| Abstract
| - The mechanism of ketene−diene reactions has been studied both experimentally and theoretically.Careful experiments of the reactions of diphenylketene (1) with cyclic (s-cis) 1,3-dienes [cyclopentadiene (2)and cyclohexa-1,3-diene (3)] lead to the first direct detection of the Diels−Alder cycloadducts (10 and 11) bylow-temperature NMR spectroscopy. The initially formed cycloadducts are converted to the final Staudingerproducts, cyclobutanones (6 and 7), by [3,3] sigmatropic (Claisen) rearrangements. In contrast, ketene 1 reactswith open-chain 1,3-dienes [2,3-dimethyl-1,3-butadiene (4) and 1-methoxy-1,3-butadiene (5)] to afford initiallyboth the Staudinger-type (8, 9) and Diels−Alder-type cycloadducts (12, 13). The Staudinger cycloadducts (8,9) are converted eventually to Diels−Alder products (12, 13) by the retro-Claisen rearrangement. Thus, ketenerecognizes dienes in cycloadditions as ketenophiles different from olefins. [4 + 2] and [2 + 2] cycloadductsare generated and can be intermediates or products flexibly according to diene structures.
|
