M. V. Zabalov

Mechanism of urethane formation from cyclocarbonates and amines: a quantum chemical study

The mechanism of the reaction of cyclic esters with amines, providing non-isocyanate urethanes, was studied in terms of DFT by the PBE/TZ2P method using as examples the reactions of ethylene carbonate and propylene carbonate with methylamine. The reaction can proceed through the one- or multistage path involving one or two amine molecules. The second amine molecule plays the role of the catalyst of the process, resulting in a substantial decrease in the activation energy of the reaction.

Nonisocyanate Polyurethanes from Amines and Cyclic Carbonates: Kinetics and Mechanism of a Model Reaction

The kinetic features of a model reaction of the formation of nonisocyanate urethanes from cyclic carbonates and amines have been determined through the use of the example of the interaction of ethylene with n-butylamine in n-butanol and dioxane at different temperatures. The reaction proceeds via two parallel routes involving one or two amine molecules with an activation energy of 7.0 or 2.5 kcal/mol, respectively, in n-butanol and 14.0 or 3.7 kcal/mol in dioxane. The relative contributions of these routes to the observed reaction rate depend on the amine concentration and the temperature. In accordance with the density functional theory, the activation barriers have been calculated and data on the structures of the transition states and the role of the proton-donor solvent in the reaction mechanism have been obtained.

Bifunctional catalysis by acetic acid in the urethane formation from cyclocarbonates and amines: quantum chemical and kinetic study

The mechanism of bifunctional catalysis by acetic acid in the addition reaction of amines with cyclocarbonates was studied by the DFT (PBE/TZ2P) method. Several one-step and multistep reaction pathways were found. The barrier for the transformation through the most favorable pathway in the presence of acetic acid was shown to substantially decrease (to ∼12 kcal mol−1) compared to the non-catalytic reaction (∼33 kcal mol−1) and the reaction involving the second amine molecule as the catalyst (∼22 kcal mol−1). The kinetics of the model reaction of ethylene carbonate with butylamine in dioxane was experimentally studied, and it was found that the effective activation energy decreases to 5.3 and 1.1 kcal mol−1 for the channels involving one and two amine molecules, respectively, compared to the reaction in the absence of acetic acid (14.0 and 3.7 kcal mol−1).

Transition metal complexes with heterocyclic analogs of fluorene 1. Synthesis, structures, and haptotropic rearrangements of tricarbonylchromium complexes of dibenzothiophene

The complex formation of dibenzothiophene with chromium carbonyl complexes of the general formula L3Cr(CO)3 (L = Py, NH3, or CO) afforded η6-C12H8SCr(CO)3 (1). In the presence of tetramethylethylenediamine, complex 1 was selectively metallated with BuLi at position 4 of the coordinated ring to form η6-4-LiC12H8SCr(CO)3 (2). In decane, the tricarbonylchromium group is reversibly and intramolecularly migrates from the unsubstituted to substituted ring due to the inter-ring haptotropic rearrangement (IRHR) at 130 °C for 100 h. In decane, the rate constant of IRHR was estimated experimentally by 1H NMR spectroscopy and theoretically by the density functional theory (DFT).

Quantum chemical study of the arylacyl azide complexes with Lewis acids and their Curtius rearrangement to isocyanates

The structures of donor-acceptor complexes of syn-benzoyl azide, its 2-methyl- and 2,6-dimethyl-substituted derivatives with BF3, AlCl3, and SbCl5, and the corresponding transition states of the rearrangement into isocyanates were studied by the PBE/TZ2P method in the framework of the density functional theory (DFT). The complexes are formed at the oxygen and nitrogen atoms of the acyl azide group and have the composition 1: 1 or 1: 2 depending on the Lewis acid (L) structure. The complexes at the oxygen atom are more stable; the most stable complexes are formed by the reactions of acyl azides with AlCl3. Complex formation with Lewis acids decreases the activation energy of the transformation of acyl azides into isocyanates owing to the +M effect and stabilization of the Ar-C(O-L(1−))=N(1)-N(2)(1+)≡N(3) mesomeric form. The activation energy decreases with an increase in the number of ortho-methyl substituents in benzoyl azide due to the +I effect of the phenyl group. The turn of the phenyl ring at almost 90° with respect to the CON3 group is needed for the rearrangement to occur, and the energy necessary for this process is ∼8 kcal mol−1.

Cyclopenta[b]thienyl ligand in organometallic chemistry. Studies of the regioselectivity of the synthesis of new σ-element-substituted cyclopenta[b]thiophene derivatives

Reactions of 2-ethyl-5-methylcyclopenta[b]thienyllithium (thiopentalenyllithium) (2) with various electrophilic reagents afford σ-element-substituted thiopentalenes. However, the reaction with Ph3SnCl yields only one of two possible isomers,viz, triphenyl(4H-cyclopenta[b] thiophen-4-yl)stannane (4c), whereas the reactions with Me3SiCl, Me3SiCl, or Ph2PCl give both possible isomers,viz., trimethyl(6H-cyclopenta[b]thiophen-6-yl)silane (3a) and trimethyl(4H-cyclopental[b]thiophen-4-yl)silane (4a), trimethyl(6H-cyclopenta[b]thiophen-6-yl)stannane (3b) and trimethyl(4H-cyclopental[b]thiophen-4-yl)stannane (4b), or diphenyl(6H-cyclopenta[b]thiophen-6-yl)phosphine (3d) and diphenyl(4H-cyclopenta[b]thiophen-4-yl)phosphine (4d) in ratios of 1∶2, 1∶2, or 1∶1, respectively. The structure of compound4c was established by X-ray diffraction analysis. The observed regioselectivity of formation of compound4c is attributed to the specific precoordination of the tin atom by the sulfur atom of the thiopentalenyl ligand and to the steric overcrowding of the Sn atom in organotin electrophiles.

The supermolecule method, as applied to studies of liquid-phase reaction mechanisms taking cyclocarbonate aminolysis in dioxane as an example: specific features

The supermolecule method was used to describe the mechanism of liquid-phase processes taking the reaction of ethylene carbonate with methylamine as an example. Specific features of the approach are considered. The problem of choosing the reference point for calculating the relative energies of individual reaction steps was solved by introducing the idea of the structure of noninteracting solvated reactants. In this case, no basis set superposition error (BSSE) correction is required because the solvated reactants, the pre-reaction complex, and the transition state have the same atomic composition and are calculated in the same basis set. To calculate the title reaction in dioxane by the supermolecule method with acceptable accuracy, it is sufficient to consider one solvent molecule.

Green chemistry of polyurethanes: The catalytic n -butylaminolysis of ethylene carbonate as a model chain-growth reaction in the formation of nonisocyanate polyurethanes

The kinetic regularities and the mechanism of the catalytic action of 1,5,7-triazabicyclo[4.4.0]dec-5-ene, which is the most active of the known catalysts for the formation of hydroxyurethanes from cyclocarbonates and amines, were studied using the example of the n-butylaminolysis of ethylene carbonate. In contrast with the noncatalytic reaction, which proceeds via two parallel pathways that involve one and two molecules of amine, the catalytic reaction follows a single pathway: the second molecule of amine is replaced by a molecule of the catalyst that accelerates the process in accordance with the mechanism of bifunctional catalysis. Different reaction pathways were studied by quantum chemical calculations based on the density functional method. It was shown that the high activity of 1,5,7-triazabicyclo[4.4.0]dec-5-ene results from the formation of a planar cation-like form of the catalyst. Moreover, the low-energy transition between the cation and the initial 1,5,7-triazabicyclo[4.4.0]dec-5-ene enables the catalyst to simultaneously be a good donor and acceptor of protons. This study presents a new way for finding among bifunctional organic compounds the catalysts that are even more active in the reaction of cyclocarbonates with amines.

Carboxylation of aromatic compounds in a supercritical carbon dioxide medium

The reaction of direct carboxylation of benzene and its derivatives PhX (X = Me, Br, Ph, OPh, OMe), as well as mesitylene, durene, and ferrocene, in a supercritical CO2 medium in the presence of various Lewis acids (AlCl3, FeCl3, ZrCl4, and ZnCl2) is studied. It is shown that, in all cases, secondary reactions proceed faster than the primary reaction of carboxylic acid formation. For the thoroughly studied AlCl3-CO2toluene system, optimal conditions of the formation of n-toluic acid are determined. For the AlCl3-CO2-benzene system, as an example, quantum-chemical calculations of the characteristics of the allowed pathways of the carboxylation reaction are performed.

Synthesis, structure, and quantum-chemical study of tricarbonyl(4-methoxybenzaldehyde)chromium(0)

The structure of tricarbonyl(4-methoxybenzaldehyde)chromium(0) was studied by X-ray crystallography. The crystals are orthorhombic, space group P212121, a = 8.040(1) Å, b = 10.510(7) Å, c = 13.279(4) Å, V = 1122.1(8) Å3, Z = 4. Quantum-chemical calculations predict that this compound can exist in the gas phase as two stable conformers with almost the same energies. Their mutual transformations are examined using the density functional theory.