J Org Chem. 2000 Apr 07;65(7):2151-62. doi: 10.1021/jo9917644.

Novel preparation of alpha,beta-unsaturated aldehydes. Benzeneselenolate promotes elimination of HBr from alpha-bromoacetals.

The Journal of organic chemistry

Vasil'ev, Engman

PMID: 10774040 DOI: 10.1021/jo9917644

Abstract

Acetalization, alpha-bromination, nucleophilic phenylselenenylation, oxidative elimination/hydrolysis was investigated as a novel protocol for the alpha,beta-dehydrogenation of aldehydes. Treatment of acetals with bromine in methylene chloride afforded the corresponding alpha-bromoacetals in 80-90% yields. Nucleophilic phenylselenenylation was then conveniently effected by treatment with benzeneselenolate generated in situ in dimethyl sulfoxide from diphenyl diselenide, hydrazine and potassium carbonate. Unbranched alpha-bromoacetals cleanly afforded substitution products whereas beta- and gamma-branched ones gave substantial amounts of alpha,beta-unsaturated acetals via formal loss of hydrogen bromide. Oxidative elimination/hydrolysis of these mixtures afforded alpha,beta-unsaturated aldehydes in 50-80% overall yields. In the case of tertiary alpha-bromoacetals, treatment with benzeneselenolate afforded only dehydrobromination products as mixtures of isomers. The presence of at least a catalytic amount of the organoselenium reagent was found to be crucial for olefin formation. A SET-mechanism, involving benzeneselenolate-induced electron transfer to the halide, loss of bromide ion, and hydrogen atom or proton/electron was proposed for the benzenselenolate-promoted elimination reaction. Experiments designed to trap carbon-centered radicals in intramolecular cyclization or ring-opening reactions failed to provide any evidence for free-radical intermediates.

Publication Types

• Journal Article