1,2‐Oxazine N‐Oxides as Catalyst Resting States in Michael Additions of Aldehydes to Nitro Olefins Organocatalyzed by α,α‐Diphenylprolinol Trimethylsilyl Ether

By combining enamines, derived from aldehydes and diphenylprolinol trimethylsilyl ether (the Hayashi catalyst), with nitroethenes ((D₆)benzene, 4‐Å molecular sieves, room temperature) intermediates of the corresponding catalytic Michael‐addition cycles were formed and characterized (IR, NMR, X‐ray analysis; Schemes 3–6 and Fig. 1–3). Besides cyclobutanes 2 , 1,2‐oxazine N‐oxide derivatives 3 – 6 and 8 have been identified for the first time, some of which are very stable compounds. It may not be a lack of reactivity (between the intermediate enamines and nitro olefins) that leads to failure of the catalytic reactions (Schemes 3–5) but the high stability of catalyst resting states. The central role zwitterions play in these processes is discussed (Schemes 1 and 2).     

Unusual Reactions of (+)‐Car‐2‐ene and (+)‐Car‐3‐ene with Aldehydes on K10 Clay

The reactions of (+)‐car‐2‐ene ( 1 ) and (+)‐car‐3‐ene ( 2 ) with aldehydes in the presence of montmorillonite clay were studied for the first time (Schemes 3 and 5). The major products of these reactions are optically active, substituted hexahydroisobenzofurans, probably formed as a result of an attack of the protonated aldehyde at the cyclopropane ring. Quite unexpectedly, the products are cis‐configured at the ring‐fusion site; the fact was established by means of quantum‐chemical calculations and NMR data. It appeared that the behavior of the 2 : 3 mixture 1 / 2 in reactions with aldehydes in the presence of K10 clay differed substantially from the reactivities of the corresponding individual monoterpenes.     

Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine – A Computational Study

It has been shown previously that the reaction of diazomethane with 5‐benzylidene‐3‐phenylrhodanine ( 1 ) in THF at ?20° occurs at the exocyclic C?C bond via cyclopropanation to give 3a and methylation to yield 4 , respectively, whereas the corresponding reaction with phenyldiazomethane in toluene at 0° leads to the cyclopropane derivative 3b exclusively. Surprisingly, under similar conditions, no reaction was observed between 1 and diphenyldiazomethane, but the 2‐diphenylmethylidene derivative 5 was formed in boiling toluene. In the present study, these results have been rationalized by calculations at the DFT B3LYP/6‐31G(d) level using PCM solvent model. In the case of diazomethane, the formation of 3a occurs via initial Michael addition, whereas 4 is formed via [3+2] cycloaddition followed by N₂ elimination and H‐migration. The preferred pathway of the reaction of 1 with phenyldiazomethane is a [3+2] cycloaddition, subsequent N₂ elimination and ring closure of an intermediate zwitterion to give 3b . Finally, the calculations show that the energetically most favorable reaction of 1 with diphenyldiazomethane is the initial formation of diphenylcarbene, which adds to the S‐atom to give a thiocarbonyl ylide, followed by 1,3‐dipolar electrocyclization and S‐elimination.     

Reactivity and Selectivity of Captodative Olefins as Dienes in Hetero‐Diels–Alder Reactions

The reactivity and selectivity of the the captodative olefins 1‐acylvinyl benzoates 1a – 1f and 3a as heterodienes in hetero‐Diels–Alder reactions in the presence of electron‐rich dienophiles is described. Heterodienes 1 undergo regioselective cycloaddition with the alkyl vinyl etherdienophiles 6a , b and 9 to give the corresponding dihydro‐2H‐pyrans 7, 8 , and 10 under thermal conditions. The reactivity of these cycloadditions depends, to a large extent, on the electronic demand of the substituent in the aroyloxy group of the heterodiene. Frontier‐molecular‐orbital (FMO; ab initio) and density‐functional‐theory (DFT) calculations of the ground and transition states account for the reactivity and regioselectivity observed in these processes.     

Structures of the Reactive Intermediates in Organocatalysis with Diarylprolinol Ethers

Structures of the reactive intermediates (enamines and iminium ions) of organocatalysis with diarylprolinol derivatives have been determined. To this end, diarylprolinol methyl and silyl ethers, 1 , and aldehydes, Ph? CH₂? CHO, ^tBu? CH₂? CHO, Ph? CH=CH? CHO, are condensed to the corresponding enamines, A and 3 (Scheme 2), and cinnamoylidene iminium salts, B and 4 (Scheme 3). These are isolated and fully characterized by melting/decomposition points, [α]_D, elemental analysis, IR and NMR spectroscopy, and high‐resolution mass spectrometry (HR‐MS). Salts with BF₄, PF₆, SbF₆, and the weakly coordinating Al[OC(CF₃)₃]₄ anion were prepared. X‐Ray crystal structures of an enamine and of six iminium salts have been obtained and are described herein (Figs. 2 and 4–8, and Tables 2 and 7) and in a previous preliminary communication (Helv. Chim. Acta 2008 , 91, 1999). According to the NMR spectra (in CDCl₃, (D₆)DMSO, (D₆)acetone, or CD₃OD; Table 1), the major isomers 4 of the iminium salts have (E)‐configuration of the exocyclic N?C(1′) bond, but there are up to 11% of the (Z)‐isomer present in these solutions (Fig. 1). In all crystal structures, the iminium ions have (E)‐configuration, and the conformation around the exocyclic N‐C? C‐O bond is synclinal‐exo (cf. C and L ), with one of the phenyl groups over the pyrrolidine ring, and the RO group over the π‐system. One of the meta‐substituents (Me in 4b , CF₃ in 4c and 4e ) on a 3,5‐disubstituted phenyl group is also located in the space above the π‐system. DFT Calculations at various levels of theory (Tables 3–6) confirm that the experimentally determined structures (cf. Fig. 10) are by far (up to 8.3 kcal/mol) the most stable ones. Implications of the results with respect to the mechanism of organocatalysis by diarylprolinol derivatives are discussed.     

Tris‐cyclometalated Iridium(III) Complexes with Three Different Ligands: a New Example with 2‐(2,4‐Difluorophenyl)pyridine‐Based Complex

An iridium(III) complex comprising three different cyclometalated phenylpyridine‐based ligands was designed and synthesized. Interestingly, mixed‐ligand complexes could be obtained by using a simple and straightforward procedure. A tris(heteroleptic) Ir^III complex was obtained as a mixture of stereoisomers that could not be separated. Photophysical properties of the tris(heteroleptic) complex was investigated by UV/VIS absorption and luminescence spectroscopy, and compared with those of the parent homoleptic complexes. Modelling by time‐dependent density functional theory (TD‐DFT) was also performed to elucidate the nature and the location of the excited state, and to support the experimental results.     

On the Solvent‐free Structure of a Jäger‐type Cobalt(II) N2O2‐Chelate Complex

A combined synchrotron X‐ray and density functional theory (DFT) study on the structure of a Jäger‐type N₂O₂ chelate complex was carried out. The ethoxy‐substituted bis(3‐oxo‐enaminato)cobalt(II) complex ( 1 ) was an original sample from the laboratory of the late Professor Ernst‐G. Jäger (University of Jena, Germany). Single‐crystal X‐ray analysis revealed essentially flat molecules of 1 , which are unsolvated and coordinatively unsaturated. The DFT calculations on the isolated molecule predict a planar structure for the non‐hydrogen atoms, which is a local minimum on the energy surface. The crystal packing is achieved through off‐set stacking (staircase arrangement), resulting in a herringbone pattern in the space group P2₁2₁2₁. The structure of 1 is compared to known structures of related bis(3‐oxo‐enaminato)cobalt(II) complexes ( 2 – 4 ). Original bulk material of 1 was investigated by scanning electron microscopy (SEM), powder X‐ray diffraction (PXRD), melting point determination, and infrared (IR) spectroscopy.     

Experimental and Theoretical Conformational Analysis of 5‐Benzylimidazolidin‐4‐one Derivatives – a ‘Playground’ for Studying Dispersion Interactions and a ‘Windshield‐Wiper’ Effect in Organocatalysis

The PF₆ salts of 5‐benzyl‐1‐isopropylidene‐ and 5‐benzyl‐1‐cinnamylidene‐3‐methylimidazolidin‐4‐ones 1 (Scheme) with various substituents in the 2‐position have been prepared, and single crystals suitable for X‐ray structure determination have been obtained of 14 such compounds, i.e., 2 – 10 and 12 – 16 (Figs. 2–5). In nine of the structures, the Ph ring of the benzyl group resides above the heterocycle, in contact with the cis‐substituent at C(2) (staggered conformation A ; Figs. 1–3); in three structures, the Ph ring lies above the iminium π‐plane (staggered conformation B ; Figs. 1 and 4); in two structures, the benzylic C? C bond has an eclipsing conformation ( C ; Figs. 1 and 5) which places the Ph ring simultaneously at a maximum distance with its neighbors, the CO group, the N?C‐π‐system, and the cis‐substituent at C(2) of the heterocycle. It is suggested by a qualitative conformational analysis (Fig. 6) that the three staggered conformations of the benzylic C? C bond are all subject to unfavorable steric interactions, so that the eclipsing conformation may be a kind of ‘escape’. State‐of‐the‐art quantum‐chemical methods, with large AO basic sets (near the limit) for the single‐point calculations, were used to compute the structures of seven of the 14 iminium ions, i.e., 3, 4 / 12, 5 – 7, 13 , and 16 (Table) in the two staggered conformations, A and B , with the benzylic Ph group above the ring and above the iminium π‐system, respectively. In all cases, the more stable computed conformer (‘isolated‐molecule’ structure) corresponds to the one present in the crystal (overlay in Fig. 7). The energy differences are small (≤2 kcal/mol) which, together with the result of a potential‐curve calculation for the rotation around the benzylic C? C bond of one of the structures, 16 (Fig. 8), suggests that the benzyl group is more or less freely rotating at ambident temperatures. The importance of intramolecular London dispersion (benzene ring in ‘contact’ with the cis‐substituent in conformation A ) for DFT and other quantum‐chemical computations is demonstrated; the benzyl‐imidazolidinones 1 appear to be ideal systems for detecting dispersion contributions between a benzene ring and alkyl or aryl CH groups. Enylidene ions of the type studied herein are the reactive intermediates of enantioselective organocatalytic conjugate additions, Diels–Alder reactions, and many other transformations involving α,β‐unsaturated carbonyl compounds. Our experimental and theoretical results are discussed in view of the performance of 5‐benzyl‐imidazolidinones as enantioselective catalysts.     

Abieseconordines A and B,Two Novel Norditerpenoids with a 18‐Nor‐5,10 : 9,10‐disecoabietane Skeleton from Abies forrestii

A systematic phytochemical investigation on Abies forrestii afforded two new and 20 known compounds. Abieseconordines A and B ( 1 and 2 ) are the first two examples of norditerpenes with a novel 18‐nor‐5,10 : 9,10‐disecoabietane skeleton. Their structures were established mainly by analysis of 1D‐ and 2D‐NMR spectroscopic data. In addition, electronic circular‐dichroism calculations and molecular‐orbital analysis were utilized to confirm the absolute configuration of 1 . Both compounds exhibited a potent effect in a bioassay inhibiting LPS‐stimulated NO production in RAW264.7 macrophages.     

N-甲基b-磺内酰胺氨解反应机理的理论研究

The aminolysis and the effect of water on the aminolysis processes of n-methyl β-sultam have been studied using density functional theory （DFF） method at the B3LYP/6-31G＊ level. The stationary structures and energies have been investigated for both reactions to find two different reaction channels. Specific and general solvent effects have been evaluated and the most favored pathway was found. The presence of solvent disfavors the reaction, whereas the participation of water in the aminolysis reaction plays a positive role and reduces the activation energy greatly. All transition states in the assisted aminolysis are 35-70 kJ/mol lower than those for the non-assisted reaction.     

Reactivity of 3‐Methylbenzenediazonium Ions with Gallic Acid. Kinetics and Mechanism of the Reaction

We investigated the kinetics and mechanism of the reaction between the 3‐methylbenzenediazonium ions (3MBD), and gallic acids (=3,4,5‐trihydroxybenzoic acid; GA) in aqueous buffer solution under acidic conditions by employing spectrometric, electrochemical, and chromatographic techniques and computational methods. To discern which of the three OH groups of GA is the first one undergoing deprotonation, the geometries of the resulting dianions were optimized by using B3LYP hybrid density‐functional theory (DFT) and a 6‐31G(++d,p) basis set, and the results suggest that the OH group at the 4‐position is the first one which is deprotonated. The variation of the observed rate constant, k_obs, with the acidity at a given [GA] follows an upward curve suggesting that the reaction takes place with the dianionic form of gallic acid, GA^2?, and rate enhancements of ca. 23000 fold are obtained on going from pH 3.5 up to pH 7.5. At relatively high acidities, the variation of k_obs with [GA] is linear with an intercept very close to the value for the thermal decomposition of 3MBD; however, a decrease in the acidity leads to saturation‐kinetics profiles with nonzero, pH‐dependent intercepts. The saturation‐kinetics patterns found suggest the formation of an intermediate in a rapid pre‐equilibrium step, but the nonzero, pH‐dependent intercepts cause the double reciprocal plots of 1/k_obs vs. 1/[GA] to curve. This prompts us to propose an alternative reaction mechanism comprising consecutive equilibrium processes involving the bimolecular, reversible formation of a highly unstable (Z)‐diazo ether which undergoes isomerization to the (E)‐isomer through a unimolecular step. The results obtained indicate the complexity of reactions of arenediazonium ions with nucleophilic arenes containing three or more OH groups.     

The Rearrangement of 2,2‐Diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane to 3,4,4‐Triphenylcyclopent‐1‐ene: a DFT Analysis

A computational study on the rearrangement of 2,2‐diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane ( 1 ) is presented, using density functional theory (DFT), (U)B3LYP with the 6‐31G* basis set (DFT1) and (U)M05‐2X with the 6‐311+G** basis set (DFT2). In agreement with a biradical character of the transition structure (TS) or intermediate, the potential‐energy hypersurface is lowered by the influence of three conjugated Ph groups. Surprisingly, two conformations of the geminal diphenyl group (different twist angles) induce two different minimum‐energy pathways for the rearrangement. Independent of the functional used, the first hypersurface harbors true biradical intermediates, whereas the second energy surface is a flat, slightly ascending slope from the starting material to the TS. The functional (U)M05‐2X with the basis set 6‐311+G** provides realistic energies which seem to be close to experiment. The activation energy for racemization of enantiomers of 1 is lower than that of rearrangement by 2.5 kcal mol^?1, in agreement with experiment.     

DFT Study for a Series of Less-Symmetrical Crown Ethers and Their Complexes with Alkali Metal Cations

A series of ring‐contracted (14‐crown‐5, 17‐crown‐6) and ring‐enlarged (16‐crown‐5, 17‐crown‐5, 19‐crown‐6, 20‐crown‐6) crown ethers and their complexes with alkali‐metal cations Na⁺ and K⁺ had been explored using density functional theory (DFT) at B3LYP/6‐31G* level in order to reveal the effects of the methylene‐chain length in a crown ether. The nucleophilicity of all crown ethers had been investigated by the Fukui functions. The quantum chemistry parameters, such as the energy gap (ΔE), the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) for less‐symmetrical crown ethers and symmetrical frameworks (15‐crown‐5, 18‐crown‐6) had been calculated. In addition, the thermodynamic energies of complexation reactions had also been studied. The results of the DFT calculations show that the methylene‐chain length plays an important role in determining the structure characters of the crown ethers and also strongly influences the properties of the ethers. Some of the calculated results are in a good agreement with the experimental values.     

DFT studies of the phosphinidene derivative HPNaF and its insertion reaction with R–H(R=F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>)

The formations of the phosphinidene derivative HPNaF and its insertion reactions with R–H (R=F, OH, NH₂, CH₃) have been systematically investigated employing the density functional theory (DFT), such as the B3LYP and MPW1PW91 methods. A comparison with the results of MP2 calculations shows that MPW1PW91 underestimates the barrier heights for the four reactions considered. Similarly, the same is also true for the B3LYP method depending on the selected reactions, but by much less than MPW1PW91, where the barrier heights of the four reactions are 25.2, 85.7, 119.0, and 142.4 kJ/mol at the B3LYP/6-311+G* level of theory, respectively. All the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been located during the insertion reaction. Then, the intermediate could dissociate to substituted phosphinidane(H₂RP) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are −92.2, −68.1, −57.2, and −44.3 kJ/mol at the B3LYP/6-311+G* level of theory, respectively, where both the B3LYP and MPW1PW91 methods underestimate the reaction energies compared with the MP2 results. The linear correlations between the calculated barrier heights and the reaction energies have also been observed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH₂ > H–CH₃.     

Theoretical study on the structures and properties of the isomers of Nn(CH)8−nH8 (n = 0–7)

With replacement of N atoms by CH groups in the most stable chain isomer of N8H8, 34 possible isomers of N_n(CH)_8−nH₈ (n = 0–7) have been designed and optimized at the B3LYP/6-311++G** level of theory. The natural bond orbital (NBO) and atoms in molecules (AIM) analysis are carried out to study the bonding nature and relative stabilities of these conformers. G3MP2 method is applied to calculate energies and heats of formation. The results indicate that the hyperconjugation effect from lone pairs of nitrogen atoms to germinal C–N bonds is the major factor which caused the change of the C–N bond length. With the more replacement of nitrogen atoms by CH groups, the heats of formation of the isomers of N_n(CH)_8−nH₈ (n = 0–7) decrease gradually, but the energies increase linearly.     

A Convenient and Versatile Method for the Preparation of α‐Hydroxymethyl Ketone Derivatives from the Corresponding Allyl Silyl Ethers or Allyl Carboxylates

The ozonolysis of 1‐substituted allyl silyl ethers or 1‐substituted allyl carboxylates followed by treatment with bases gave the corresponding α‐silyloxymethyl‐ or α‐acyloxymethyl‐ketones in good yields. It is proposed to proceed via the corresponding α‐silyloxy‐ or α‐acyloxyaldehydes intermediates followed by 1,4‐group migration. The results of theoretical calculations are applicable to explain the experimental results.     

The distribution pattern of squares on the stability of F4F6-(BN)n (n = 10–22) polyhedrons

To gain an insight into the structures and stability of F₄F₆-(BN)_n polyhedrons with alternation of B and N atoms, a density functional theory study was performed on all isomers of F₄F₆-(BN)_n polyhedrons with n between 10 and 22. The calculation results demonstrate that the lowest energy isomers do not contain B₄₄ bonds (the bonds shared by two squares) and the energies of those isomers containing B₄₄ bonds increase with the number of B₄₄ bonds linearly, indicating that the energetically favored structures of F₄F₆-(BN)_n polyhedrons satisfy the isolated square rule and square adjacency penalty rule. The structural analysis reveals that the stability is determined by the pyramidalization of B and N atoms at the square–square fusion. The binding energy is fitted to the numbers of edges and a model is proposed for predicting the relative stability of these B–N polyhedral molecules.     

Computational Study of the Substitution Effect on the Mechanism for the Aza‐ and Arsa‐Wittig Reactions

The aza‐ and arsa‐Wittig reactions HM=PH₃ + O=CHX → HM=CHX + O=PH₃ (M = N, As; X = H, F, Cl, Me, OMe, NMe₂, CMe₃) were examined using the density functional theory calculations. All of the structures were completely optimized at the B3LYP/6‐311++G** level of theory. The main finding of this work is that the difference between singlet‐triplet splitting of O=CHX and HM=PH₃ play an important role in determining the kinetic and thermodynamic stability of the aza‐ and arsa‐Wittig reactions. When HM=PH₃ with more ylidic character is utilized, the reaction has a smaller activation energy and a larger exothermicity.