Covalent Assistance in Metal-Mediated [4 + 2] Cycloadditions of Butadiene and Acetylene in the Gas Phase

Abstract

The reactions of butadiene and acetylene, both complexed to atomic metal cations M + (M = Cr, Mn, Fe, Co, Cu), have been investigated using ion-beam four-sector and Fourier transform ion cyclotron resonance mass spectrometry. For all metal ions studied, with the exception of Cu +, the reaction proceeds as a formal [4 + 2] cycloaddition involving 1,4-cyclohexadiene/M + as an intermediate. This subsequently eliminates molecular hydrogen to generate the corresponding benzene/M + complexes or bare M + and C 6 H 6 . The Fe + -mediated reaction has been analyzed in detail, and isotopic labeling reveals that the cyclization step is rate-determining and that dehydrogenation of the six-membered ring occurs specifically from the C(1)/C(4)-positions of the butadiene building block. In conjunction with literature thermochemistry, qualitative potential-energy surfaces for the [4 + 2] cycloaddition are derived for M = Cr, Mn, Fe, and Co. The reactions are very efficient for Fe + and Co +, while Cr + and Mn + are less capable of inducing C−C bond formation. Finally, Cu + with its closed-shell s 0 d 10 electronic ground state does not mediate the [4 + 2] cycloaddition at all. These differences are explained in terms of a model which invokes the active participation of the transition metal's d orbitals (covalent assistance), rather than mere Lewis-acid catalysis, which is known to catalyze many Diels−Alder reactions in the condensed phase.