In our laboratory we are investigating the pathways for reactions of singlet carbenes with a variety of nucleophiles, including ethers and alcohols. We have studied solvent effects (ref 1) and the directing effects of nucleophilic substituents attached to substrates (ref 2) in an effort to detect reversible interactions between carbenes and nucleophiles. We have also used an intramolecular rearrangement to "trap" an ylide intermediate in the reaction of :CH2 with an alcohol (ref 3).
Here we describe the trapping of an ylide intermediate in the reaction of carboethoxycarbene with an allylic alcohol.
Considerable effort has gone into determining the pathways by which a variety of carbenes insert into the O-H bonds of alcohols (ref 4). Three pathways have been considered:
Carboethoxycarbene is an electrophilic carbene and, as such, would be expected to react via the ylide pathway (ref 4). Consistent with this expectation, previous experimental studies have shown that carboethoxycarbene reacts with ethers and alcohols at similar rates (suggesting similar reaction paths), and that the rate of reaction is insensitive to the acidity of the alcohol or to deuteriation of the hydroxyl group (ref 5). It should be noted, however, that this carbene is quite indiscriminate and reacts with a variety of substrates including dialkyl sulfides, alkenes, ethers and alcohols at comparable rates (k = 1 - 8 x 10e9)(ref 5a).
Here we report a different type of evidence for the formation of ylides in the reaction of carboethoxycarbene with alcohols: the trapping, through intramolecular rearrangement, of an ylide intermediate in the reaction with an allylic alcohol.
Rearrangement of allylic ylides as shown below may occur as either a concerted 2,3-sigmatropic shift or via homolytic cleavage of the heteroatom-carbon bond to form a radical pair (Stevens rearrangement) (ref 6).
Although the involvement of ylide intermediates in reactions of singlet carbenes with allylic substrates has often been inferred from the formation of rearrangement products (ref 7), this approach had not (to the best of our knowledge) been successfully applied to reactions of carbenes with alcohols until recently, when we trapped an ylide intermediate in the reaction of singlet :CH2 with an allylic alcohol (ref 3).