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

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.