Generation and reactions of thionitroso and thioxophosphine intermediates
A series of alkylthionitroso compounds have been generated via thermal fragmentation of N-chlorothio-N-trimethylsilylalkylamines and intercepted with dienes to form 1,2-thiazine heterocycles (via Diels-Alder reaction) and N-alkylsulfenamides (via Ene reaction). Both types of product were found to be unstable, decomposing in ca. 5h (at 20ºC). The reaction of these thionitroso compounds with dienes was temperature dependant, higher temperature (e.g. 90ºC) favoured the Ene pathway whilst lower temperature (e.g. 50ºC) favoured the Diels-Alder route.Thionitroso compounds 125a-e involving 1,5 and 1,6 S-O or S--S nonbonded interactions were generated and reacted with 2,3-dimethyl-1,3-butadiene to afford 1,2-thiazines 126a-e via Diels-Alder reaction. Compounds 126a-e exhibited remarkable chemical stability relative to 1,2-thiazines without such a nonbonded interaction. This enhanced stability was thought to arise due to the prevention of ring opening by the aforementioned interaction. Low temperature (^1)H NMR and Nuclear Overhauser Enhancement spectroscopy provided physical evidence for the existence of the nonbonded interaction in compounds 126. Reaction of thionitroso compounds with dienes in the presence of metal fluorides (AgF, CsF, KF) resulted in a promotion of the Diels-Alder reaction at the expense of the Ene reaction. When unsymmetrical dienes (1 -methyl-1,3-butadiene, 2-methyl-1,3-butadiene) were used a dramatic increase in the regioselectivity of the cycloaddition reaction was observed. Arylthioxophosphines have been generated, by magnesium induced dechlorination of arylthiophosphonic dichlorides, and intercepted with unsymmetrical dienes (1-methyl-1,3- butadiene, 2-methyl-1,3-butadiene) to afford 1,2-thiaphosphorin heterocycles. The reactions were found to proceed with complete regioselectivity in favour of the C6 and C5 methyl isomers respectively.