Highly Effective Catalytic Methods for Ylide Generation from Diazo Compounds. Mechanism of the Rhodium- and Copper-Catalyzed Reactions with Allylic Compounds

Article abstract of DOI:10.1021/jo00338a008

The catalytic effectiveness of rhodium(II) acetate and hexadecacarbonylhexarhodium for ylide generation in reactions of diazo compounds with allyl methyl sulfide, representative allylic tertiary amines, and allyl halides is described.Exclusive formation of products derived from the <2,3> sigmatropic rearrangement of ylide intermediates generated from ethyl diazoacetate and allyl sulfides, amines, or iodides is observed.Cyclopropanation is dominant in reactions of either diethyl diazomalonate or ethyl diazoacetate with allyl chloride, and competition between cyclopropanation and ylide rearrangement is exhibited in reactions with allyl bromide.Allyl iodide and allyl methyl sulfide are very similar in their ability to intercept the presumed carbenoid intermediates.Copper catalysis results in significantly higher yields of ylide-derived products, relative to cyclopropane products, than does rhodium catalysis in reactions with allyl bromides and chlorides.Reactions with crotyl bromide further exemplify the catalytic dissimilarities of rhodium and copper compounds and identify alternative transformations of metal-associated ylide intermediates.Cyclopropanation and metal ylide formation are represented as competing processes.Metal dissociation from the metal-associated ylide intermediate produces a reactive allylic ylide that is subject to the <2,3> sigmatropic rearrangement.However, products derived from rearrangement of the metal-associated ylide by allyl group transfer to the metal are observed to predominate in copper-catalyzed reactions of ethyl diazoacetate with crotyl bromide.These same products are not observed in rhodium(II) acetate catalyzed reactions performed at or below 25 deg C.