Carbene deinsertion

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

O Connor proposed a mechanism involving deinsertion of carbon monoxide from the vinylketene complex 106 to form the new cobaltacyclobutene 109. The cobalt may then undergo a 1,3-shift to the carbonyl of the ester group to create the oxycobaltacycle 110, before deinsertion of the cobalt moiety forms the furan 108. Alternatively, 109 may rearrange to the vinyl-carbene 111, which then undergoes ester-carbonyl attack on the carbene carbon to form the zwitterionic species 112, which finally aromatizes to yield the furan 108. Notice that this latter postulate is identical to the final steps of the mechanism formulated by Wulff (see Section V,B) for the reaction between a cobalt carbene and an alkyne, in which a cobaltacyclobutene is a key intermediate.51... 

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