Keto carbenoids

There are two important rhodium-catalyzed transformations that are broadly used in domino processes as the primary step. The first route is the formation of keto carbenoids by treatment of diazo keto compounds with Rh11 salts. This is then followed by the generation of a 1,3-dipole by an intramolecular cyclization of the keto carbenoid onto an oxygen atom of a neighboring keto group and an inter- or intramolecular 1,3-dipolar cycloaddition. A noteworthy point here is that the insertion can also take place onto carbonyl groups of aldehydes, esters, and amides. Moreover, cycloadditions of Rh-carbenes and ring chain isomerizations will also be discussed in this section. 

The proposed reaction pathway invokes initial formation of carbonyl ylide 100 by intramolecular cyclization of the intermediate keto carbenoid onto the oxygen atom of the amide. Subsequent isomerization to the azomethine ylide is followed by 1,3-dipolar cycloaddition to DMAD to furnish the intermediate cycloadduct 101, which undergoes in situ alkoxy 1,3-shift to the final drhydropyrrolizine 102 (Scheme 3.28). 

The 1,3-dipolar cycloaddition of a-keto carbenoids to the polar double bond of heterocumulenes provides a direct access to five-membered heterocycles. The reaction of a-diazo ketones 132 with phenyl isocyanate in the presence of a Rh2(OAc)4 catalyst affords the 1,3-cycloadduct, 3-phenyl-2(3//)-oxazolones 133 (Fig. 5.32). ... 

Azomethine ylide cycloadducts derived from keto carbenoid cycliza-tion onto a thiobenzoxazole have also been encountered in our studies. When l-diazo-3-[2-(benzoxazolyl)thio]-2-propanone (235) was used, the initially formed cycloadduct 237 undergoes a subsequent [1,3]-... 

A related cyclization occurred using l-diazo-3-[(2-(pyridyl)thio]-2-pro-panone (146). The initial reaction involves generation of the expected pyridinium ion 147 by intramolecular cyclization of the keto carbenoid onto the nitrogen atom of the pyridine ring. Dipolar cycloaddition of 147 with DMAD affords... 

Carbenoids, derived from diazo ketones RCOCN2R (R = R = Ph R = Me, R = Ph R = Ph, R = Me) react with enaminones to give pyrroles, e.g., 144. The mechanism presumably involves the electrophilic attack of a keto carbenoid on N and/or at the C-a-position of an enaminone system, followed by cyclization and loss of water (88JOC2084). 

The procedure described here provides a direct synthesis of highly substituted furans (see Table). Reaction of keto carbenoids with acetylenes is normally an efficient method to prepare cyclopropenes. In numerous systems, however, the formation of furans was observed as a competing side reaction. Furan formation is particuiariy favored when the carbenoid is a pyruvate or contains two electron-withdrawing groups,and when eiectron-donating groups are present on the acetyiene. >8... 

Rhodium(II) catalyzed reaction of a-diazoazetophenone in the presence of 2-(methylthio)pyridine and DMAD gave 3-benzoyl-l,2-dicarbomethoxy-5-(methylthio)indolizine. The formation of the product proceeds via a pyridinium ylide formed by attack of the nitrogen lone pair on the electrophilic keto carbenoid <93JOCll44>. 

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