166 It was postulated that in the presence of the rhodium catalyst dediazotization leads to a rhodium–carbenoid intermediate resulting in a more selective carbenoid type reaction as opposed to the highly reactive carbene intermediate. Recently, Rh 2(OAc) 4 has been used as a catalyst to decompose N-aziridinyl imine 95 to give ( Z)-selective alkene 96 via a rhodium–carbenoid intermediate ( Scheme 27). 171 In general the more highly substituted alkene is the favored product, but the selectivity is often low, with high selectivity only observed in a few cases. Whereas five- and six-membered rings give predominantly the alkene product, 170 medium-sized rings tend to result in bicyclic products via transannular carbene insertion.

In some reactions other products such as the β-insertion product and skeletal rearrangement products are observed. Decomposition normally requires heating and lead directly to the alkene. Only relatively weak bases need to be employed to deprotonate the NH proton of sulfonylhydrazones and initiate decomposition, but high temperatures are normally required ( Scheme 27). The aprotic Bamford–Stevens reaction, most commonly carried out in diethylene glycol dimethyl ether, proceeds via carbene intermediates to give alkenes in good yields, and has found particular use in the synthesis of multicyclic, strained hydrocarbons. In contrast, ( 64), subjected to the aprotic condition (excess sodium methoxide in diglyme at 135–140 ☌), gives diazetine ( 69) via carbene intermediate ( 68 Scheme 6). For example, in the case of the protic medium, reaction of oxabicyclic ketone tosylhydrazone ( 64) with sodium in ethylene glycol at 115–120 ☌ yields hydroxy ketone ( 67), mainly via the intermediate diazo compound ( 65) followed by the exo-diazonium cation ( 66). 53 Very often different behavior is observed in the decomposition of tosylhydrazones of aliphatic ketones under protic or aprotic Bamford–Stevens conditions. 56 Possible mechanisms in the formation of these products are in agreement with previous mechanistic conclusions concerning tosylhydrazone decompositions.

The tosylhydrazone ( 63), bearing an adjacent cyclopropyl group, decomposes on treatment with sodium methoxide in ethylene glycol to give four alkenic products (equation 44). Ethylene glycol is the most popular and reliable solvent. The original Bamford–Stevens reaction employs protic solvents in the decomposition of a tosylhydra- zone with a strong base.

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