Addition of linear methanol associates at the C=N and C=O bonds of methyl isocyanate was studied in terms of the B3LYP/6-311++G(df,p) hybrid quantum-chemical method. The addition at the C=N bond is more favorable than the reaction at the carbonyl group. All reactions involve late asymmetric cyclic transition states. The activity of the reacting system increases in parallel with the degree of methanol association. Isomerization of methyl hydrogen methylcarbonimidate into carbamate is catalyzed by methanol associates. Thermal decomposition of carbamates with formation of isocyanates can occur in autocatalytic mode. * For communication II, see [1].We previously performed quantum-chemical studies on noncatalytic reactions of phenyl isocyanate with linear methanol associates [1, 2]. The obtained results allowed us to conclude that the addition of alcohols at the C=N bond of isocyanates is considerably more favorable than the reaction at the carbonyl group. The reactions involved late concerted asymmetric transition states in which the degree of formation of new C-O bond was higher than the degree of formation of new N-H bond. Alcohol associates were more reactive than monomeric species, which was attributed to enhanced electron-donating properties of alcohol associates. Reactions of isocyanates with alcohol associates were both kinetically and thermodynamically preferable as compared to monomeric species. Reactions of isocyanates with alcohol associates were found to be thermodynamically preferable not only in the gas phase but also in the liquid phase [3].Reactions of alcohols with aliphatic isocyanates are also important from the viewpoint of large-scale applications [4,5]. Therefore, it seemed reasonable to elucidate whether the relations revealed previously by quantum-chemical methods for reactions of aromatic isocyanates with alcohols could be extended to analogous transformations with participation of aliphatic isocyanates. In the present article we report on the results of our quantum-chemical study on the mechanisms of noncatalytic reactions of methyl isocyanate with linear methanol associates (monomer, dimer, and trimer).Quantum-chemical calculations were performed at the B3LYP/6-311++G(df,p) level of theory using Gaussian 03 software package [6,7]. The geometric parameters of the reacting species were preliminary optimized in terms of the density functional theory using Priroda 6 software [8] with L11 basis set (an analog of cc-pCVDZ [9]). Transition states on the potential energy surface were identified by the first negative vibration frequency in the Hessian matrix and were checked by descend toward both sides using IRC (internal reaction coordinate) procedure followed by optimization of geometric parameters of pre-and postreaction complexes. The structures of the most favorable conformers were determined by energy minimization as described in [2].Noncatalytic paths of the reaction of methyl isocyanate with linear methanol associates. We examined reaction paths involving addition of linear methanol associa...

The mechanism of addition of linear methanol associates (monomer, dimer, trimer) to aryl isocyanates at their C=N and С=О bonds was investigated applying the quantum-chemical method B3LYP/6-311++G(df,p). Notwithstanding the electronic character of substituents in the aromatic ring of the isocyanates all reactions proceed through concerted asymmetric late transition states. The addition to the C=N bond is considerably more preferable than to the С=О bond. In the transformations under consideration the intermolecular donor-acceptor interactions between the reagents result in the appearance of abnormal selectivity.Formerly we carried out a quantum-chemical investigation of the reaction mechanism between phenyl isocyanate with methanol linear associates (monomer, dimer, trimer) [2, 3]. It was established that the reactions proceeded through concerted cyclic asymmetric late transition states. With the growing degree of the alcohol association the activation barriers on the conversion route decrease due to the growing electron-donor property of the alcohol, and increases the thermodynamic feasibility of their reaction with isocyanates [1,4].In order to reveal whether the mechanism of the alcohol reaction with isocyanates might change with the change of the electronic character of substituents in the isocyanates we supplementary to the previously obtained data [2-4] investigated reactions of p-nitro-and ----------------*For Communication IV, see [1].p-methoxyphenyl isocyanate with the methanol linear associates.The electronic character of the substituents in the aryl isocyanates in question is essentially different. It was therefore expectable that if the substituents in the aromatic ring of isocyanates might cause a change in the reaction mechanism, it would be observed in the reactions under study.Quantum-chemical calculations were performed by the method B3LYP/6-311++G(df,p) employing the package of the applied programs GAUSSIAN 03 [5,6]. The preliminary geometry optimization was carried out by the density functional method with the help of PRIRODA 6 program [7] using the basic L11 (analog of cc-pCVDZ [8]). The search for the transition state was fulfi lled with respect to the fi rst negative oscillation frequency in the Hesse matrix. The verity of the transition state was determined by the procedure of the decent from the transition state point in two directions using the IRC procedure with the subsequent geometry optimization of the obtained prereaction and postreaction complexes. The principles of the estimation of conformer structures corresponding to the minimal energy are described in [2].

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It was shown by quantum-chemical method B3LYP/6-311++G(df,p) that in the autocatalytic reaction the molecules of methyl carbamate formed prereaction complexes with the monomer and dimer of methanol. The complexes possess higher electron-donor properties than free alcohol molecules thus increasing the activity of complexes in the reaction with isocyanates. The products of the autocatalytic reaction are new molecules of carbamate and azomethinenol. The conversions occur through concerted asymmetric late transition states. The isomerization of azomethinenols into carbamates is catalyzed by molecules of alcohols and their associates. The autocatalytic reaction of isocyanates with alcohols becomes possible owing to the ability of alcohols to catalyze the mentioned isomerization.