Metalloporphyrin-mediated asymmetric nitrogen-atom transfer to hydrocarbons: Aziridination of alkenes and amidation of saturated C-H bonds catalyzed by chiral ruthenium and manganese porphyrins

Abstract

Chiral metalloporphyrins [Mn(Por*)(OH)(MeOH)] (1) and [Ru(Por*) (CO)(EtOH))] (2) catalyze asymmetric aziridination of aromatic alkenes and asymmetric amidation of benzylic hydrocarbons to give moderate enantiomeric excesses. The mass balance in these nitrogen-atom-transfer processes has been examined. With PhI=NTs as the nitrogen source, the aziridination of styrenes, trans-stilbene, 2-vinylnaphthalene, indene, and 2,2-dimethylchromene catalyzed by complex 1 or 2 resulted in up to 99% substrate conversions and up to 94% aziridine selectivities, whereas the amidation of ethylbenzenes, indan, tetralin, 1-, and 2-ethylnaphthalene catalyzed by complex 2 led to substrate conversions of up to 32% and amide selectivities of up to 91%. Complex 1 or 2 can also catalyze the asymmetric amidation of 4-methoxyethylbenzene, tetralin, and 2-ethylnaphthalene with "PhI(OAc)2" + NH2SO2Me", affording the N-substituted methanesulfonamides in up to 56% ee with substrate conversions of up to 34% and amide selectivities of up to 92%. Extension of the "complex 1 + PhI=NTs" or "complex 1 + PhI(OAc)2 + NH2R (R = Ts, Ns)" amidation protocol to a steroid resulted in diastereoselective amidation of cholesteryl acetate at the allylic C-H bonds at C-7 with substrate conversions of up to 49% and amide selectivities of up to 90% (α:β ratio: up to 4.2:1). An aziridination- and amidation-active chiral bis-(tosylimido)ruthenium(VI) porphyrin, [Ru(Por*)(NTs)2] (3), and a ruthenium porphyrin aziridine adduct, [Ru(Por*)(CO)(TsAz)] (4, TsAz=N-tosyl-2-(4-chlorophenyl)aziridine), have been isolated from the reaction of 2 with PhI=NTs and N-tosyl-2-(4-chlorophenyl)aziridine, respectively. The imidoruthenium porphyrin 3 could be an active species in the aziridination or amidation catalyzed by complex 2 described above. The second-order rate constants for the reactions of 3 with styrenes, 2-vinylnaphthalene, indene, ethylbenzenes, and 2-ethylnaphthalene range from 3.7-42.5 × 10-3 dm3mol-1 s-1. An X-ray structure determination of complex 4 reveals an O- rather than N-coordination of the aziridine axial ligand. The fact that the N-tosylaziridine in 4 does not adopt an N-coordination mode disfavors a concerted pathway in the aziridination by a tosylimido ruthenium porphyrin active species.