Investigations on the radical chemistry of thionocarbonates of alcohols and acyl derivatives of hydroxamic and thiohydroxamic acids

Abstract

The radical deoxygenation of primary alcohols has been achieved using derivatives of perhalophenyl chlorothionocarbonates. Following the tradition of the Barton-McCombie reaction the deoxygenation proceeds via an organotin radical attack at the thiocarbonyl group. This forms an intermediate radical which rapidly decomposes to furnish the primary carbon radical. This is smoothly reduced by tributylin hydride and forms the chain carrying organotin radical. This is a clean and fast reaction and deoxygenation is achieved in a matter of minutes with high yields. Utilzing organotin radicals, a radical chain has been achieved with acyl derivatives of hydioxamic acids. This is the first example of a chain reaction via decarboxylation for these derivatives. Encouraging results were also obtained when the hydrogen source and chain carrier radical was replaced by an organosilane. Three acyl derivatives of hydroxamic acids are not photoactive to visible light the same way thiohydroxamic counterparts are. A new class of acyl derivatives of 2-aryl-3-hydroxy-3,4-dihydro-quinazoline-4(3H)-thione and its benzoquinazoline analogue has been introduced which were found to be more photolabelle than their N-hydroxy-2- thiopyridone oounterpart. In preliminary results of trapping experiments these new derivatives oompare well with the N-hythoxy-2-thiopyridone counterpart and are even less sensitive to lower temperatures. Quantum yields with various traps (CCI4. CBrCb. CBt4, (PhS)2. (PhSe)a and CH2-CHBO2Ph) were determined for primary, secondary and tertiary radicals produced by the action of light on the corresponding acyl derivatives of N4tydroxy-2-thiopyridone. These were in the range expected (10-30) for good radical chain reactions. Values for the new acyl derivatives were much higher (with CBrCis as a trap), sometimes even double. This corresponds to lower half-life values, which are an order of magnitude lower for these new acyl derivatives.