Comparative molecular field analysis and comparative molecular similarity index analysis studies on 1H NMR chemical shift of NH group of diaryl triazene derivatives

Comparative molecular field analysis (CoMFA), comparative molecular field analysis region focusing (CoMFA‐RF) for optimizing the region for the final partial least square analysis, and comparative molecular similarity indices analysis (CoMSIA) methods were employed to develop three‐dimensional quantitative structure–activity relationship (3D‐QSAR) models of ¹H NMR chemical shift of NH proton of diaryl triazene derivatives. The best orientation was searched by all‐orientation search (AOS) strategy to minimize the effect of the initial orientation of the structures. The predictive abilities of CoMFA‐RF and CoMSIA models were determined using a test set of ten compounds affording predictive correlation coefficients of 0.721 and 0.754, respectively, indicating good predictive power. For further model validation, cross validation (leave one out), progressive scrambling, and bootstrapping were also applied. The accuracy and speed of obtained 3D‐QSAR models for the prediction of ¹H NMR chemical shifts of NH group of diaryl triazene derivatives were greater compared to some computational well‐known procedures. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantum chemical study of the nature of regioselectivity in reactions of 2,4,6-triazidopyridines withtert-butylphosphaacetylene

We have performed PM3 calculations for 2,4,6-triazido-3,5-dicyanopyridine and 2,4,6-triazido-3-chloro-5-cyanopyridine, and also the cycloadducts of these compounds with one and two tert-butylphosphaacetylene molecules. We have established that the selectivity of addition of a phosphaacetylene molecule at the -azido group of triazidopyridines is due to the specifics of the electronic properties and geometry of these groups, characterized by enhanced electrophilicity of the terminal nitrogen atoms, a greater contribution from atomic orbitals to the LUMO (lowest unoccupied molecular orbital), and greater bending of the chain of N–N–N atoms. The cycloaddition itself corresponds to the dipole-LUMO-controlled reaction type, and sequential addition of phosphaacetylene molecules at the azido groups of pyridines leads to formation of cycloadducts having lower LUMO energies. Nevertheless, the -azido groups of the triazidopyridines can remain stronger dipoles than the azide groups of the cycloadducts when the difference between the LUMO energies in the triazidopyridines and cycloadducts is no greater than 10 kcal/mole.Institute of Chemical Physics at Chernogolovka, Russian Academy of Sciences, Chernogolovka 142432. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 676–686, May, 1997.     

Iminonitroso ene reactions: experimental studies on reactivity, regioselectivity, and enantioselectivity

Ene reactions of iminonitroso agents with olefins were investigated in both solution and solid phase. Reactions afforded allyl hydroxylamine products in up to 99% yield and with high regioselectivity. A Cu(I)-mediated enantioselective nitroso ene reaction gave an ene product with up to 40% ee.     

Chemo- and regioselectivity in the reactions of polyfunctional pyrroles

The chemo- and regioselectivity of the reduction, oxidation and Wittig reaction of polyfunctional pyrroles, containing a variety of reactive centres was investigated. The reaction of 3,5-dichloropyrrole-2,4-dicarboxaldehydes with potassium permanganate leads to regioselective oxidation of the 2-formyl group, while the Wittig reaction with 1 equiv of a triphenylphosphorane produced the 2-alkenyl substituted pyrroles.     

Effect of methoxyl group position on the regioselectivity of ammonia substitution reactions involving 3,3'-dichloro-2,2'-binaphthoquinones

A series of methoxyl-substituted 3,3'-dichloro-2,2'-binaphthoquinones 2 were prepared and evaluated for regioselectivity in ammonia substitution reactions. Biquinone 2b underwent regiospecific amination at the unsubstituted chloronaphthoquinone unit whereas the isomeric biquinone, 2c, produced moderate regioselectivity (85:15). Biquinone 2d, however, showed no level of regioselectivity demonstrating that the position of the methoxyl group influences the regiochemistry. The intramolecular hydrogen bond in biquinone 5 altered the regioselectivity. Semiempirical calculations revealed comparatively larger LUMO coefficients at the chlorinated carbons that underwent preferential substitution.     

Understanding the unusual regioselectivity in the nucleophilic ring-opening reactions of gem-disubstituted cyclic sulfates. Experimental and theoretical studies

The regioselectivity of the nucleophilic ring-opening reactions of three gem-disubstituted cyclic sulfates with sodium azide has been studied from both experimental and theoretical viewpoints. It is found that, depending on the substituent present in the cyclic sulfate, the reaction displays reversed regioselectivity, which allows one or another regioisomer to be obtained with selectivities greater than 4:1. The theoretical calculations show that, contrary to previous understanding, the intrinsic preference in all cases is azide attack at the less-substituted C(beta) position, a consequence of similar stereoelectronic effects in the three sulfates considered. The observed preference for C(alpha) attack in the case of the ester sulfate is explained in terms of differential solvent effects, which are in turn due to subtle differences in the charge transfer in the different transition structures.     

On the regioselectivity of elimination reactions in terpene derivatives

Tertiary alcohols with prenyl or geranyl moities, bearing a second oxygen. function in the 3, 4 or 5 position were dehydrated under various conditions : the composition of the olefinic mixture obtained was accounted for, by an intramolecular base relay effect.Basic elimination of analogous dimethylsutfonium salts gave results that could be related to the inductive effect : a correlation between regioselectivity and the Taft constantσ_I of the second oxygen function was found, except for the very basic alkoxide groups where an alternative regioselective elimination took place.     

Expedient synthesis of chiral 1,2- and 1,4-diamines: protecting group dependent regioselectivity in direct organocatalytic asymmetric Mannich reactions

[reaction: see text] Organocatalytic asymmetric Mannich reaction of protected amino ketones with imines in the presence of an L-proline-derived tetrazole catalyst afforded diamines with excellent yields and enantioselectivities of up to 99%. The amino ketone protecting group controlled the regioselectivity of the reaction providing access to chiral 1,2-diamines from azido ketones and 1,4-diamines from phthalimido ketones.     

Electronic regioselectivity in the reactions of pyridinium ylides with tetracyanoethylene

Highly regioselective reactions of pyridinium ylides with tetracyanoethylene occur via an Ad_N-E mechanism to form Z-isomers of 3-aroyl-3-(1-pyridinio)-1,1,2-tricyano-2-propenides-1, corresponding in structure to 1,4-ylides with maximum charge separation. If the unsaturated nitrite starting material contains a nucleofugal group 1,2-elimination predominates over 1,3-elimination.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1087–1094, August, 1991.     

Indium-mediated propargylation of aldehydes: regioselectivity and enantioselectivity studies

An enantioselective propargylation reaction of aldehydes with enantioselectivity up to 85% has been achieved in organic solvents by using stoichiometric amounts of (−)-cinchonidine as the chiral source.     

Acidic rearrangement of benzyl group in flavone benzyl ethers and its regioselectivity

The benzyl-substituted flavone compounds are rare in nature, while some of which have interesting biological activities. The total synthesis of benzyl-substituted flavone derivatives via the acidic rearrangement of benzyl groups in flavone benzyl ethers, and the complicated regioselectivity of the rearrangement were reported. The regioselectivity was proposed to be determined by the steric hindrance as well as the ease of electrophilic substitution reaction for benzyl cations at different positions of corresponding debenzylated flavone compounds.     

The regioselectivity in the reactions of N-aryl-C-ethoxycarbonylnitrilimines with acrylic acid derivatives

The cycloaddition of a series of C-ethoxycarbonyl-N-arylnitrilimines 5a-f to acrylic acid derivatives namely acrylamide, acrylonitrile and ethyl acrylate has been studied. Under thermal conditions 1,3-dipolar cycloadditions proceed with complete regioselectivity to give 5-R substituted 2-pyrazolines 8–10 in high yield. The structures of the cycloadducts 8–10 were confirmed by ¹³C nmr, ¹H nmr and ir spectra. The regioselectivity is interpreted in terms of HOMO(nitrilimine)-LUMO(dipolarophile) interaction.     

Dynamical correlations and long-time tails in chemical reactions. A molecular dynamics study

A simple hard-sphere and disk model of a chemical reaction of the type A + B ⇌ C + D is studied by molecular dynamics (MD). Ensemble and time averages are carried out so that the MD calculation simulates chemical relaxation in a homogeneous system which is in equilibrium with respect to the mechanical degrees of freedom. For large times, the decay of the concentrations of the different species is not given by the expected exponential law but shows a long-time tail instead. A simple explanation of the effect is given in terms of recollision events (dynamical correlations) between pairs of particles. It is argued that non-exponential decay of the kind observed might be a general phenomenon in chemical reactions.