Urethane, catalysis

Although reactions of aromatic isocyanates with alcohols generally follow second-order kinetics or modified second-order kinetics as outlined by Baker and Holdsworth [19], Sato [126] reported that aliphatic and edkenyl isocyanates were much more subject to alcohol and urethane catalysis. The reactions of these compounds were found to ee with the expression... 

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Schollenberger added 2% of a polycarbodiimide additive to the same poly(tetra-methylene adipate) urethane with the high level of acid (AN = 3.66). After 9 weeks of 70°C water immersion, the urethane was reported to retain 84% of its original strength. Carbodiimides react quickly with residual acid to form an acyl urea, removing the acid catalysis contributing to the hydrolysis. New carbodiimides have been developed to prevent hydrolysis of polyester thermoplastics. Carbodiimides are also reported to react with residual water, which may contribute to hydrolysis when the urethane is exposed to high temperatures in an extruder [90]. 

Condensation of methyl urethane, H2NC06CH3j with CH20 gives rise to the methylene diurethane (III) and this compd, under different specific conditions of refluxing, resulta in the various specific linear (IV V) and cyclic products (I II) as shown. The observations of relevance here are that the various products, both linear and cyclic, are interconvertible under acid catalysis including / into IF and conversely. The... 

I. S. Bechara, The Mechanism of Tin-Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols, in Urethane Chemistry and Applications, ACS Symposium Series 172, K. N. Edwards, (Ed.), American Chemical Society, Washington, DC, 1981. 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

The catalysts of reactions between 4,4 -diphenylmethane diisocyanate (MDI) and alcohols in N,N-dimethylformamide (DMF) by dibutylin dilaurate has been investigated. The reaction rate of the catalyzed urethane formation in DMF is proportional to the square root of dibutylin dilaurate concentration. This result differs from that of similar studies on apolar solvents. The catalysis in DMF can be explained very well by a mechanism in which a small amount of the dibutylin dilaurate dissociates into a catalytic active species. 

T n continuation of a study of the uncatalyzed reactions between MDI (4,4 -diphenylmethane diisocyanate) and alcohols in DMF (N,N-dimethylformamide) (J), the effect of dibutyltin dilaurate on the same reactions has been studied. The results were compared with those found in studies on the mechanism of catalysis of urethane formation in apolar solvents (2-6). 

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. 

Cameron, J.F. and Frechet, J.M.J. (1990) Base catalysis in imaging materials. 1. Design and synthesis of novel light-sensitive urethanes as photoprecursors of amines. Journal of Organic Chemistry, 55, 5919-5922. 

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 . 

It is now evident that the order of the initial reaction rate constant will lie between 2 and 3, depending on the relative importance of the first two terms on the right-hand side of Equation (17). In solvents which form hydrogen bonds with the alcohol, the second term will predominate, and at low extent of reaction and low alcohol concentration a second-order reaction will be found. In either case beyond the initial stage the reaction rate is increased by auto-catalysis due to the urethane complex catalysis. 

The most important example of a combination of addition and substitution processes is the catalysis by organotin compounds of the formation of a urethane from an alcohol and isocyanate.49 The mechanism which appears to be generally accepted42 is that the... 

The second part of the book is devoted to the polymer chemistry and physics of urethane RIM systems. In this section, variables affecting the morphology, structure, properties, and catalysis of urethane RIM systems are discussed. 

Based on the presented data, the probable mechanism of the catalysis of urethane reaction by the DBTDL catalyst is depicted in Figures 6 and 7. 

F. Florio, Catalysis of urethane systems, in Handbook of Coatings Additives, 2nd Edn 1. Florio and D. Miller (Eds), Marcel Dekker, New York (2004). 

If the mechanism of the catalysis of isocyanate-alcohol reaction by tin carboxylates does indeed proceed via the alkoxide as proposed by Bloodworth and Davies (11), then the synergism of the amine to tin can readily be explained from the above equilibrium. The amine will assist in the alcoholysis step and speed up the decomposition of the tin-carbamate complex by the alcohol to the urethane and tin alkoxide. 

---