Transition metal complexes with isocyanates

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Several groups have screened a variety of transition metal complexes for activity in the double silylation system, but only compounds of nickel, palladium, and platinum appear to be viable catalysts. The key factor appears to be the involvement of a M(0) species, although certain M(II) complexes can also be used, presumably with in situ reduction to M(0). Generalizations regarding the activity of the different transition metal complexes are difficult, as many variables exist in each system. However, the most active complexes seem to combine palladium metal centers with dba, small basic phosphine, or isocyanate ligands. 

Adhesives which are meant to cure at temperatures of 120 or 171°C require curatives which are latent at room temperature, but react quickly at the cure temperatures. Dicyanodiamide [461-58-5], (TH INI is one such latent curative for epoxy resins. It is insoluble in the epoxy at room temperature but rapidly solubilizes at elevated temperatures. Other latent curatives for 171°C are complexes of imidazoles with transition metals, complexes of Lewis acids (eg, boron trifluoride and amines), and diaminodiphenylsulfone, which is also used as a curing agent in high performance composites. For materials which cure at lower temperatures (120°C), these curing agents can be made more soluble by alkylation of dicyanodiamide. Other materials providing latency at room temperature but rapid cure at 120°C are the blocked isocyanates, such as the reaction products of toluene diisocyanate and amines. At 120°C the blocked isocyanate decomposes to regenerate the isocyanate and liberate an amine which can initiate polymerization of the epoxy resin. Materials such as Monuron can also be used to accelerate the cure of dicyanodiamide so that it takes place at 120°C. 

The formal [2+2+1] cocyclization of alkenes, alkynes and CO by transition metal complexes has been used as a powerM tool for the synthesis of cyclopentenones (Scheme 10.1a) [15]. Other variants of C=C bond like allene, carbonyl or imino moiety can also be used in place of alkene or as a partner of alkene to synthesize 4-or 5-alkylidene cyclopentenones, [16] y-lactones [17] or y-lactams, [18] respectively. Catalytic [2+2+1] cocyclization can also be used with full efficiency to construct heterocyclic compounds using het-erocumulenic compounds such as carbodiimides and/or isocyanates with alkynes and CO (Scheme 10.1c). 

The reversed reaction, i.e. attack of CO on the corresponding transition metal azido complex, sometimes also provides an attractive route for the synthesis of isocyanate complexes, e.g. the reaction of CO with, for example, Co(N3)(DH)2(PPh3),344 Rh(f/5-C5Me5)(N3)4276 or Rh(N3)(cod).344... 

In conclusion it is necessary to note the considerable change in chemical activity occurring on transformation from the alkoxides into oxocomplexes. An example is the synthesis of a bimetallic Bi-Ti complex. The complex formation of 2 isopropoxides occurs only in the presence of water (h = 0.2-0.7), which leads to the formation of Bi-oxoisopropoxide, which then reacts with Ti(OPr )4 already at room temperature providing BiTi20(0Pr% [447] (see also Chapter 8). Teyssie et al. [760] have proposed a large group of alkoxides of 3d-transition metals, and also those of Zn, Al, and Mo as highly effective selective catalysts for polymerization of lactones, isocyanates, and so on. 

Heterounsaturated monomers that undergo coordination polymerisation or copolymerisation with other monomers can be divided into two classes monomers with a carbene-like structure such as isocyanides and carbon monoxide which are coordinated by n complex formation with the transition metal atom at the catalyst active site, and monomers such as isocyanates, aldehydes, ketones and ketenes which are coordinated via 5-bond formation with the metal atom at the catalyst active site. 

The reductive carbonylation of nitroarenes with transition metal catalysts is a very important process in industry, as the development of a phosgene-free method for preparing isocyanate is required. Ruthenium, rhodium, and palladium complex catalysts have all been well studied, and ruthenium catalysts have been shown to be both highly active and attractive. The reduction of nitroarene with CO in the presence of alcohol and amine gives urethanes and ureas [95], respectively, both of which can be easily converted into isocyanates [3,96]. 

TABLE 8. Complex of nitrile oxides, organic thiocyanates, isocyanates and isothiocyanates with transition metals... 

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). ... 

Thus the urea and other by-products would not be formed from an amine derived from reaction of the intermediate isocyanate with adventitious moisture, but via side reactions of nitrene-like intermediates. However, this view considers the nitrene species as a pure organic entity, whereas, in the presence of transition metals, nitrene complexes can be formed (Chapter 1) and the reactivity of the bound nitrene ligand is entirely different from the one of the free species (Chapter 6) [14-16]. 

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