Mechanism and structural aspects of thermal Curtius rearrangement. Quantum chemical study

The electronic and geometric structures of formyl, acetyl, and benzoyl azides were studied and fragments of the potential surfaces for the thermal Curtius rearrangement of these azides into the corresponding isocyanates were calculated by density functional theory at the PBE/TZ2P level. Acyl azides adopt two stable, conformations syn and anti, with respect to the C-N bond. The syn conformers are more stable than their anti analogs. The activation energies of the syn-anti isomerization in the series under study are 9.4, 7.0, and 9.2 kcal mol⁻¹, respectively, and the activation energies of the reverse reaction are 8.5, 6.1, and 2.5 kcal mol⁻¹. The rearrangement of syn-acyl azides is a one-step process, in which elimination of N₂ occurs synchronously with the rearrangement of atoms and bonds to form isocyanates. The activation energies of the rearrangements of syn-HC(O)N₃, syn-MeC(O)N₃, and syn-PhC(O)N₃ are 28.0, 32.9, and 34.5 kcal mol⁻¹, respectively. The rearrangement of the anti conformers of the above-mentioned azides involves the formation of singlet acylnitrene. The activation energies of the latter process are 34.6, 32.9, and 32.3 kcal mol⁻¹, respectively. The activation energies of the rearrangement of acylnitrenes into isocyanates are 20.9, 18.9, and 13.6 kcal mol⁻¹, respectively. The energy characteristics of the process and the structural data for the starting compounds, final products, and transition states provide evidence that the thermal Curtius rearrangement occurs predominantly by a concerted mechanism. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2200–2209, October, 2005.     

Curtius rearrangement of aromatic carboxylic acids to access protected anilines and aromatic ureas

The reaction of a chloroformate or di-tert-butyl dicarbonate and sodium azide with an aromatic carboxylic acid produces the corresponding acyl azide, presumably through the formation of an azidoformate. The acyl azide undergoes a Curtius rearrangement to form an isocyanate derivative which is trapped either by an alkoxide or by an amine to form the aromatic carbamate or urea. The reaction conditions are compatible with a variety of functional groups and allow the synthesis of a number of aniline derivatives containing alkyl, halide, nitro, ketone, ether, and thioether substituents. [reaction: see text]     

Quantum chemical study of the rearrangement of phenoxyl-hydroxyphenyl radicals

The absorption spectra and decomposition kinetics of intermediates formed upon the photolysis of p-iodophenol are studied via flash photolysis. The extinction coefficient of the p-iodophenoxyl radical is calculated. It is found that p-iodophenol acts as an inhibitor of light-independent liquid-phase oxidation reactions.     

Quantum chemical study of the mechanism of the Claisen amino rearrangement

Using the LCAO MO method in the MNDO approximation, we have compared the concerted [3, 3]-sigmatropic and stepwise mechanisms for the acid-catalyzed Claisen amino rearrangement. The latter is preferred.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 634–639, March, 1991.     

Quantum chemical studies of hydrogenation of borazine and polyborazines in the presence of Lewis acids

Thermodynamic parameters and activation energies of hydrogenation processes of borazine, polyborazines, and their donor-acceptor complexes were calculated by the B3LYP/TZVP quantum chemical method. Formation of donor-acceptor complexes of borazine and polyborazines with Lewis acids leads to a considerable decrease in endothermicity and activation energy of their hydrogenation. This allows us to recommend Lewis acids for the use as catalysts in hydrogenation of borazine and polyborazines. Hydrogenation of polyborazines primarily occurs at the heterocycle periphery. The reactivity of polyborazines toward hydrogenation decreases with their increasing size.     

Quantum chemical study of Lewis acid catalyzed allylboration of aldehydes

The reaction rate enhancement in the reaction of allylboronate with benzaldehyde in the presence of AlCl3 has been studied theoretically. B3LYP calculations find a relatively high activation barrier for the reaction of pinacol allylboronate with benzaldehyde in the absence of the Lewis acid. The reaction paths that go through the transition states coordinated by an AlCl3 molecule at one of the two oxygen atoms in the boronate give significantly lower values of activation energy. An analysis of electron populations and orbitals taking part in bond formation indicates that the AlCl3 molecule attached to the boronate oxygen atom strengthens the electrophilicity of the boron center, while it weakens the nucleophilicity of the C(gamma)-C(beta) bond. The result supports the electrophilic boronate activation mechanism proposed by Rauniyar and Hall on the basis of experiments and kinetic studies. In contrast, the reaction path in which AlCl3 is coordinated to the carbonyl oxygen of benzaldehyde shows a higher activation barrier, though the initial reactant complex is more stable than those in other reaction paths. The AlCl3 molecule reduces the reactivity of aldehyde by depressing the nucleophilicity of the sigma-type lone pair of electrons on the carbonyl oxygen, though the electrophilicity of the carbonyl pi orbital is strengthened to some extent. The significance of charge polarization within allylboronate in enhancing the reactivity of boron by the Lewis acid is discussed.     

Quantum chemical methods and their applications to chemical reactions

This review is concerned with the impact of quantum chemistry on chemical reactions. Starting from the mid-sixties it focusses on those developments which have enabled us to predict essential features of simple chemical reactions. Thus model theories and computational methods are presented which provide the tools for these predictions. Then procedures to characterize potential surfaces and search methods for reaction paths are described. It is also attempted to relate these features to the terminology of the experimentalist. Finally a systematic survey of the main types of reaction (rearrangement, addition, elimination, substitution) is given.     

A convenient synthesis of sulfonylureas from carboxylic acids and sulfonamides via an in situ Curtius rearrangement

A one-pot reaction of carboxylic acids with sulfonamides to afford sulfonylureas is described.     

Quantum chemical study of the adducts of azomethine cobalt complexes with acenaphthene-1,2-diimines

Adducts based on the cobalt azomethine bis-chelates and acenaphthene-1,2-diimines have been studied using the density functional theory method [DFT B3LYP*/6-311++G(d,p)]. It has been shown that the stability of the formed complexes with respect to dissociation and the ability to demonstrate valence tautomeric properties can be controlled by functionalization of the ligands. Compounds have been revealed whose magnetic properties vary due to intramolecular redox process.     

Quantum chemical study of the molecular structure of the sodium dodecylsulfate complexes with glycine and cysteine

The quantum chemical modeling of sodium dodecylsulfate (DDSNa) complexes with glycine (Gly) and cysteine (Cys) is carried out at the DFT/B97D level using the Grimme hybrid exchange-correlation functional with a dispersion correction and the 6-311++G(2d,2p) basis set. The structure of the Gly and Cys molecular and zwitterionic forms is examined. The structures and the formation energies of DDSNa…Gly and DDSNa…Cys complexes are determined. The effect of the amino acid structural features on the stability of the complexes they form with DDSNa is analyzed. The NBO analysis of the electron density distribution in the DDSNa molecule is performed.     

Quantum chemical simulation of excited states of chlorophylls, bacteriochlorophylls and their complexes

The present review describes the use of quantum chemical methods in estimation of structures and electronic transition energies of photosynthetic pigments in vacuum, in solution and imbedded in proteins. Monomeric Mg-porphyrins, chlorophylls and bacteriochlorophylls and their solvent 1:1 and 1:2 complexes were studied. Calculations were performed for Mg-porphyrin, Mg-chlorin, Mg-bacteriochlorin, mesochlorophyll a, chlorophylls a, b, c(1), c(2), c(3), d and bacteriochlorophylls a, b, c, d, e, f, g, h, plus several homologues. Geometries were optimised with PM3, PM3/CISD, PM5, ab initio HF (6-31G*/6-311G**) and density functional B3LYP (6-31G*/6-311G**) methods. Spectroscopic transition energies were calculated with ZINDO/S CIS, PM3 CIS, PM3 CISD, ab initio CIS, time-dependent HF and time-dependent B3LYP methods. Estimates for experimental transition energies were obtained from linear correlations of the calculated transition energies of 1:1 solvent complexes against experimentally recorded solution energies (scaling). According to the calculations in five-coordinated solvent complexes the magnesium atom lies out of the porphyrin plane, while in six-coordinated complexes the porphyrin is nearly planar. Charge densities on magnesium and nitrogen atoms were strongly dependent on the computational method deployed. Several dark states of low oscillator strength below the main Soret band were predicted for solvent complexes and chlorophylls and bacteriochlorophylls in protein environment. Such states, though not yet identified experimentally, might serve as intermediate states for excitation energy transfer in photosynthetic complexes. Q(y), Q(x) and Soret transition energies were found to depend on the orientation of the acetyl group and external pressure. A method to estimate site energies and dimeric interaction energies and to simulate absorption and CD spectra of photosynthetic complexes is described. Simulations for the light harvesting complexes Rhodospirillum molischianum, chlorosomes of Chlorobium tepidum and Chloroflexus aurantiacus, and LHC-II of Spinacia oleracea are presented as examples.     

Quantum chemical study of molecular ion complexes with hydrogen bonds (Review)

Institute of Bioorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 6, pp. 157–183, November–December, 1992.     

Quantum chemical study of ferrocene derivatives 1. Arylation reactions with aminobenzoic acids

Russian Chemical Bulletin - Quantum chemical calculations of arylation reactions of ferrocene with three aminobenzoic acid isomers were carried out. Based on the results of the calculations using...     

Theoretical study on the rearrangement of metallabenzenes to cyclopentadienyl complexes

The thermodynamic and kinetic aspects of the rearrangement reactions of a series of metallabenzenes to cyclopentadienyl complexes have been investigated by DFT computational study in order to reveal how substituents on the metallacycle, ligands around the metal center, and metals affect the transformation. We found that substitutents and their locations on the metallacycle have a significant effect on the thermodynamics and kinetics of the rearrangement reactions.     

Quantum chemical study of binuclear adducts of cobalt azomethine complexes with pyrene-4,5,9,10-tetraimine

A computer simulation of 2 : 1 adducts of cobalt azomethine complexes with pyrene-4,5,9,10-tetraimine was carried out within the framework of the density functional theory (DFT B3LYP*/6-311++G(d,p)). It was shown that the variation of the substituents at the nitrogen atoms of cobalt bischelates, as well as the annulation of the five-membered ring to the azomethine fragment, considerably affects the relative energies of the isomers with different spin states. Compounds, which can manifest one- and two-step redox-isomeric rearrangements, accompanied by the migration of paramagnetic centers, were revealed.     

Quantum chemical study of free and coordinate molecules and complexes of amides with mercuric acetate propionamide

Electron structures and reactivities of free and coordinated molecules of fatty acid amides and complex of mercury acetate with propionamide have been studied using quantum chemical methods. The geometry and energy parameters have been determined. The conformational change in the molecules of fatty acid amides and complex of mercury acetate with propionamide at complexation has been observed.     

The Curtius rearrangement of cyclopropyl and cyclopropenoyl azides. A combined theoretical and experimental mechanistic study

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.     

Quantum chemical calculations on the interaction of diazomethane with proton acids

Diazoalkanes may form H-bonded associates with weak proton acids, whereas with strong acids proton transfer leads to the diazonium ion. In order to get information about protonation and association at and N, the corresponding energy balances, electron charge distributions and bond strengths have been calculated by means of quantum chemical ab initio methods with diazomethane as substrate and HF,NH₄⁺, OH₃⁺ as acids.