Vinylic to aryl rhodium migration

A few examples of vinylic to aryl rhodium migration have also been observed. One such example was discovered independently by two groups almost at the same time [77,78], In this reaction, aryl propargyl alcohols undergo a rhodium-catalyzed isomerization to furnish indanone products (Table 12). The reaction proceeds by a fairly complicated mechanism shown in Scheme 19. Thus, after base-promoted ligand... 

Scheme 20 Rhodium-catalyzed hydroarylation of alkynes with vinylic to aryl rhodium migration...

In addition to palladium, rhodium is also known to migrate from one carbon to another under suitable conditions. However, compared with palladium migration processes, rhodium migration is scarce. To date, only a few examples are reported, all involving 1,4-alkyl/vinylic to aryl migrations. 

Scheme 19 Proposed mechanism for the rhodium-catalyzed isomerization with vinylic to aryl migration...

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

Eilbracht et al. have developed rhodium- or ruthenium-catalyzed one-pot synthesis of cyclopentanones from allyl vinyl ether via tandem Claisen rearrangement and hydroacylation [109-111]. This protocol requires elevated temperature (140-220°C) and also requires alkyl or aryl substituents at the terminal position of the allylic double bond to prevent undesirable double bond migration in the intermediary formed, unsaturated aldehyde. 

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