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

Abstract

Bis(tosylimido)ruthenium(VI) porphyrins, [Ru(VI)(Por)(NTs)2] (Por = TPP, TTP, 4-C1-TPP, 4-MeOTPP, OEP), were prepared in 60-74% yields by treatment of [Ru(II)(Por)(CO)(MeOH)] with (N-(p-tolylsulfonyl)- imino)phenyliodinane (PhI=NTS) 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, norbornene, cyclooctene, and β- methylstyrene afforded the corresponding N-tosylaziridines in 66-85% yields. The aziridination of cis-stilbene and cis-β-methylstyrene by [Ru(VI)(Por)(NTs)2] is nonstereospecific with a partial loss of the alkene stereochemistry. Kinetic studies on the reactions between [Ru(VI)(TPP)(NTs)2] and 16 alkenes (cyclooctene, norbornene, 2,3-dimethyl-2- butene, styrene, para-substituted styrenes, α and β-methylstyrene, and α- and β-deuteriostyrene) gave the second-order rate constants (k2) ranging from (1.60 ± 0.06) x 10-3 to (90 ± 4) x 10-3 dm3 mo1-1 s-1 at 298 K. The slope of the linear plot of log k2 vs E(l/2) for eight representative alkenes was found to be -1.7 V-1. In the case of para-substituted styrenes, linear correlation between log k(R) (k(R) = relative rate) and σ+ gives a ρ+ value as small as -1.1. However, the effect of para substituents on k(R) can be best accounted for by considering both the polar and spin delocalization effect. Measurements on the secondary deuterium isotope effect revealed that only the β-carbon atom of styrene 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 corresponding amides in 52-88% yields. For cyclohexane and toluene, the tosylamidation products were formed in poor yields (ca. 10%). Kinetic studies on the reactions between [Ru(VI-) (TPP)(NTs)2] and nine hydrocarbons (cumene, ethylbenzene, cyclohexene, and para-substituted ethylbenzenes) gave the second-order rate constants (k2) in the range of (0.330 ± 0.008) x 10-3 to (16.5 ± 0.3) x 10-3 dm3 mol-1 s-1. These reactions exhibit a large primary deuterium isotope effect, with a k(H)/k(D) ratio of 11 for the tosylamidation of ethylbenzene. In the case of para-substituted ethylbenzenes, both electron-donating and -withdrawing substituents moderately promote the reaction. There is an excellent linear correlation between log k(R) and a related carboradical parameter. On the basis of these observations, a mechanism involving the rate-limiting formation of a carboradical intermediate is postulated.