Isolation of a Diazoalkane Intermediate in the Photic Bamford-Stevens Reaction

Article abstract of DOI:10.1021/jo00336a012

Arylsulfonylhydrazones decompose in alkaline media on thermolysis (Bamford-Stevens reaction) or on UV irradiation.The thermal method is known to produce a diazo hydrocarbon, which gives products by carbenic and/or cationic routes according to the reaction conditions.In contrast, mechanistic information on the photic decomposition is lacking.We now report a successful isolation of a diazo hydrocarbon from direct photolysis of a tosylhydrazone sodium salt and present evidence that the diazo compound is an intermediate in the overall photic Bamford-Stevens reaction.Our key substrate was spiro<12.11>tetracosan-13-one (5a), synthesized from cyclododecanone by a three-step sequence involving reductive coupling, epoxidation of the derived olefin, and rearrangement of the oxirane.Photic Bamford-Stevens reaction of the tosylhydrazone sodium salt of 5a under different alkaline conditions was brought about by UV irradiation through a Vycor filter (i.e., λ > ca. 220 nm).The products were (E)-spiroalkene 11 (75-80percent), (Z)-spiroalkene 12 (10-14percent), and saturated hydrocarbons (10-11percent).The alkanes were not individually identified but are expected products of intramolecular C-H insertions by a carbene.By using lower energy light (i.e., Pyrex filter; λ > ca. 270 nm), we isolated instead 13-diazospiro<12.11>tetracosane (14) in 95percent yield.This diazo hydrocarbon was characterized spectroscopically, by elemental analysis, and by an independent synthesis.Then, on separate irradiation with Vycor-filtered light, 14 gave a product mixture, which, after correction for a small contribution from cationic pathways, contained E olefin 11, Z olefin 12, and saturated hydrocarbons in a ratio of 78:12:11, the same as that from the uninterrupted photic Bamford-Stevens reactions.We also found that thermal Bamford-Stevens conditions give markedly different product ratios, viz., 90percent 11, ca. 0percent 12, and 10percent alkanes.These differences suggest that the "carbene" (or diazo hydrocarbon) derived thermally and the one derived photically behave differently with respect to the stereoselectivity of the 1,2 H shift that produces olefin.