The Curtius rearrangement of some organic azides: A DFT mechanistic study

The reaction mechanism for the thermal Curtius reaction of formyl azide has been investigated using B3LYP/6‐311+G(d,p). It is found that, while the synisomer undergoes nitrogen elimination via a concerted mechanism, yielding isocyanic acid directly, the anti‐isomer cannot undergo reaction via the concerted mechanism and first eliminates nitrogen, yielding oxazirene, via a transition state which is higher in energy than that for the concerted mechanism. Singlet formyl nitrene does not exist as an independent moiety. Rather, the strong N? O interaction yields the cyclic isomer oxazirene. The isomerization of oxazirene to isocyanic acid goes through a transition state which is even higher in energy than that for nitrogen elimination. It is hence proposed that this reaction should take place via the concerted mechanism only, the anti‐isomer undergoing isomerization first to the syn isomer since the activation barrier for this step is very small. The same mechanism is found to prevail for acetyl and benzoyl azide. These findings are in accord with all experimental data. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A combined experimental and theoretical study of the thermal cycloaddition of aryl azides with activated alkenes

Reactions were performed from aryl azides on the one hand, and activated alkenes coming from β-dicarbonyl compounds or malonodinitrile on the other hand, either with recourse to conventional heating or to microwave activation, to afford 1-aryl-1H-1,2,3-triazoles. The mechanism and the regioselectivity of the reactions involving β-dicarbonyl compounds have been theoretically studied using DFT methods at the B3LYP/6-31G* level: they are domino processes comprising a tautomeric equilibrium of the β-dicarbonyl compounds with their enol forms, a 1,3-dipolar cycloaddition of the enol forms with the aryl azides (high activation energy), and a dehydration process (lower activation energy). The effect of non-conventional activation methods on the degradation of 1,2,3-triazolines was next studied experimentally. Finally, some of the 1,2,3-triazoles such synthesized were evaluated for their bactericidal and cytotoxic activities.     

An experimental and theoretical study of the type C enone rearrangement: mechanistic and exploratory organic photochemistry

We recently described a new photochemical rearrangement which we termed a Type C process. The reaction involves a delta to alpha aryl migration in 5-disubstituted cyclohexenones also having bulky C-3 substituents. In contrast to most cyclohexenone rearrangements, the reaction occurs via a twisted pi-pi excited triplet rather than the usual n-pi state. The electronic nature of the rearrangement was assessed using migration selectivity with p-anisyl and p-cyanophenyl groups. A synthesis of the reactants was elaborated, and the product structures were established by X-ray and NMR analysis. The reaction mechanism was established further with DFT and CASSCF computations. In the latter, localized NBO basis orbitals permitted proper selection of the active space. The nature of the diradical intermediates as well as the transition states was established computationally. Sensitization experiments with regioselectivities the same as those in direct irradiation confirmed the triplet multiplicity of the process.     

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.     

Thermochemistry of biphenylcarboxylic and dicarboxylic acids. A combined experimental and theoretical study

The standard molar enthalpies of combustion and sublimation of 2- and 4-biphenylcarboxylic acid, 2,2'- and 4,4'-biphenyldicarboxylic acid were measured and the gas-phase enthalpies of formation, at T= 298.15 K, were determined. Ab initio calculations were performed and a theoretical study on molecular structure of all the biphenyl acid isomers has been carried out. Calculated enthalpies of formation using appropriate isodesmic reactions are compared with experimental values, and a good agreement is observed. Estimates of enthalpies of formation for the isomers, which were not studied experimentally, are presented. All the acids containing at least one ortho COOH are comparatively less stable than their isomers having just meta or para COOH group(s).     

Curtius rearrangement and Wolff homologation of functionalized peroxides

The Curtius and Wolff rearrangements of peroxide-containing alkanoyl azides and diazoketones provide an efficient entry to peroxy-substituted amines, isocyanates, carbamates, and peroxyalkanoates.     

The role of cesium fluoride in aryl propargyl ether Claisen rearrangement and its mechanistic elucidation: a theoretical study

The role of cesium fluoride (CsF) in aryl propargyl ether Claisen rearrangement and its mechanistic pathway have been investigated in gas and solvent phase using the density functional theory implemented in Gaussian 09. Our results indicate that the [3,3]-sigmatropic rearrangement is the rate-limiting step with ΔG ^? value of 37.1 kcal/mol in solvent phase. Furthermore, the results show that the enolization of α-allenylketone intermediate (Int1-CsF) has a higher free energy barrier, which implies that the formation of benzopyran is not favored in the presence of CsF. However, the abstraction of the α-hydrogen atom in Int1-CsF with CsF shows a very low free energy barrier and is the most favored pathway for aryl propargyl ether Claisen rearrangement in the presence of CsF to form benzofuran. In the case of substituted aryl propargyl ethers, a methoxy group on the benzene ring lowers the activation barrier. The HOMO–LUMO, conformational and NBO analysis indicate that increasing methyl substitution on the propargyl residue enhances the rearrangement reaction.     

The electronic spectrum of AuO: a combined theoretical and experimental study

The near-infrared electronic spectrum of AuO(1) has been re-examined in light of the new microwave data on the v = 0 and v = 1 vibrations of the X(2)Pi(3/2) state of AuO. The two observed bands in the spectrum, with red-degraded bandheads located at 10726 and 10665 cm(-1), have been reanalyzed. New theoretical work on AuO clarifies the electronic structure, and the bands in the infrared are now assigned as the (0,1) and (1,2) bands of the a(4)Sigma(-)(3/2) - X(2)Pi(3/2) transition, respectively.     

Azo-hydrazo conversion via [1,5]-hydrogen shifts. A combined experimental and theoretical study

Azoalkenes 6e, 6g, 6h, and 8c underwent an easy azo-hydrazo conversion via a [1,5]-hydrogen shift yielding α,β-unsaturated hydrazones. The isomerization products were characterized through spectroscopic and spectrometric techniques. In order to understand the nature of the mechanism of these [1,5]-hydrogen shifts, the transition state structures of the reactions were theoretically studied at the B3LYP/6-31G(d,p) level. Substitution effects in the propenylazo system on the kinetic and thermodynamic parameters were analyzed. An electron localization function (ELF) analysis of the electronic structure of the transition state structure associated with the azo-hydrazo conversion of the simplest 1-azopropene 6a indicates that these [1,5]-hydrogen shifts have a two-stage one-step mechanism via pseudodiradical transition states, in which a formal hydrogen atom is transferred. This finding allows us to reject the pericyclic reaction model for these [1,5]-hydrogen shift reactions.     

Large blue-shift in the optical spectra of fluorinated polyphenylenevinylenes. A combined theoretical and experimental study

Modifications of the optical properties of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] induced by fluorination of the vinylene units are investigated by means of time dependent density functional theory (TD-DFT) calculations and spectroscopic measurements in solution. The energy of the main absorption peak is blue-shifted by more than 0.8 eV in the fluorinated polymers. TD-DFT excitation energies for non-fluorinated and fluorinated oligomer structures of increasing number of monomers, employing fully relaxed geometries, are compared to the experimental absorption energies of the polymers. We found that the measured large blue-shift induced by the fluorination of the vinylene units is not caused by the electron-withdrawing effect of the fluorine substituents but it is related to a steric effect. The inter-monomer torsional angle of the fluorinated structures increases above 50 degrees , while in the non-fluorinated systems it is below 20 degrees . Further insight into the origin of the large blue-shift of the excitation energies is gained by a detailed analysis of the torsional potentials of non-fluorinated and fluorinated dihydroxystilbene. While for planar geometries the energy gap increases due to fluorination, it decreases for highly distorted geometries. In addition, we found that the torsional potential of dihydroxystilbene is rather flat, meaning that different isomers might, e.g., in the solid state, coexist.     

Aromatic hydrocarbon nitration under tropospheric and combustion conditions. A theoretical mechanistic study

The viability of some nitration pathways is explored for benzene (B), naphthalene (N), and in part pyrene (P). In principle, functionalization can either take place by direct nitration (NO2 or N2O5 attack) or be initiated by more reactive species, as the nitrate and hydroxyl radicals. The direct attack of the NO2 radical on B and N, followed by abstraction of the H geminal to the nitro group (most likely accomplished by 3O2) could yield the final nitro-derivatives. Nevertheless, the initial step (NO2 attack) involves significant free energy barriers. N2O5 proves to be an even worst nitrating agent. These results rule out direct nitration at room temperature. Instead, NO3 and, even more easily, HO can form pi-delocalized nitroxy- or hydroxycyclohexadienyl radicals. A subsequent NO2 attack can produce several regio- and diastereoisomers of nitroxy-nitro or hydroxy-nitro cyclohexadienes. In this respect, the competition between NO2 and O2 is considered: the rate ratios are such to indicate that the NO3 and HO initiated pathways are the major source of nitroarenes. Finally, if the two substituents are 1,2-trans, either a HNO3 or a H2O concerted elimination can give the nitro-derivatives. Whereas HNO3 elimination is feasible, H2O elimination presents, by contrast, a high barrier. Under combustion conditions the NO2 direct nitration pathway is more feasible, but remains a minor channel.     

Growth of polyaromatic molecules via ion-molecule reactions: an experimental and theoretical mechanistic study

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C(10)H(7)(+) with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C(16)H(13)(+), C(16)H(n)(+) (with n=10-12), and C(15)H(10)(+). From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 A?(2) (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C(16)H(12)(+) and C(15)H(10)(+) ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)×10(-11) cm(3) s(-1) at an average collision energy of about 0.27 eV can be estimated for the process C(10)H(7)(+)+C(6)H(6)→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H(2), CH(3)). A major signal relevant to the fragmentation of the initial adduct C(16)H(13)(+) belongs to C(15)H(10)(+). Since it is not obvious how CH(3) loss from C(16)H(13)(+) can take place to form the C(15)H(10)(+) radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol(-1). From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol(-1) with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the "naphthyl" part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.     

Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study

The experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans-stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO CI level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the 1H-NMR experiments, the ab initio geometry of ZE-2-pyridylphenylbutadiene is consistent with the presence of the N.H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (piH, piL*) character in all compounds except for E-4,4'-dipyridylethene, where S1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the 1(n, piL*) and 1(piH, piL*) states which occurs in solvents of high polarity. The inclusion of the most important doubly- and triply-excited configurations in the CI calculations shows that the 1A(g)- excited state is above the spectral region analyzed.     

A theoretical study of α-substituted isopropyl and cyclopropyl anions

Ab initio molecular orbital theory has been used to probe the effect of the substituent X on the structures, strain energies, stabilization energies, inversion barriers, and proton affinities of carbanions CH₃CX CH and cis-C₃H₄X^?, where X = H, F, CN, and NC. All geometries have been optimized with a 3-21G basis set, and the parent anions (X = H) were also optimized with the same basis set with a diffuse function added (i.e. the 3-21 + G basis set). The anions, with the exception of the α-cyanoisopropyl anion, are pyramidal. The out-of-plane angle, α, for the pyramidal anions decreases in the order F > H ≈ NC > CN, and the barriers to inversion follow the same order with the cyclopropyl anions consistently having higher barriers than the isopropyl anions. The substituents strongly stabilize the anions with the stabilization energy following the order CN > NC > F. The cyano group slightly reduces the strain energy of cyclopropane, but the isocyano and fluoro substituents are weakly and strongly destabilizing, respectively. The pyramidal cyclopropyl anions are less strained than the cyclopropanes except when the substituent is a cyano group where the strain energies are reversed but are very similar. The planar anions all have higher strain energies than the cyclopropanes.     

A theoretical study of α-substituted cyclopropyl and isopropyl radicals

The structures of α-X-cyclopropyl and α-X-isopropyl radicals (X = H, CH₃, NH₂, OH, F, CN, and NC) are reported at the RHF 3-21G level of theory. The isopropyl radicals are pyramidal with out-of-plane angles varying from 12° (X = CN) to 39° (X = NH₂), and barriers to inversion ranging from 0.4 kcal/mol (X = H) to 4.0 kcal/mol (X = NH₂). The cyclopropyl radicals have larger out-of-plane angles, from 39.9° (X = CN) to 49.4° (X = NH₂), and their barriers to inversion, which increase with the inclusion of polarization functions, vary from 5.5 kcal/mol (X = H) to 16.7 kcal/mol (X = F). In both types of radicals the amino group is the most stabilizing substituent, while the α-fluoro has little effect. The β-fluoro group is weakly destabilizing in the cyclopropyl radical. The strain energies of the cyclopropyl radicals (36–43 kcal/mol) are compared with those of similarly substituted anions, cations, and cyclopropanes.     

The nature of unsupported uranium-ruthenium bonds: a combined experimental and theoretical study

Four new uranium-ruthenium complexes, [(Tren(TMS))URu(η(5)-C(5)H(5))(CO)(2)] (9), [(Tren(DMSB))URu(η(5)-C(5)H(5))(CO)(2)] (10), [(Ts(Tolyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (11), and [(Ts(Xylyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (12) [Tren(TMS)=N(CH(2)CH(2)NSiMe(3))(3); Tren(DMSB)=N(CH(2)CH(2)NSiMe(2)tBu)(3)]; Ts(Tolyl)=HC(SiMe(2)NC(6)H(4)-4-Me)(3); Ts(Xylyl)=HC(SiMe(2)NC(6)H(3)-3,5-Me(2))(3)], were prepared by a salt-elimination strategy. Structural, spectroscopic, and computational analyses of 9-12 shows: i) the formation of unsupported uranium-ruthenium bonds with no isocarbonyl linkages in the solid state; ii) ruthenium-carbonyl backbonding in the [Ru(η(5)-C(5)H(5))(CO)(2)](-) ions that is tempered by polarization of charge within the ruthenium fragments towards uranium; iii) closed-shell uranium-ruthenium interactions that can be classified as predominantly ionic with little covalent character. Comparison of the calculated U-Ru bond interaction energies (BIEs) of 9-12 with the BIE of [(η(5)-C(5)H(5))(3)URu(η(5)-C(5)H(5))(CO)(2)], for which an experimentally determined U-Ru bond disruption enthalpy (BDE) has been reported, suggests BDEs of approximately 150 kJ mol(-1) for 9-12.     

A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.