No description available.

Thesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009.

A mechanism for alkene metathesis has been proposed by Chauvin, wherein metal carbenes act as catalysts for the reaction. The use and discovery of Fischer-, Tebbe-, Grubbs- and Schrock-type metal carbenes have to a certain extent proven the general mechanism. These metal carbenes showed different activity for alkene metathesis. Only Grubbs- and Schrock-type carbenes proved to be highly active for metathesis. A lot of studies have been done on the reasons for the activity, but still the main factors are unknown. In this study a molecular modelling investigation into the mechanism of the alkene metathesis reaction is done in an attempt to identify a factor(s) that can predict activity. By defining and knowing factors that contribute to activity, new catalysts can be designed that are truly active and selective. Fischer-, Tebbe-, Grubbs- and Schrocktype metal carbenes are investigated in this regard. The results of the investigation indicate that the frontier molecular orbital theory shows a possibility for prediction of alkene metathesis activity. By observing the size and location of the atomic orbital coefficients of the molecular orbital, the site of primary overlap for formation of metathesis products could be identified. The largest atomic orbital coefficient of the LUMO should be located on the metal atom. An atomic orbital coefficient should also be present on the carbene carbon for secondary overlap for formation of the metallacyclobutane intermediate. By exchanging the ruthenium in the second generation Grubbs catalyst framework the effect of the metal could be elucidated. The results clearly showed the important influence the metal atom has on the electronic properties of the catalyst complex. The results of frontier molecular orbital calculations supported the general activity trend of the four main types of metal carbenes for the metathesis of linear alkenes. By changing the metal in known catalyst frameworks a deeper understanding can be gained for the design of new alkene metathesis catalysts. / Thesis (PhD (Chemistry))--North-West University, Potchefstroom Campus, 2013

A mechanism for alkene metathesis has been proposed by Chauvin, wherein metal carbenes act as catalysts for the reaction. The use and discovery of Fischer-, Tebbe-, Grubbs- and Schrock-type metal carbenes have to a certain extent proven the general mechanism. These metal carbenes showed different activity for alkene metathesis. Only Grubbs- and Schrock-type carbenes proved to be highly active for metathesis. A lot of studies have been done on the reasons for the activity, but still the main factors are unknown. In this study a molecular modelling investigation into the mechanism of the alkene metathesis reaction is done in an attempt to identify a factor(s) that can predict activity. By defining and knowing factors that contribute to activity, new catalysts can be designed that are truly active and selective. Fischer-, Tebbe-, Grubbs- and Schrocktype metal carbenes are investigated in this regard. The results of the investigation indicate that the frontier molecular orbital theory shows a possibility for prediction of alkene metathesis activity. By observing the size and location of the atomic orbital coefficients of the molecular orbital, the site of primary overlap for formation of metathesis products could be identified. The largest atomic orbital coefficient of the LUMO should be located on the metal atom. An atomic orbital coefficient should also be present on the carbene carbon for secondary overlap for formation of the metallacyclobutane intermediate. By exchanging the ruthenium in the second generation Grubbs catalyst framework the effect of the metal could be elucidated. The results clearly showed the important influence the metal atom has on the electronic properties of the catalyst complex. The results of frontier molecular orbital calculations supported the general activity trend of the four main types of metal carbenes for the metathesis of linear alkenes. By changing the metal in known catalyst frameworks a deeper understanding can be gained for the design of new alkene metathesis catalysts. / Thesis (PhD (Chemistry))--North-West University, Potchefstroom Campus, 2013

Thesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009.

Thesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009.

Thesis advisor: Amir H. Hoveyda / Chapter One: Stereoretentive Cross-Metathesis of Trisubstituted Olefins The development of stereoretentive olefin metathesis catalysts has solved a long-standing problem in the field, allowing for trisubstituted alkenes to be synthesized in high stereochemical purity and under kinetic control. E- as well as Z-isomers of trisubstituted alkenyl halides, nitriles, and allylic alcohols can be accessed through cross-metathesis of commercially available and easily accessible alkenes. Through the use of the same strategy, macrocyclic trisubstituted alkenes have been accessed in either isomeric form through stereoretentive ring-closing metathesis of the corresponding diene starting materials. Thus, for the first time, a wide range of E- and Z-trisubstituted alkenes can be obtained selectively through olefin metathesis, regardless of the underlying thermodynamic preferences. Chapter Two: Development of Catalytic Stereoretentive Cross-Metathesis of Trisubstituted Alkenyl Bromides We have introduced a general and widely applicable strategy for the synthesis of E- and Z-trisubstituted alkenyl bromides through cross-metathesis. The reaction is applicable to terminal, disubstituted, and trisubstituted olefins bearing a variety of functional groups including alkenes with α-, or β-branches. The requisite stereodefined cross-partners, E- and Z-2-bromo-2-butene are commercially available and can be synthesized with ease in one step from abundant starting materials. This represents a notable improvement over our previous approach, where the non-halogenated alkene starting material had to be prepared through cross-coupling in high stereochemical purity to ensure high stereoretention in the subsequent cross-metathesis. Catalysts derived from Mo monoaryloxide pyrrolide complexes, some of which are commercially available, are optimal for this transformation. The applicability of the approach is underscored through the formal synthesis of phomactin A with improved overall yield and step count. Chapter Three: Total Synthesis of Ambrein We have completed a total synthesis of ambrein, a terpenoid isolated from whale secretion, a much sought perfume ingredient. The approach involved joining two fragments through formation of the central trisubstituted alkene. Our route entailed a sequence of cross-metathesis of alkenyl bromides and cross-coupling, providing access to a previously difficult-to-access trisubstituted olefin with high efficiency and selectivity. One fragment was generated from a readily accessible enantiomerically enriched compound, sclareolide, and the other from inexpensive methylcyclohexenone. The stereogenic center of the latter was established through a NHC-Cu-catalyzed enantioselective allylic substitution, which was followed by differentiation of these alkenes through site-selective epoxidation. The total synthesis is more efficient and offers a more practical route to ambrein. Chapter Four: Stereoretentive Cross-Metathesis of Trisubstituted α,β-Unsaturated Carbonyl Compounds We have developed a strategy for the synthesis of Z- and E-Trisubstituted α,β-unsaturated carbonyl compounds through stereoretentive CM involving commercially available or easily accessible alkene substrates. The method is applicable to a variety of α,β-unsaturated esters, thioesters, and acyl fluorides. Furthermore, mono-, di-, and trisubstituted alkenes can be used as starting materials. Transformations may be carried out on gram scale and, in some cases, with commercially available Mo catalysts. The utility of the catalytic approach was highlighted through synthesis of previously accessed intermediates more directly and with improved efficiency. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

No description available.

Metathesis is a valuable reaction for the production of new alkenes. In the last 50 years, heterogeneous as well as homogeneous catalysts have been used for this reaction. In the homogeneous category are the very successful catalysts designed by the Grubbs group. The first generation Grubbs precatalyst (Gr1) bearing two phosphine ligands was followed after extensive studies by the more active second generation Grubbs precatalyst (Gr2). In Gr2, one of the phosphine ligands is replaced with an N-heterocyclic carbene. Grubbs type precatalysts bearing pyridynyl-alcoholato chelating ligands are pertinent to this study. Scheme 1: The synthesis of Grubbs type precatalysts bearing a pyridynyl-alcoholato ligand. In two previous studies, both supported by computational methods, Grubbs type precatalysts with N^O chelating ligands were synthesised. These investigations were motivated by the fact that chelating ligands bearing different donor atoms can display hemilability. The loosely bound donor atom can de-coordinate to make available a coordination site to an incoming substrate “on demand”, whilst occupying the site otherwise and hence preventing decomposition via open coordination sites. In the first investigation, the incorporation of an O,N-ligand with both R1 and R2 being phenyl groups into the Gr2 precatalyst, resulted in an increase in activity, selectivity and lifetime of the precatalyst in comparison to Gr2 in the metathesis reaction with 1- octene. In the second study, three synthesised complexes were found to be active for the metathesis of 1-octene. This computational study sought to better understand the structural differences and thermodynamic properties of these Grubbs type precatalysts with bidentate/hemilabile ligands. A large number of structures were constructed in Materials Studio by varying the R groups of the bidentate/hemilabile ligand attached to both the Gr1 and Gr2 catalysts. The majority of structures were Gr1-type complexes. For each ligand selected, a group of structures consisting of closed precatalyst, open precatalyst, and where applicable a precatalyst less PCy3, closed metallacycle, open metallacycle and where applicable a metallacycle less PCy3, was constructed and optimised using DMol3. Bond lengths, bond angles, HOMO and LUMO energies and Hirshveld charges of structures were compared with one another. PES scans were performed on the metallacycles of four groups. The purpose of the PES scans was to ascertain whether these bidentate ligands were hemilabile and to illuminate the preferred reaction mechanism for these types of precatalysts. The major finding of this study was that the possibility of an associative mechanism cannot be ruled out for some Gr2-type precatalysts with bidentate ligand. For some precatalysts hemilability is energetically expensive and possibly not viable. No evidence of a concerted mechanism was found. The dissociative mechanism was found to be the preferred mechanism for most of the structures that were subjected to PES scans. The HOMO-LUMO energies of a complex can be used, as a predictive tool, to assess the reactivity and stability of a complex, as well as its preference for substrates. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 2013.

Metathesis is a valuable reaction for the production of new alkenes. In the last 50 years, heterogeneous as well as homogeneous catalysts have been used for this reaction. In the homogeneous category are the very successful catalysts designed by the Grubbs group. The first generation Grubbs precatalyst (Gr1) bearing two phosphine ligands was followed after extensive studies by the more active second generation Grubbs precatalyst (Gr2). In Gr2, one of the phosphine ligands is replaced with an N-heterocyclic carbene. Grubbs type precatalysts bearing pyridynyl-alcoholato chelating ligands are pertinent to this study. Scheme 1: The synthesis of Grubbs type precatalysts bearing a pyridynyl-alcoholato ligand. In two previous studies, both supported by computational methods, Grubbs type precatalysts with N^O chelating ligands were synthesised. These investigations were motivated by the fact that chelating ligands bearing different donor atoms can display hemilability. The loosely bound donor atom can de-coordinate to make available a coordination site to an incoming substrate “on demand”, whilst occupying the site otherwise and hence preventing decomposition via open coordination sites. In the first investigation, the incorporation of an O,N-ligand with both R1 and R2 being phenyl groups into the Gr2 precatalyst, resulted in an increase in activity, selectivity and lifetime of the precatalyst in comparison to Gr2 in the metathesis reaction with 1- octene. In the second study, three synthesised complexes were found to be active for the metathesis of 1-octene. This computational study sought to better understand the structural differences and thermodynamic properties of these Grubbs type precatalysts with bidentate/hemilabile ligands. A large number of structures were constructed in Materials Studio by varying the R groups of the bidentate/hemilabile ligand attached to both the Gr1 and Gr2 catalysts. The majority of structures were Gr1-type complexes. For each ligand selected, a group of structures consisting of closed precatalyst, open precatalyst, and where applicable a precatalyst less PCy3, closed metallacycle, open metallacycle and where applicable a metallacycle less PCy3, was constructed and optimised using DMol3. Bond lengths, bond angles, HOMO and LUMO energies and Hirshveld charges of structures were compared with one another. PES scans were performed on the metallacycles of four groups. The purpose of the PES scans was to ascertain whether these bidentate ligands were hemilabile and to illuminate the preferred reaction mechanism for these types of precatalysts. The major finding of this study was that the possibility of an associative mechanism cannot be ruled out for some Gr2-type precatalysts with bidentate ligand. For some precatalysts hemilability is energetically expensive and possibly not viable. No evidence of a concerted mechanism was found. The dissociative mechanism was found to be the preferred mechanism for most of the structures that were subjected to PES scans. The HOMO-LUMO energies of a complex can be used, as a predictive tool, to assess the reactivity and stability of a complex, as well as its preference for substrates. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 2013.