Experimental and theoretical studies of molecular complexes.

Thesis (Ph.D.)-University of Natal, Durban, 1999.The binary complexes comprising the molecules hydrogen fluoride, hydrogen chloride, water, hydrogen sulphide, ammonia, phosphine, methane and silane have been studied by ab initio molecular orbital theory thus generating a matrix of eight by eight molecular complexes. The structures, energetics and vibrational spectra of these molecular complexes, both homodimers and heterodimers, have been studied at the second order level of Møller-Plesset perturbation theory using the 6-31G** basis set, by means of the Gaussian 92 computer program. The self consistent field interaction energies have been further dissected into their various components according to the Morokuma scheme, using the Monstergauss computer program. An analysis of the computed interaction energies of the complexes has also been carried out. A complete normal mode analysis, identifying the normal modes of Vibration, has been carried out using the Vibra program. The infrared spectra of some of the associated species have also been determined experimentally by means of the matrix isolation technique using argon and nitrogen as matrix gases. The combined use of the theoretical and experimental approaches has enabled reliable vibrational assignments to be made, which have been used in determining the molecular structures of the aggregates formed on complexation

Synthesis of Disila-Crown Ether Complexes and Heteroatomic Bridged Paracyclophanes

Der erste Abschnitt dieser kumulativen Dissertation behandelt die Synthese von hybriden Kronenethern, in denen Tetramethyldisilan-Gruppen sowie Glycol-Gruppen vorliegen. Umsetzungen mit Alkali- und Erdalkalimetallsalzen führen zu Koordinaitonsverbindungen, die unter anderem kristallographisch untersucht wurden. DFT-Rechnungen und dynamische Proton-NMR Experimente zeigten, dass die Lithium-Komplexe von 1,2-Disila-12-Krone-4 und 1,2,7,8-Tetrasila-12-Krone-4 eine ähnliche Komplexstabilität aufweisen im Vergleich zu dem Lithium-Komplex des organischen 12-Krone-4. Im zweiten Abschnitt der Arbeit werden diverse Synthesewege zu heteroatomverbrückten Paracyclophane vorgestellt. Hierbei fanden insbesondere Silicium, Phosphor, Stickstoff und Gallium Anwendung als verbrückende Atome

Novel Molecular Si(II) Precursors for Synthesis of the First Compounds with Metal-Silicon Triple Bonds

NHC-adducts of reactive silylenes were isolated as crystalline thermally stable solids. The NHC-adducts of SiBr2 and SiI2 were prepared upon reduction of the corresponding NHC adducts of silicon tetrahalides. The first NHC-adducts of arylchlorosilylenes were also prepared upon dehydrochlorination of the corresponding aryldichlorosilanes with two equivalents of N-heterocyclic carbene. Starting from these Si(II) compounds, a unique approach to silylidene complexes of Group 6 transition-metals was achieved by metathetical exchange with carbonyl metallates was developed. Thereby, a series of zwitterionic silylidene complexes unprecedented up to date was obtained. NHC abstraction from the complex [Cp(CO)2Mo=SiAr(NHC)] (Ar = -C6H3-2,6-Trip2) afforded the first silylidyne complex. Furthemore, halide abstraction from halosilylidene complexes afforded unprecedented cationic silylidyne complexes of Cr, Mo and W. Reactivity of the silylidyne complexes towards polar molecules, nucleophiles and alkynes has been investigated

Some kinetic and equilibrium studies of σ-adduct formation and proton transfer in the reactions of aromatic nitro-compounds with bases

Comparison of kinetic and equilibrium data for the cyclisation of 1-(2,2-dimethyl-3-hydroxypropoxy)-2,4-dinitro- naphthalene in alkaline media with those for the cyclisation of 1-(3-hydroxypropoxy)-2,4-dinitronaphthalene indicates the absence of a marked gem-dimethyl effect. (^1)H n.m.r. and visible spectral measurements show that alkoxide addition to 2,2’4,4’,6,6’-hexanitrobibenzyl (HNBB) and 2,2’,4,4’,6,6’-hexanitrostilbene (HNS) gives σ-adducts. Formation of the 3-(3’-) adduct is kinetically preferred but the 1-(1’-) adduct is thermodynamically more stable. In media of high basicity the 1:2 adduct with alkoxide addition at the 1- and 1’- positions is observed. For HNS a third interaction occurs which may be alkoxide addition at the olefinic bond. Kinetic and equilibrium data are reported for the reactions with methoxide ions in methanol and ethoxide ions in ethanol and compared with data for related compounds. The interactions of aliphatic amines with 2,4,6-trinitro- benzyl chloride (TNBCl), HNBB, HNS and 2,4,6-trinitrophenetole (TNP) in dimethyl sulphoxide have been investigated using visible and (^1)H n.m.r. spectroscopic methods. Kinetic and equilibrium data are reported for the various processes observed. The reversible reactions of TNBCl with primary amines are found to be: rapid formation of the 3-adduct, followed by isomerisation to the thermodynamically more stable 1-adduct, followed by equilibration with the conjugate base formed by transfer of a side-chain proton. With the secondary amines piperidine and pyrrolidine, σ-adduct formation at the 1-position is not observed because the presence of two bulky groups at the 1-position is sterically unfavourable. The σ-adduct forming reactions occur via zwitterionic intermediates and it is shown that proton transfers between these species and amines may be kinetically significant. The reactions of HNBB and HNS with amine also involve the initial formation of 3-adducts and 1-adducts. At high amine concentrations di-adducts may be formed by reaction of the 1- and 1’- or 3- and 3’- positions. A slow reaction of HNBB with amines gives a blue species which is shown to be a dianion formed by loss of two methylene protons. The slow step-in this reaction is shown to be rate limiting proton transfer from the substrate or from l:lσ-adducts. TNP reacts with primary and secondary amines to give isomeric σ-adducts at the 3-position and 1-position. Nucleophilic substitution involves general acid catalysed expulsion of the ethoxy group and yields N-substituted picramides. Data are also reported for the reactions of TNBCl with hydroxide ions in water and 30:70 (v/v) DMSO-water, of TNBCl with hydroxide ions in mixed (methanol-water-tetrahydrofuran) solvents, of HNBB with sulphite ions and of 1,3,5-trinitrobenzene with thioglycollic acid in water

Ab initio modeling of complex aqueous and gaseous systems containing nitrogen

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2008.Includes bibliographical references (p. 305-312).Nitrogen chemistry is ubiquitous in everyday life, from biological processes at ambient conditions to atmospheric chemistry at low pressures and temperatures to high-temperature combustion. Understanding the chemical behavior of nitrogen-containing species under a variety of conditions and in multiple phases is critical to accurately modeling system behavior. The further ability to model system behavior based solely on a first principles approach would be a boon to researchers attempting to design and understand technologies utilizing complex systems. This work attempts to further these abilities for both solution-phase and gas-phase predictions from ab initio calculations. An overview of solvation thermodynamics is given that relates computational chemistry to phenomenological thermodynamics for common equilibrium expressions. Special attention is paid to fully understanding the role of activity coefficients, standard states, and reference states and how these affect the subsequent expressions. A procedure is outlined for estimating the thermochemical properties of small molecules in aqueous solution based on computational chemistry calculations utilizing continuum solvation models. The partitioning of the entropic and enthalpic contributions is of the utmost importance if one is to accurately estimate the enthalpy of formation and entropy in solution. Procedures for rate coefficient estimation via solution-phase transition state theory, simple electron transfer theory, and dissociative isomerizations within a solvent cage are also discussed. The oxidation of hydroxylamine in aqueous nitric acid was chosen as a test system. A detailed chemical mechanism was constructed and thermochemical and rate parameters from computational chemistry calculations were used to model the behavior of the system. Using current continuum solvation models, it does not appear possible to build reliable predictive models of complex aqueous systems, particular those with a high ionic strength. However, the present semi-quantitative models may be helpful in focusing attention on the key unknowns.(cont.) Group additivity values were estimated for more than 50 new functional groups containing nitrogen based on high-level computational chemistry estimates of the thermochemical parameters of 105 non-cyclic C/H/N/O species. The thermochemical and kinetics databases of the group's Reaction Mechanism Generator software were restructured to be more extensible and to explicitly include nitrogen chemistry. This allows new chemistry to be added to the software more easily and will allow predictions for gas-phase nitrogencontaining systems in the very near future.by Robert Wilson Ashcraft.Ph.D

Synthesis, Characterization and Reactivity of Ylidyne and μ-Ylido Complexes Supported by Scorpionato Ligands

The chemistry of compounds featuring triple bonds of the heavier Group-14 elements Si–Pb with transition metals is a very challenging research area, which combines modern molecular main-group element with transition-metal chemistry, and is of fundamental importance for the understanding of chemical bonding. During the last 15 years, the research in this area has witnessed considerable progress in isolating a series of closed-shell tetrylidyne complexes. However, despite numerous attempts, open-shell tetrylidyne complexes and heavier group 14 element congeners of metallacarbynes and carbide complexes remained inaccessible. This PhD thesis therefore focused on the development of first open-shell heavier tetrylidyne complexes featuring a tetrel-centered unpaired electron, and unprecedented metallatetrylidynes containing a multiply-bonded, linear-coordinated single heavier tetrel atom embedded between two metal centers. Initial reactivity studies of these novel complexes uncovered a plethora of exceptional products. Representative examples include a novel m3-silicido complex, the first dimetallasilacumulene with a linear, two-coordinated single silicon atom and the first compounds of planar tetracoordinated silicon (ptSi) (Anti-van’t Hoff-Le Bell Silicon). Alongside, isolation and full characterization of the first room-temperature stable disilavinylidene (Z)-(SIdipp)Si=Si(Br)Tbb, a silicon analogue of the very reactive vinylidenes (R2C=C:), is also reported in this thesis. The highly reactive and extremely air-sensitive compounds presented in this thesis were thoroughly analyzed by single-crystal X-ray diffraction and several spectroscopic techniques, which in combination with quantum chemical studies provided a comprehensive insight into the unusual electronic structures and properties of these novel compounds

Synthèse chimioenzymatique et énantiosélective de produits naturels

La synthèse énantiosélective, c'est-à-dire la synthèse de composés chiraux énantiopurs, est un thème central de la chimie organique. La biocatalyse à l'aide d'enzymes est un procédé efficace pour contrôler la chiralité. Les enzymes les plus employées pour ce type de transformations sont les lipases et les esterases de la classe des hydrolases. Dans cette thèse, ce sont d'ailleurs les procédés chimioenzymatiques qui sont privilégiés, tels les résolutions cinétiques (dynamiques) de mélanges racémiques et les désymétrisations de composés prochiraux ou méso. Dans l'introduction, un bref historique de la biocatalyse à l'aide d'enzymes est décrit en plus de donner des précisions sur les six classes d'enzymes et les trois grands types de biotransformations pouvant être réalisées à l'aide de ces enzymes. Des exemples d'applications récentes en industrie et en milieu académique y seront présentés. Le chapitre 1 est un article de revue de la littérature portant sur les acylations stéréosélectives (transestérifîcations) à l'aide de diverses lipases et esterases en milieu organique. Divers concepts généraux sur l'emploi des hydrolases y sont également présentés. Cet article de revue a été publié dans le journal Current Organic Synthesis : Chênevert, R.; Pelchat, N.; Jacques, F., Stereoselective Enzymatic Acylations (Transestérifîcations). Curr. Org. Chem. 2006,10, 1067-1094. Le chapitre 2 porte sur une étude de la tolérance de diverses lipases à l'emploi de divers agents acylants fonctionnalisés sur un substrat modèle, le 1-phényléthanol. Le concept a ensuite été appliqué à la synthèse de deux produits naturels, soit les phoracantholides I et J. L'étape clé est une acylation stereoselective (résolution cinétique) du 6-heptèn-2-ol par le pent-1-ènoate de 1-éthoxyvinyle (ee = 95 %). Des travaux ont également été réalisés afin de rendre dynamique cette résolution cinétique à l'aide de la lipase B de Candida antarctica. Cet article a été publié dans le journal Tetrahedron : Asymmetry : Chênevert, R.; Pelchat, N.; Morin, P., Lipase-Mediated Enantiosélective Acylation of Alcohols with Functionalized Vinyl Esters: Acyl Donor Tolerance and Applications., Tetrahedron : Asymmetry 2009, 20, 1191-1196. Ul Le chapitre 3 porte sur la première synthèse chimioenzymatique et énantiosélective des deux énantiomères du a-tocotriènol, un des huit membres de la famille des vitamines E. L'étape clé a été la désymétrisation chimioenzymatique d'un noyau chromanediméthanol prochiral (ee > 98 %) par l'acétate de vinyle en présence de la lipase B de Candida antarctica et l'inversion de la configuration du centre chiral sur le noyau chromane par le truc méso. Ces travaux ont été publiés dans le journal Bioorganic & Medicinal Chemistry : Chênevert, R.; Courchesne, G.; Pelchat, N., Chemoenzymatic Synthesis of Both Enantiomers of a-Tocotrienol.. Bioorg. Med. Chem. 2006,14, 5389-5396. Le chapitre 4 porte sur la synthèse chimioenzymatique de la portion alcool du dolabriférol, un polypropionate d'origine marine. Le dolabriférol est composé de deux unités polypropionates reliées par un lien ester. L'étape clé consiste en une acylation stereoselective (désymétrisation) du composé méso-(anti-anti)-2,4-diméthyl-1,3,5-pentanetriol par l'acétate de vinyle en présence de la lipase de Candida rugosa (ee = 95 %). Ces travaux ont été publiés dans le Journal of Organic Chemistry : Pelchat, N.; Caron, D.; Chênevert, R., Enantiosélective Synthesis of the Alcohol Moiety of Dolabriférol.. J. Org. Chem. 2007, 72, 8484-8488. En annexe se trouve une revue de la littérature pâme sous forme d'un chapitre de livre sur la transestérification et l'hydrolyse de dérivés d'acides carboxyliques, d'alcools et d'époxides à l'aide d'hydrolases. Cette revue de la littérature constitue un chapitre d'un livre : Chênevert, R.; Morin, P., Pelchat, N., Transestérification and Hydrolysis of Carboxylic Acid Derivatives, Alcohols, and Epoxides., dans Asymmetric Organic Synthesis with Enzymes (Éditeurs : Gotor, V.; Alfonso, I.; Garcia-Urdiales, E.), Wiley-VCH, 2008, chapitre 6, 133-169