Reactions of alkyl and hydride derivatives of permethylzirconocene and permethylhafnocene with carbon monoxide. Synthesis and reactivity studies of aldehyde complexes of zirconium and hafnium

The alkyl hydride derivatives Cp2*M(H)(CH2CHMe2) (Cp* = η5‐C5Me5, M = Zr, Hf) react with carbon monoxide at low temperature to yield the η2‐acyl hydride complexes Cp M(H)(η2‐COCH2CHMe2) (1a, b), which on warming to room temperature under CO afford the carbonyl/isovaleraldehyde adducts Cp M(CO)(η2‐OCHCH2CHMe2) (4a, b). The zirconium derivative 4a slowly loses its coordinated CO and rearranges to the enolate hydride Cp Zr(H)(OCHCHCHMe2) (3). By contrast Cp Hf‐(CO)(η2‐OCHCH2CHMe2) (4b) rearranges without loss of CO to the cyclic enediolate tautomer (5), most likely by a 1,2H shift from the cyclic acyl derivative . The coordinatively unsaturated η2‐isovaleraldehyde adduct of permethylzirconocene [Cp Zr(η2‐OCHCH2CHMe2)] is implicated in reactions of the acyl hydride Cp Zr(H)(η2‐COCH2CHMe2) (1a) with the trapping substrates ethylene, 2‐butyne, dihydrogen, and tert‐butylacetylene to yield (12), (13), Cp Zr(H)(OCH2CH2CHMe2) (14), and Cp2*Zr(CCCMe3)(OCH2CH2CHMe2) (15). Carbonylations of the hafnacyclopentane (9) and the alkenyl hydride Cp Hf(H)(trans‐CHCHCMe3) do not show direct evidence for η2‐cyclopentanone or η2‐aldehyde intermediates. The former does, however, proceed directly to the bicyclic enediolate complex 11 most likely by the subsequent carbonylation of the cyclopentanone adduct. The latter undergoes carbonylation first to the hafna‐oxacyclopentene , which reacts further with CO to yield hafnadioxo‐cycloheptadiene (19). Possible mechanisms for these processes are discussed. The general natures of this chemistry complement theoretical and experiment results from the Hofmann group presented in the preceding article17.