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The (E)- and (Z)-geometrical isomers of fluorinated propenes (CWZ = CY−CX3, W, X, Y, and Z = H or F) are computationally investigated. Our calculations show that geometrical isomers with two fluorine atoms located at the cis positions of the C C bond tend to undergo additional energy lowering compared to the trans isomers. Natural bond orbital analysis suggests that the origin of this lowering of energy is the same as that of the typical fluorine cis effects observed for 1,2-dihaloethenes. The barrier height magnitude of the internal rotation of the methyl or trifluoromethyl group is shown to be sensitive to the substitution patterns in the fluorinated propenes. The change in the rotational barrier heights is closely related to the differences in the magnitude of intramolecular repulsive non-bonded steric interactions. Furthermore, the π-bonds in fluorinated propenes are shown to be weaker for the derivatives possessing two fluorine atoms at both ends of the C C bond. The OH radical reactions of the fluorinated propenes are initiated by the attack of the OH radicals on the π-bond. Nevertheless, no obvious correlation is found between the rate constants for the OH radical reactions and the π-bond strengths of the fluorinated propenes.

The strengths of π-bonds in a series of fluorinated ethylenes have been computationally investigated, and the potential energy surfaces for thermal C C bond rotation in fluorinated ethylenes, as well as in ethylene, have been explored. The computed thermal rotational barrier values and the π-bond strengths obtained using Benson’s procedure show parallel trends. Our calculations indicate that the π-bond in CH2 = CHF is slightly stronger, and the one in CF2 = CF2 is noticeably weaker than that in CH2 = CH2. In addition, the stabilization effects of fluorine substitution have been evaluated for fluorinated ethylene molecules and for singlet biradical thermal rotation transition states. Considering these fluorine substituent effects, the change in the π-bond strength of a series of fluorinated ethylenes can be qualitatively interpreted.

The CCSD(T) level intermolecular interaction energies calculated for the CF4-CH4 complex are compared with those for the CF4-CF4 and CH4-CH4 complexes. The interactions in the CF4-CH4 complex is stronger than the average of the interactions in the CF4-CF4 and CH4-CH4 complexes. DFT-SAPT calculations show that the dispersion interactions in the CF4-CH4 complex, which are the primary source of the attraction, are nearly identical to the average of those in the CF4-CF4 and CH4-CH4 complexes, while the attractive electrostatic interactions in the CF4-CH4 complex is stronger than the average of those in the CF4-CF4 and CH4-CH4 complexes.

Rate constants for the reactions of OH and NO3 radicals with CH2CHF (k1 and k4), CH2CF2 (k2 and k5), and CHFCF2 (k3 and k6) were determined by means of a relative rate method. The rate constants for OH radical reactions at 253–328 K were k1 = (1.20 ± 0.37) × 10−12 exp[(410 ± 90)/T], k2 = (1.51 ± 0.37) × 10−12 exp[(190 ± 70)/T], and k3 = (2.53 ± 0.60) × 10−12 exp[(340 ± 70)/T] cm3 molecule−1 s−1. The rate constants for NO3 radical reactions at 298 K were k4 = (1.78 ± 0.12) × 10−16 (CH2CHF), k5 = (1.23 ± 0.02) × 10−16 (CH2CF2), and k6 = (1.86 ± 0.09) × 10−16 (CHFCF2) cm3 molecule−1 s−1. The rate constants for O3 reactions with CH2CHF (k7), CH2CF2 (k8), and CHFCF2 (k9) were determined by means of an absolute rate method: k7 = (1.52 ± 0.22) × 10−15 exp[−(2280 ± 40)/T], k8 = (4.91 ± 2.30) × 10−16 exp[−(3360 ± 130)/T], and k9 = (5.70 ± 4.04) × 10−16 exp[−(2580 ± 200)/T] cm3 molecule−1 s−1 at 236–308 K. The errors reported are ±2 standard deviations and represent precision only. The tropospheric lifetimes of CH2CHF, CH2CF2, and CHFCF2 with respect to reaction with OH radicals, NO3 radicals, and O3 were calculated to be 2.3, 4.4, and 1.6 days, respectively. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 619–628, 2010

The structures of iminium salts formed from diarylprolinol or imidazolidinone derivatives and α,β-unsaturated aldehydes have been studied by X-ray powder diffraction (Fig. 1), single-crystal X-ray analyses (Table 1), NMR spectroscopy (Tables 2 and 3, Figs. 2–7), and DFT calculations (Helv. Chim. Acta2009, 92, 1, 1225, 2010, 93, 1; Angew. Chem., Int. Ed.2009, 48, 3065). Almost all iminium salts of this type exist in solution as diastereoisomeric mixtures with (E)- and (Z)-configured +NC bond geometries. In this study, (E)/(Z) ratios ranging from 88 : 12 up to 98 : 2 (Tables 2 and 3) and (E)/(Z) interconversions (Figs. 2–7) were observed. Furthermore, the relative rates, at which the (E)- and (Z)-isomers are formed from ammonium salts and α,β-unsaturated aldehydes, were found to differ from the (E)/(Z) equilibrium ratio in at least two cases (Figs. 4 and 5, a, and Fig. 6, a); more (Z)-isomer is formed kinetically than corresponding to its equilibrium fraction. Given that the enantiomeric product ratios observed in reactions mediated by organocatalysts of this type are often ≥99 : 1, the (E)-iminium-ion intermediates are proposed to react with nucleophiles faster than the (Z)-isomers (Scheme 5 and Fig. 8). Possible reasons for the higher reactivity of (E)-iminium ions (Figs. 8 and 9) and for the kinetic preference of (Z)-iminium-ion formation are discussed (Scheme 4). The results of related density functional theory (DFT) calculations are also reported (Figs. 10–13 and Table 4).

Rate constants were determined for the reactions of OH radicals with halogenated cyclobutanes cyclo-CF2CF2CHFCH2(k1), trans-cyclo-CF2CF2CHClCHF(k2), cyclo-CF2CFClCH2CH2(k3), trans-cyclo-CF2CFClCHClCH2(k4), and cis-cyclo-CF2CFClCHClCH2(k5) by using a relative rate method. OH radicals were prepared by photolysis of ozone at a UV wavelength (254 nm) in 200 Torr of a sample reference H2OO3O2He gas mixture in an 11.5-dm3 temperature-controlled reaction chamber. Rate constants of k1 = (5.52 ± 1.32) × 10−13 exp[–(1050 ± 70)/T], k2 = (3.37 ± 0.88) × 10−13 exp[–(850 ± 80)/T], k3 = (9.54 ± 4.34) × 10−13 exp[–(1000 ± 140)/T], k4 = (5.47 ± 0.90) × 10−13 exp[–(720 ± 50)/T], and k5 = (5.21 ± 0.88) × 10−13 exp[–(630 ± 50)/T] cm3 molecule−1 s−1 were obtained at 253–328 K. The errors reported are ± 2 standard deviations, and represent precision only. Potential systematic errors associated with uncertainties in the reference rate constants could add an additional 10%–15% uncertainty to the uncertainty of k1–k5. The reactivity trends of these OH radical reactions were analyzed by using a collision theory–based kinetic equation. The rate constants k1–k5 as well as those of related halogenated cyclobutane analogues were found to be strongly correlated with their CH bond dissociation enthalpies. We consider the dominant tropospheric loss process for the halogenated cyclobutanes studied here to be by reaction with the OH radicals, and atmospheric lifetimes of 3.2, 2.5, 1.5, 0.9, and 0.7 years are calculated for cyclo-CF2CF2CHFCH2, trans-cyclo-CF2CF2CHClCHF, cyclo-CF2CFClCH2CH2, trans-cyclo-CF2CFClCHClCH2, and cis-cyclo-CF2CFClCHClCH2, respectively, by scaling from the lifetime of CH3CCl3. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 532–542, 2009

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, PhCH2CHO, tBuCH2CHO, PhCH=CHCHO, 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. Acta2008, 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 NC(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-CC-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.

This paper describes the results of ab initio investigations on the reactions of OH radical/O (3P) atom addition to halogen-substituted ethenes (CH2 CHX, CH2 CX2; X=F, Cl, Br) and propene. Two reaction pathways, addition to the substituted carbon atom (α-site) and addition to the non-substituted carbon atom (β-site), have been examined. The energetic of each reaction coordinate has been evaluated at the computational level of PMP2/aug-cc-pVTZ//MP2/6-311+G(2d,p). Our calculations suggested that the reactions of OH radical addition should be more exothermic than those of the O (3P) atom addition and that the α-addition products should be lower in energy than the corresponding β-addition products. Relative stabilities of the transition states for the α- and β-addition channels can be discussed in terms of the exothermicities of the reaction channels and the relative spin densities in the 3ππ* states of the reactant ethene molecules. For the reactions of the chloro- and bromo-substituted ethenes, the transition states for the α-addition were found to be higher in energy than those for the β-addition, but for the reactions of the fluoro-substituted ethenes and propene, the differences in energy between the transition states for the α- and β-addition channels were found to be rather small.

The electronic structures and energetics of the reactants, products, transition states, and association complexes involved in the reaction of dimethyl ether (DME) and dimethyl sulfide (DMS) with OH radical have been investigated at the MP2(FC) and DFT (BHH one represents a direct hydrogen-abstraction and the other one for the abstraction through prereactive complex. Among the computational levels employed in the present work, the PMP2/6-311++G(2df,2pd)//M2/6-311++G(d,p) level was found to reproduce best the experimentally measured rate constants. Our best estimates of the rate constants at 298 K for the reactions of OH radical with DME and DMS are 5.9×10−13 and 1.1×10−12 cm3 molecule−1 s−1, respectively, which are underestimated by 4 times as compared to the experimentally derived values. At low temperatures, the direct H-abstraction predominates over the H-abstraction through the prereactive complex, while both channels compete at elevated temperatures.

The rate constants for the reactions of OH radicals with CF 3 OCHFCF 3 , and CF 3 CHFCF 3 have been measured over the temperature range 250–430 K. Kinetic measurements have been carried out using the flash photolysis, and laser photolysis methods combined, respectively, with the laser induced fluorescence technique. The influence of impurities in the samples has been investigated by using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these fluorinated compounds, if the purified samples were used in the measurements. The following Arrhenius expressions were determined: k (CF 3 OCHFCF 3 ) = (4.39 ± 1.38) × 10 −13 exp[−(1780 ± 100)/ T ] cm 3 molecule −1 s −1 , and k (CF 3 CHFCF 3 ) = (6.19 ± 2.07) × 10 −13 exp[−(1830 ± 100)/ T ] cm 3 molecule −1 s −1 .

trans-Hydrogen-bond hyperfine splitting via magnetic interaction, which is observed as J-coupling in NMR experiments, was theoretically studied. trans-Hydrogen-bond hyperfine splitting should be closely related to the orbital interaction between the lone-pair orbital of the H-bond acceptor and the antibond orbital of the H-bond donor. A linear relationship was observed between magnetic interaction hyperfine splitting through a H-bond and the H-bond strength. The relationship was dependent on the type of the nucleus forming the H-bond; linear correlation was observed in NH⋅⋅⋅O/N type or OH⋅⋅⋅N type H-bonded complexes, but not in OH⋅⋅⋅O type H-bonded complexes.

To investigate the fluorine substitution effect on the rate for ester hydrolysis, the energies of the tetrahedral intermediates, as well as those of the reactants and products, were evaluated with ab initio calculations. The CBS-4 and CBS-Q energies suggested significant acceleration of the hydrolysis rate of fluoro-substituted esters: not only the trifluoromethyl group in the acyl portion of esters but also that in the alcohol portion will accelerate significantly the ester hydrolysis rate as compared to non-substituted counterparts. This implies that perfluoroalkyl esters resulting from the atmospheric degradation of hydrofluoroethers (HFEs) will be rather unstable even in almost neutral hydrolytic conditions.

We propose a semiempirical procedure for the estimation of the rate constants for hydrogen atom abstraction reactions of OH radicals with haloalkanes and haloethers. Our procedure is derived from the collision theory based kinetic equation, which was originally proposed by Heicklen (Int. J. Chem. Kinet. 1981, 13, 651). This equation provides the estimates for the rate constants of hydrogen abstraction from the CH bond dissociation enthalpy for each potential hydrogen atom abstraction site. We reparameterized the equation and then applied this procedure to a series of haloalkane and haloether molecules. The results obtained from the new equations are found to be quite satisfactory. In addition, we also report highly reliable calculated values of the CH bond dissociation enthalpies for six environmentally important haloether molecules (CH2FOCH2F, CHF2CF2OCH2CF3, CF3CH2OCH2CF3, CF3CF2CH2OCHF2, CHF2OCF2CHFCl, and CHF2OCHClCF3). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 130–138, 2003

The three-layered feed-forward type artificial neural network (ANN) was applied to estimate the bond dissociation enthalpies (BDEs) of the C–H bonds in partially halogenated alkanes and ethers. The training values consisted of BDEs of the 93 C–H bonds in 53 alkanes and 24 ether molecules, which were computed by using density functional theory at the (RO)B3LYP/6-311G ∗∗ level. As input parameters of ANN, we defined 14 kinds of topological descriptors, such as H–C⋯X (X=H, F, Cl, Br, O), H–C–C⋯X (X=H, F, Cl, O), H–C–C–C⋯X (X=H, F, O), H–C–O–C⋯X (X=H, F). Consequently, the average of absolute deviation in correlation and estimation were 0.70 and 0.84 kcal/mol, respectively. It was possible to construct a reliable and useful BDE estimation method.

Basis set effect in evaluation of base pair formation energy of the Watson–Crick type base pair between substituted 1-methyluracil and 9-methyladenine was studied in the Hartree–Fock and second-order Moller–Plesset level of theory. The same trend was observed in the substitution effect in all basis sets attempted; thus, the error depending on the basis set incompleteness is minimal in the evaluation of the substitution effect of the base pair formation energy. Neither the hydrogen bond distance nor the atomic charge was a valid index for the hydrogen bond status in base pairing.

Kinetics and mechanism of the hydrogen abstraction reaction between trifluoromethyl formate, CF3OCHO, and OH radical have been investigated by using ab initio molecular orbital theory up to G2(MP2) level. The hydrogen abstraction rate constant has been calculated for the first time over a temperature range of 250–450 K by using standard transition state theory including the tunneling correction. Arrhenius parameters of the reaction have been estimated from the temperature dependence of the calculated rate constant. The calculated value for the rate constant (2.0 × 10−14 cm3 molecule−1 s−1) at 298 K is found to be in very good agreement with the recent experimental results. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 500–507, 2002

The self-consistent charge and configuration method for subsystems (SCCCMS) is used to study the cycloaddition reactions of 1,3-dipoles such as R–N 3 (R=H and CH 3 ) azides with H 2 C CHX (X=H, F, Cl, CH 3 , OH, and CN) ethylenes acting as dipolarophiles. All calculations are performed for reactive systems (1,3-dipole/dipolarophile) in transition-state (TS) geometries. Special emphasis is put on the components of interaction energy and on DFT-based reactivity indices. It is shown that charge-transfer (CT) energy is not responsible for the observed trends in regioselectivity though its stabilizing influence cannot be disregarded. The CT energy is the only term, which is very sensitive to the type of substituent in azide and ethylene molecules. The main factor governing the orientation in 1,3-dipolar cycloaddition is the Heitler–London energy. Present analysis also confirms adequacy of a simple two-reactant regional softness matching rule, called the maximum complementarity rule, for predicting more stable configurations.

High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and ammonia as a model of NH/π interaction. The intermolecular interaction energy was calculated from the extrapolated MP2 interaction energy at the basis set limit and a CCSD(T) correction term. The calculated interaction energy (−2.22 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The monodentate complex is slightly more stable than the bidentate and tridentate complexes. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the monodentate complex (0.13 kcal/mol) is repulsive. The large gain in the attraction by electron correlation correction (−2.36 kcal/mol) indicates that the dispersion interaction is significantly important for the attraction. The electrostatic energy (−1.01 kcal/mol) is also important for the attraction. The benzene−water (OH/...

The substitution effect on hydrogen bond energies of the mismatched base pairs between some uracil derivatives (U X ) and guanine (G) was studied in molecular orbital theory (MP2/6-31G ∗ //HF/6-31G ∗ level). The substituent effect of U X on G–U X mismatched base pair formation energy was small, though the base pair formation energy between adenine (A) and U X , which is a normal type of the Watson–Crick type base pair, was enforced by 1.5 kcal/mol upon introducing an electron-withdrawing group (EWG) into U. Thus, U X having EWG is more likely to selectively form a base pair with A. Namely, introduction of EWG does not change selectivity of the Watson–Crick type base pair formation of U.

High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and hydrocarbon molecules (methane, ethane, ethylene, and acetylene). Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated benzene−methane interaction energy (−1.45 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The benzene−methane complex prefers a geometry in which the C−H bond points toward the benzene ring. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the complex (0.85 kcal/mol) is repulsive. The large gain of the attraction energy (−2.30 kcal/mol) by electron correlation correction indicates that dispersion interaction is the major source of the attraction. Although the electrostatic energy (−0.25 kcal/mol) is small, a highly ori...

We present results of solution-phase ab initio and DFT investigations on the trigonal bipyramidal (TBP) intermediates postulated for the hydrolysis of methyl ethylene phosphate (MEP). A key intermediate for the hydrolysis would not be a TBP species with the hydroxyl group axial, but would be a TBP intermediate with the hydroxyl group equatorial. Considering the reaction pathways through this intermediate, not only exclusive ring opening in the dilute alkaline hydrolysis of MEP but also significant amount of exocyclic cleavage in the strong alkali can be rationalized. This interpretation is in accord with the mechanisms proposed by Lim et al. and by Lonnberg et al.

Chiral polycyclic aromatic hydrocarbons containing bis(1-adamantyl) groups at the peri-positions, named equatorenes, were synthesized in optically pure form starting from optically pure 4,5-bis(1-adamantyl)-8-methoxy-1-naphthol. A sequential Diels-Alder reaction of furan and arynes generated from 1,2-bromotriflates provided tricyclic and tetracyclic epoxides, and acid-catalyzed aromatization gave phenanthrol and chrysenol. Deoxygenation reactions involving the hydrogenolysis of triflates gave 1,8-bis(1-adamantyl)naphthalene, 1,10-bis(1-adamantyl)phenanthrene, and 1,12-bis(1-adamantyl)chrysene. 3,4-Bis(1-adamantyl)pyrene was synthesized from phenanthrol by Sonogashira coupling and Pt-catalyzed cyclization. Essentially no racemization occurred during the synthesis. X-ray analysis indicated the distorted naphthalene moiety possessing the peri-diadamantyl groups and the flat structure of the other benzene rings. UV-vis analysis of the equatorenes showed considerable redshifts compared with that of the corresponding achiral arenes. Electrochemical analysis of the naphthalene and pyrene indicated that the distortion decreased the highest occupied molecular orbital stability with no marked effect on the lowest unoccupied molecular orbital energy level, and the origin was discussed on the basis of calculation results.

The complexation of magnesium with hydrogen malonate and formate anions was studied with ab initio calculations. At least HF/6-31G ∗ level of accuracy is necessary for a qualitatively correct description of the complexes. The coordination space of magnesium has to be filled to attain reliable results. If crystal structures are used as experimental references, at least part of the crystal interactions must be properly included in the calculations.

The relative energies of the two minima and the two transition states on the torsional potential of the title compounds were obtained by ab initio molecular orbital calculations with electron correlation energy correction by the Moller-Plesset methods (MP2, MP3 and MP4(SDTQ)), coupled cluster calculations (CCD, ST4CCD, CCSD and CCSD(T)) and configuration interaction energy calculations (CISD, QCISD and QCISD(T)) using the 6-31G ∗ basis set. The calculated trans-gauche energy differences and the rotational barrier heights by the MP2 and CISD methods are slightly different from those with other methods. The calculated relative energies with other electron correlation correction methods are close to each other and close to the experimental values.

The atomic charge distributions of dimethyl ether, ethyl methyl ether, methyl alcohol and ethyl alcohol are obtained by Mulliken and natural population analyses and by electrostatic potential (ESP) fitting methods from HF and MP2 wavefunctions with several basis sets up to the 6-311G (2d,2p) basis set. To evaluate the atomic charge distributions, the dipole moments are calculated from the atomic charge distributions. The dipole moments from the atomic charge distributions, which are obtained by the fitting of the electrostatic potentials, are close to those directly obtained from the wavefunctions. They are also close to the experimental dipole moments, if the wavefunctions are calculated with a polarized basis set and electron correlation correction. The dipole moments obtained by the Mulliken and natural population charges are strongly basis set dependent and do not agree with the experimental dipole moments. The same tendency is observed from the calculations of other ethers and alcohols.

The chiral titanium complex-catalyzed ene-type reaction and the Mukaiyama aldol reaction with fluoral are shown to serve as an efficient route to the enantioselective and diastercoselective synthesis of CF 3 -substituted components of biological and synthetic importance.

The intermolecular interaction potentials of methane, ethane, ethylene and benzene dimers were calculated using several basis sets [up to 6–311G(3d,4p)] with electron correlation correction by the Moller-Plesset perturbation method and basis set superposition error (BSSE) correction. The calculated interaction energies considerably depend on the basis set used. Whereas the interaction energies of the repulsive and Coulombic energy components calculated at the HF level are not affected by the change of the basis set, the dispersion energy component, calculated as the electron correlation energy, greatly depends on the basis set used. A basis set with multiple polarized functions is necessary to calculate the dispersion energy correctly. The use of small basis sets greatly underestimates the dispersion energy.

Diphenylprolinol silyl ether was found to be an effective organocatalyst for promoting the asymmetric, catalytic, intramolecular [6 + 2] cycloaddition reactions of fulvenes substituted at the exocyclic 6-position with a δ-formylalkyl group to afford synthetically useful linear triquinane derivatives in good yields and excellent enantioselectivities. The cis-fused triquinane derivatives were obtained exclusively; the trans-fused isomers were not detected among the reaction products. The intramolecular [6 + 2] cycloaddition occurs between the fulvene functionality (6π) and the enamine double bond (2π) generated from the formyl group in the substrates and the diphenylprolinol silyl ether. The absolute configuration of the reaction products was determined by vibrational circular dichroism. The reaction mechanism was investigated using molecular orbital calculations, B3LYP and MP2 geometry optimizations, and subsequent single-point energy evaluations on model reaction sequences. These calculations revealed the following: (i) The intermolecular [6 + 2] cycloaddition of a fulvene and an enamine double bond proceeds in a stepwise mechanism via a zwitterionic intermediate. (ii) On the other hand, the intramolecular [6 + 2] cycloaddition leading to the cis-fused triquinane skeleton proceeds in a concerted mechanism via a highly asynchronous transition state. (iii) The fulvene functionality and the enamine double bond adopt the gauche-syn conformation during the C-C bond formation processes in the [6 + 2] cycloaddition. (iv) The energy profiles calculated for the intramolecular reaction explain the observed exclusive formation of the cis-fused triquinane derivatives in the [6 + 2] cycloaddition reactions. The reasons for the enantioselectivity seen in these [6 + 2] cycloaddition reactions are also discussed.

The chiral titanium complex-catalyzed ene-type reaction with fluoral is shown to serve as an efficient route for the asymmetric synthesis of CF 3 -substituted compounds of biological and synthetic importance in extremely high enantiomeric excesses (>95% ee). The ene reactivity of trihaloacetaldehyde including fluoral is estimated in terms of the balance of LUMO energy level vs . electron density on the carbonyl carbon.

The intermolecular interaction potential of the H2-H2 system was calculated by an ab initio molecular orbital method using several basis sets (up to 6-31 lG(3pd)) with inclusion of the electron correlation correction of the Moller-Plesset perturbation method and the basis set superposition error (BSSE) correction of the counterpoise method in order to evaluate the basis set effect. The calculated interaction energies depend strongly on the basis set used. Whereas the interaction energies of the repulsive and coulombic energy components calculated at the Hartree-Fock level are not affected by a change of basis set, the dispersion energy component depends strongly on the basis set used. Parameters of an exp-6-1 type non-bonding interaction potential were optimized on the basis of the MP4(SDTQ)/6-311G(3p) level intermolecular interaction energies of the H2-H2 system.

In the attack of OCH 3 − on ethylene phosphate monoanion, a dianionic pentacoordinate intermediate has been found by ab initio calculations at the 3-21 G* level. Although only marginally stable, this contrasts with the results of Lim and Karplus (Nonexistence of Dianionic Pentacovalent Intermediates in the ab Initio Study of the Base-Catalyzed Hydrolysis of Ethylene Phosphate. J. Am. Chem. Soc. 1990, 112, 5872-5873) for attack by OH − on ethylene phosphate monoanion where there is only a pentacoordinate transition state. Together with the results of another paper, it can be concluded that the cause of the discrepancy is attributable to the difference in axial substituents (OCH 3 vs OH)

trans-tert-Butyldimethylsiloxy-L-proline displays greater catalytic activity and affords higher enantioselectivity than the parent proline in the alpha-amination reaction of carbonyl compounds with azodicarboxylate. A quantum mechanical calculation reveals the structure of the transition state. In the presence of a catalytic amount of siloxyproline and water (3-9 equiv), alpha-amino carbonyl derivatives, which are important synthetic intermediates, are obtained in good yield and with excellent enantioselectivity.

[reaction: see text] The key [4 + 4] cycloaddition in the biosynthesis of epoxytwinol A has been established by theoretical calculations to comprise of three processes. The first step is formation of the C8-C8' bond generating a biradical intermediate. Next, rotation about the C8-C8' bond occurs, and finally the C1-C1' bond is formed. Biradicals stabilized by conjugation and two hydrogen bonds are essential for realization of this rare thermal [4 + 4] cycloaddition.

A model chemistry for the evaluation of intermolecular interaction between aromatic molecules (AIMI Model) has been developed. The CCSD(T) interaction energy at the basis set limit has been estimated from the MP2 interaction energy near the basis set limit and the CCSD(T) correction term obtained by using a medium size basis set. The calculated interaction energies of the parallel, T-shaped,and slipped-parallel benzene dimers are -1.48, -2.46, and -2.48 kcal/mol, respectively. The substantial attractive interaction in benzene dimer, even where the molecules are well separated, shows that the major source of attraction is not short-range interactions such as charge-transfer but long-range interactions such as electrostatic and dispersion. The inclusion of electron correlation increases attraction significantly. The dispersion interaction is found to be the major source of attraction in the benzene dimer. The orientation dependence of the dimer interaction is mainly controlled by long-range interactions. Although electrostatic interaction is considerably weaker than dispersion interaction, it is highly orientation dependent. Dispersion and electrostatic interactions are both important for the directionality of the benzene dimer interaction.

The oxidized octaethyltetraphenylporphyrin (1, OETPP) and the corresponding newly prepared octaisobutyltetraphenylporphyrin (3, OisoBuTPP) could be isolated from the reaction of OETPPLi2 (or OisoBuTPPLi2) with SOCl2. The X-ray analysis and the characteristic UV-vis spectra of 1 and 3 revealed that these are the first examples of 16 pi nonaromatic porphyrins.

To elucidate the role of the Mg2+ ion in ribozyme reactions, we carried out ab initio molecular orbital investigations on dianionic trimethoxyphosphorane A and its Mg2+ complex (overall a neutral molecule) as a model system for the reaction center of Tetrahymena-type ribozyme. Although dianionic oxyphosphorane A concentrates its negative charges on the equatorial phosphoryl oxygens, the coordination of the Mg2+ ion between these two oxygens is unlikely. Geometry optimizations of the complex and the electrostatic potential of A both suggest that Mg2+ coordination preferably occurs in the region between the axial oxygen and the equatorial phosphoryl oxygen. The considerations of electrostatic potential rationalize the geometries of carboxylate-metal and phosphate-metal interactions extracted from the Cambridge Structural Database as well. Consequently, the Mg2+ ion at the active site of Tetrahymena-type ribozyme most likely lies in the regions between the axial and equatorial oxygens. The axial-equatorial coordinations of Mg2+ ions conceivably increase the electronegativities of the axial oxygens and facilitate cleavage of the phosphodiester bond located at the junction of the intron and the exon. It is thus likely that the Mg2+ ions play the key role in the phosphodiester cleavage reactions mediated by ribozymes.

From calculations of a model reaction scheme for base-catalyzed RNA hydrolysis, a pentacoodinate dianionic intermediate 2a (Storer, et al., J. Am. Chem. Soc. , 1991 , 113 , 5216–5219) as well as two transition state, TS1 and TS2 , to the intermediate have been locateed by ab initio calculations at the 3–21G ∗ level. Although the intermediate, which has the well depth on the order of k B T, is unlikely to be kinetically significant, the overall rate-limiting transition state structure TS2 obtained at 3–21G ∗ level is very close to the corresponding structure at the STO-3G level; it has an extended P-O(5′) bond breaking character. These gas-phase calculation results are used to qualitatively interpret mutagenesis results of Barnase and RNase T 1 where water molecules are absent from the active site.

Synthese a partir de retronecine et d'acide d'integerrinecique en utilisant le groupe methylthiomethyl comme groupe protecteur activable de la fonction acide