Insights into the Mechanism of the Ruthenium–Porphyrin‐Catalysed Allylic Amination of Olefins by Aryl Azides

Abstract

This paper describes the synthesis of allylic amines by aryl azides (ArN 3 ) catalysed by [Ru(TPP)CO] (TPP = dianion of tetraphenylporphyrin). The employment of aryl azides renders the methodology sustainable as the formation of molecular nitrogen is the only stoichiometric byproduct. The isolation of catalytic intermediates and spectroscopic and kinetic studies revealed interesting information about the reaction mechanism, which could improve its catalytic efficiency in future research. An important result is the X-ray characteri-

Resonance Raman spectroscopy as an in situ probe for monitoring catalytic events in a Ru–porphyrin mediated amination reaction

“…The mechanism of the ruthenium-catalysed amination reaction proposed, 37 and illustrated in and  (Cm-Ph)) in the Raman spectra cannot be made. These bands are relatively broad in the measured spectra and, in the course of the reaction, must contain unresolved contributions from different species.…”

“…At high cyclohexene concentration the active species was considered to be the mono-imido species B which is highly reactive and would be immediately trapped by a substrate molecule giving the aminated product E whilst re-generating the starting catalyst A, Ru(TPP)CO. At low cyclohexene concentration, B undergoes a further reaction with the aryl azide giving, via an unstable intermediate C, the bis-imido complex D which is now the active species in allylic amination. 37 Complex D is stable and can be isolated from the reaction of A with 3,5-bis(trifluoromethyl)phenyl azide; however, the isolation of bis-imido complexes from aryl azides bearing other electron-withdrawing groups was less successful, and the formation of bis-imido complexes from aryl azides bearing electron-donating groups has never been observed. 37 In the presence of these other aryl azides, the catalytic cycle was assumed to be restricted to the example involving the mono-imido intermediate B due the poor stability of imido derivatives.…”

“…37 Complex D is stable and can be isolated from the reaction of A with 3,5-bis(trifluoromethyl)phenyl azide; however, the isolation of bis-imido complexes from aryl azides bearing other electron-withdrawing groups was less successful, and the formation of bis-imido complexes from aryl azides bearing electron-donating groups has never been observed. 37 In the presence of these other aryl azides, the catalytic cycle was assumed to be restricted to the example involving the mono-imido intermediate B due the poor stability of imido derivatives. Direct experimental observation of the mono-imido complex B has not been made before.…”

“…metal-catalysed nitrene transfer) that are currently receiving considerable attention as a strategy for preparing desirable nitrogencontaining organic compounds from hydrocarbons. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Notably the mechanism for this specific amination reaction has been the subject of both a kinetic 37 and a theoretical investigation 38 with the former indicating at least two independent catalytic cycles that depend on the ratio of substrate to aryl azide (see Figure 1). At high cyclohexene concentration the active species was considered to be the mono-imido species B which is highly reactive and would be immediately trapped by a substrate molecule giving the aminated product E whilst re-generating the starting catalyst A, Ru(TPP)CO. At low cyclohexene concentration, B undergoes a further reaction with the aryl azide giving, via an unstable intermediate C, the bis-imido complex D which is now the active species in allylic amination.…”

“…The allylic amine product E, C6H9NHAr (Ar is 3,5-bis(trifluoromethyl)phenyl), was synthesised using a literature procedure 37 and a solution of E (7.7 mg, 2.5×10 -2 mmol) was prepared in benzene (5 ml). Compound A (3.7 mg, 5.0×10 mmol) was suspended in benzene (2 ml) under a nitrogen atmosphere, at room temperature, in a glove box, and an excess of 3,5-bis(trifluoromethyl)phenyl azide (12.6 mg, 4.9×10 -2 mmol) was added.…”