working also with directing groups other than the N-methoxy amide. In fact, arenes 1l and 1m with directing groups of pyridine and pyrazole could also react with 2a forming tetrasubstituted allenes 3la and 3ma. Thus, such a protocol may be applied to other type of substrates with functionalizable CH bonds (Fig. 6).

Synthetic applications. The synthetic applications for the formed products were also conducted: 3la could react with an extra propargyl carbonate, affording bis-allene 5, which showed the possibility of a stepwise allenylation (Fig. 7, equation (a)). The

NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 3

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946

O

OMe

O

2 mol% [Cp*RhCl2]2a

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C

R3

R

N H

OMe

N H

R2

R3

R1

R

+ R1

OCO2Me

1 2

R2

3

O

OMe

O

N H

OMe

Br

3fa, R1 = Bu, R2 = Me, 79% 3ff, R1 = Ph, R2 = Et, 79%b

R1

N H

3fa

MeO2C

Bu

R2

3ga, 76%

O

O

OMe

O

OMe

N H

OMe

N H

Cl

C5H11

N H

NC

OH

Et

3hi, 76%c

OTHP

3al, 70%

3ij, 58%

O

OMe

N H

Bu

O O

Bu

2m

3am, 50%

COOH

Figure 4 | Functional group compatibility. Many synthetically useful yet sensitive functional groups survived without any protection. aThe reaction was conducted with 1 (1.2 mmol), 2 (1 mmol), [Cp*RhCl2]2 (0.02 mmol), NaOAc (0.3 mmol), MeOH (6 ml) and H2O (0.3 ml) and monitored by TLC.

b1f (1.5 mmol), [Cp*RhCl2]2 (0.04 mmol) was used. c [Cp*RhCl2]2 (0.04 mmol) was used.

+ Bu

a H

O

H

O

OMe

OMe

N H

N H

Bu

H

OCO2Me

2 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C 23 h, 81%

1j 2a

1.2 equiv.

1.2 equiv.

3ja

b

OMe

H

O NH

H

O NH

OMe

Bu

H

+ Bu

S

OCO2Me

2 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C 36 h, 81%

2a

S

1k 3ka

Figure 5 | Regioselectivity of the reaction. (a) The less hindered CH bond was exclusively functionalized. (b) The thiophene CH bond was exclusively functionalized.

N-methoxyamide moiety in 3aa could direct a well-known [4 2]

addition with 1,2-diphenylethyne giving isoquinolinone 6 with allene intact (Fig. 7, equation (b))18.

DiscussionIn order to further study the mechanism, following experiments have been conducted. Firstly, N-methoxy-2-naphthamide 1j was

4 NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946 ARTICLE

a

N N Bu

+ Bu

Bu

N Bu

+ Bu

+

OCO2Me

4 mol% [Cp*RhCl2]2 30 mol% NaOAc

MeOH/H2O = 20/1, 10 C 60 h

2a

1l 3la

1m 3ma

3 equiv., 2.3276 g

0.9923 g

82%

4la

4%

b

5 mol% [Cp*RhCl2]2 30 mol% NaOAc

MeOH/H2O = 20/1, room temperature

65 h, 85%

N N

N N Bu

OCO2Me

2a

3 equiv.

Figure 6 | The scope of directing groups. Pyridine and pyrazole could also be suitable directing groups.

a

N Bu

2 equiv.

C8H17

N Bu

+ C8H17

OCO2Me

4 mol% [Cp*RhCl2]2 30 mol% NaOAc

2.5 mol% [Cp*RhCl2]2

30 mol% CsOAc

MeOH/H2O = 20/1, 60 C,20 h 70%

2n

3la

5

Ph

b

O

Ph

NH

N

H

OMe

O

Bu

+ Ph Ph(1.1 equiv.)

Bu

MeOH, 60 C, 24 h 60%

3aa 6

Figure 7 | The synthetic applications. (a) A stepwise allenylation. (b) A [4 2] addition affording isoquinolinone.

O

N H

OMe

+

O

Ph

20 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, room temperature, 44.5 h 12%, after recrystallization

Ph

NH

Ph

OCO2Me

MeO

7

Ph

1j 2o

O

Ph

O

O

NHOMe

NHOMe

NHOMe

Ph

Rh

Ph

Rh

MeO2CO

Ph

Ph

OCO2Me

Ph A-type

B-type

Figure 8 | Striking tertiary carbon atom effect. Secondary propargylic carbonate 2o only demonstrated the opposite regioselectivity.

treated with 20 mol% of [Cp*RhCl2]2 to in situ generate the naphthyl Rh species, its reaction with secondary propargylic carbonate 2o demonstrated the opposite regioselectivity, leading to the undesired reported B-type insertion regioisomer exclusively18, nally yielding the [4 2] product 7 observed by

Fagnou, Glorius, Rovis, Cramer and other groups (Fig. 8; for details of the X-ray diffraction study of 7, see Supplementary

Dataset 2 and page 108 in the Supplementary File)1722: the desired A-type insertion regioisomer, which would have led to the formation of allene via b-elimination, was not formed! Thus, we reasoned that there is a striking tertiary carbon atom effect here: in order to have the carbonyl group acts the directing group in this linear CC triple bond environment, the tertiary carbon atom in the carbonate may push the carbonyl group for its easier

NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946

coordination with Rh while having a weaker interaction with the to-be-inserted CC triple bond.

In the second place, to our delight, when chiral carbonate S-2f (98% ee) was applied, axial chiral allenes were formed with an excellent ee (98% ee) with a surprisingly high efciency of chirality transfer (Fig. 9). The absolute conguration of chiral 3af was determined as S by X-ray diffraction study and the absolute congurations of other chiral allenes S-3bf, S-3cf and S-3ff were dened by analogy. (For details of the X-ray diffraction study of S-3af, see Supplementary Dataset 3 and page 110 in the Supplementary File.) These results further excluded the possibility of oxidative addition of propargylic carbonate with Rh to form allenyl rhodium intermediate (leading to racemization)24,26,27, which would react with arene via a CMD process followed by reductive elimination to afford the allene.

The kinetic isotope effect was measured by parallel experiments of 1a and 1a-D5 with 2a. The kinetic isotope effect value of 2.1 demonstrated that CH activation should be the rate-determining step (Fig. 10a)28.

According to these results, a plausible mechanism has been proposed as shown in Fig. 10b. The rst step of the rate-determining CMD of arenes would form rhodabicyclic intermediate Int 1 (ref. 29), which is followed by coordination with the carbonyl oxygen of the carbonate forming Int 2. Subsequent reversed regiospecic insertion of the alkyne moiety18 directed by the carbonyl group in propargyl carbonate S-2f afforded with the carbonyl oxygen-coordinated rhoda-tricyclic Int 3 (refs 1114). The steric effect may set the carbonyl group for its better coordination with Rh and the subsequent dened coordination/insertion with CC triple bond leading to the formation of Int 3-type tricyclic intermediate, which directly underwent syn-b-oxygen elimination to form the nal product

S-3af and relieves the catalytically active Rh(III) species3037. It should be noted that usually anti-b-oxygen elimination was observed2325. This syn-b-oxygen elimination explains the excellent efciency for chirality transfer since it was reported that the Rh-catalysed reaction of optically active propargylic acetates with aryl boronic acids forming allenes in an extremely low efciency of chirality transfer2325.

In conclusion, a completely new protocol has been established for the efcient synthesis of allenes using the readily available arenes and 2-alkynylic carbonates as the starting materials at room temperature with very good yields. The reaction is

compatible with ambient air, moisture and a broad array of synthetically useful functional groups such as Cl, Br, CO2Me, CN, free alcohol and even acid. The formed tetrasubstituted allenes could be transformed to isoquinolinone as while as bis-allene products. In fact, in some cases, very minor amount of cycloisomerisation products were observed in the NMR analysis of the crude products. Excellent selectivity for alkyne insertion must be induced by combination of carbonyl directing group as well as the steric effect of tertiary carbon centre. The surprisingly high efciency of chirality transfer addresses the challenge for the synthesis of not-readily available highly optically active fully substituted allenes from a highly optically active carbonate. This protocol for the synthesis of allenes will trigger such reactions using other types of substrates with functionalizable CH bonds and propargylic alcohol derivatives, thus, will be of high interest in organic chemistry and related disciplines. Further studies in this area are being pursued in our laboratory.

Methods

Materials. [Cp*RhCl2]2 was purchased from Strem Chemicals. N-methoxybenzamides22 and known propargylic carbonates38 were prepared according to the literature procedures. Other commercially available chemicals were purchased and used without additional purication unless noted otherwise.

General spectroscopic methods. 1H NMR spectra were recorded on a Bruker-300 MHz spectrometer and 13C NMR spectra were recorded at 75 MHz. All 1H NMR experiments were measured with tetramethylsilane (0 p.p.m.) or the signal of residual CHCl3 (7.26 p.p.m.) in CDCl3 as the internal reference, 13C NMR experiments were measured in relative to the signal of CDCl3 (77.0 p.p.m.), and 19F NMR experiments were measured in relative to the signal of residual CFCl3(0 p.p.m.) in CDCl3. Infrared spectra were recorded from this lms of pure samples on sodium chloride plates for liquid or in the form of KBr discs for the solid samples. Mass and high-resolution mass spectrometry (HRMS) spectra were carried out in Electron Ionization (EI) mode. Thin-layer chromatography (TLC) was performed on pre-coated glass-back plates and visualized with ultraviolet light at 254 nm. Flash column chromatography was performed on silica gel. 1H NMR,

13C NMR and High Performance Liquid Chromatography (HPLC) spectra (for chiral compounds) are supplied for all compounds: see Supplementary Figs 182. See Supplementary Methods for the characterization data of compounds not listed in this part.

Synthesis of 3aa. To a dried Schlenk tube equipped with a Teon-coated magnetic stirring bar were added N-methoxybenzamide 1a (182.1 mg, 1.2 mmol), [Cp*RhCl2]2 (12.6 mg, 0.02 mmol), NaOAc (24.3 mg, 0.3 mmol), methyl (2-methyloct-3-yn-2-yl) carbonate 2a (198.5 mg, 1 mmol), MeOH (6 ml), and H2O

(0.3 ml) sequentially at room temperature. After being stirred for 19 h at 0 C, the reaction was complete as monitored by TLC. Filtration through a short column of

O

O

4 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C

OMe

OMe

N H

N H

OCO2Me

Et

+ Ph

R

Ph

R

1.5 equiv.

S-2f (98% ee)

Et

1

S-3

O

O

O

O

OMe

OMe

OMe

OMe

N H

N H

NH N

H

Ph

t-Bu

Ph

MeO

Ph

Br

Ph

Et

Et

Et

Et

S-3af, 77%, ee = 98% S-3bf, 76%, ee = 98% S-3cf, 72%, ee = 98% S-3ff, 77%, ee = 98%

Figure 9 | Highly entioselective synthesis of tetra-substituted allenes by chirality transfer. Excellent efciency of chirality transfer was found when using chiral carbonate S-2f. The reaction was conducted with 1 (0.3 mmol), 2 (0.2 mmol), [Cp*RhCl2]2 (0.008 mmol), NaOAc (0.06 mmol), MeOH (1.2 ml), and

H2O (0.06 ml) and monitored by TLC. The ees were determined by HPLC.

6 NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946 ARTICLE

a

O

OMe

OMe

+ Bu

N H

D /H D /H

1.2 equiv.

N H

OCO Me

2 mol% [Cp*RhCl2]2

30 mol% NaOAc

MeOH/H2O = 20/1, 0 C, 10 h

O

Bu

2a

1a and 1a-D5

3aa and 3aa-D4

Parallel experment

KIE2.1

O

b

OMe

N H

1a

OAc

HOAc

OCO Me

Et

Ph

S-2f

O

O

NOMe Rh

Cp*

NH

OMe

Et

Int 1

Int 3

CH metalation

Ph

O

NOMe

Rh

Int 2

S-3af

syn--oxygen elimination

*Cp

O

OMe

Ph O

Et

1a

O

NOMe

Rh

O

*Cp

OMe

O

Ph

Et

Figure 10 | Mechanistic issues (a) The kinetic isotope effect. A kinetic isotope effect value of 2.1 was measured. (b) Plausible mechanism.

The carbonate carbonyl and tertiary carbon atom effect accounts for the reversed regioselectivity and the excellent efciency of chirality was realized via syn-b-elimination.

silica gel (eluent: ethyl acetate 20 ml 3) and evaporation afforded the crude

product, which was puried by ash column chromatography on silica gel (eluent: petroleum/ethyl acetate/dichloromethane 10/1/0.1/ to 5/1/0.5) to afford 3aa

(218.6 mg, 80%): solid; melting point (m.p.) 66.968.1 C (hexane/ethyl acetate);

1H NMR (300 MHz, CDCl3) d 8.74 (brs, 1 H, NH), 7.58 (d, J 7.2 Hz, 1 H, ArH),

7.39 (td, J1 7.5 Hz, J2 1.3 Hz, 1 H, ArH), 7.327.21 (m, 2 H, ArH), 3.86 (s, 3

H, OCH3), 2.29 (t, J 7.1 Hz, 2 H, CH2), 1.76 (s, 6 H, 2 CH3), 1.491.27 (m, 4 H,

2 CH2), 0.89 (t, J 6.9 Hz, 3 H, CH3); 13C NMR (75 MHz, CDCl3) d 201.3, 167.9,

138.3, 131.5, 130.5, 129.1, 126.7, 102.9, 97.6, 64.3, 33.6, 30.0, 22.1, 20.3, 13.9; IR (neat, cm 1) 3,188, 2,956, 2,931, 2,875, 2,859, 1,953, 1,659, 1,590, 1,495, 1,465, 1,440, 1,299, 1,156, 1,035; MS (EI, 70 eV) m/z (%) 273 (M , 4.18), 242 (100); Anal.

Calcd for C17H23NO2: C 74.69, H 8.48, N 5.12. Found: C 74.89, H 8.62, N 4.89.

Synthesis of 5. To a dried Schlenk tube equipped with a Teon-coated magnetic stirring bar were added [Cp*RhCl2]2 (12.5 mg, 0.02 mmol), NaOAc (12.8 mg,0.15 mmol), 3la (277.5 mg, 1.0 mmol), 2n (126.8 mg, 0.5 mmol), MeOH (3 ml), and H2O (0.15 ml) sequentially at room temperature. The Schlenk tube was then equipped with a condenser. After being stirred for 20 h at 60 C, the reaction was complete as monitored by TLC (eluent: petroleum ether/ethyl acetate 20/1).

Filtration through a short column of silica gel (eluent: ethyl acetate 20 ml 3)

and evaporation afforded the crude product, which was puried by ash column chromatography on silica gel (eluent: hexane/ethyl acetate 100/1) to afford 5

(158.4 mg, 70%): oil; 1H NMR (300 MHz, CDCl3) d 8.61 (d, J 4.2 Hz, 1 H, ArH),

7.62 (td, J1 7.8 Hz, J2 1.8 Hz, 1 H, ArH), 7.307.22 (m, 2 H, ArH), 7.217.11

(m, 3 H, ArH), 2.011.90 (m, 4 H, CH2 2), 1.39 (s, 12 H, CH3 4), 1.321.10

(m, 16 H, CH2 8), 0.87 (t, J 6.8 Hz, 3 H, CH3), 0.80 (t, J 7.1 Hz, 3 H, CH3);

13C NMR (75 MHz, CDCl3) d 201.2, 160.0, 148.6, 139.92, 139.89, 138.1, 134.9, 127.5, 127.4, 125.9, 121.1, 103.3, 103.2, 95.6, 34.2, 33.9, 31.8, 29.8, 29.5, 29.3, 29.0,27.6, 22.6, 22.1, 20.4, 14.1, 14.0; IR (neat, cm 1) 3,059, 2,955, 2,926, 2,854, 1,962, 1,932, 1,588, 1,571, 1,561, 1,452, 1,419, 1,377, 1,361, 1,188, 1,023; MS (EI, 70 eV)

m/z (%) 455 (M , 18.96), 84 (100); HRMS Calcd for C33H45N (M ): 455.3552. Found: 455.3553.

Synthesis of 6. To a dried Schlenk tube equipped with a Teon-coated magnetic stirring bar were added 3aa (109.3 mg, 0.4 mmol), 1,2-diphenylethyne (78.6 mg,0.44 mmol), [Cp*RhCl2]2 (6.2 mg, 0.01 mmol), CsOAc (22.4 mg, 0.12 mmol), and MeOH (2 ml) sequentially at room temperature. The Schlenk tube was then equipped with a condenser. After being stirred for 24 h at 60 C, the reaction was complete as monitored by TLC (eluent: petroleum ether/ethyl acetate 3/1).

Filtration through a short column of silica gel (eluent: (dichloromethane/ethyl acetate 1/1) (20 ml 3)) and evaporation afforded the crude product, which was

puried by ash column chromatography on silica gel (eluent: dichloromethane/ ethyl acetate 20/1) to afford 6 (101.1 mg, 60%): solid; m.p. 199.0200.4 C

(hexane/ethyl acetate); 1H NMR (300 MHz, CDCl3) d 9.02 (s, 1 H, ArH), 7.44 (t, J 7.8 Hz, 1 H, ArH), 7.347.10 (m, 12 H, ArH), 2.33 (t, J 7.1 Hz, 2 H,

CH2), 1.76 (s, 6 H, CH3 2), 1.531.29 (m, 4 H, CH2 2), 0.90 (t, J 7.1 Hz, 3 H,

CH3); 13C NMR (75 MHz, CDCl3) d 199.0, 161.6, 142.7, 140.2, 137.2, 136.4, 135.1, 131.9, 131.7, 129.8, 129.0, 128.5, 128.3, 127.1, 124.9, 122.5, 116.8, 106.9, 95.6, 34.7,30.5, 22.5, 20.9, 14.2; IR (neat, cm 1) 3,453, 3,165, 3,027, 2,947, 2,929, 2,869, 2,849, 1,947, 1,642, 1,596, 1,584, 1,486, 1,462, 1,441, 1,311, 1,144; MS (EI, 70 eV) m/z (%)

NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 7

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946

419 (M , 31.3), 376 (100); HRMS Calcd for C30H29NO (M ): 419.2249. Found: 419.2247. Anal. Calcd for C30H29NO: C 85.88, H 6.97, N 3.34. Found: C 84.90, H 6.96, N 3.31.

Synthesis of 7. Following procedure for the synthesis of 3aa, the reaction of 1j (303.1 mg, 1.5 mmol), [Cp*RhCl2]2 (185.5 mg, 0.3 mmol), NaOAc (36.9 mg,0.45 mmol), 2o (400.1 mg, 1.5 mmol), MeOH (9 ml), and H2O (0.45 ml) at room temperature afforded impure 7 with impurities (136.8 mg) (eluent: Hexane/ethyl acetate/dichloromethane 10/1/0.2 to 8/1/0.5), which was further puried by

recrystallization (hexane/THF) afford pure 7 (69.7 mg, 12%): solid; m.p. 117.5119.0 C (hexane/THF); 1H NMR (300 MHz, CDCl3) d 9.02 (s, 1 H, ArH),8.91 (bs, 1 H, NH), 8.32 (s, 1 H, ArH), 8.067.94 (m, 1 H, ArH), 7.757.66 (m, 1 H, ArH), 7.557.40 (m, 8 H, ArH), 7.367.17 (m, 4 H, ArH), 5.57 (s, 1 H, CH), 3.31 (s, 3 H, CH3); 13C NMR (75 MHz, CDCl3) d 163.2, 141.6, 139.4, 135.1, 134.8, 131.2, 131.1, 129.6, 129.04, 128.97, 128.81, 128.77, 128.5, 128.3, 127.8, 127.1, 126.6, 126.2, 124.5, 111.1, 80.3, 56.5; IR (neat, cm 1) 3,181, 3,057, 3,027, 2,927, 2,890, 2,819, 1,659, 1,625, 1,600, 1,493, 1,448, 1,355, 1,312, 1,089, 1022; MS(EI, 70 eV) m/z (%) 392 (M 1, 30.89), 391 (M , 100); HRMS Calcd for

C27H21NO2 (M ): 391.1572. Found: 391.1571.

Synthesis of S-3af. Following procedure for the synthesis of 3aa, the reaction of 1a (45.6 mg, 0.3 mmol), [Cp*RhCl2]2 (4.8 mg, 0.008 mmol), NaOAc (5.3 mg,0.06 mmol), S-2f (98% ee, 45.9 mg, 0.2 mmol), MeOH (1.2 ml), and H2O (0.06 ml) at 0 C afforded S-3af (46.6 mg, 77%) (eluent: petroleum/ethyl acetate/ dichloromethane 5/1/0.5): 98% ee (HPLC conditions: Chiralcel AD-H column,

hexane/i-PrOH 10/1, 1.0 ml min 1, l 207 nm, t

R(minor)

21.9 min,

tR(major) 24.5 min); solid; m.p. 99.9101.1 C (hexane/ethyl acetate); 1H NMR

(300 MHz, CDCl3) d 8.66 (s, 1 H, NH), 7.78 (d, J 7.5 Hz, 1 H, ArH), 7.537.14

(m, 8 H, ArH), 3.45 (s, 3 H, OCH3), 2.272.06 (m, 2 H, CH2), 1.90 (s, 3 H, CH3),1.13 (t, J 7.4 Hz, 3 H, CH3); 13C NMR (75 MHz, CDCl3) d 202.0, 166.6, 137.2,

135.6, 132.5, 131.20, 131.18, 129.6, 128.6, 127.9, 127.1, 126.5, 107.5, 106.5, 63.9,27.4, 18.7, 12.3; IR (neat, cm 1) 3,199, 3,058, 3,018, 2,966, 2,932, 1,947, 1,659, 1,595, 1,491, 1,456, 1,439, 1,310, 1,159, 1,031; MS (EI, 70 eV) m/z (%) 307(M , 1.69), 246 (100); Anal. Calcd for C20H21NO2: C 78.15, H 6.89, N 4.56. Found:

C 77.98, H 6.88, N 4.34.

References

1. Ma, S. Some typical advances in the synthetic applications of allenes. Chem. Rev. 105, 28292871 (2005).

2. Alcaide, B. & Almendros, P. Progress in allene chemistry Special issue, Eds. Chem. Soc. Rev. 43, 28793206 (2014).

3. Hoffmann-Rder, A. & Krause, N. Synthesis and properties of allenic natural products and pharmaceuticals. Angew. Chem. Int. Ed. 43, 11961216 (2004).

4. Yu, S. & Ma, S. Allenes in catalytic asymmetric synthesis and natural product syntheses. Angew. Chem., Int. Ed. 51, 30743112 (2012).

5. Yu, S. & Ma, S. How easy are the syntheses of allenes? Chem. Commun. 47, 53845418 (2011).

6. Ye, J. & Ma, S. Conquering three-carbon axial chirality of allenes. Org. Chem. Front. 1, 12101224 (2014).

7. Miura, T., Shimada, M., Ku, S.-Y., Tamai, T. & Murakami, M. Stereoselective synthesis of a-allenols by rhodium-catalyzed reaction of alkynyl oxiranes with arylboronic acids. Angew. Chem. Int. Ed. 46, 71017103 (2007).

8. Hayashi, S., Hirano, K., Yorimitsu, H. & Oshima, K. Synthesis of arylallenes by palladium-catalyzed retro-propargylation of homopropargyl alcohols. J. Am. Chem. Soc. 130, 50485049 (2008).

9. Shu, W., Jia, G. & Ma, S. Palladium-catalyzed regioselective cyclopropanating allenylation of (2,3-butadienyl)malonates with propargylic carbonates and their application to synthesize cyclopentenones. Org. Lett. 11, 117120 (2009).

10. Hashimoto, T., Sakata, K., Tamakuni, F., Dutton, M. J. & Maruoka, K. Phase-transfer-catalysed asymmetric synthesis of tetrasubstituted allenes. Nat. Chem. 5, 240244 (2013).

11. Satoh, T. & Miura, M. Oxidative coupling of aromatic substrates with alkynes and alkenes under rhodium catalysis. Chem. Eur. J. 16, 1121211222 (2010).

12. Colby, D. A., Bergman, R. G. & Ellman, J. A. Rhodium-catalyzed C-C bond formation via heteroatom-directed C-H bond activation. Chem. Rev. 110, 624655 (2010).

13. Song, G., Wang, F. & Li, X. C-C, C-O and C-N bond formation via rhodium(III)-catalyzed oxidative C-H activation. Chem. Soc. Rev. 41, 36513678 (2012).

14. Kuhl, N., Schrder, N. & Glorius, F. Formal SN-type reactions in rhodium(III)-catalyzed C-H Bond activation. Adv. Synth. Catal. 356, 14431460 (2014).15. Ackermann, L. Carboxylate-assisted transition-metal-catalyzed C-H bond functionalizations: mechanism and scope. Chem. Rev. 111, 13151345 (2011).

16. Engle, K. M., Mei, T.-S., Wasa, M. & Yu, J.-Q. Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions. Acc. Chem. Res. 45, 788802 (2011).

17. Ackermann, L. & Fenner, S. Ruthenium-catalyzed C-H/N-O bond functionalization: green isoquinolone syntheses in water. Org. Lett. 13, 65486551 (2011).

18. Guimond, N., Gouliaras, C. & Fagnou, K. Rhodium(III)-catalyzed isoquinolone synthesis: the N-O bond as a handle for C-N bond formation and catalyst turnover. J. Am. Chem. Soc. 132, 69086909 (2010).

19. Schipper, D. J., Hutchinson, M. & Fagnou, K. Rhodium(III)-catalyzed intermolecular hydroarylation of alkynes. J. Am. Chem. Soc. 132, 69106911 (2010).

20. Rakshit, S., Patureau, F. W. & Glorius, F. Pyrrole synthesis via allylic sp3 C-H activation of enamines followed by intermolecular coupling with unactivated alkynes. J. Am. Chem. Soc. 132, 95859587 (2010).

21. Hyster, T. K. & Rovis, T. Rhodium-catalyzed oxidative cycloaddition of benzamides and alkynes via C-H/N-H activation. J. Am. Chem. Soc. 132, 1056510569 (2010).

22. Pham, M. V., Ye, B. & Cramer, N. Access to sultams by rhodium(III)-catalyzed directed C-H activation. Angew. Chem. Int. Ed. 51, 1061010614 (2012).

23. Murakami, M. & Igawa, H. A study of the stereochemical course of b-oxygen elimination with a rhodium(I) complex. Helv. Chim. Acta 85, 41824188 (2002).

24. Xu, L., Zhu, Q., Huang, G., Cheng, B. & Xia, Y. Computational elucidation of the internal oxidant-controlled reaction pathways in Rh(III)-catalyzed aromatic C-H functionalization. J. Org. Chem. 77, 30173024 (2012).

25. Alexakis, A., Marek, I., Mangeney, P. & Normant, J. F. Mechanistic aspects on the formation of chiral allenes from propargylic ethers and organocopper reagents. J. Am. Chem. Soc. 112, 80428047 (1990).

26. Xie, F., Qi, Z., Yu, S. & Li, X. Rh(III)- and Ir(III)-catalyzed C H alkynylation

of arenes under chelation assistance. J. Am. Chem. Soc. 136, 47804787 (2014).27. Schrder, N., Wencel-Delord, J. & Glorius, F. High-yielding, versatile, and practical [Rh(III)Cp*]-catalyzed ortho bromination and iodination of arenes.J. Am. Chem. Soc 134, 82988301 (2012).28. Simmons, E. M. & Hartwig, J. F. On the interpretation of deuterium kinetic isotope effects in C-H bond functionalizations by transition-metal complexes. Angew. Chem. Int. Ed. 51, 30663072 (2012).

29. Wu, S., Zeng, R., Fu, C., Yu, Y., Zhang, X. & Ma, S. Rhodium-catalyzed C-H functionalization-based approach to eight-membered lactams. Chem. Sci 6, 22752285 (2015).

30. Ma, S. & Yu, S. Palladium-catalyzed functionalization of indoles with 2-acetoxymethyl substituted electron-decient alkenes. Tetrahedron Lett. 45, 84198422 (2004).

31. Oi, S., Tanaka, Y. & Inoue, Y. Ortho-selective allylation of 2-pyridylarenes with allyl acetates catalyzed by ruthenium complexes. Organometallics 25, 47734778 (2006).

32. Kuninobu, Y., Ohtan, K. & Takai, K. Rhenium-catalyzed allylation of CH bonds of benzoic and acrylic acids. Chem. Commun. 47, 1079110793 (2011).

33. Yao, T., Hirano, K., Satoh, T. & Miura, M. Stereospecic copper-catalyzed C-H allylation of electron-decient arenes with allyl phosphates. Angew. Chem. Int. Ed. 50, 29902994 (2011).

34. Makida, Y., Ohmiya, H. & Sawamura, M. Regio- and stereocontrolled introduction of secondary alkyl groups to electron-decient arenes through copper-catalyzed allylic alkylation. Angew. Chem. Int. Ed. 51, 41224127 (2012).

35. Fan, S., Chen, F. & Zhang, X. Direct palladium-catalyzed intermolecular allylation of highly electron-decient polyuoroarenes. Angew. Chem. Int. Ed. 50, 59185923 (2011).

36. Yu, Y.-B., Fan, S. & Zhang, X. Copper- and phosphine-ligand-free palladium-catalyzed direct allylation of electron-decient polyuoroarenes with allylic chlorides. Chem. Eur. J 18, 1464314648 (2012).

37. Wang, H., Schrder, N. & Glorius, F. Mild rhodium(III)-catalyzed direct C-H allylation of arenes with allyl carbonates. Angew. Chem. Int. Ed. 52, 53865389 (2013).

38. Shu, W. Studies on Palladium-Catalyzed Two- or Three-Component Cyclization Reactions Involving Allenes. PhD thesis, Shanghai Institute of Organic Chemistry, China (2010).

Acknowledgements

Financial support is acknowledged from National Basic Research Program (2015CB856600) of China. S.M. is a Qiu Shi Adjunct Professor at Zhejiang University. We thank Mr. Weilong Lin in this group for reproducing the results for synthesis of 3ah, 3ga and S-3cf.

Author contributions

S.M. directed the research and developed the concept of the reaction with S.W. S.W. performed the experiments and data analysis. X.H. performed some experiments. W.W. and P.L. contributed in helping collecting some experimental data. S.W., C.F. and S.M. checked the experimental data with the help of X.H. The paper was written by S.W. and S.M. with assistance from the other authors.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Web End =http://www.nature.com/ http://www.nature.com/naturecommunications

Web End =naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

8 NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8946 ARTICLE

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

Web End =http://npg.nature.com/ http://npg.nature.com/reprintsandpermissions/

Web End =reprintsandpermissions/

How to cite this article: Shangze, W. et al. A CH bond activation-based catalytic approach to tetrasubstituted chiral allenes. Nat. Commun. 6:7946 doi: 10.1038/ ncomms8946 (2015).

This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the articles Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/

NATURE COMMUNICATIONS | 6:7946 | DOI: 10.1038/ncomms8946 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 9

& 2015 Macmillan Publishers Limited. All rights reserved.