BETA EFFECT OF PHOSPHORUS FUNCTIONALITIES

Diphelnylphosphinoyl (-P(=O)Ph(2)) and diphenylthiophosphinoyl (-P(=S) Ph(2)) respectively are modest and strong p-effect functionalities. Wh en these groups are antiperiplanar to mesylate, the substrates solvoly ze through unimolecular ionization. In contrast, substrates with the s ynclinal (gauche) arrangement react bimolecularly with solvent. The an ti/gauche rate ratio is 440 for the oxide and 3.2 x 10(6) for the sulf ide at 25 degrees C. The antiperiplanar sulfide in fact reacts 220 tim es more rapidly than the analogous cyclohexyl substrate at 25 degrees C, despite the strong electron-withdrawing nature of diphenylthiophosp hinoyl. The a-secondary hydrogen/deuterium kinetic isotope effects of 1.21 for the anti oxide and 1.26 for the anti sulfide suggest kigh sp( 2) 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 emp ty carbocation p orbital is more stable than the perpendicular geometr y by 22.94 kcal mol(-1) for PH2, 8.80 kcal mol(-1) for P(O)H-2, and 10 .54 kcal mol(-1) for P(S)H-2, confirming significant hyperconjugation for these substituents. The global minimum, however, is the bridged st ructure (four-membered rings for the oxide and sulfide, three-membered ring for the simple phosphine), so that the mechanistic choice betwee n a vertical mechanism (hyperconjugation) and a nonvertical mechanism (bridging) is not clear-cut.