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 inverted if the identities of all hydrogen and fluorine atoms are reversed. A similar and somewhat smaller alteration is observed in the reduction of 5-chloroperfluoroadamantan-2-one. The interpretation is that the periplanar bonds are then no longer capable of stabilizing the transition state by electron donation, and instead do so by accepting electron density from the incipient bond into the σ* orbitals. The configurations of the reduction products were deduced from their ¹⁹F NMR spectra by means of a chemical shift additivity procedure.