THE KINETICS AND MECHANISM OF THE REACTION OF TRIMETHYL PHOSPHITE WITH BENZYLIDENEACETOPHENONES

The kinetics and mechanism of the equilibrium reaction of benzylidenea cetophenones 4 with trimethyl phosphite (TMP) to yield, 2,2,2-trimetho xy-2,3-dihydro-1,2 lambda(5)-oxaphospholes 5 are reported. The second- order rate constants for the formation of 5 were determined using a po larographic method and the rate constants for the decomposition reacti on were calculated from the equilibrium constants. The linear Hammett plots in some cases and the curved ones in others indicate that the ra te-determining step depends on the character of the ring substituents in 4 and 5. A thermodynamic study of the forward reaction showed a rel atively small enthalpy and a large negative entropy of activation in a greement with the existence of a strongly polarised and/or crowded str ucture in the transition state. The proposed mechanism, in cases where electron-donating substituents (Y) or hydrogen are on the aromatic ri ng next to the carbonyl carbon or where electron-donating substituents (X) are on the aromatic ring next to the double bond, involves nucleo philic attack of phosphite on the carbonyl carbon atom in the rate-lim iting step to form intermediate I-1 followed by a ring closure to an o xaphosphirane I-2 and ring enlargement to the oxaphospholene 5. The us e of electron-withdrawing substituents (both for X and Y) results in a change in the reaction pathway in which phosphite attacks the carbon atom beta to the carbonyl group in the rate-limiting step followed by a fast ring closure of I-3 to oxaphospholene 5.