Novel alkylidenation, epimetallation and hydrometallation reactions of Group 4 metal alkyls.

摘要

The decomposition pathways of Group 4 metal alkyls leading to three main types of reagents, namely an alkylidenating agent, an epimetallating agent and a hydrometallating agent, have proved useful in many organic transformations, especially those employed in the pharmaceutical and fine chemical industry. The study of the reactions of these agents, which do not involving cyclopentadienyl ligands, has been the principal focus of this research. In the current investigation alkylidenating agents, prepared in situ in hydrocarbon solvents such as toluene, from two equivalents of methyllithium and one equivalent of the tetrachlorides of titanium, zirconium or hafnium at −78°C, were observed to methylenate carbonyl compounds. The presence of the alkylidenating agents prepared by the above method was confirmed by trapping reactions with propiophenone and with diphenylacetylene. Moreover, such zirconium methylidene and titanium methylidene agents were observed to catalyze the ring opening metathesis polymerization (ROMP) of norbornene.;Epimetallation reactions were explored through a numbers of approaches, prominent among them having been: (1) the elucidation of the role of transfer-epimetallation in the mechanism for the formation of the key titanacyclopropane intermediate in the Kulinkovich cyclopropanol synthesis; (2) the generation of authentic titanocene, Cp2Ti, from titanium dichloride and cyclopentadienyl sodium (CpNa), and its subsequent trapping with diphenylacetylene to afford 1,1-biscyclopentadienyl-2,3,4,5-tetraphenyltitanacyclopentadiene in high yield; (3) reaction of titanium dialkoxides, such as Ti(OMe)2 or Ti(OiPr)2 to effect the reductive dimerization of methyl benzoate to give benzoin or benzil upon hydrolysis; and (4) the catalytic role of MAO in Ziegler-Natta polymerization through the intiation of the epimetallation process of the alkene monomer by Cp2TiMe 2.;The present investigation of hydrometallation also uncovered that in a non-coordinating solvent like toluene, BunZrCl2 or BunHfCl2 prepared from ZrCL4 or HfCL 4 and two equivalents of n-butyllithium at −78°C, respectively, was found to provide hydride of the type, H2M tCl2, which were able to hydrometallate unsaturated organic substrates like 1-octene, 1-decene, cyclooctene, allylbenzene, diphenylacetylene and cis-stilbene to give their corresponding monomolecular reduction products or bimolecular reduction products (in case of diphenylacetylene) upon hydrolytic work-up. Isomerization of 1-decene or 4-phenyl-1-butene to internal olefins was also observed in some of these reactions. These findings permit the conclusion that these R2MtCl2 reagents can also undergo transfer-epimetallation depending on the solvents used.