Rhodium-catalyzed enantioselective alkynylative cyclization of allenyl aldehydes with terminal alkynes

Dedicated to Henri Kagan on the occasion of his 80th birthday
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Abstract

Asymmetric addition of terminal alkynes to allenyl aldehydes took place in the presence of an acetylacetonate complex [Rh(acac)L∗] (L∗ = binap or segphos) as a catalyst to give indanol derivatives in high yields with high regio- and enantioselectivity. The acetylacetonate (acac) ligand on the rhodium is important for the selective formation of the indanol derivatives.

Introduction

Transition metal-catalyzed addition of terminal alkynes to unsaturated bonds has provided a useful methodology for the synthesis of internal alkynes in view of high atom efficiency,1 and a recent growing attention has been paid to asymmetric alkynylations with diverse transition-metal catalysts.2 Rhodium is one of the most versatile metals, catalyzing the addition of terminal alkynes to alkynes,3 allenes,4 vinyl ketones,5 aldehydes,6 and ketones.6 In this context, we recently reported rhodium-catalyzed asymmetric alkynylation7 of conjugate enones,8 conjugate enals,9 allenes,10 and azabenzonorbornadienes.11, 12 Herein, we report rhodium-catalyzed asymmetric alkynylative cyclization of allenyl aldehydes with terminal alkynes giving indanol derivatives in high yields with high diastereo- and enantioselectivity.

In our previous studies on the asymmetric addition of terminal alkynes to diphenylphosphinylallenes, the phosphinyl substituent on the 1,1-disubstituted allenes was essential for selective formation of exo-enynes, which is formed by regioselective protonolysis of the π–allylrhodium(I) intermediate I involved in the catalytic cycle (Scheme 1).10 The other substituent (R) is also important for the formation of the addition product. For a mono-substituted phosphinyl allene (R = H), the reaction with a terminal alkyne did not give the corresponding hydroalkynylation product, which is due to the oligomerization caused by the reaction of allene with the reactive π–allylrhodium(I) intermediate.13 These results prompted us to examine the alkynylation of an allenyl aldehyde 1,14, 15 which has a formyl group at an appropriate position to undergo the intramolecular reaction with the nucleophilic π–allylrhodium(I) intermediate giving a chiral cycloalkanol (Scheme 2).

Section snippets

Results and discussion

Our initial studies were focused on the addition of (triphenylsilyl)acetylene 2m to 2-(buta-2,3-dienyl)benzaldehyde 1a in the presence of a chiral phosphine–rhodium complex as a catalyst to find suitable reaction conditions for the formation of 1-(silylethynyl)ethenyl-substituted indanol 3am (Table 1). The reaction in the presence of 5 mol % of rhodium catalyst Rh(acac)((R)-binap)16 (acac = acetylacetonate), generated in situ from Rh(acac)(C2H4)2 and (R)-binap, at 80 °C for 24 h gave 63% yield of 3am

Conclusion

In conclusion, we developed a rhodium-catalyzed asymmetric addition of terminal alkynes to allenyl aldehydes giving indanol derivatives in high yields with high regio- and enantioselectivity. It was found that the use of a rhodium catalyst with acac ligand is important for the selective formation of the indanol derivatives without isomerization into the indanones.

General

All anaerobic and moisture-sensitive manipulations were carried out with standard Schlenk techniques under predried nitrogen. NMR spectra were recorded on a JEOL JNM LA-500 spectrometer (500 MHz for 1H, 125 MHz for 13C, and 202 MHz for 31P). Chemical shifts are reported in δ (ppm) referenced to an internal SiMe4 standard for 1H NMR, chloroform-d (δ 77.16) for 13C NMR, and external H3PO4 standard for 31P NMR. The following abbreviations are used; s, singlet, d, doublet, t, triplet, q, quartet,

Acknowledgment

This work was supported by Grant-in-Aid for Scientific Research (S) (19105002) from MEXT, Japan.

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