Remote Stereochemistry of a Frustrated Lewis Pair Provides Thermal and Photochemical Control of Reactivity

Hydroboration of tBuCCSR with
(C6F5)2BH generate the B/S FLPs,
(C6F5)2B­(RS)­CCH­(tBu) (R = ptol, Me). For R = ptol
the product Z-1 exhibits Z stereochemistry, is monomeric in solution and is capable
of both classical Lewis acid–base and FLP reactivity. Photoisomerization
of Z-1 affords E-1, which features an intramolecular
S–B interaction promoted by the remote steric influence of
the tert-butyl group. E-1 is not active as an FLP under ambient conditions
but reacts with phenylacetylene when activated with either heat or
light. The stereochemical outcome of the product is dependent on the
stimulus employed.