The reactivity patterns of late-metal metallacyclobutene complexes are much less developed than those of metallacyclopentadiene complexes, despite frequent literature suggestions that metallacyclobutenes are key intermediates in a number of important metal-catalyzed reactions of alkynes and carbenes. Here we report the first thermolysis studies on metallacyclobutene complexes, including the reversible formation of dinuclear vinylcarbene complexes. The cobaltacyclobutene complexes, (C5H5)(PPh3)Co{κ2-[C(R1)=C(R2)CH(COR3)]} (R1 = SO2Ph, R2 = SiMe3, R3 = OEt; R1 = SO2Tol, R2 = SiiPr3, R3 = OEt; R1 = Ph, R2 = COCF3, R3 = OEt; are readily prepared from the corresponding cobalt-alkyne complexes,(C5H5)(PPh3)Co{η2-[C(R1)≡C(R2)]} upon reaction with ethyl diazoacetate. Thermolysis of the cobaltacyclobutenes in the presence of CpCo(PPh3)2 generates dicobalt-vinylcarbene complexes, which have been characterized by X-ray crystallography. Mechanistic and computational studies support the rate-limiting loss of triphenylphosphine to form η3-vinylcarbene intermediates which are trapped by "CpCo" to give the observed dicobalt products. Air oxidation of one of these dicobalt complexes in the presence of triphenylphosphine regenerates the starting metallacyclobutene complex. The implications of these reactions for understanding the chemical reactivity patterns of late-metal metallacyclobutene complexes will be discussed.

Thermal reactivity of late-metal metallacyclobutene complexes:  Reversible formation of dicobalt-vinylcarbene complexes

254th National Meeting (2017)

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