1,2-Shifts addition-fragmentation mechanism

Unsaturated groups, however, can shift by a two-step addition-fragmentation mechanism via a discrete cyclopropane radical intermediate. Phenyl groups are especially good at undergoing radical 1,2-shifts 1,2-shifts of acyl, alkenyl, and, to a lesser extent, alkynyl and cyano groups are also seen. 

Heavy atoms such as Br and Se can also undergo a radical 1,2-shift by an addition-fragmentation mechanism. An intermediate that features a heavy atom with a nonet of electrons intervenes. First-row elements cannot accommodate a nonet of electrons like heavy elements can, so the addition-fragmentation mechanism is disallowed for them. 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

The formation of the methyl radical (reaction 2 in Scheme 15) is believed to be caused by the primary split of a C-N bond, followed by a hydrogen shift and further fragmentation. This mechanism is backed by the observation that addition of NH to ethylene yields methyl radicals (244). CH3 formation could also be a secondary process, reaction 70 (245-247). The precursor of some of the acetylene is thought to be an excited ethylene... 

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