Toward an understanding of the factors responsible for the 1,2-migration of alkyl groups in borate complexes

The anionotropic 1,2-migration of an organic substituent from a tetrasubstituted borate ion, often referred to as an "ate" complex, to an acceptor atom is at the basis of the most useful application of organoboranes in organic synthesis. We recently showed that chiral sulfur ylides react with boranes to give homologated products with high enantiomeric excess. In considering reactions with mixed boranes, the issue of which group would migrate arises. Although we are primarily interested in sulfur ylide reactions with boranes. in this review we have summarized the most important factors that are responsible for which group migrates from a broad spectrum of reactions involving borate complexes. We also discuss the use of blocking/nonmigrating groups and highlight when they are effective and not effective. Consideration of the most important factors that affect the outcome of which group migrates and understanding how and why blocking groups work, provides a strategy for designing boranes with nonmigrating groups for use in new reactions of these useful synthetic intermediates.