Isocyanates imido metal complexes

In general high pressure and temperature are required for these reactions to occur. However there are significant examples of reactions catalysed at atmospheric pressure, in part icular for the synthesis of isocyanates (4.2.5.). In the majority of cases the most important steps of these reactions are supposed to be the deoxygenation of the nitro function by carbon monoxide iving a nitrene residue bound to the metal centre, followed by insertion of carbon monoxide into the metal-nitrene bond. This is a likely hyphotesis since nitrene complexes can be obtained by stoichiometric reactions of nitro compounds with metal carbonyls. Conversion of the imido metal complex to the observed... 

A [3,3]-sigmatropic cyanate-isocyanate rearrangement has been identified as a route to a-isocyanato allylboronic esters that can be trapped with nucleophiles (Scheme 2). A DFT (density functional theory) investigation of [3,3]-sigmatropic rearrangement versus a [2 4-2]-cycloaddition of a formal 5 2 substitution of imido metal complexes with allylic electrophiles has been reported (Scheme 3). ... 

In several cases [16], the CO molecule derived from the starting isocyanate does not leave the coordination sphere of the metal and imido-carbonyl complexes are formed. 

While all of the transformations detailed above take place at low-valent metal centers, a double sulfur-carbon bond cleavage process has also been postulated to occur at the molybdenum(VI) center (527, 528, 839). Thus, thermolysis of organic isocyanates with [Mo02(S2CNR2)2] affords imido-disulfide complexes, [Mo(NR )(S2)(S2CNR2)2] (Fig. 289), in moderate yields. The fate of the aminocarbyne fragment remains unknown and further, since monosulfido... 

The metathetical reaction of C02 with metal (Ti(IV), U(V)) arylimido-complexes led to an arylisocyanate and a metal-oxo species [4g, 115a]. In these processes, which take place under very mild conditions (ambient temperature and pressure), the metal center acts as the sink for unrequired oxygen. A common feature which characterizes these transformations from a mechanistic point of view is the intermediate formation of a metal carbimato-species through the [2+2] cycloaddition of C02 with the metal-imido complex (Scheme 6.23). By cycloreversion, the four-membered aza-metalla-cycle then converts into isocyanate and a stable metal-oxo complex. These processes, at least formally, are reminiscent of the reaction of imino-phosphoranes with C02 to give isocyanates and carbodiimides [115b,c]. 

Hydrolysis of the vinyl groups from the metal center affords olefins. Weiss, Schubert, and Schrock investigated the reaction of the alkylidyne complex W(CCMe3)Cl3(dme) with cyclohexyl isocyanate (208). Two isocyanate molecules are incorporated into the complex as shown in Eq. (212). The reaction was postulated to proceed via a cycloaddition adduct of isocyanate to the metal-carbon triple bond and cleavage of the four-mem-bered ring into metal imido and ketenyl species. Subsequent insertion of a second isocyanate into the metal-ketenyl bond would then give the observed product. 

Despite several experimental facts [3] rendering a transient metal-imido species a likely source for many products of the carbonylation reaction, its role as an actual intermediate in the catalytic transformation of simple nitroaromatic substrates has never been proven. Accordingly, a type-5 mechanism (Scheme 3, involving no such intermediate) could also be operative for the formation of isocyanate. In this case, an imido complex could also be generated by a parallel minor pathway... 

Treatment of Cp W(NO)Cl2 with (aryl)2Mg (aryl = Ph, o- and jo-tolyl) affords 16e Cp W(NO)(aryl)2 complexes that react with CO, CO2, CS2, or aryl isocyanates to produce rf--diCy, -carboxylate, -thiocarboxylate, and -amide-containing complexes, respectively. In a related system, the water-induced isomerization of CpW(NO)(o-tolyl)2 affords the isomeric oxo(imido) complex CpW(0)(N-o-tolyl)(o-tolyl). Furthermore, the ortho-metalated phosphine complex, Cp W(NO)(H)(j7 -PPh2C6H4) (27), undergoes... 

Metathesis reactions have also been used to prepare imido compounds. In one such metathesis reaction, an 0x0 complex reacts with an isocyanate to form an imido complex (Equation 13.59). In another case, a carbene complex reacts with an organic imine to form the corresponding olefin and a transition metal-imido complex (Equation 13.60). ... 

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] ... 

However, CO2 can react with metal-imido bonds through a [2+2] cycloaddition reaction to give N,0-bound metal carbamates [118]. The latter species can react further and either extrude the corresponding isocyanate with formation of a metal-0x0 complex [83, 119] or also insert a second CO2... 

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