ACTIVATION OF C-C AND C-H BONDS OF CIS- AND TRANS-1-ACETYL-2-METHYLCYCLOPROPANE BY BARE METAL(I) CATIONS IN THE GAS PHASE - COMPARATIVE STUDY OF THE FIRST-ROW TRANSITION-METAL IONS CR+-CU+

摘要

Stereochemical effects on transition-metal-mediated C-H and C-C bond activation are probed by examining unimolecular fragmentation reactions of 1-acetyl-2-methylcyclopropane/M(+) complexes (M = Cr+-Cu+) in the gas phase. Basically, three general reaction types can be distinguished: (i) metal-induced ring cleavage leading to losses of molecular hydrogen, ethene, and acetaldehyde (these C-H and C-C bond activation processes can be ascribed to remote functionalization of an acyclic hexenone/M(+) intermediate), (ii) insertion of the metal in an or-C-C bond and subsequent decarbonylation to yield the corresponding olefin/M(+) complexes, and (iii) geminal C-C bond activation leading to the formation of metal carbene complexes as intermediates for M = Cr, Mn, Co, and Ni. These carbene complexes eventually give rise to unimolecular losses of ethene or propene. Direct C-H bond activation of the terminal methyl group is observed for none of the metals. Stereochemical differences for cis and trans isomers are only observed when the rate-determining steps for C-C bond activation leading to different products, i.e. remote functionalization and decarbonylation, compete directly with each other, and distinct stereochemical effects are observed for the Mn+, Fe+, and Cu+ complexes. The experimental data and the trends within the late first-row transition metals are compared with previous findings and discussed in terms of activation barriers associated with C-C and C-H bond activation processes and thermodynamics. [References: 53]