| Research Abstract |
Non-Recognition for 1,3-dienes in ketene reactions has long been an important problem in organic chemistry. We have solved the problem in this research. The mechanism of ketene-diene cycloadditions has been studied both theoretically and experimentally. The paths of the cycloadditions of diphenylketene to cyclic and linear dienes are traced by precise experiments and ab initio calculations. The obtained results show that the addition paths and final products depend upon the structures of dienes, cyclic (with s-cisform) or linear (with s-cis and s-trans forms). The cycloadditions of diphenylketene to cyclic dienes are two-step processes, composed of [4 + 2] addition and Clasien rearrangement, leading to [2 + 2] cycloadducts. On the other hand, those of diphenylketene to linear dienes are two-step processes, composed of [2 + 2] addition and retro-Claisen rearrangement, [4 + 2] cycloadducts.
Careful experiments in the reactions of diphenylketene with cyclic (s-cis) 1,3-dienes lead to the f
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irst detection of the Diels-Alder cycloadducts (α-methylenedihydropyrans) by low-temperature NMR spectroscopy. The initially formed cycloadducts are converted by the final [2 + 2]-type products, cyclobutanones, by a [3,3] sigmatropic (Claisen) rearrangement. In contrast, the ketene reacts with open-chain 1,3-dienes (with s-cis and s-trans forms) to form initially both the [2 + 2]-type and [4 + 2]-type cycloadducts. The formers are converted to [4 + 2]-type cycloadducts by retro-Clasien rearrangement. Thus, ketene distinguishes s-cis 1,3-dienes from s-trans ones leading to the initially formed [4 + 2] and [2 + 2] cycloadducts, respectively. The [2 + 2] cycloadduct is converted to the final [4 + 2] product through retro-Claisen rearrangement. Those experimental results are consistent with theoretical ones perfectly. We discovered that ketene is a dienophile not well-known [2 + 2] cycloaddition but for [4 + 2] (Diels-Alder) reactions across its C=O bond. Thus, we have made an innovation in ketene chemistry. Less
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