Formation urethane

Urethane formation. See (C) below. This reaction should be used for crystalline derivative formation, and not as a general reaction for phenols. 

Two major mechanisms for thermal degradation and one minor mechanism are shown in Fig. 9. The first mechanism is the reverse of urethane formation. The second mechanism, which was proposed by Fabris, forms a primary amine and an olefin. It involves a six-member intermediate, as shown in Fig. 10. A thermal... 

Usually no hardeners are added during the produetion of wood-based panels (PB, MDF, OSB, engineered wood produets). With speeial additives, an aecel-eration of the hardening reaction and hence shorter press times or lower press temperatures ean be aehieved. This is espeeially interesting for eold-setting systems as well as for the produetion of partieleboards. Possible catalysts are those traditionally used for the promotion of urethane formation [17,143],... 

Unfortunately, the presence of the benzylic alcohol moiety at the focal point of the dendrimer, along with the catalyst (used in the urethane formation) led to the formation of undesired side products, presumably due to carbamate interchange. These side reactions were avoided by switching to monomer 19, methyl-3,5-dihydroxybenzoate. While the carbamate linkages of dendrons 53 and 54 were too unstable under the alkylation conditions required to afford larger dendrons, the merits of the concept was adequately demonstrated for this accelerated synthesis of [G-3] dendrons. 

FMF Chen, Y Lee, R Steinauer, NL Benoiton. Mixed anhydrides in peptide synthesis. Reduction of urethane formation and racemization using A-methylpiperidine as tertiary amine base. J Org Chem 48, 2939, 1983. 

SECONDARY REACTIONS OF MIXED ANHYDRIDES URETHANE FORMATION... 

There have been reports that urethane was produced when the mixed-anhydride method was employed for the coupling of segments. However, studies on urethane formation during the aminolysis of mixed anhydrides of peptides have never been carried out. The anhydrides are too unstable to be isolated. The activated moiety of the peptide cyclizes too quickly to the 2,4-dialkyl-5(4//)-oxazolonc (see Section 2.23), and since the time allowed to generate the anhydride in segment couplings is always limited to avoid epimerization, one cannot exclude the possibility that the urethane that was produced originated by aminolysis of unconsumed chloroformate. 

In step growth reactions, on the other hand, neither are specific activated centers present to force the connection of the monomers, nor does the process occur as a cascade reaction. Instead, the monomers are tied together in discreet, independent steps via conventional organic reactions such as ester-, ether-, amide-, or urethane formation. Depending on whether small molecules are set free in the connection step, one distinguishes between polycondensations (2.9) and polyadditions (2.10) ... 

Dicarboxylic acid halogen amides, their salts and use in crosslinking of cellulose fibers thru urethane formation 5 D1202... 

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. 

From measurements at different temperatures, the activation parameters AS and Afor the uncatalyzed and the catalyzed urethane formation were calculated. 

From the pseudo-first-order rate constants ku the second-order rate constants k2 are obtained by dividing kx by the alcohol concentration. It was found that the reaction rate constant kx is proportional to the alcohol concentration (at the same catalyst concentration). Table I gives the k2 values for the reaction between methanol and MDI catalyzed by dibutyltin dilaurate at 25.1°C. A plot of the k2 values vs. the dibutyltin dilaurate concentration (Figure 2) apparently deviates from a straight line, indicating that the mechanism of the catalyzed urethane formation in DMF differs from the mechanisms observed in apolar solvents (2-6). Most workers have assumed that in apolar solvents the mechanism involves formation of a complex between alcohol and dibutyltin dilaurate or the formation of a ternary complex between alcohol, isocyanate, and catalyst. In these cases, the relation between k2 and catalyst concentration differs from the relation observed in DMF. 

For dissociation reactions in DMF, AH° and AS0 values are not known. In general, the AH0 values are between —6 and 3 kcal/mole and the AS0 values are always negative (9) (from 0 to —50 eu). When AS0 is negative, it follows that AS caf is much less negative than AS Uncat- This is important for the mechanism of the catalytic reaction. Probably the transition state of the catalyzed urethane formation in DMF is much less rigid than the transition state in the uncatalyzed urethane formation. 

The kinetics and mechanism of the phosphorus-catalysed dimerization of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied.114 The reactions of aryhminodimagnesium (138) with //-substituted p-cyanobenzophenones, l-cyano-9-fluorenenone, o-, m-, and p-dicyanobcnzcnes, and o-, m-, and p-nitrobenzonitriles have been examined.115 The effect of pressure on the reaction of 3 -methyl- l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied.116 The effect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined.117 A Hammett a value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifunctional amidines.118... 

Aminodextran and dextran can be coupled rapidly to the urethanated EVAL film through urea or urethane formation, as evidenced by contact angle and zeta potential. IR assay shows that the coupling reaction has taken place not only at the film surface but also in the interior. 

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