β Effect of Phosphorus Functionalities

Diphenylphosphinoyl (−P(O)Ph2)
and diphenylthiophosphinoyl (−P(S)Ph2)
respectively are modest
and strong β-effect functionalities. When these groups are
antiperiplanar to mesylate, the substrates solvolyze
through
unimolecular ionization. In contrast, substrates with the
synclinal (gauche) arrangement react bimolecularly with
solvent. The anti/gauche rate ratio is 440 for the oxide and 3.2
× 106 for the sulfide at 25 °C. The
antiperiplanar
sulfide in fact reacts 220 times more rapidly than the analogous
cyclohexyl substrate at 25 °C, despite the strong
electron-withdrawing nature of diphenylthiophosphinoyl. The
α-secondary hydrogen/deuterium kinetic isotope effects
of 1.21 for the anti oxide and 1.26 for the anti sulfide suggest high
sp2 character in the transition state, as
expected
for stabilization by hyperconjugation in a vertical mechanism.
Energies calculated at the MP2 level show that the
geometry with the P−C bond parallel to the empty carbocation p
orbital is more stable than the perpendicular geometry
by 22.94 kcal mol-1 for PH2, 8.80
kcal mol-1 for P(O)H2, and
10.54 kcal mol-1 for P(S)H2,
confirming significant
hyperconjugation for these substituents. The global minimum,
however, is the bridged structure (four-membered
rings for the oxide and sulfide, three-membered ring for the simple
phosphine), so that the mechanistic choice between
a vertical mechanism (hyperconjugation) and a nonvertical mechanism
(bridging) is not clear-cut.