Asymmetric Sulfur Ylide Reactions with Boranes: Scope and Limitations, Mechanism and Understanding

GY Fang, OA Wallner, N Di Blasio, X Ginesta, JN Harvey, VK Aggarwal

Research output: Contribution to journalArticle (Academic Journal)

46 Citations (Scopus)

Abstract

The reactions of aryl-stabilized sulfur ylides with organoboranes has been studied under a variety of conditions. At 5 or -78 C, the reaction with Et3B gave a mixture of the first and second homologation products, but at -100 C, only the first homologation product was obtained even with just 1.1 equiv of Et3B. Under these optimized conditions, the chiral sulfur ylides (derived from camphor sulfonic acid) with different aryl groups were reacted with Et3B to give the corresponding alcohols (95-98% yield, 96-98% ee) and amines (74-77% yield, >98% ee). The origin of the high enantioselectivity is discussed. The use of nonsymmetrical 9-BBN derivatives was also explored. It was found that whereas primary alkyl substituents gave mixtures of products derived from competing migration of the boron substituent and the boracycle, all other groups resulted in either exclusive migration of the boron substituent (Ph, hexenyl, i-Pr) or exclusive migration of the boracycle (hexynyl, cyclopropyl). The factors responsible for the outcome of the reactions involving a hindered (i-Pr) and an unhindered (propynyl) substituent were studied by DFT calculations. This revealed that, in the case of an unhindered substituent, the conformation of the ate complex is the dominant factor whereas, in the case of a hindered substituent, the barriers to interconversion between the conformers of the ate complex and subsequent migration control the outcome of the reaction.
Translated title of the contributionAsymmetric Sulfur Ylide Reactions with Boranes: Scope and Limitations, Mechanism and Understanding
Original languageEnglish
Pages (from-to)14632 - 14639
Number of pages8
JournalJournal of the American Chemical Society
Volume129 (47)
DOIs
Publication statusPublished - Nov 2007

Bibliographical note

Publisher: American Chemical Society

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