Alan T. Dronsfield

THE THERMOLYSIS OF BENZYL COBALOXIMES : A NEW ONE STEP SYNTHESIS OF 5-ARYLISOXAZOLES

Abstract When benzyl cobaloximes are either dry-distilled or boiled in xylene solution they afford the corresponding 5-arylisoxazoles in moderate to good yields.

Antabuse's diamond anniversary: still sparkling on?

Seventy-five years after its first medical use was described, disulfirams place in treatment is being favourably re-evaluated. This paper reviews its discovery and subsequent development.

Thermochemical studies of benzyl cobaloximes: a decomposition pathway different from that reported for the alkyl analogues

Abstract Differential scanning calorimetry, differential thermal analysis and thermogravimetric analysis indicate that, like their alkyl analogues, benzyl cobaloximes decompose via a three-step mechanism. However mass spectroscopic studies reveal that the sequence of steps is different and involves compounds which do not feature in the decomposition of the alkyl counterparts. These include both a monoxime derived from NOH displacement from the dimethylglyoxime monoanion of the equatorial plane of the complex and 3,4-dimethyl-5-phenylisoxazole resulting from oxidative ring closure of the monoxime.

Cobaloximes as Environmentally Advantageous Alternatives to Organotin Hydrides in Iodine Atom Abstraction Routes to Benzyl Radicals

High yields of alkyl radicals derived from alkylcobaloximes have been achieved using tungsten light (or in some cases ultrasound) radiation in both organic and aqueous media. These improved yields are obtained when pyridine (the usual base ligand) is replaced by suitably bulky lone-pair donors or water. The alkyl radicals so generated take part in iodine abstraction reactions with benzyl iodides giving benzyl radicals which may be trapped in near-quantitative yield with 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO) or in good yield with lepidinium (4-methylquinolinium) triefluoroethanoate or lepidinium camphor-10-sulphonate. The usual drawbacks of cost and tedious work-up procedures associated with the more commonly used organotin hydride reagents are avoided

Pyridines from allyl cobaloximes: a new decomposition pathway

Abstract Thermochemical and mass spectrometric studies on allyl cobaloxime indicate that it thermally decomposes in a stepwise manner. In comparison with the behaviour of benzyl cobaloximes, which are also precursors to stabilised carbon radicals, there are superficial similarities but also significant differences. Both show a final highly exothermic transition and both produce a heterocyclic ring during thermolysis. Whereas benzyl cobaloximes produce the corresponding 3,4-dimethyl-5-arylisoxazole in moderate to good yields, allyl cobaloximes provide low yields of the corresponding 2,3-dimethylpyridines. Each route involves the cyclisation of an analogous monoxime. The allyl series favours cyclisation through dehydration whereas the benzyl series follows an oxidative ring closure. It is proposed that the low yields of pyridines are due to the relatively ineffective capture of allyl radicals by the dimethylglyoxime monoanion.

The thermal decompositions of alkyl cobaloximes: a correction to the literature

Abstract Mass spectrometric studies have clarified the stages of thermal decomposition of alkyl cobaloximes. For the thermolysis of complexes containing pyridine as the base ligand and primary and secondary alkyl as the top axial ligand, all examples examined lose the top axial alkyl ligand first, followed by pyridine, sometimes at a significantly higher temperature. For examples with water as base ligand, primary alkyl compounds lose water first, but in the cases of secondary alkyl complexes, the hydrocarbon group leaves at the lower temperature. In the final stage of thermal disruption, equatorial ligands such as dimethylglyoxime and cyclohexane-1,2-dione dioxime undergo symmetrical cleavage and dehydration to form the corresponding nitriles, acetonitrile and hexanedinitrile.

Generation and trapping of benzyl radicals from benzyl iodides by cobaloximemediated iodine atom abstractions

Ethylcobaloxime has been demonstrated to function as a convenient source of ethyl radicals on white light photolysis in ethanol. The ethyl radicals so generated take part in iodine abstractions with benzyl iodides giving benzyl radicals which may be trapped with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) or lepidinium camphor-10-sulfonate.