HOMOGENEOUS CATALYSIS - CATALYTIC INTRAMOLECULAR CONVERSION OF 1,4-DIALDEHYDES TO GAMMA-LACTONES

A series of catalysts of the type [Rh(diphosphine)(solvent)2]+ (solvent = weakly coordinating solvent) were investigated for converting 1,4-dialdehydes and 1,4-keto aldehydes to the corresponding γ-lactones, the equivalent of the intramolecular Cannizzaro reaction. It was found that a number of these species were very effective catalysts which tolerated a high degree of steric hindrance and transformed the substrates without any detectable enolization. The carbonylated species [Rh(diphosphine)(CO)2]+ and [Rh(diphosphine) (CO) (solvent)]+ are ineffective in catalysis and are the side products of intramolecular lactonization. Decarbonylation of the substrate to give these inactive carbonylated species is a minor component of the catalysis with 1,4-dialdehydes but is the major pathway for the lactonization of 1,4-ketoaldehydes. These catalysts provide a practical, mild catalytic method of converting 1,4-dialdehydes to 7-lactones. The catalysis is very rapid, occurring even at −60 °C. At −78 °C a stable substrate (η2-aldehyde)rhodium adduct was identified and is possibly the precursor to aldehyde C–H activation. No other catalytic intermediates could be intercepted at low temperatures. The catalysis appears to be a “black box” system where, once the C–H bond is activated, none of the putative intermediates exist in detectable concentrations. © 1990, American Chemical Society. All rights reserved.