SYNTHESIS AND STRUCTURE OF MODERATELY STABLE METALLAOXETANES - (ETA-5-C5ME5)2(CH3)TAOCHRCH2 (R = H, C6H5) - INVESTIGATIONS OF THEIR DECOMPOSITION TO OLEFIN AND (ETA-5-C5ME5)2TA(=O)CH3 AND EVIDENCE REVEALING THAT THEY ARE NOT INTERMEDIATES IN THE DEOXYGENATION OF EPOXIDES BY [(ETA-5-C5ME5)2TA(CH3)]

Metallaoxetanes Cp*2(CH3)TaOCHRCH2 (Cp* = (eta-5-C5Me5); R = H, p-C6H4X; X = H, Cl, CF3, CN, NO2) have been prepared by reaction of Cp*2Ta(= CH2)CH3 with the appropriate aldehyde RCHO. The kinetic products of the 2 + 2 reaction between the benzaldehydes with Cp*2Ta(= CH2)CH3 are O-syn-Cp*2(CH3)TaCH2CH(p-C6H4X)O, which rearrange in benzene solution at 25-degrees-C to the more stable O-anti-Cp*2(CH3)TaOCH(p-C6H4X)CH2. An X-ray crystal structure determination for O-anti-Cp*2(CH3)TaOCH(C6H5)CH2 has been carried out (space group P2(1)/c (No. 14) with cell parameters a = 15.677 (10) angstrom, b = 9.502 (4) angstrom, c = 18.315 (18) angstrom, beta = 110.81 (7)-degrees, V = 2550.3 (32) angstrom 3, and Z = 4), revealing a slightly puckered four-membered ring. The distortion from planarity likely arises from close contacts between a pentamethylcyclopentadienyl ligand and the phenyl ring. Decomposition of O-anti-Cp*2(CH3)TaOCHRCH2 proceeds smoothly at 80-degrees-C to afford Cp*2Ta(= O)CH3 and RCH = CH2. The rates for styrene formation depend only slightly on the para substituent and do not correlate with sigma or sigma+. In a related transformation, Cp*2Ta(= CH2)H reacts with epoxides in THF even at -50-degrees-C to yield Cp*2Ta(= O)CH3 and olefin. Significantly, neither O-syn-Cp*2(CH3)TaCH2CH2O nor O-anti-Cp*2(CH3)TaOCH2CH2 is observed as an intermediate for the deoxygenation of ethylene oxide, and deoxygenation of trans-styrene-d1 oxide, trans-ethylene-d2 oxide and cis-ethylene-d2 oxide proceed with > 95% retention of stereochemistry. These results strongly implicate a concerted process and indicate that epoxide deoxygenation and (more importantly) olefin epoxidation with transition-metal oxo derivatives need not involve metallaoxetane intermediates.