Regioselectivity in the addition of vinylmagnesium bromide to heteroarylic ketones: C- versus O-alkylation

The reactivity of heteroarylic ketones toward vinylmagnesium bromide (2) and the regiochemistry of the addition were investigated. The reactivity drastically increases when the carbonyl is conjugated with at least one aza group and the regiochemistry of the addition of the vinyl Grignard reagent depends on the carbonyl compound: in the series of di(heteroazolyl) ketones the O-alkylation product was observed as unique with di(1,3-benzothiazol-2-yl) ketone, and in different relative ratios with respect to the classic C-alkylation product with di(1,3-thiazol-2-yl) ketone, (1,3-benzothiazol-2-yl) (1,3-thiazol-2-yl) ketone, and di(1,3-benzoxazol-2-yl) ketone, whereas di(N-methylbenzimidazol-2-yl) ketone gave the exclusive formation of the carbinol. This behavior can be explained by the intervention of the delocalization power of the heterocyclic ring and this was confirmed by the results obtained from the reaction between vinylmagnesium bromide and a series of mixed (1,3-benzothiazol-2-yl) (para-substituted phenyl) ketones, that showed a relative O-/C-alkylation ratio dependent on the nature and on the electronic effect of the substituent on the phenyl ring. The results are in agreement with the existence of intermediate species bearing a negative charge on the benzylic carbonyl carbon atom, and make the O-alkylation reaction between vinyl Grignard reagents and carbonyl compounds no longer a rare case, since it was observed with a number of heterocyclic carbonyl compounds, such as (1,3-benzothiazol-2-yl) aryl ketones and di(heteroaryl) ketones of the pentatomic 1,3-heteroazolic series.     

Chiral phosphoproline-catalyzed asymmetric Michael addition of ketones to nitroolefins: an experimental and theoretical study

A novel pyrrolidine-based chiral phosphoproline is an effective bifunctional organocatalyst for the asymmetric Michael addition of ketones to nitroolefins giving high levels of diastereo- and enantio-selectivities (up to > 99 : 1 dr and 96% ee). anti-SR Transition state has the lowest barrier which controls the stereoselectivity, in agreement with experimental results.     

Silylium-arene adducts: an experimental and theoretical study

The solvent-coordinated [Me(3)Si·arene][B(C(6)F(5))(4)] salts (arene = benzene, toluene, ethylbenzene, n-propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene) are prepared and fully characterized. As an interesting decomposition product the formation of bissilylated fluoronium ion [Me(3)Si-F-SiMe(3)](+) was observed and even cocrystallized with [Me(3)Si·arene][B(C(6)F(5))(4)] (arene = benzene and toluene). Investigation of the degradation of [Me(3)Si·arene][B(C(6)F(5))(4)] reveals the formation of fluoronium salt [Me(3)Si-F-SiMe(3)][B(C(6)F(5))(4)], B(C(6)F(5))(3), and a reactive "C(6)F(4)" species which could be trapped with CS(2). Upon addition of CS(2), the formation of a formal S-heterocyclic carbene adduct, C(6)F(4)CS(2)-B(C(6)F(5))(3), was observed. The structure and bonding of substituted [Me(3)Si·arene][B(C(6)F(5))(4)] with arene = R(n)C(6)H(6-n) (R = H, Me, Et, Pr, and Bu; n = 0-6) is discussed on the basis of experimental and theoretical data. X-ray data of [Me(3)Si·arene][B(C(6)F(5))(4)] salts reveal nonplanar arene species with significant cation···anion interactions. As shown by different theoretical approaches (charge transfer, partial charges, trimethylsilyl affinity values) stabilizing inductive effects occur; however, the magnitude of such effects differs depending on the degree of substitution and the substitution pattern.     

The electrophilic addition of thiols to olefins: A theoretical and experimental study

Density functional theory with the B3LYP hybrid functional and 6–31G* basis set was used to study the geometric and electronic structure of H₂C = CHR (R = H, CH₃, C₂H₅, C₃H₇, C₄H₉, and C₅H₁₁) olefins, their carbocations formed in the addition of the proton to the olefins, R′-S-H aliphatic thiols (R′ = H, CH₃, C₂H₅, and C₃H₇), the products of the addition of thiols to carbocations, and the final products of the addition of thiols to olefins. The proton affinity of the olefins and the products of the addition of thiols to olefins was calculated. The conclusion was drawn that the limiting stage in the nonradical addition of thiols to olefins catalyzed by acids was proton transfer from the protonated reaction product to the olefin. The theoretical results were compared with the experimental data on the electrophilic addition of polymercaptan to heptene-1.     

The role of fluorine in the stereoselective tandem aza-Michael addition to acrylamide acceptors: an experimental and theoretical mechanistic study

Aza-Michael additions of alpha-amino esters to fluorinated acceptors take place in a highly stereoselective manner, to give partially modified Psi[NHCH2]retropeptides incorporating a hydrolytically stable trifluoroalanine mimic. The reaction mechanism has been investigated experimentally and theoretically, in order to explain the effect of the trifluoromethyl group on the reactivity and the origins of the experimentally observed stereocontrol. The reaction is a two-step process, involving a tandem aza-Michael addition followed by a stereoselective hydrogen transfer. Both steps are base-catalyzed. The high level of stereocontrol is the result of a combination of electrostatic interactions and steric effects.     

Amine exchange in formamidines: an experimental and theoretical study

N-H-containing formamidines combine a reasonably strong association to carboxylic acids to form complexes of well-defined geometries with a simultaneous proton-induced electrophilicity enhancement that allows for the exchange of their amine portion. The N=C(H)-NH fragment, therefore, undergoes "imine-like" exchange with N-containing nucleophiles. Because of the prototropic equilibrium, the N=C(H)-NH fragment may behave as a "bisimine" centred on the same carbon, in which both N-containing fragments can be exchanged. Considering the proton-induced sensitisation of both C-N units and the well-defined formamidine-carboxylic acid complex geometry, it should be possible to use carboxylic acids as templates for the synthesis of defined architectures by dynamic amine exchange within formamidines. This study highlights three exchange regimes based on the nature of the incoming amine (aliphatic amines, aromatic amines and alkoxyamines), as well as exchange rules based on the amine leaving groups. Following this analysis, a proof of concept for carboxylic acid templated macrocycle formation through dynamic exchange is provided.     

Enantioselective Michael addition of aldehydes to maleimides organocatalysed by chiral 1,2-diamines: an experimental and theoretical study

Simple and commercially available chiral 1,2-diamines were used as organocatalysts for the enantioselective conjugate addition of aldehydes, including α,α-disubstituted, to maleimides. The reaction was carried out in the presence of hexanedioic acid as an additive in aqueous solvents at room temperature. By employing (1S,2S)- and (1R,2R)-cyclohexane-1,2-diamine as organocatalysts, the corresponding Michael adducts bearing new stereocenters were obtained in high or quantitative yields with enantioselectivities of up to 92%, whereas the use of (1S,2S)-1,2-diphenylethane-1,2-diamine gave a much lower ee. Theoretical calculations were used to justify the observed sense of the stereoinduction.     

Fluorine-fluorine interactions in the solid state: an experimental and theoretical study

The solid state structures of three compounds that contain a perfluorinated chain, CF(3)(CF(2))(5)CH(2)CH(CH(3))CO(2)H, CF(3)(CF(2))(5)(CH(2))(4)(CF(2))(5)CF(3) and {CF(3)(CF(2))(5)CH(2)CH(2)}(3)P═O have been compared and a number of C-F···F-C and C-F···H-C interactions that are closer than the sum of the van der Waals radii have been identified. These interactions have been probed by a comprehensive computational chemistry investigation and the stabilizing energy between dimeric fragments was found to be 0.26-29.64 kcal/mol, depending on the type of interaction. An Atoms-in-Molecules (AIM) study has confirmed that specific C-F···F-C interactions are indeed present, and are not due simply to crystal packing. The weakly stabilizing nature of these interactions has been utilized in the physisorption of a selected number of compounds containing long chain perfluorinated ponytails onto a perfluorinated self-assembled monolayer, which has been characterized by IRRAS (Infrared Reflection Absorption Spectroscopy).     

3-bromozinc propenyl esters: an experimental and theoretical study of the unique stereocrossover observed in their addition to aromatic and aliphatic aldehydes

We report the results of a combined experimental and theoretical study on the reaction of 3-bromopropenyl acetate in the presence of zinc with three different aldehydes (i.e., benzaldehyde, 2-methylpropanal, and cyclohexanecarboxaldehyde). A 80% de in favor of the anti product has been experimentally observed with both saturated aldehydes, while for benzaldehyde, a 1:1 syn/anti ratio has been found. DFT computations show the existence of three eta1-allylic organozinc complexes [gamma-(Z)-5a, gamma-(E)-5a, and alpha-5a], very close in energy. Only gamma-(Z)-5a and gamma-(E)-5a lead to the observed product. The computational investigation of the reaction of these allylic organozinc complexes with benzaldehyde and 2-methylpropanal demonstrates in both cases the existence of two competitive reaction paths leading to the syn and anti adducts, respectively. An anti preference has been found for 2-methylpropanal with both gamma-(Z)-5a and gamma-(E)-5a species (a diastereoselectivity larger than 80% is predicted), in agreement with the experiment. With benzaldehyde, while the reaction of gamma-(Z)-5a retains an anti-stereopreference (de = 70%), that involving gamma-(E)-5a is characterized by two degenerate transition states. In this case, the agreement between computations and experiments would be satisfactory under the assumption that the initial oxidative addition affords the gamma-(E)-5a zinc complex only. Additional MP2 computations have demonstrated that pi-stacking interactions can play a significant role in determining the relative energy of the transition states leading to the syn and anti products.     

Solvent effects on the thioamide rotational barrier: an experimental and theoretical study.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.     

Intermediates of CO oxidation on iron oxides: an experimental and theoretical study

Reactions of laser-ablated iron oxides with CO in excess argon are investigated by infrared adsorption spectroscopy and density functional theoretical calculations. The carbonyl iron oxides OFe(CO)(n) (n = 1-3) and O(2)Fe(CO)(m) (m = 1, 2) are generated during sample deposition or annealing, whereas CO(2) is greatly produced at the expense of these carbonyl iron oxides upon UV irradiation, showing the formation of intermediate carbonyl iron oxides in the oxidation of carbon monoxide to carbon dioxide. These intermediate carbonyl iron oxides are characterized on the basis of isotopic substitution, stepwise annealing, change of CO concentration and laser energy, and comparison with theoretical calculations. The overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these complexes from the matrix infrared spectra. The reaction pathways for the formation of the products are proposed based on the experimental and theoretical results presented.     

Cationic prevesicle and vesicle nanoaggregates: an experimental and theoretical study

The formation of spontaneous mixed prevesicles and vesicles consisting of a cationic double-chain surfactant, didecyldimethylammonium bromide (di-C(10)DMAB), and a cationic single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C(10)TAB, and/or tetradecyltrimethylammonium bromide, C(14)TAB) has been investigated by means of a series of (i) highly precise experimental techniques, such as conductometry, transmission electronic microscopies (TEM and cryo-TEM), laser Doppler electrophoresis (LDE), and steady-state fluorescence spectroscopy and (ii) theoretical models, such as the DLVO theory and two of its main further modifications, Inoues's and Sogami's models. Two new potentials, based on the combination of DLVO or Inoue potentials with that of Sogami, have been proposed and checked. This theoretical analysis has been carried out not only for the aggregates studied in this work but also for other di-C(m)DMAB + C(n)TAB (m = 10, 12 and n = 10, 12, 14) systems previously reported by us. In respect to the experimental study, special emphasis has been devoted to the prevesicle domain. We have confirmed the existence of two critical aggregation concentrations in the very diluted concentration domain, where the conductivity plot shows a zigzag pattern: the so-called mixed critical aggregate concentration, CAC* and the mixed critical vesicle concentration, CVC*. Contrarily, only CVC* is detected. The pre-CAC* nanoaggregates, with a variety of sizes and shapes, do not show a clear aggregation pattern, but even at such low concentrations a small number of nanoaggregates with a clear and ordered aggregation pattern has been visualized. In the postvesicle domain, the aggregates (vesicles) are unilamellar and spherical with a medium polidispersity and a net averaged surface density charge of around 14 x 10(-3) (pure vesicles) and 24 x 10(-3) C m(-2) (mixed vesicles). The hydrophobicities of the lipidic bilayer and the surface of the vesicles resemble those of media with dielectric constants of around 30 and 75, respectively. Finally, theoretical predictions confirm the stability of the pure and mixed vesicles studied in this work and in other works previously reported.     

Phosphorescent binuclear iridium complexes based on terpyridine-carboxylate: an experimental and theoretical study

The phosphorescent binuclear iridium(III) complexes tetrakis(2-phenylpyridine)μ-(2,2':6',2'-terpyridine-6,6'-dicarboxylic acid)diiridium (Ir1) and tetrakis(2-(2,4-difluorophenyl) pyridine))μ-(2,2':6',2'-terpyridine-6,6'-dicarboxylic acid)diiridium (Ir2) were synthesized in a straightforward manner and characterized using X-ray diffraction, NMR, UV-vis absorption, and emission spectroscopy. The complexes have similar solution structures in which the two iridium centers are equivalent. This is further confirmed by the solid state structure of Ir2. The newly reported complexes display intense luminescence in dichloromethane solutions with maxima at 538 (Ir1) and 477 nm (Ir2) at 298 K (496 and 468 nm at 77 K, respectively) and emission quantum yields reaching ~18% for Ir1. The emission quantum yield for Ir1 is among the highest values reported for dinuclear iridium complexes. It shows only a 11% decrease with respect to the emission quantum yield reported for its mononuclear analogue, while the molar extinction coefficient is roughly doubled. This suggests that such architectures are of potential interest for the development of polymetallic assemblies showing improved optical properties. DFT and time-dependent-DFT calculations were performed on the ground and excited states of the complexes to provide insights into their structural, electronic, and photophysical properties.     

A study on theoretical predictive model and experimental findings of melt-electrospinning process

The current study extends a previous work published by the authors in which an analytical predictive model is proposed to simulate the melt-electrospinning process. The analytical model is specifically designed to predict the various behaviors of the melt-electrospun fiber under different material and processing conditions. A brief discussion of this model is presented here to establish context and help the reader capture the modeling philosophy employed. The current study complements the previous work by focusing on the experimental aspects of the research. Correlations between the independent process parameters and the topological attributes of the melt-electrospun fibers are investigated and compared with findings from the theoretical model. The effects of changes in the process parameters on average fiber diameters and the collection diameter are experimentally analyzed using the design of experiments (DOE) techniques. Toward this end, polylactic acid (PLA) is melt-electrospun at different treatment levels of the processing parameters in a controlled environment. Two regression-based models—one for predicting the collection diameter and the other for the fiber diameter—are derived from the DOE data for benchmarking and quantitative evaluation of the predictive performance of the theoretical model. The theoretical model is run based on the same treatment levels as the experiment. The elastic parameter values used in the theoretical simulation are extracted from rheological tests. Comparison between the simulated and the observed fiber characteristics revealed that the collector diameter predictions by the theoretical model exhibited approximately a 16.7% difference compared to 24.2% for the average fiber diameter. Finally, a discussion is presented on the challenges and potential factors contributing to the observed differences. Overall, given the identified challenges and gaps in material characterization, the results of the theoretical predictive model are encouraging.     

Untangling the excited states of DR1 in solution: an experimental and theoretical study

We report the experimental observations and the theoretical predictions of the fully separated n-pi* and pi-pi* bands of Disperse Red One in acidified methanol solution. The analysis of the linear and two-photon absorption spectra is presented in four specific solvents. We demonstrate the possibility to establish the position of the first two excited states combining linear and nonlinear spectroscopy. Calculations using density functional theory at TD-B3LYP/6-31G*//HF/6-31G* level accurately predict the spectral region of the n-pi* and pi-pi* transitions of DR1 in all solvents.     

Dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone: an experimental and theoretical study

The photoionization and dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone (1,8-DHAQ) have been investigated by infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (IR LD/VUV PIMS) technique and theoretical calculations. Consecutive losses of two carbon monoxides and elimination of hydroxyl group are found to be the major fragmentation channels in low photon energy range. Photoionization efficiency (PIE) spectrum of 1,8-DHAQ was measured in the photon energy range of 8.2-15.0 eV. Adiabatic ionization energy (IE) of 1,8-DAHQ (M) and appearance energies (AEs) of the major fragments (M-CO) (+), (M-C 2O 2) (+), and (M-OH) (+) are determined to be 8.54 +/- 0.05, 10.8 +/- 0.1, 11.0 +/- 0.1, and 13.1 +/- 0.1 eV, respectively, which are in fair agreement with calculated results. The B3LYP method with the 6-31+G(d) basis set was used to study fragmentation of 1,8-DHAQ. Theoretical calculations indicate that five lowest-energy isomers of 1,8-DHAQ cations can coexist by virtue of bond rotation and intramolecular proton transfer. A number of decarbonylation and dehydroxylation processes of 1,8-DHAQ cations are well established.     

Templating Schiff-base lateral macrobicycles: an experimental and theoretical structural study of the intermediates

In this paper, we report a structural study both in the solid state and in solution of barium complexes with the diamine N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)), that allows us to rationalize the template effect of the metal ion in the synthesis of Schiff-base lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. The X-ray crystal structures of [Ba(L(2))(ClO(4))](ClO(4)) (3) [triclinic space group P1 with Z = 2, a = 10.467(2) A, b = 10.4755(2) A, c = 16.9911(3) A, alpha = 85.075(1) degrees, beta = 80.907(1) degrees, and gamma = 61.627(4) degrees ] and [Ba(L(2))(NCS)(H(2)O)](SCN) (4) [monoclinic space group P2(1)/n with Z = 4, a = 9.954(5) A, b = 29.193(5) A, c = 11.313(5) A, and beta = 91.371(5) degrees ] demonstrate that in the solid state the barium(II) ion induces an anti conformation of the receptor in the complexes. Variable temperature (1)H and (13)C NMR data point out that in solution compounds 3 and 4 exist as a mixture of syn and anti isomers. The presence of the syn isomer in solution, independent of the counterion employed (perchlorate or thiocyanate), accounts for the effectiveness of the barium(II) ion as a template agent in the synthesis of the lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. Density functional theory calculations (at the B3LYP/LanL2DZ level) for [Ba(L(2))](2+) predict the syn conformation to be more stable both in vacuo and in solution (PCM model). In order to asses which of the two isomers predominates in acetonitrile solution, the (13)C NMR shielding tensors of the two isomers of [Ba(L(2))](2+) were calculated for the in vacuo optimized structures by using the GIAO method, and the results were compared with the experimental ones. According to these analyses, a syn stereochemistry is assigned to the major species in solution.