2-Nitroethanal conformations in solution: AnAb initio SCRF study

The structure of 2-nitroethanal has been studied at the MP2/6-31G^* level in the gas phase and in acetonitrile using a continuum model to represent the electrostatic solute-solvent interactions. The relative energies of the two stable conformations obtained are quite dependent on the media. Indeed, our computations predict a change of the most stable conformation from gas to polar solvent. These results are in agreement with experimental data for the axial/equatorial conformational equilibrium of 2-nitrocyclohexanone.     

Origins of the diastereoselectivity in hydrogen bonding directed Diels-Alder reactions of chiral dienes with achiral dienophiles: a computational study

In this study, the origins of diastereoselectivity in the hydrogen bonding assisted Diels-Alder reactions of chiral dienes with achiral dienophiles have been investigated with density functional methods. The distortion/interaction model has been applied to shed light on the origins of selectivity. C9-Substituted chiral anthracene templates (R = (CH(3))(OCH(3))(H), R = (CH(3))(OH)(H), R = (CH(3))(CH(2)CH(3))(H) and R = (-CH(2)-C(CH(3))(OCH(3))(H)) are used to rationalize the role of a stereogenic center and H-bonding on the product distribution ratio. Even though hydrogen bonding increases the reactivity of the diene, the stereoselectivity is reduced because of the hydrogen bonding capacity of both diastereomeric transition states. The interaction energies of the studied anthracene templates with N-methyl maleimide at the transition state correlate linearly with an increase in reactivity. The selectivity is determined by both favorable distortion and interaction energies. The π-facial selectivity induced by the presence of a chiral auxiliary in 1-substituted 1,3-pentadienes (R1 = (CH(3))(OCH(3))(H) and R1 = (CH(3))(OH)(H)) has also been modeled in order to rationalize the role of the stereogenic center and H-bonding on the stereoselectivity of an aliphatic diene. In both parts, the product distribution ratios calculated from Boltzmann distributions based on Gibbs free energies are in reasonable agreement with the experimental results. Finally the role of OH-substituted five-membered pyrrolidine on C9 of anthracene is investigated since the successful usage of the conformationally rigid pyrrolidines in asymmetric synthesis is well known. Overall, both in the acyclic system and in anthracene, the facilitation due to H-bonding is reflected in the interaction energies: the higher the difference in interaction energies in the transition structures of the two diastereomers, the more selective the H-bonding assisted Diels-Alder reaction is.     

A Computational Study of the Wallach Rearrangement

Treatment of azoxybenzene and its derivatives with acids is known to result in the Wallach rearrangement, which leads to 2- or4-hydroxyazobenzenes. Starting in the 1960s, experimental findings have lead to the proposal of several mechanisms for this rearrangement. In this work, molecular orbital theory employing the semiempirical AM1 method is used to locate and discuss the energetics of the intermediates and the transition states for this rearrangement. Based on the results of AM1 calculations in vacuum and in solution, the most plausible mechanistic pathways are proposed and discussed.     

Experimental and modeling approach to decolorization of azo dyes by ultrasound: degradation of the hydrazone tautomer

Sonochemical bleaching of monoazo dyes C.I. Acid Orange 7 and C.I Acid Orange 8, which exist in their hydrazone forms in dye solutions, was investigated by irradiating 40 microM dye solutions using a 300 kHz emitter. It was found that the rate of bleaching was first-order with respect to the maximum absorption of the dye in the visible band and accelerated with increased acidity. Decolorization of Acid Orange 7 was slightly faster than that of Acid Orange 8 at equivalent test conditions. The oxidative degradation of Acid Orange 7 and Acid Orange 8 were modeled by means of density functional theory calculations. The adduct formation by hydroxyl radical attack to the carbon atom bearing the azo linkage was more preferred over the attack on the nitrogen atom. A competing reaction of hydrogen abstraction from the CH3 group in C.I Acid Orange 8 was found responsible for the difference in color removal rates.     

Modeling the cyclopolymerization of diallyl ether and methyl α‐[(allyloxy)methyl]acrylate

The free‐radical cyclopolymerization of diallyl ether (1) and methyl α‐(allyloxymethyl)acrylate (2) has been modeled with the B3LYP/6‐31G* methodology, by making use of model compounds for the growing radicals. The cyclization of both monomers is exo, with activation barriers of 5.33 and 9.82 kcal/mol, respectively. To account for the polymerizabilities of these monomers, competing reactions have also been modeled. Although both monomers have a lower barrier for homopolymerization than for cyclization, cyclization dominates due to entropy. This explains the high cyclopolymerization vs. homopolymerization of monomer 2, although its monofunctional counterpart has been reported to homopolymerize well. It has also been shown that the degradative chain transfer by H‐abstraction from the allylic carbon is not effective with this monomer. Poor cyclopolymerization of the monomer 1 has been demonstrated by modeling the degradative chain transfer by H‐abstraction from the allylic carbon, which has been shown to compete very efficiently with polymerization reactions. Additionally, intermolecular propagation reaction has been shown to be facile due to cyclization, since the attacking monomer adopts a cyclic structure. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Solvent effect on the synthesis of clarithromycin: A molecular dynamics study

Clarithromycin (6-O-methylerythromycin A) is a 14-membered macrolide antibiotic which is active in vitro against clinically important gram-positive and gram-negative bacteria. The selectivity of the methylation of the C-6 OH group is studied on erythromycin A derivatives. To understand the effect of the solvent on the methylation process, detailed molecular dynamics (MD) simulations are performed in pure DMSO, pure THF and DMSO:THF (1:1) mixture by using the anions at the C-6, C-11 and C-12 positions of 2',4"-[O-bis(TMS)]erythromycin A 9-[O-(dimethylthexylsilyl)oxime] under the assumption that the anions are stable on the sub-nanosecond time scale. The conformations of the anions are not affected by the presence of the solvent mixture. The radial distribution functions are computed for the distribution of different solvent molecules around the 'O-' of the anions. At distances shorter than 5 A, DMSO molecules are found to cluster around the C-11 anion, whereas the anion at the C-12 position is surrounded by the THF molecules. The anion at the C-6 position is not blocked by the solvent molecules. The results are consistent with the experimental finding that the methylation yield at the latter position is increased in the presence of a DMSO:THF (1:1) solvent mixture. Thus, the effect of the solvent in enhancing the yield during the synthesis is not by changing the conformational properties of the anions, but rather by creating a suitable environment for methylation at the C-6 position.     

Computational study of alkyllithium/pyridine derivative systems as initiators for the living anionic polymerization of methyl methacrylates

The reactivity of n‐butyllithium (n‐BuLi) toward pyridine derivatives (pyridine, pyridazine, pyrimidine, and 1,3,5‐triazine) was subjected to a computational study to determine the most suitable n‐BuLi/heterocyclic ring system as an initiator for the anionic polymerization of methyl methacrylate (MMA). These systems were suggested to prevent side reactions occurring through n‐BuLi attack on the carbonyl carbon of MMA by sterically blocking the initiator. The initiation reaction was modeled with the B3LYP methodology 6‐31+G*. Activation barriers were used to analyze the reactivity of each n‐BuLi/heterocyclic ring system. Computational results showed that n‐BuLi/triazine had a significantly lower activation barrier. Therefore, n‐BuLi/triazine was the suggested initiator system for the anionic polymerization of poly(methyl methacrylate). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 455–467, 2005     

Transition Density Estimates for a Class of Lévy and?Lévy-Type Processes

We show on- and off-diagonal upper estimates for the transition densities of symmetric Lévy and Lévy-type processes. To get the on-diagonal estimates, we prove a Nash-type inequality for the related Dirichlet form. For the off-diagonal estimates, we assume that the characteristic function of a Lévy(-type) process is analytic, which allows us to apply the complex analysis technique.     

Solvent‐Catalyzed Ring–Chain–Ring Tautomerization in Axially Chiral Compounds

The mechanism of ring–chain–ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2‐oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring–chain–ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring–chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers.