Benzenes, reactions with vinylcarbenoids

Reaction with benzene follows a similar pathway, yielding a bicyclo[3.2.2]nonatriene structure. Vinylcarbenoids also react with pyrroles to give tropanes via a cyclopropanation-Cope rearrangement route. The direct addition of carbenes to acetylenes does not give satisfactory yields of cyclopropanes, but the rhodium carboxylate catalysed reaction of diazo compounds with acetylenes is a useful source of cyclopro-panes. Carbenoids can also attack a carbonyl oxygen atom, giving rise to a zwitterion (249). An excellent review of intramolecular carbenoid reactions has appeared. ... 

The reaction of vinylcarbenoids is not limited to dienes but may also be extended to a variety of electron-rich aromatics. Rhodium(II) trifluoroacetate catalyzed decomposition of 4 in the presence of benzene results in the formation of the unstable bicyclo[3.2.2] system 63 (Scheme 23). Direct hydrogenation of 63 results in the formation of 64 from 4 in 29% overall yield. Similar reactions occur with other alkyl benzenes, although these reactions are not of practical utility because mixtures of unstable isomeric products are formed. 

The intramolecular version of the reaction of vinylcarbenoids with a benzene ring is interesting because several isomeric structures can be formed, and the isolated product is very dependent on the reaction conditions. Rhodium(II) octanoate catalyzed decomposition of 118 at 0 °C, results in forming the norcaradiene 119 in 48% yield (Scheme... 

An alternative approach for intramolecular reactions between vinylcarbenoids and benzenes was reported by Kohmoto and co-workers starting with the (5H)-pyrazole 122 (Scheme 42). Photolysis of 122 followed by thermolysis at 110 °C results in the formation of the norcaradiene 123 as a mixture of stereoisomers. The norcaradiene 123... 

A successful [3+4] annulation between vinylcarbenoids and pyrroles would be a very useful transformation because it would result in a direct access to the tropane skeleton. The direct reaction of the vinyldiazomethane 4 with pyrrole does not lead to the formation of [3+4] annulation products. Instead, the alkylation product 87 is formed in 71% yield (Scheme 31). This result parallels that observed with electron-rich benzene derivatives. 

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See also in sourсe #XX -- [ ]

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