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DOI: 10.1055/sos-SD-046-00090

Gandon, V.; Thorimbert, S.; Malacria, M.Science of Synthesis, (200946181.

Linear CH activation products are encountered during attempted cocyclizations of alkynes with a double bond of an indole,[‌34‌] pyrimidine,[‌35‌] pyridin-2-one,[‌36‌,‌37‌] furan, or thiophene.[‌38‌] For instance, the N-alkynylated pyridin-2-one 27 undergoes [2+2+2] cocyclization with bis(trimethylsilyl)acetylene to give the expected (η5-cyclopentadienyl)cobalt-complexed cyclohexadiene 28 (Scheme 11) (see also Section[‌36‌] However, the length of the tether attached to N1 of the heterocycle is important and, in the case of the homologue 29, the formation of the (η5-cyclopentadienyl)cobalt-complexed 1,3-diene 30 is observed. Oxidative demetalation of the complex 30 using iron(III) nitrate may then be achieved to give the free diene 31 in quantitative yield. When heated in toluene at 110°C for 12hours the diene 31 undergoes cyclization and desilylation to give the 2,3-dihydro-1H,5H-pyrido[ij]quinolizin-5-one 32 in 95% yield (Scheme 11). The mechanism of cobalt-mediated CH activation has been studied using density functional theory (DFT) computational methods;[‌37‌,‌39‌] moreover, such processes are not restricted to hetarenes as alkenylcarbonitriles also provide linear 1,3-dienes in the presence of a cobalt complex.[‌40‌]

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