Metallacyclobutene complexes (e.g. 1) have been proposed as reactive intermediates in a number of important metal-mediated reactions of alkynes. A particularly fascinating aspect of metallacyclobutene reactivity stems from the fact that the metallacycle ring contains both metal-(sp3)carbon and metal-(sp2)carbon bonds. Thus, reactions of small molecules with the propendiyl ligand may, in principle, lead to bond formation at either alpha metallacycle carbon. For example, cobaltacyclobutene complexes (1), readily undergo reaction with additional ethyl diazoacetate to give 1,3-diene complex (2) for which the carbene has coupled to the C1 carbon of the propendiyl ligand. Alternatively, alkenes undergo reaction with 1 to give 1,4-diene complexes, indicative of alkene coupling to the C3 carbon of 1. The reactions of alkynes with 1 lead to highly substituted cobalt-cyclopentadiene complexes (4), for which the structure does not reveal which propendiyl carbon (C1 or C3) initially coupled to the alkyne. Herein, we report experimental and computational studies which support a mechanism for cyclopentadiene formation that involves intial coupling of the alkyne to C1 of the metallacyclobutene, followed by coupling at C3.

Mechanistic studies on the conversion of metallacyclobutenes to highly substituted cyclopentadienes

254th National Meeting (2017)

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