Carbonylation of the alkyl compounds [U(η-C5H4R)3R′] and rearrangement of the acyl derivatives [U(η-C5H4R)3-(η2-COR′)] into C6H4RR′. Influence of R and R′(Me, Prior But) and role of the solvent (benzene or totrahydrofuran)

Abstract

Carbonylation of the alkyl compounds [U(η-C5H4R)3R′]1 into the acyl derivatives [U(η-C5H4R)3(η2-COR′)]2 exhibited pseudo first-order kinetics (pCO = 1 atm). The rate constants were dependent on R, following the order But < Pri < Me < H and varied with R′ in the unusual order Pri < Me < But < Bun; for [U(η-C5H5)3R′], k(Bun)= 2k(But)= 4k(Pri). The rates were similar in benzene and in tetrahydrofuran. These results suggest that the migratory insertion of CO into the metal–carbon bond is not the rate-determining step. The rearrangement of the acyl complexes 2 afforded the alkylbenzene molecules C6H4RR′, resulting from ring enlargement of a C5H4R ligand by incorporation of the CR′ fragment. This reaction exhibited first-order kinetics. Whatever the solvent, the rate constants were found to depend markedly on R′, increasing in the order But < Pri < Bun < Me. In benzene, the rates varied with R in the sequence H < Me < Pri < But whereas the opposite trend was observed in tetrahydrofuran. For a given solvent, the relative proportions of meta- and para-isomers of C6H4RR′ were practically constant, whatever R and R′. These facts are best explained by a mechanism which involves a cyclopropyl intermediate resulting from addition of the oxycarbene group to the cyclopentadienyl ligand.