SYNTHESIS AND CHARACTERIZATION OF THIOALDEHYDE HYDRIDE DERIVATIVES OF PERMETHYLTANTALOCENE - INVESTIGATIONS OF THEIR EQUILIBRATION WITH THIOLATES AND THE STEREOCHEMISTRY OF ALKYL MIGRATIONS FROM SULFUR TO TANTALUM

Permethyltantalocene thioaldehyde hydride complexes, Cp*2Ta(eta-2-S-CHR)H (Cp* = eta-5,-C5Me5; R = H, C6H5, CH2C6H5, CH2CMe3), have been prepared by treating precursors to [Cp*2Ta-R'] (e.g. Cp*2Ta(=C=CH2)H for R' = CH=CH2) with the appropriate thiol RCH2SH. Oxidative addition of the S-H bond leads to the unstable Ta(V) derivatives Cp*2Ta(R')(H)(SCH2R). Reductive elimination of R'H is facile, forming [Cp*2Ta-SCH2R], which subsequently undergoes beta-H elimination to yield the thioaldehyde hydrides Cp*2Ta(eta-2-S-CHR)H. The results of an X-ray structure determination for Cp*2Ta(eta-2-S-CHCH2C6H5) H are summarized. These permethyltantalocene thioaldehyde hydride complexes are shown to be in rapid equilibrium with the corresponding 16-electron thiolate species [Cp*2Ta-SCH2R] through a beta-H migratory insertion/elimination process. When they are heated, the permethyltantalocene thioaldehyde hydride complexes undergo rearrangement to the thermodynamically favored tautomer, the permethyltantalocene sulfido alkyl Cp*2Ta(=S)CH2R. An inverse kinetic deuterium isotope effect observed for Cp*2Ta(eta-2-S-CY2)Y (Y = H, D) (k(H)/k(D) = 0.72 (3) at 138-degrees-C) is indicative of a stepwise process involving fast preequilibrium of Cp*2Ta(eta-2-S-CY2)Y with [Cp*2Ta-SCY3] and rate-determining alpha-CY3 elimination to Cp*2Ta(=S)CY3. Derivatives of the permethyltantalocene phenylthioacetaldehyde hydride have been prepared from the erythro- and threo-phenethyl-d2 mercaptan, C6H5CHDCHDSH, to elucidate the mechanism of alkyl transfer. The migration has been found to proceed with greater-than-or-equal-to 85% retention of stereochemistry at carbon for the migrating phenethyl-d2 group. The sulfido methyl complex Cp*2Ta(=S)CH3 is hydrogenated under forcing conditions to yield methane and Cp*2Ta(=S)H, but this product resists final hydrogenation to Cp*2TaH3 and H2S.