On the mechanism of oxygen atom or nitrene group transfer in reactions of epoxides and aziridines with tungsten(II) compounds

Article abstract of DOI:10.1021/ja00003a021

The tungsten(II) complexes WCl₂(PMePh₂)₄ (1) and WCl₂(CH₂=CH₂)₂(PMePh ₂)₂ (2) react with epoxides and aziridines to form tungsten(IV)-oxo and -imido complexes. The relative reactivities of epoxides with 2 have been determined from competition experiments. More substituted epoxides are harder to deoxygenate: the reactivities of ethylene, isobutylene, and tetramethylethylene oxides fall in the geometric progression 100:10:1. cis-2-Butene oxide is deoxygenated faster than its trans isomer. Reaction occurs with predominant (≥85%) retention of configuration (e.g. cis epoxides to cis olefins). The reaction of 2 with ethylene-d₄ oxide yields W(O)Cl₂(CH₂=CH₂)(PMePh₂)₂ (4) and uncoordinated CD₂=CD₂. The data suggest that de-epoxidation occurs via oxygen atom abstraction, and not via an oxametallacyclobutane which rearranges to an oxo-ethylene complex. Similarly, the tungsten center is suggested to attack the nitrogen atom of the aziridines, rather than react by initial oxidative addition of a C-N bond. This is indicated by the observation of an N-bound complex of aziridine and by the much slower rates of reaction for aziridines with bulky substituents on the nitrogen. The reactivities of para-substituted styrene epoxides are not strongly affected by the nature of the substituent (a ρ⁺ value of -0.5 is calculated with Hammett σ⁺ parameters) indicating that the transition state is not very polar, although there appears to be some conjugation between the phenyl ring and the epoxide. In sum, the data are most consistent with either concerted oxygen or nitrene transfer to tungsten or a mechanism involving a short-lived radical intermediate.