Rhodium Alkenylidenes in Catalysis

When the electrophile is an alkyl halide, a C—C a-bond is forged thus, alkenylidene formation is irreversible. Vinylidene formation by 1,2-migration, on the other hand, is generally reversible. Because of this contrast, alkenylidenes can offer access to new catalytic reaction manifolds, in addition to unique molecular architecture. 

Lee and coworkers went on to show that the concept of alkenylidene formation and functionalization by a single catalyst can be applied to other transformations. Under conditions similar to those reported by Trost and coworkers for vmylidene-mediated catalytic intramolecular hydroalkoxylation (see Section 9.2.3), alcohol 111 was transformed into a mixture of enol ethers with moderate selectivity for three-component coupling (Equation 9.9). 

Like alcohols, arenes can attack the electrophilic a-position of metal vinylidenes (see Section 9.4.6). Substrate IIS was transformed into tetracycle 117 in high yield, presumably via 6it-electrocyclization and subsequent rearomatization (Equation 9.10). To date, no intermolecular examples of metal alkenylidene-mediated catalysis have come to light. The extension of Lee s alkylative approach to catalysis by other metals may prove fmitfiil in this regard. 

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