Journal of the American Chemical Society, Vol.120, No.34, 8692-8701, 1998
New strategies in carbonylation chemistry: The synthesis of delta-lactones from saturated alcohols and CO
This paper describes the delta-carbonylation of saturated alcohols which uses a 1,5-hydrogen-transfer reaction of alkoxyl radicals and subsequent carbonylation at the delta-carbon atoms as the key. The carbonylation reactions of five classes of saturated alcohols, namely, primary alcohols having primary delta-carbons, primary alcohols having secondary delta-carbons, primary alcohols having tertiary delta-carbons, secondary alcohols having primary delta-carbons, secondary alcohols having secondary delta-carbons, were carried out,in which lead tetraacetate (LTA) was used as a one-electron oxidant to generate the alkoxyl radicals. Carbonylation of these saturated alcohols, except for primary alcohols having tertiary delta-carbons, took place to afford delta-lactones in moderate to good yields. The mechanism of the remote carbonylation likely involves (1) alkoxyl radical generation via LTA oxidation of a saturated alcohol, (2) conversion of this alkoxyl radical to a delta-hydroxyalkyl radical by a 1,5-hydrogen-transfer reaction, (3) CO trapping of the delta-hydroxyalkyl radical yielding an acyl radical, and (4) oxidation and cyclization of the acyl radical to give a delta-lactone. A metal salt-free system was also tested for a substrate derived from a tertiary alcohol having a secondary delta-carbon; the photolysis of an alkyl 4-nitrobenzenesulfenate under CO pressures gave a delta-lactone in moderate yield.