Dienophilic behavior of the vinylic (C=C) and the carbonyl (C=O) bonds of ketenes in reactions with 1,3-diazabuta-1,3-dienes

Ab initio and density functional studies (DFT) on cycloaddition reactions o f 1,3-diazabuta-1,3-dienes with ketenes are reported. The vinylic (C = C) a nd the carbonyl (C = O) units of the ketenes are found to participate in co ncerted asynchronous [4 + 2] cycloaddition reactions. The transition states (3t, 4t, and 7t) for these paths have been located on the RE surface at th e correlated levels of ab initio calculations. A reasonable mechanism for t he formation of [4 + 2] and [2 + 2] adducts is presented.