Synthesis of phenylsulfinylcyclopropanes and studies of alpha-diazosulfoxides and phenylsulfinyl carbene

Abstract

Phenylsulfinylcyclopropanes have been prepared by the reaction of phenylsulfinyldiazomethane with isobutene, Z-2-butene, E-2-butene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, cyclohexene, ethyl vinyl ether and cyclopentadiene. Two methods of synthesis of these cyclopropanes were developed. The first involved preforming the unstable phenylsulfinyldiazomethane by the reaction of benzenesulfinyl chloride with diazomethane at -78° in THF to give a 2:1 mixture of the a-diazo sulfoxide and phenylchloromethyl sulfoxide. This was followed by addition of this mixture to olefin at 25º. The second method involved forming the alpha-diazo sulfoxide in situ by adding benzenesulfinyl chloride to a mixture of olefin and diazomethane. The in situ method gave lower yields due to the formation of a by-product, phenyl vinyl sulfoxide, which was formed by the reaction of the cyclopropanating agent, phenylsulfinyl carbene, and diazomethane. The phenyl vinyl sulfoxide reacted further with diazomethane to form 3-phenylsulfinyl-1-pyrazoline. The yields from the preformed method of synthesis ranged from 35% to 60% while the yields from the in situ preparations ranged from 15% to 35% with all yields based on starting benzenesulfinyl chloride. Nitrogen evolution kinetics shows the rate of decomposition of phenylsulfinyldiazomethane to be independent of the presence or absence of olefin establishing that phenylsulfinyl carbene is formed. The rate constant in cyclohexene/THF 1:1 was found to be 2.2 X 10^-4 sec^-1 and the activation parameters determined are Delta H‡ 20.0 ± 0.8 kcal/mol and Delta S‡= -1.7 ± 2.5 e.u. The Hammett rho for the reaction is -0.1. The stereoselectivity and relative rates of addition of phenylsulfinyl carbene to olefins were determined by quantitative carbon-13 NMR spectroscopy. The anti/syn ratios determined for cyclohexene, Z-2-butene and 2-methyl-2-butene are 20:1, 12:1, and 3.5:1 respectively. In order to ascertain the carbene selectivity index for phenylsulfinyl carbene, mPhS(O)CH, a separation of the steric and electronic effects for carbene addition was made by assuming that the two effects are independent. The value of mphS(O)CH, -0.51, was used to calculate a value of -0.50 for sigma R+ of the phenylsulfinyl group using the equation developed by Moss. This means that, under conditions of high electron demand, when direct resonance donation of the sulfinyl lone electron pair is possible (sigma R+cases), the sulfinyl function can supply electrons by resonance with a facility intermediate between that of sulfenyl sulfur and chlorine or bromine.