The photolysis of 1-phosphabarrelenes, generated from 3,5-diarylphosphinines and benzyne in a [4+2] cycloaddition reaction, affords hithertoe unknown 2-phosphasemibullvalenes via di-π-methane rearrangement reaction. These compounds occur only as intermediates, while subsequent and rapid dimerization to 6-membered, cyclic diphosphanes with a P-P bond was observed. The results are in stark contrast to the photochemical conversion of 1-phosphabarrelenes, obtained from 2,4,6-triarylphosphinines and a strong dienophile. In this case, the corresponding 5-phosphasemibullvalenes are formed selectively and exclusively. Our results nicely demonstrate the strong impact of the substitution pattern of the starting material on the outcome of the di-π-methane rearrangement reaction.

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