Isocyanates catalytic mechanisms

Catalysis in isocyanate chemistry is of broad interest from a theoretical viewpoint. It is possible to vary the structure of each of the reactants and of the catalyst and to observe profound effects. As for the catalyst, it has been established that both basicity and steric properties are of fundamental importance. Kinetic measurements have proved most useful in investigations, carried out for the most part since 1945. Knowledge gained has permitted consideration of the steric and electronic factors of the active reactant-catalyst complex, key to the catalytic mechanisms. 

Burkus s conclusion is that the catalytic mechanism consistent with the experimental data involves a base-isocyanate complex formed by the direct... 

Isocyanates and isothiocyanates are reduced to methylamines on treatment with LiAlH4. LiAIH4 does not usually reduce azo compounds622 (indeed these are the products from LiAIH4 reduction of nitro compounds, 9-67), but these can be reduced to hydrazo compounds by catalytic hydrogenation or with diimide623 (see 5-9). Diazonium salts are reduced to hydrazines by sodium sulfite. This reaction probably has a nucleophilic mechanism.624... 

The use of polar solvents or ligands such as 2,2 -bipyridyl decreased the yield. The complex [Pd(diphos)2] (104) was inactive, as were [Ni(PPh3)4] and [Pt(PPh3)4]. Complex (103) was believed to give the carbonyl [Pd(CO)(PPh3)3] prior to commencement of catalytic action. When [Pd(DBA)2](105) was used as catalyst, reaction (139) did not occur at all, the sole product being the vinyl isocyanate formed according to equation (140). A possible mechanism for these reactions is shown in Scheme 51. 

For the catalysis of isocyanate-alcohol reactions in apolar solvents, several mechanisms have been proposed. However, the results of the kinetic measurements in DMF could not be explained with these mechanisms. So we concluded that, in the polar solvent DMF, the mechanism of the catalyzed urethane formation differs from the published mechanisms in apolar solvents. The behavior in DMF can be explained from a mechanism in which dibutyltin dilaurate dissociates into a catalytic active species. 

Several phosphorous compounds having a P=S bond are also catalysts for this reaction. For example, heating of phenyl isocyanate with triphenylphosphine sulfide at 160 °C affords diphenylcarbodiimide. Since triphenylphosphine sulfide was recovered unchanged, a different mechanism seems to be operative. A tricyclic P S compound, S=P[N(Me)CH2CH2l3N, also catalyzes the transformation of isocyanates to carbodiimides. Triphenylarsine oxide and triphenylantimony oxide also catalyze the conversion of isocyanates into carbodiimides. The catalytic activity of the oxides of phosphorous, arsenic and antimony are in agreement with the dipole moments of... 

A similar strategy for the preparation of substituted imidazolidin-2-ones in two steps from readily available A -allylamines has been developed. Addition of amines 1152 to isocyanates affords A -allylureas 1153, which are converted to imidazolidin-2-one 1154 with the formation of two bonds and up to two stereocenters when treated with aryl bromides and catalytic amounts of Pd2(dba)3/Xantphos in the presence of NaO/-Bu. Based on the stereochemical and regiochemical outcomes, a mechanism for the transformation has been proposed, as illustrated in Scheme 280 <20060L2531>. 

According to the first hypothesis, the amine complexes with the alcohol and both the amine and this complex can function as catalysts for the reaction, the same isocyanate-base complex resulting from attack by either catalytic species. Therefore, the original mechanism is really a composite one of the following two steps ... 

For explaining the catalytic effect of acids (A), two mechanisms were considered one of which involves a complex formation with the isocyanate and the other an alcohol-acid complex. 

Both of these mechanisms are compatible with the experimental results. At low alcohol/isocyanate ratios both give a catalytic effect which is proportional to the acid concentration. Also, both predict that at high alcohol excess the catalytic effect disappears. According to mechanism I this would be due to the formation of the complex becoming the rate determining step at high alcohol excess. On the other hand, in the case of mechanism II,... 

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... 

In Scheme 5 (a) and (b) for instance, the alcohol intervenes very early in the catalytic cycle and it is essential for the efficient carbonylation/deoxygenation of the substrate [56, 62-64]. Among the mechanisms proposed, only Scheme 5 (c) [3] remains somewhat related to the simple scheme mentioned earlier invoking isocyanate as the primary reaction product, subsequently trapped by alcohol. 

The presently proposed mechanism of synergism implies that ligands other than amine which can coordinate with the tin ion should also synergize its catalytic activity toward alcohol-isocyanate reactions. This apparently is the case for triphenyl phosphine-DBTDL combination. When triphenyl phosphine was added to DBTDL, it accelerated the rate of reaction of isocyanate with alcohol and water (Figure 6). Although triphenyl phosphine is known to dimerize isocyanates, (12), the dimerization... 

We conclude that the intramolecular elimination of aryl isocyanate is the mechanism which releases the organic product from the catalyst. In all of the stoichiometric reactions this isocyanate is immediately trapped by ArNH2 to form the observed product, diarylurea. When the reaction was conducted imder catalytic conditions either diarylurea or the carbamate was observed depending upon the conditions. Even in those runs that produced carbamates, urea was still proposed to be the intermediate.f5)... 

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