Cooperative Catalytic Reactions Using Organocatalysts and Transition

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Cooperative Catalytic Reactions Using Organocatalysts and Transition Metal Catalysts: Propargylic Allylation of Propargylic Alcohols with α,β-Unsaturated Aldehydes Masahiro Ikeda, Yoshihiro Miyake, and Yoshiaki Nishibayashi* Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Propargylic allylation of propargylic alcohols with α,β-unsaturated aldehydes in the presence of a thiolatebridged diruthenium complex and a secondary amine as cocatalysts has been found to give the corresponding propargylic allylated products, where the γ-propargylation occurs selectively at the α,β-unsaturated aldehydes, in high yields as a mixture of two diastereoisomers.

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Scheme 1. Dienamine-Catalyzed γ-Amination of α,βUnsaturated Aldehydes

INTRODUCTION Quite recently, we found the enantioselective propargylic alkylation of propargylic alcohols with aldehydes in the presence of a thiolate-bridged diruthenium complex and an optically active secondary amine as cocatalysts to give the corresponding propargylic alkylated products in excellent yields as a mixture of two diastereoisomers with a high enantioselectivity (up to 99% ee).1 This catalytic reaction is considered to provide a new type of enantioselective propargylic substitution reaction,2 where the enamines generated in situ from aldehydes enantioselectively attack the γ-carbon atom of the ruthenium−allenylidene complexes.3 In this reaction system, both the transition metal catalyst (ruthenium complex) and the organocatalyst4 (secondary amine) activate propargylic alcohols and aldehydes, respectively, and both catalysts cooperatively and simultaneously work to promote the propargylic alkylation enantioselectively. The enamine intermediates generated from aminocatalysts and carbonyl compounds are known to work as carbon-centered nucleophiles like this reaction. On the other hand, the iminium intermediates, generated from α,β-unsaturated carbonyl compounds and amine, are well known to work as suitable electrophiles. However, Jørgensen and co-workers disclosed that aminocatalysts can invert the usual electrophilic reactivity of α,βunsaturated aldehydes, where the dienamine intermediates generated from aminocatalysts and α,β-unsaturated aldehydes work as carbon-centered nucleophiles. In 2006, Jørgensen and co-workers reported the direct asymmetric γ-functionalization of α,β-unsaturated aldehydes with azodicarboxylate as an electrophile (Scheme 1).5 This is the first successful example of the use of dienamines as carbon-centered nucleophiles for the direct γ-functionalization of α,β-unsaturated aldehydes. However, the successful examples of dienamine catalysis are limited to only a few cases until now.6 As an extension of our continuous study on the propargylic substitution reactions7,8 by the cooperative catalytic reaction using distinct catalysts, we have now envisaged cooperative catalytic reactions of propargylic alcohols with α,β-unsaturated © 2012 American Chemical Society

aldehydes using a ruthenium complex and organocatalyst. In fact, we have found that reactions of propargylic alcohols with α,β-unsaturated aldehydes in the presence of a thiolate-bridged diruthenium complex and a secondary amine as cocatalysts gave the corresponding propargylic allylated products as a mixture of two diastereoisomers. We believe that the method described in this article may provide a new type of dual catalytic reactions using both transition metal catalysts and organocatalysts.9

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RESULTS AND DISCUSSION Treatment of 1-(1-naphthyl)-2-propyn-1-ol (1a) with 2 equiv of trans-2-pentenal (2a) in the presence of catalytic amounts of α,α-bis[3,5-bis(trifluoromethyl)phenyl]-2-pyrrolidinemethanol trimethyl silyl ether (3a), benzoic acid, methanethiolate-bridged diruthenium complex10,11 [Cp*RuCl(μ2-SMe)]2 (Cp* = η5C5Me5; 4a), and NH4BF4 in toluene at room temperature for 40 h gave (E)-4-methyl-5-(1-naphthyl)-2-hepten-6-ynal (5a) exclusively (Table 1, entry 1). (E)-4-Methyl-5-(1-naphthyl)-2hepten-6-yn-1-ol (6a) was isolated in 49% yield as a mixture of two diastereoisomers (1.1:1) after the reduction of 5a with NaBH4 at 0 °C for 30 min. When other solvents such as CH2Cl2 and THF were used in place of toluene, the catalytic Received: April 9, 2012 Published: May 2, 2012 3810

dx.doi.org/10.1021/om300286b | Organometallics 2012, 31, 3810−3813

Organometallics

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Table 1. Propargylic Allylation of Propargylic Alcohol (1a) with α,β-Unsaturated Aldehyde (2a)a

entry

3

additive

solvent

time (h)

yield of 6a (%)b

major:minor of 6ac

1 2 3 4 5 6d

3a 3a 3a 3a 3b 3a

PhCO2H PhCO2H PhCO2H none PhCO2H PhCO2H

toluene CH2Cl2 THF toluene toluene toluene

40 40 40 140 40 40

49 16 22 59 0 58

Table 2. Propargylic Allylation of Propargylic Alcohols (1) with α,β-Unsaturated Aldehydes (2)a

entry

1

1.1:1 1.1:1 1:1.7 1:1

1

1.1:1

4

R1 = 1-naphthyl (1a) R1 = 1-naphthyl (1a) R1 = 1-naphthyl (1a) R1 = 2-naphthyl (1b) R1 = p-PhC6H4 (1c) R1 = p-MeC6H4 (1d) R1 = p-ClC6H4 (1e) R1 = p-FC6H4 (1f) R1 = p-MeOC6H4 (1g) R1 = o-MeOC6H4 (1h)

2 3

a All reactions of 1a (0.20 mmol) with 2a (0.40 mmol) were carried out in the presence of 3 (0.010 mmol), additive (0.010 mmol), 4a (0.010 mmol), and NH4BF4 (0.020 mmol) at room temperature in toluene (4 mL). bIsolated yield. cDetermined by 1H NMR. d3 equiv of 2a was used.

5 6 7 8 9 10

2 R2 = Me (2a) R2 = nPr (2b) R2 = Bn (2c) R2 = nPr (2b) R2 = nPr (2b) R2 = nPr (2b) R2 = nPr (2b) R2 = nPr (2b) R2 = nPr (2b) R2 = nPr (2b)

time (h)

yield of 6 (%)b

major:minor of 6c

40

58 (6a)

1.1:1

40

71 (6b)

1.3:1

90

64 (6c)

1.7:1

120

60 (6d)

1.2:1

120

63 (6e)

1.1:1

120

69 (6f)

1.0:1

90

61 (6g)

1.0:1

90

69 (6h)

1.0:1

90

65 (6i)

1.1:1

90

82 (6j)

1.2:1

a

All reactions of 1 (0.20 mmol) with 2 (0.60 mmol) were carried out in the presence of 3a (0.010 mmol), PhCO2H (0.010 mmol), 4a (0.010 mmol), and NH4BF4 (0.020 mmol) at room temperature in toluene (4 mL). bIsolated yield. cDetermined by 1H NMR.

reactions did not proceed smoothly under the same reaction conditions (Table 1, entries 2 and 3). The presence of benzoic acid is required to proceed smoothly (Table 1, entry 4). No reaction occurred at all when α,α-diphenyl-2-pyrrolidinemethanol trimethyl silyl ether (3b) was used in place of 3a (Table 1, entry 5). The reaction proceeded more smoothly when 3 equiv of 2a to 1a was used under the same reaction conditions (Table 1, entry 6). Separately, we confirmed that the use of either 3a or 4a did not promote the propargylic allylation. This result indicates that both 3a and 4a cooperatively work as catalysts to promote the catalytic reaction enantioselectively. Next, propargylic allylation of a variety of propargylic alcohols with aldehydes was carried out by using 3a and 4a as cocatalysts. Typical results are shown in Table 2. The corresponding propargylic allylated products were obtained in higher yields when trans-2-heptenal (2b) and trans-5-phenylpent-2-enal (2c) were used as substrates (Table 2, entries 2 and 3). A longer reaction time was required when 1-(2-naphthyl)-2propyn-1-ol (1b) and propargylic alcohols bearing a phenyl, methyl, chloro, fluoro, or methoxy group at the para-position of the benzene ring were used as substrates (Table 2, entries 5− 9). Interestingly, the introduction of a methoxy group at the ortho-position of the benzene ring of propargylic alcohol substantially increased the reactivity (Table 2, entry 10). This is the first successful example of the use of dienamines as carboncentered nucleophiles toward propargylic alkylation. Enantioselective propargylic allylation of propargylic alcohol 1a with α,β-unsaturated aldehyde 2c was also carried out by using (S)-α,α-bis[3,5-bis(trifluoromethyl)phenyl]-2-pyrrolidinemethanol trimethyl silyl ether (3c) and diruthenium complex 4a as cocatalysts. Typical results are shown in Table 3. When benzoic acid was used as an additive, only a moderate enantioselectivity was observed in the major isomer, but a high enantioselectivity was observed in the minor isomer (Table 3, entry 1). The enantioselectivity dramatically dropped

Table 3. Propargylic Allylation of Propargylic Alcohol (1a) with α,β-Unsaturated Aldehyde (2c)a

ee (%)d entry

3

additive

yield of 6c (%)b

major:minor of 6cc

major6c

minor6c

1 2e 3 4 5 6

3c 3c 3c 3c 3d 3e

PhCO2H PhCO2H TFA C6F5CO2H C6F5CO2H C6F5CO2H

64 59 66 45 75 70

1.7:1 1.3:1 1.9:1 2.0:1 2.5:1 3.2:1

52 12 42 67 74 74

88 40 61 52 47 44

a All reactions of 1a (0.20 mmol) with 2c (0.60 mmol) were carried out in the presence of 3 (0.010 mmol), additive (0.010 mmol), 4a (0.010 mmol), and NH4BF4 (0.020 mmol) at room temperature in toluene (4 mL). bIsolated yield. cDetermined by 1H NMR. d Determined by HPLC. e4b was used in place of 4a.

when 2-propanethiolate-bridged diruthenium complex [Cp*RuCl(μ2-SiPr)]2 (4b) was used as a catalyst in place of 3811

dx.doi.org/10.1021/om300286b | Organometallics 2012, 31, 3810−3813

Organometallics

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Scheme 2. Reaction Pathway for the Propargylic Allylation of Propargylic Alcohols with α,β-Unsaturated Aldehydes

currently in progress to clarify the scope and limitations of the present propargylic alkylation.13

4a (Table 3, entry 2). The use of trifluoroacetic acid (TFA) as an additive instead of benzoic acid also decreased the enantioselectivity (Table 3, entry 3). However, the use of C6F5CO2H as an additive slightly increased the enantioselectivity of the major isomer (Table 3, entry 4). When other secondary amines bearing more sterically bulky substitution (3d and 3e) were used as organocatalysts, enantioselectivity of the major isomer increased to 74% ee (Table 3, entries 5 and 6). Unfortunately, we have not yet determined the absolute configurations of the main products because diastereomerically pure compounds were not isolated from the mixtures of two diastereoisomers. A proposed reaction pathway is shown in Scheme 2. The initial step is the formation of an allenylidene complex (B)3,12 by the reaction of propargylic alcohol 1 with 4a via a vinylidene complex (A). Subsequent attack of an dienamine (E) generated in situ from α,β-unsaturated aldehyde 2 and amine 3 on the γcarbon of B results in the formation of another vinylidene complex (D) via an alkynyl complex (C). Then, the alkylated products 5 are formed from D by ligand exchange with another propargylic alcohol 1. In summary, we have found the propargylic allylation of propargylic alcohols with α,β-unsaturated aldehydes in the presence of a thiolate-bridged diruthenium complex and a secondary amine as cocatalysts to give the corresponding propargylic allylated products, where the γ-propargylation occurs selectively at α,β-unsaturated aldehydes, in moderate to high yields as a mixture of two diastereoisomers. In the present reaction system, both the transition metal catalyst (ruthenium complex) and the organocatalyst (secondary amine) activate propargylic alcohols and α,β-unsaturated aldehydes, respectively, and both catalysts cooperatively and simultaneously work to promote the propargylic allylation. We believe that the finding described in this paper will open up a new aspect of not only dual catalytic reactions using both organocatalysts and transition metal catalysts but also the γpropargylation of α,β-unsaturated aldehydes.5,6 Further work is

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Funding Program for Next Generation World-Leading Researchers (GR025) and a Grantin-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformations by Organocatalyst” from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.I. is a recipient of the JSPS Predoctoral Fellowships for Young Scientists and acknowledges the Global COE Program for Chemistry Innovation. We also thank the Research Hub for Advanced Nano Characterization at The University of Tokyo for X-ray analysis.

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REFERENCES

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