ONSET OF ANH HYPERCONJUGATION IN THE TRANSITION-STATE OF CARBONYL ADDITION BY DEACTIVATION OF PERIPLANAR VICINAL BONDS

The previously observed preferential syn delivery of-hydride anion in the reduction of the carbonyl group of 5-fluoroadamantan-2-one is inve rted if the identities of all hydrogen and fluorine atoms are reversed . A similar and somewhat smaller alteration is observed in the reducti on of 5-chloroperfluoroadamantan-2-one. The interpretation is that the periplanar bonds are then no longer capable of stabilizing the transi tion state by electron donation, and instead do so by accepting electr on density from the incipient bond into the sigma orbitals. The confi gurations of the reduction products were deduced from their F-19 NMR s pectra by means of a chemical shift additivity procedure.