Revisiting the stability of endo/exo Diels-Alder adducts between cyclopentadiene and 1,4-benzoquinone

In this work it is presented a detailed theoretical analysis of the relative stability of endo/exo Diels-Alder adducts formed by the reaction between cyclopentadiene (1) and 1,4-benzoquinone (2). The intrinsic reaction coordinate (IRC) showed the existence of only one transition state for the reaction studied, for both endo 3 and exo 4 adducts. The energies of both adducts were obtained at high level of theory (CBS-Q) confirming that the endo adduct is more stable than exo, which is in the opposite way to the observed in reactions that usually follow Alder's rule. An electronic structure analysis was performed through NBO methodology, indicating that the attractive delocalization interaction predominates over the steric repulsive interaction in the endo adducts. In summary, for the studied cycloaddition reaction the endo adduct is the thermodynamic and kinetic product, which can be also confirmed by experimental data mentioned in this work

Determining the Absolute Configuration of Small Molecules by Diffusion NMR Experiments

Enantiomers are ubiquitous in many areas of science, such as pharmaceuticals, agriculture, and food. Nuclear magnetic resonance (NMR) alone is not able to differentiate enantiomers as their spectra are identical. However, these can be distinguished using chiral auxiliaries (such as chiral complexing agents) that form diastereomeric complexes, but absolute identification is still troublesome, usually requiring a chemical reaction with a chiral derivatizing agent. Here, we propose a new method that uses a hybrid mixture of solvating agents in a simple comparison of diffusion NMR experiments, which can discriminate enantiomers in both frequency and diffusion domains, dubbed CHIMERA (CHIral Micelle Enantiomer Resolving Agent). The new method was assessed for twenty-three small chiral molecules using a combination of BINOL and (-)-DMEB, a chiral surfactant, and initial results indicate that absolute configuration can be obtained from a simple experiment

Counterintuitive deshielding on the C-13 NMR chemical shift for the trifluoromethyl anion

The trifluoromethyl anion (CF3-) displays C-13 NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF3, 122.2 ppm) and cation (CF3+, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the C-13 shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleusCNPQ - Conselho Nacional de Desenvolvimento Científico e TecnológicoFAPESP – Fundação de Amparo à Pesquisa Do Estado De São Paulosem informação2015/08541-6 ; 2017/20890-

Kinetic resolution of alpha-bromophenylacetamides using quinine or Cinchona alkaloid salts

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The kinetic resolution of racemic alpha-bromophenylacetamides 1 was achieved in the presence of benzenethiolate and Cinchona alkaloid salts as phase-transfer catalysts or benzenethiol and quinine, yielding (S)-enantioenriched alpha-sulfanylated products. The observed stereoselection was rationalized on the basis of the best fitting of 1 and the resolving agent in the ternary complexes. (C) 2012 Elsevier Ltd. All rights reserved.2310748753Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Kinetic resolution of α-bromophenylacetamides using quinine or cinchona alkaloid salts

The kinetic resolution of racemic α-bromophenylacetamides 1 was achieved in the presence of benzenethiolate and Cinchona alkaloid salts as phase-transfer catalysts or benzenethiol and quinine, yielding (S)-enantioenriched α-sulfanylated products. The observed stereoselection was rationalized on the basis of the best fitting of 1 and the resolving agent in the ternary complexes2310748753CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPSem informaçãoSem informaçã

Kinetic resolution of alpha-bromophenylacetamides using quinine or Cinchona alkaloid salts

The kinetic resolution of racemic alpha-bromophenylacetamides 1 was achieved in the presence of benzenethiolate and Cinchona alkaloid salts as phase-transfer catalysts or benzenethiol and quinine, yielding (S)-enantioenriched alpha-sulfanylated products. The observed stereoselection was rationalized on the basis of the best fitting of 1 and the resolving agent in the ternary complexes. (C) 2012 Elsevier Ltd. All rights reserved.FAPESPFAPESPCNPqCNP

The electronic origin of unusually large nJFN coupling constants in some Fluoroximes

SOPPA(CCSD) calculations show that the FC term is the most important contribution to the through-space transmission of JFN coupling constants for the fluoroximes studied in this work. Because of the well-known behavior of FC term, a new rationalization for the experimental TSJFN SSCC is presented. It is mainly based on the overlap matrix (Sij) between fluorine and nitrogen lone pairs obtained from NBO analyses. An expression is proposed to take into account the influence of the electronic density (Dij) between coupled nuclei as well as the s% character at the site of the coupling nuclei of bonds and non-bonding electron pairs involved in Dij. In using this approach, a linear correlation between TSJFN versus Dij is obtained. The most important aspect of this rationalization is related to the facility for understanding the behavior of some unusual experimental coupling constants. It is shown that, at least in this case, the electronic origin of the so-called through-space coupling is transmitted through to the overlap of orbitals on the coupled atoms, suggesting that, at least for these compounds, instead of through-space coupling, it should better be dubbed as ‘through overlapping orbital coupling’.Fil: Favaro, Denize C.. Universidade Estadual de Campinas; BrasilFil: Contreras, Ruben Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Tormena, Claudio F.. Universidade Estadual de Campinas; Brasi

Unexpected behavior of the (3)J(CH) coupling constant in unsaturated compounds

The experimental and theoretical behavior of the (O(sic)C)-C(sic)C-H (3)J(CH) coupling constant is investigated for a series of alpha,beta-unsaturated compounds (1 to 8), and for some of them, the well-known relationship ((3)J(CHcis) J(CHtrans)) was observed. However, for some compounds, close values for (3)J(CHcis) and (3)J(CHtrans) couplings were observed, and for aldehydes group containing compounds (7 and 8E), an inversion order is observed ((3)J(CHcis) > (3)J(CHtrans)). In all cases where the (3)J(CHcis) J(CHtrans) relationship it is not followed, a polar group or electronegative atom oriented in opposite direction (s-trans) to the H-C(sic)C hydrogen is present, suggesting that conformational preference of such polar group or atoms are important factor on the behavior of (3)J(CH) couplings. Taking all of these in consideration, a new Karplus-type equation was proposed for (3)J(CH) couplings in alpha,beta-unsaturated compounds, which can be used for configurational and conformational assignment on trisubstituted double bond derivatives5711939945CNPQ - Conselho Nacional de Desenvolvimento Científico e TecnológicoFACEPE - Fundação de Amparo à ciência E Tecnologia Do Estado De PernambucoFAPESP – Fundação de Amparo à Pesquisa Do Estado De São Paulo426216/2018-0APQ-0507-1.06/152015/08541-6; 2016/24331-