Formation of cis-Enediyne Complexes from Rhenium Alkynylcarbene Complexes

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Abstract
Dimerization of the alkynylcarbene complex Cp(CO)2ReC(Tol)C⋮CCH3 (8) occurs at 100 °C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)2Re]2[η2,η2-TolC⋮CC(CH3)C(CH3)C⋮CTol] (10- E and 10- Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)2ReC(Tol)C⋮CC3H5 (11) dimerizes at 120 °C to give a 5:1 ratio of enediyne complexes [Cp(CO)2Re]2[η2,η2-TolC⋮C(C3H5)CC(C3H5)C⋮CTol] (12- E and 12- Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)2ReC(Tol)C⋮CCH2CH2CH2C⋮CC(Tol) Re(CO)2Cp (13) dimerizes rapidly at 12 °C to give the cyclic cis-enediyne complex [Cp(CO)2Re]2[η2,η2-TolC⋮CC(CH2CH2CH2)CC⋮CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)2ReC(Tol)C⋮C]2C10H6 (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)2Re]2(η2,η2-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.

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