18207-17-5Relevant articles and documents
Novel Preparation of α,β-Unsaturated Aldehydes. Benzeneselenolate Promotes Elimination of HBr from α-Bromoacetals
Vasil'ev, Andrei,Engman, Lars
, p. 2151 - 2162 (2007/10/03)
Acetalization, α-bromination, nucleophilic phenylselenenylation, oxidative elimination/hydrolysis was investigated as a novel protocol for the α,β-dehydrogenation of aldehydes. Treatment of acetals with bromine in methylene chloride afforded the corresponding α-bromoacetals in 80-90% yields. Nucleophilic phenylselenenylation was then conveniently effected by treatment with benzenese-lenolate generated in situ in dimethyl sulfoxide from diphenyl diselenide, hydrazine and potassium carbonate. Unbranched α-bromoacetals cleanly afforded substitution products whereas β- and γ-branched ones gave substantial amounts of α,β-unsaturated acetals via formal loss of hydrogen bromide. Oxidative elimination/hydrolysis of these mixtures afforded α,β-unsaturated aldehydes in 50-80% overall yields. In the case of tertiary α-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.
