Nitrenes organic reaction mechanisms

Recent editions of Organic Reaction Mechanisms have highlighted a number of carbene and nitrene CH-insertion reactions. This field has now been reviewed with a focus on enantioselective reactions catalysed typically by dirhodium species.5 The use of C2-symmetric box ligands in asymmetric cyclopropanation reactions has been discussed in the context of a wider review of these ligands as a source of asymmetry.6... 

Attempted reactions of 7d with Ru3(CO)i2 in order to isolate a possible nitrene complex intermediately formed were unsuccessful [9], Thus, we can only speculate about the reaction mechanism. Formation of a nitrene complex analogous to compound 4 is a reasonable hypothesis. However, when the carbonylation of some organic nitro derivatives was conducted in c/.y-cyclooctene as solvent and with Ru3(CO)i2 as catalyst, nitrones have been detected among the products [14]. Under these conditions, the intermediate nitroso derivative reacts with the alkene solvent to yield the corresponding nitrone. The reaction of nitroso derivatives with alkenes is in fact one of the methods for the preparation of nitrones [28]. Thus it could be that in the case of the carbonylation of ortho-nitrostyrenes, the reaction follows the path depicted in Scheme 4 ... 

The addition reactions of organic azides to unsaturated systems can occur by two mechanisms.1 If the reaction temperature is lower than the decomposition temperature of the azide, a 1,3-dipolar addition is observed. When, however, the reaction is carried out at the wavelength or at the decomposition temperature of the azide, the addition proceeds through an intermediate nitrene. 

The thermolysis of methylazide was studied first by Ramsperger 21a> who found it to be a homogenous first-order reaction. Studies of thermolyses of several other organic azides in various solvents and at different concentrations also show first-order kinetics 22-28). This led to the conclusion that free nitrene intermediates are involved. There are however thermal decompositions of azides where nitrene intermediates are not involved. For example ortho-substituted arylazides like o-nitrophenylazide 2, undergo formation of benzofurazan 3 by a concerted mechanism according to l.c.21b). 

Imidoiodanes and especially A-tosyliminoiodanes, ArINTs (Section 2.1.12.4), have found broad synthetic application as useful nitrene precursors in transition metal catalyzed aziridination of aUcenes and amidation of various organic substrates [584, 761]. Mansuy and coworkers in 1984 first reported the aziridination of alkenes with tosyliminoiodane PhINTs in the presence of iron- or manganese-porphyrins [762]. This reaction has a mechanism similar to the metal-catalyzed oxygen atom transfer reactions of iodosylbenzene (Section 3.1.20) and involves a metal-nitrene complex as the intermediate. 

The mechanism by which isocyanates are formed by catalytic carbonylation of organic nitro compoimds is reasonably well understood only in a few cases (see Chapter 6). The intermediate formation of a nitrene (imido) complex has been very often proposed to intermediately occur, being these derivatives formed in some cases from nitro compounds and transition metals carbonyl complexes (2.1.3.). Moreover the synthesis and reactivity of this class of compounds is now well developed [14-16]. However, only in a few cases a clean reaction between a metal-nitrene complex and carbon monoxide giving an isocyanate derivative has been reported. For example, the monomeric terminal imido complex Cp IrNR (Cp = q -CsMes, R = Bu ) reacts under 600 torr of carbon monoxide, affording the corresponding r -isocyanate complex (eq. 2) [17, 18] ... 

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