FACE SELECTIVITY OF THE C-H BOND ACTIVATION OF CYCLOHEXANE BY THE BARE FIRST-ROW TRANSITION-METAL CATIONS SC-ZN+()

The ability of the first-row, atomic transition-metal cations from Sc through Zn+ to activate C-H and C-C bonds in cyclohexane has been exa mined by means of ion cyclotron resonance and sector-field mass spectr ometry. Three types of reactions can be distinguished. (i) The early-t ransition-metal cations Sc+, Ti+, and V+ mediate multiple dehydrogenat ion of cyclohexane to yield benzene/M(+) as the major product ion. (ii ) For the late, group 8 transition-metal cations Fe+, Co+, and Ni+ sin gle dehydrogenation is observed preferentially, and for Co+ and Ni+ th e formation of M(C3H6)(+) complexes demonstrates that these metal ions also activate C-C bonds of cyclohexane. (iii) For Cr+, Mn+, Cu+, and Zn+ there is no indication of any C-H or C-C bond activation. The ster eochemistry of the C-H bond activation of cyclohexane/M(+) was explore d by investigating specifically labeled [all-cis-1,2,3,4,5,6-D-6]-cycl ohexane. For all transition-metal cations studied here, the first dehy drogenation occurs stereospecifically in terms of a syn elimination. I n the subsequent dehydrogenations this regiospecificity is maintained for the early transition metals Sc+, Ti+, and V+, whereas the group 8 transition metals Fe+, Co+, and Ni+ induce H/D exchange processes invo lving both faces of the cycloalkane ring; most probably, acyclic inter mediates are involved.