Reaction of aromatic N-oxides with dipolarophiles. Part 18. Formation mechanism and X-ray structure of the cycloadduct from sequential pericyclic reaction of pyridine N-oxides with phenylsulfonylallene

Abstract

Sequential pericyclic reactions of pyridine N-oxides (1) with phenylsulfonylpropadiene (2) and 1-phenylsulfonylpropyne (3) were investigated. 3,5-Dimethylpyridine N-oxide (la) was allowed to react with 2 in CHCl₃ at room temperature to give a mixture of the [1,5]-sigmatropic rearrangement product (4a) of the 1 : 1 cycloadduct and the 1 : 2 azetidine-type cycloadduct (5a). The structure of 5a was established by single crystal X-ray analysis. The reaction rate of 1a with 2 was about 50 000 times that of 1a with N-phenylmaleimide. The reaction of 1a with 3 did not give 5a but 4a as the sole product. The reactivity, regio- and peri-selectivity and formation mechanism of 4 and 5 are discussed in terms of the frontier molecular orbital consideration based on kinetic and molecular orbital calculation data.