Ruthenium-nitrene complexes

By reaction in benzene of P 3aromatic nitro compounds, derivatives such as <1) and <3) reacts with Ru3reaction with excess nitrobenzene in boiling benzene [653. On the other hand even the thermal disproportionation of <5) (M=sRu R=Ph) under an inert atmosphere and in solution leads to the formation of <1), and other as yet uncharacterised products[663. Complexes following sequence of reactions]603s... 

In addition to epoxides, three-membered nitrogen heterocycles, aziridines, can be obtained by means of catalytic asymmetric aziridinations (Eq. 30). To this aim, chiral ruthenium(salen) complexes 67 [56] and 68 [57] were useful (Fig. 1). The former phosphine complexes 67 gave the aziridine from two cy-cloalkenes with 19-83% ee [56]. On the other hand, terminal alkenes selectively underwent aziridination in the presence of the latter carbonyl complex 68 with 87-95% ee [57]. In these examples, N-tosyliminophenyliodinane or N-tosyl azide were used as nitrene sources. Quite recently, catalytic intramolecular ami-dation of saturated C-H bonds was achieved by the use of a ruthenium(por-phyrin) complex (Eq. 31) [58]. In the presence of the ruthenium catalyst and 2 equiv iodosobenzene diacetate, sulfamate esters 69 were converted into cyclic sulfamidates 70 in moderate-to-good yields. 

Mechanistic studies on the reaction involving mthenium-nitrene complexes [100] or ruthenium-nitroso complexes [95] have also been reported. A stoichiometric reaction of Ru(dppe)(CO)3 (18) with ArNO gives Ru(dppe)(CO)2[CON(Ar)0] (19) (Eq. 11.51). In the first step of the catalytic reaction, nitroarene is reduced to nitrosoar-ene, while in the second step the complex 19 reacts with methanol and CO to give a... 

An intermediate nitrene complexed on ruthenium was suggested, with -formation o-f the urea by attack o-f aniline on the coordinated phenyl isocyanate -formed by insertion o-f CO into the metal-ni trene bond. However another mechanism could be operative, on considering the reactivity, o-f nitro compounds with Fe3using aniline as substrate no urea was obtained. 

The ensuing decomposition reactions are light-catalysed and show a first-order dependence on the ruthenium(m) complex. Although back-bonding (dn pn) is important in metal-nitrene chemistry, the trend in rates observed with the complexes [Ru(bipy)aL(N8)] + (L = Ng", MeCN, or py) does not reflect the ability of the metal centre to donate ar-electron density but instead correlates with the oxidizing power of the complexed ruthenium(ni). No free azide radicals were detected in solution and it is suggested that the rate-determining step involves either metal-nitrene formation,... 

At 220 °C and 50 atm CO, Ru3(CO)i2 is a catalyst in CH3CN for the conversion of 1 into 2 and 3 [9]. The selectivity in the synthesis of 2 was similar to the one observed with Fe(CO)s. As it is known that aromatic nitro compounds, and more easily nitroso compounds, react with Ru3(CO)i2 to give imido complexes in which the nitrogen atom is coordinated to all of the three ruthenium atoms of the cluster skeleton [10-12], the nitrene complex 4, possible intermediate in the reaction, has been synthesised (eq. 2) and its crystal and molecular structure determined [9] ... 

It is not clear why the ruthenium catalyst is not able to induce the heterocyclisation from the presumably intermediate nitrene complex (Scheme 6). The benzamides 32 could be formed via the insertion of an intermediate isocyanate in the aromatic C-H bond of the solvent. This reaction has some precedents, where benzanilides were obtained by reaction of PhNCO with benzene, or directly from nitrobenzene, carbon monoxide and benzene, catalysed by rhodium carbonyl clusters [56-58], However, the reluctance of Ru3(CO)i2 to catalyse the reduction of nitrobenzene to phenylisocyanate in solvents such as benzene [22, 23] does not support this hypothesis. 

Cycloaddition reactions across metal/nitrogen bonds are also observed. Nitrene complexes, formed in the reaction of the ruthenium complex 148 with an isocyanate, add a second molecule to give the [2+2] cycloadduct 149. ... 

The ruthenium complex [Ru(NH3)5NH] + is not considered to be a nitrene complex but rather a species where an imido-group is co-ordinated. The mechanism accounts for the fact that no free ammonia was observed, so that N—N bond cleavage must have occurred after the Ru—N bond was formed. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

Among other convenient nitrene precursors are chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide), bromamine-T, sulfonamides in the presence of (diacetoxyiodo)benzene and various transition metal catalysts, and sulfonyl azides in the presence of ruthenium complexes . 

Highly chemoselective intramolecular allyhc C(sp )—H amination reactions with nitrenes have been reported through the development of ruthenium and iron complexes.It is worth mentioning that the reaction proceeds in a stepwise manner via a hydrogen atom abstraction with the... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

Co-ordinated Nitrenes.— The complex rm -[RuCl(Ng)(diars)2] reacts in hydrochloric acid to produce approximately equal numbers of molecules of [RuCl(NH3)(diars)g]+ and [RuCl(N2)(diars)a]+. When this reaction is carried out with N-labelled complex containing specifically Ru( N N N), the dinitrogen complex produced contains exclusively Ru( N N). This result is consistent with a co-ordinated nitrene (Ru N) intermediate, similar to those previously reported for reactions of ruthenium(m)-azide complexes. However, product and product-distribution comparisons between the ruthenium(ii)-and ruthenium(m)-azide plus hydrochloric acid reactions suggest that there must be some difference in mechanism between the complexes of the two different oxidation states. It is therefore proposed that for the rra j-[RuCl(N3> (diars)a] reaction the first step is protonation of the co-ordinated azide, which may occur at either end, followed by a split into Ng plus NH, which gives an ammonia ligand one way and a dinitrogen ligand the other ... 

The reaction of [Ru(NH3)s(OHa)] + with hydrazoic acid has as its first step an easy substitution at ruthenium to give [Ru(NH3)5(N3H)] +. This decomposes to [Ru(NH3)6(NH)] + plus dinitrogen. The main interest here, and in other similar cases, is whether this species behaves as an imido (19) or a nitrene (20) derivative. The chemical characteristics of the present intermediate indicate that it acts as an imido-complex. ... 

Photochemical reactions of the iridium(m) and rhodium(m) complexes [M(NH3)6(N3)] + result in the production of a co-ordinated nitrene intermediate. In concentrated hydrochloric add the iridium(m) product is [Ir(NH3)6(NH2Cl)] +, as in the equivalent thermal reaction. These iridium(ra) and rhodium(m) complexes thus behave photochemically in a similar mmmer to hydrazoic acid and to organic azides. Their behaviour contrasts with that of some other transition-metal azide complexes, e.g. those of ruthenium(n), where an azido-radical is the photochemical intermediate. One can indeed group transition-metal azide complexes into three groups, with their thermal and photochemical reactions depending on the relative ease of oxidation or reduction (or neither) of the transition-metal centre. ... 

---