Aziridination of alkenes and amidation of alkanes by bis(tosylimido)ruthenium(VI) porphyrins. A mechanistic study

Bis(tosylimido)ruthenium(VI) porphyrins, [Ru-VI(Por)(NTs)(2)] (Por TPP. TTP 4-Cl-TPP 4-MeO-TPP. OEP), were prepared in 60-74% yields by treatment of [ Ru-II(Por)(CO)(MeOH)] with (N-(p-tolylsulfonyl)imino)phenyliodinane (PhI=NT s) in dichloromethane. In dichloromethane containing pyrazole, they reacted with alkenes or alkanes to give tosylamidoruthenium(IV) porphyrins, [Ru-IV (Por)(NHTs)(pz)], in about 75% yields. The reactions of [Ru-VI(TPP)(NTs)(2) ] and [Ru-VI(OEP)(NTs)(2)] with styrene, para-substituted styrenes, norborn ene, cyclooctene, and beta-methylstyrene afforded the corresponding N-tosyl aziridines in 66-85% yields. The aziridination of cis-stilbene and cis-beta -methylstyrene by [Ru-VI(Por)(NTs)(2)] is nonstereospecific with a partial loss of the alkene stereochemistry. Kinetic studies on the reactions betwee n [Ru-VI(TPP)(NTs)(2)] and 16 alkenes (cyclooctene, norbornene, 2,3-dimethy l-2-butene, styrene, para-substituted styrenes, alpha- and beta-methylstyre ne, and alpha- and beta-deuteriostyrene) gave the second-order rate constan ts (k(2)) ranging from (1.60 +/- 0.06) x 10(-3) to (90 +/- 4) x 10(-3) dm(3 ) mol(-1) s(-1) at 298 K. The slope of the linear plot of log k(2) vs E-1/2 for eight representative alkenes was found to be -1.7 V-1. Tn the case of para-substituted styrenes, linear correlation between log k(R) (k(R) = rela tive rate) and sigma(+) gives a rho(+) value as small as -1.1. However, the effect of para substituents on k(R) can be best accounted for by consideri ng both the polar and spin delocalization effect. Measurements on the secon dary deuterium isotope effect revealed that only the beta-carbon atom of st yrene experienced a significant change in its hybridization in reaching the transition state. All these are consistent with rate-determining formation of a carboradical intermediate. The reactions of [Ru-VI(TPP)(NTs)(2)] and [Ru-VI(OEP)(NTs)(2)] with adamantane, cyclohexene, ethylbenzene, and cumene resulted in tosylamidation of these hydrocarbons and afforded the correspo nding amides in 52-88% yields. For cyclohexane and toluene, the tosylamidat ion products were formed in poor yields (ca. 10%). Kinetic studies on the r eactions between [Ru-VI(TPP)(NTs)(2)] and nine hydrocarbons (cumene, ethylb enzene, cyclohexene, and para-substituted ethylbenzenes) gave the second-or der rate constants (k(2)) in the range of (0.330 +/- 0.008) x 10(-3) to (16 .5 +/- 0.3) x 10(-3) dm(3) mol(-1) s(-1). These reactions exhibit a large p rimary deuterium isotope effect, with a k(H)/k(D) ratio of 11 for the tosyl amidation of ethylbenzene. In the case of para-substituted ethylbenzenes, b oth electron-donating and -withdrawing substituents moderately promote the reaction. There is an excellent linear correlation between log k(R) and a r elated carboradical parameter. On the basis of these observations, a mechan ism involving the rate-limiting formation of a carboradical intermediate is postulated.