THE REACTION OF PENTASUBSTITUTED 3-HYDROXY-1,2-DIOXOLANES (HEMIPERKETALS) WITH TRIVALENT PHOSPHORUS-COMPOUNDS - SYNTHESIS OF BETA-HYDROXY KETONES

The reaction of a series of 4,4-dimethyl-3,5,5-(R3, R2, R1)-3-hydroxy-1,2-dioxolanes 1a-d [la (R1 = R2 = R3 = Me); lb (R1 = R2 = Me, R3 = Ph); 1c (R1 = Ph, R2 = R3 = Me); 1d (R1 = R3 = Ph, R2 = Me)] with trimethyl phosphite or triphenylphosphine produced the beta-hydroxy ketones, 2a-d [(R1R2C(OH)CMe2-COR3], and the corresponding phosphoryl compounds in high yield. The reactions were slow (approximately 24 hours), requiring excess of the trivalent phosphorus compounds for optimum results. The reaction was found to be of the second order overall, first order in peroxide and in phosphorus reagent. Triphenylphosphine was found to be more reactive than trimethyl phosphite. Formal substitution of a phenyl group for a methyl group on the hemiperketals yielded a reduction of reactivity of approximately 2.4-fold at the 3-position but an increase in reactivity (approximately 1.5-fold) at the 5-position. The rate constants for tautomerization of Ic in benzene were determined. The activitation parameters for reaction of ic with triphenylphosphine in benzene-d6 were determined [DELTAH(double dagger) = 15.0 +/- 1.5 kcal/mol;,DELTAS(double dagger) = -21.7 eu; DELTAG(double dagger) = 21.7 kcal/mol; k34-degrees = 2.4 x 10(-3) M-1 s-1]. The data are consistent with direct insertion of the phosphorus compounds into the peroxide bond to yield phosphorane intermediates. Rapid intramolecular hydrolysis of the intermediates would produce the beta-hydroxy ketones and phosphoryl compounds.