Reaction Optimization and Scope of the [4+2+1] reaction. Table lists our optimization efforts of the [4+2+1] reaction using CP-capped diene-yne 1a and CO. In the presence of 5 mol% [Rh(CO)2Cl]2 as the catalyst in toluene at 80 °C, the [4+2+1] product 2a was obtained in 60% NMR yield . Neither iridium nor cobalt catalyst can achieve the [4+2+1] reaction in an acceptable yield (Table and). We then screened other solvents , finding that benzene had the best performance, giving rise to 2a in 71% yield (Table ). Replacing benzene by the less toxic solvent of toluene was tested, finding that higher reaction temperature (100 °C) and longer reaction time (2 hours) were necessary to produce 2a in 55% yield (Table ). To our delight, 2a was obtained in 81% NMR yield and 76% isolated yield when the reaction was carried out at 40 °C for 24 hours (Table ). Performing the [4+2+1] reaction at 30 °C failed to improve this cycloaddition reaction (Table ). Neither increasing nor reducing the concentration of substrate could get a better yield of the [4+2+1] reaction (Table , entries 10 and 11). As a result, we chose the conditions shown in entry 8 of Table for further investigation of the reaction scope.