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

Abstract

Permethyltantalocene thioaldehyde hydride complexes, Cp*_2Ta(η^2-S-CHR)H (Cp* = η^5-C_5Me_5; R = H, C_6H_5, CH_2C_6H_5, CH_2CMe_3), have been prepared by treating precursors to [Cp*_2Ta-R'] (e.g. Cp*_2Ta(=C=CH_2)H for R' = CH=CH_22) with the appropriate thiol RCH_2SH. Oxidative addition of the S-H bond leads to the unstable Ta(V) derivatives Cp*_2Ta(R'(H)(SCH_2R). Reductive elimination of R'H is facile, forming [Cp*_2Ta-ScH_2R], which subsequently undergoes β-He limination to yield the thiddehyde hydrides Cp*_2Ta( η^2-S-CHR)H. The results of an X-ray structure determination for Cp*_2Ta( η_2-SC-HCH_2C_6H_5)H are summarized. These permethyltantalocene thioaldehyde hydride complexes are shown to be in rapid equilibrium with the corresponding 16-electron thiolate species [CP*_2Ta-SCH_3R] through a β-H migratory inaertion/elimination process. When they are heated, the permethyltantalocene thiddehyde hydride complexes undergo rearrangement to the thermodynamically favored tautomer, the permethyltantalocene sulfido alkyl Cp*_2Ta(=S)CH_2R. An inverse kinetic deuterium isotope effect observed for Cp*_2Ta(η^2-S-CY_2)Y (Y = H, D) ((k_H)/(k_D) = 0.72 (3) at 138 °C) is indicative of a stepwise process involving to Cp*_2Ta(=S)CY_3. Derivatives of the permethyltantalocene phenylthioacetaldehyde hydride have been prepared from the erythro- and threo-phenethyl-d_2 mercaptan, C_6H_5CHDCHDSH, to elucidate the mechanism of alkyl transfer. The migration has been found to proceed with ≳ 85% retention of stereochemistry at carbon for the migrating phenethyl-d_2 group. The sulfido methyl complex Cp*_2Ta(=S)CH_3 is hydrogenated under forcing conditions to yield methane and CP*~T~(=S)bHu,t this product resists final hydrogenation to Cp*_2TaH_3 and H_2S.