Silver carbene intermediacy

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

On the basis of these results, a mechanism (Scheme 8.10) involving the intermediacy of a silver-carbene 54 was proposed in which the insertion product arises from the formation of the halonium ylide 55, followed by a 1,2 shift (55 —> 26, or 51 or 52). Alternatively, if the substrate and thus the halonium ylide 56 contain a (3-hydrogen, this could be removed by an intramolecular deprotonation with concomitant loss of halide resulting in formation of the olefin 57 and the a-haloacetate 53. At this stage, no independent evidence has been obtained to support this pathway thus this mechanism is purely speculative (see text below). Indeed, although the pathway has been depicted as involving metal-free intermediates, it is quite likely that this is not the case, but this awaits independent experimental verification. 

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. 

Mechanistically, most alkyne cyclizations of this type have been interpreted in terms of the catalyst serving as a Lewis acid, activating the n bond to nucleophilic attack,53-57 and while this pathway is feasible, the authors preferred one involving the intermediacy of a silver carbene.52 Control studies ruled out a pathway involving... 

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