Alkenyl indoles, cyclization carboxylation

Scheme 44 Widenhoefer s cyclization/carboxylation of alkenyl indoles...

Interestingly, Widenhoefer reported a similar palladium(II) catalyzed cycliza-tion of indoles onto alkenes (Scheme 58) [72]. This mild protocol for cyclization/ carboxylation of 2-alkenyl indoles makes possible catalytic addition of a carbon-nucleophile and carbonyl group across a C-C bond. The mechanism, however, is thought to involve outer-sphere attack of indole onto a palladium-olefin complex rather than the electrophilic C-H activation of the indole C(3)-H bond, exhibited by the Stoltz carbocyclization. 

Recently, palladium(II)-catalyzed arylation/carboxylation reactions were reported for the synthesis of carbazoles starting from 2-alkenyl indoles. These reactions employ the electron-rich C-3 carbon atom of the indole nucleus as carbon nucleophile in cyclization/carboalkoxylation reactions. The conditions that proved effective for 2-(2,3 or 5-alkenyl)-3-unsubstituted indoles are similar to those reported for the cyclization ofalkenes and alkynes bearing tethered heteronucleophiles. In particular, the best results has been obtained using PdCl2(CH3CN)2, CuCl2 in methanol under CO (latm). Under these conditions methyl (9-methyl-2,3,4,9-tetrahydro-4-carbazolyl)acetate 86 was isolated in 94% yield (Scheme 13.53) (100,101]. 

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