Demystifying CpTi(H)Cl and its Enigmatic Role in the Reactions of Epoxides with CpTiCl.

The role of CpTi(H)Cl in the reactions of CpTiCl with trisubstituted epoxides has been investigated in a combined experimental and computational study. Although CpTi(H)Cl has generally been regarded as a robust species, its decomposition to CpTiCl and molecular hydrogen was found to be exothermic (ΔG = -11 kcal/mol when the effects of THF solvation are considered). In laboratory studies, CpTi(H)Cl was generated using the reaction of 1,2-epoxy-1-methylcyclohexane with CpTiCl as a model. Rapid evolution of hydrogen gas was demonstrated, indicating that CpTi(H)Cl is indeed a thermally unstable molecule, which undergoes intermolecular reductive elimination of hydrogen under the reaction conditions. The stoichiometry of the reaction (CpTiCl:epoxide = 1:1) and the quantity of hydrogen produced (1 mole per 2 moles of epoxide) is consistent with this assertion. The diminished yield of allylic alcohol from these reactions under the conditions of protic versus aprotic catalysis can be understood in terms of the predominant titanium(III) present in solution. Under the conditions of protic catalysis, CpTiCl complexes with collidine hydrochloride and the titanium(III) center is less available for "cross-disproportionation" with carbon-centered radicals; this leads to by-products from radical capture by hydrogen atom transfer, resulting in a saturated alcohol.