Cobalt-Catalyzed Homo-benzannulation Reaction

Complementary to the palladium-catalyzed version, the [4 - - 2] homo-benzannulation of enynes under a cobalt catalysis was reported by Hilt group [12]. It was found that 4-substituted enynes formed the corresponding 1,6-disubstituted styrenes in moderate 

TABLE 14.2 Co-catalyzed [4 -I- 2] Homo-benzannulation Reaction Toward 2 -Styrenes 

SCHEME 14.15 Co-catalyzed [4 - - 2] homo-benzannulation reaction toward 2,6-styrenes. 

Remarkably, a change of solvent from DCM to tetrahydrofuran (THF) resulted in an unprecedented switch of reaction regioselectivity, leading to the formation of 2,3-substituted styrenes 8. A variety of 4-aryl- or heteroaryl-substituted enynes was shown to react with good efficiency (Table 14.2, entries 1 to 8). Notably, in the case of bromo-substituted enyne, a clean reaction without protodebromination or formation of crosscoupling products occurred (entry 4). For a dienyne substrate, having both terminal and endocyclic double bonds, the former double bond reacted regioselectively, while the double bond of the cyclohexene subunit remained untouched (entry 9). 

SCHEME 14.16 Co-catalyzed cyclolrimelization of an electron-deficient conjugated enyne. 

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