The kinetics and mechanism of the reaction of trimethyl phosphate with benzylideneacetophenones

Abstract

The kinetics and mechanism of the equilibrium reaction of benzylideneacetophenones 4 with trimethyl phosphate (TMP) to yield, 2,2,2-trimethoxy-2,3-dihydro-1,2λ⁵-oxaphospholes 5 are reported. The secondorder rate constants for the formation of 5 were determined using a polarographic method and the rate constants for the decomposition reaction were calculated from the equilibrium constants. The linear Hammett plots in some cases and the curved ones in others indicate that the rate-determining step depends on the character of the ring substituents in 4 and 5. A thermodynamic study of the forward reaction showed a relatively small enthalpy and a large negative entropy of activation in agreement with the existence of a strongly polarised and/or crowded structure in the transition state. The proposed mechanism, in cases where electron-donating substituents (Y) or hydrogen are on the aromatic ring next to the carbonyl carbon or where electron-donating substituents (X) are on the aromatic ring next to the double bond, involves nucleophilic attack of phosphate on the carbonyl carbon atom in the rate-limiting step to form intermediate I₁ followed by a ring closure to an oxaphosphirane I₂ and ring enlargement to the oxaphospholene 5. The use of electron-withdrawing substituents (both for X and Y) results in a change in the reaction pathway in which phosphate attacks the carbon atom β to the carbonyl group in the rate-limiting step followed by a fast ring closure of I₃ to oxaphospholene 5.