PHOTOINDUCED [6 + 2] CYCLOADDITIONS OF ALKYNES TO TRICARBONYL(CYCLOHEPTATRIENE)CHROMIUM(0)

Photolysis of [(eta(6)-C7H8)Cr(CO)3] (1) and RC = CR (2a, R = Ph; 2b, R = Tol) in toluene or n-hexane results in the formation of [(eta(2): eta(4)-C9H8R2)Cr(CO)3] (3a,b) in 70-85% isolated yield via a [6 + 2] cycloaddition reaction. Heating 3a,b in toluene gives the bicyclo [4.2.1]-nona-2,4,7-trienes 4a,b and [(eta-C6H6Me)Cr(CO)3]. Photolysis of 1 and excess DMAD (dimethyl acetylenedicarboxylate, 5) in toluene at 0-degrees-C yields the bicyclotriene 4c as the major product and small amounts of the arene C6(CO2Me)6 and the [4 + 2] cycloadduct between 4c and 5.