Catalyses with phenyl isocyanate

A similar substitution on anilines causes the reverse effect. Nitro groups in ortho position either in the isocyanate or the aniline lower the reactivity by steric hindrance. These authors also reported that the reaction is subject to catalysis by pyridine, tertiary bases, and certain carboxylic acids but is unaffected by water, inorganic acids, bases, or salts. Relative rates for the reactions of some primary aliphatic amines with phenyl isocyanates have been determined by Davis and Ebersole (52). 

TABLE I. SYNERGISTIC EFFECT OF DABCO CATALYST AND DIBUTYLTIN DILAURATE FOR THE CATALYSIS OF PHENYL ISOCYANATE (0.07M) REACTION WITH BUTANOL-2 (0.07H) IN DIOXANE AT 25°C... 

Another supporting evidence for complex formation as a prerequisite to synergism was obtained from the study of the catalysis of phenyl isocyanate-butanol reaction by soluble organic cobalt compounds in presence and absence of DABCO catalyst. The results obtained are presented in Figures 4 and 5. It is evident that the combination of DABCO catalyst with divalent cobalt compounds shows synergistic effects while the trivalent cobalt acetylacetonate shows relatively low activity. The explanation of these observations is the structure of these compounds. 

Phenyl isocyanates are generally more reactive than alkyl isocyanates in their reactions with alcohols, but with CuCl catalysis even alkyl isocyanates will react readily with primary, secondary, or tertiary alcohols (45-95% yield). ... 

The kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

The reaction of phenyl isocyanate with methanol in dibutyl ether at 20°C was found to be second order, but the value of the rate coefficient increased slowly with time, indicating catalysis by the product of the reaction. The addition of methyl carbanilate to the initial reaction mixture increased the rate of reaction, confirming catalysis by the urethane, a weak base. The rate coefficient for the initial uncatalysed reaction was 0.28 x lO" 1 mole sec . 

We next analyzed the role of the chloride anion. We have already mentioned that chloride has been reported to accelerate the reaction of nitrosoarenes with Ru3(CO)i2 and also to promote the formation of phenyl isocyanate from a trinu-clear imido-halide cluster. However, neither of these effects seems relevant to the effect observed in catalysis. In fact, apart from the problem of the cluster nature of the compounds involved, nitrosoarenes are always much more reactive than their nitro counterparts and the reaction of a free nitroso intermediate should be very fast relative to the other steps in the catalytic cycle, so that its further acceleration should not be influential. The promotion effect on the isocyanate-producing step, on the other hand, cannot be relevant to the catalytic cycle because it has been shown that the eorresponding imido clusters are not involved. So we must look for other effects. 

Some data are available about catalysis in 1,2-cycloadditions. Tributyl phosphine catalyses dimerisation of phenyl isocyanate to uretidinedione in toluene . The reaction is kinetically of first order with respect to catalyst and overall third order the reverse process is first order with respect to catalyst and overall second order. The mechanism is complex, as revealed by the value of the apparent activation energy of the forward reaction (E= l.l 0.7 kcal.mole" ), which presumably results from the combined temperature dependence of two or more steps, including formation of an isocyanate-phosphine complex (see eqn. (13), p. 113). 

The nickel (o) catalysis proceeds via a flve-membered ring heterocycle, which is isolated and characterized. The intermediate catalyzes the reaction of phenyl isocyanate with ben-zaldehyde. In the reaction of chlorosulfonyl isocyanate with aldehydes, Af-sulfonyl imines... 

If one uses monoisocyanate (e.g. phenyl-isocyanate) instead of the aliphatic diisocyanate, catalysis does not take place, just as with toluidine-diisocyanate. In contrast, the use of isophorone-diisocyanate shows catalysis. 

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