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Covalent assistance in metal-mediated [4+2]cycloadditions of butadiene and acetylene in the gas phase

K. Schroeter, C. A. Schalley, R. Wesendrup, D. Schr”öder, H. Schwarz— 1997

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 C6H6. 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.

Title012
Covalent assistance in metal-mediated [4+2]cycloadditions of butadiene and acetylene in the gas phase
AuthorK. Schroeter, C. A. Schalley, R. Wesendrup, D. Schr”öder, H. Schwarz
Date19970304
IdentifierDOI 10.1021/om9609372
Source(s)
CitationOrganometallics 1997, 16, 986-994
TypeText