Nature of the species present in the zirconocene dichloride butyllithium reaction mixture

The thermal decomposition of dibutylzirconocene (1) at room temperature affords paramagnetic butylzirconocene(III) (11), zirconocene(III) hydride (12), the diamagnetic butenylzirconocene(IV) hydride dimer 5a, and the 1,1-bis(cyclopentadienyl)-2-methyl-3-(zirconocenyl hydride)-1-zirconacyclobutane(IV) dimer 9. Initially, decomposition furnishes crotylzirconocene(IV) hydride (3a), followed by 1,1-bis(cyclopentadienyl)-2-ethyl-1-zirconacyclopropane(IV) (2a) and 1,1-bis(cyclopentadienyl)-3,4-diethyl-1-zirconacyclopentane(IV) (4a), listed in the order of appearance. This order suggests that the primary decomposition reaction is a gamma-H abstraction, which leads to the formation of 1,1-bis(cyclopentadienyl)-2-methyl-1-zirconacyclobutane(IV) (6a). The latter was not observed experimentally but is postulated on the basis of secondary products. Reactions leading to the above compounds are discussed from mechanistic and thermochemical points of view. The reported compounds have been characterized by either EPR or multidimensional, multinuclear NMR spectroscopy. Compound 3a has also been synthesized independently from zirconocene chloride hydride and crotylmagnesium bromide and undergoes the same rearrangements into 2a and then 4a. The allyl analog 3b, synthesized from zirconocene chloride hydride and allylmagnesium bromide, exhibits the same behavior and rearranges into 1,1-bis(cyclopentadienyl)-3,4-dimethyl-1-zirconacyclopentane (4b).