Carl LeBlond - Academia.edu (original) (raw)
Papers by Carl LeBlond
Thermochimica Acta, 1996
ABSTRACT Due to its ability to provide directly reaction rate data and its in-situ nature, reacti... more ABSTRACT Due to its ability to provide directly reaction rate data and its in-situ nature, reaction calorimetry has become one of the most powerful probes of reaction pathways and mechanisms of chemical reactions by virtue of providing high-quality kinetic data. In this paper, examples of enantioselective hydrogenation and selective consecutive hydrogenation reactions are presented to demonstrate the high quality of kinetic data obtainable from the reaction calorimetry. They are also used to illustrate the use of reaction calorimetry for elucidating reaction pathways and mechanisms from detailed kinetic and thermodynamic information about individual step involved in multi-step reactions that is otherwise difficult to obtain without calorimetry.
Journal of The American Chemical Society, 1996
Marked shifts in enantioselectivity in the asymmetric hydrogenation of several prochiral substrat... more Marked shifts in enantioselectivity in the asymmetric hydrogenation of several prochiral substrates were observed as a function of the availability of hydrogen to the catalyst in both heterogeneous and homogeneous catalytic reactions. The key kinetic parameter affecting ...
Studies in Surface Science and Catalysis, 1997
ABSTRACT A two-site, two-step kinetic model is proposed to rationalize the observed effects of so... more ABSTRACT A two-site, two-step kinetic model is proposed to rationalize the observed effects of solution hydrogen concentration on enantioselectivity in the asymmetric hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified heterogeneous Pt catalyst. The model successfully predicted enantioselectivity at a hydrogen concentration outside the range used in the kinetic fit. This work demonstrates how the perturbation from equilibrium adsorption of the organic substrate on a heterogeneous catalyst may account for the observed effects of pressure on enant ioselectivity. Both positive and negative hydrogen dependences on enantioselectivity may be rationalized using the same model.
Topics in Catalysis, 1998
Several in situ probes for continuously monitoring rate of catalytic reactions under reaction con... more Several in situ probes for continuously monitoring rate of catalytic reactions under reaction conditions are described. They are reaction calorimetry, measurements of hydrogen uptake in the case of hydrogenation, and infrared spectroscopy. In studying catalytic hydrogenation reactions, for example, these in situ probes provide kinetic details of the reactions from different perspectives over the entire course of the reaction. The reaction calorimetry and the hydrogen uptake measure directly, continuously, and in a non-invasive manner the rate of reaction, while the in situ infrared spectroscopy provides time-resolved compositional information in the liquid phase. A combination of the information thus obtained leads to a clear and coherent kinetic picture of the reaction under study which can greatly facilitate pathway analysis and mechanistic description of the catalytic reaction. In this report, the usefulness of the combination of these in situ probes is illustrated with two examples of heterogeneously-catalyzed hydrogenation reactions.
Studies in Surface Science and Catalysis, 1997
Cheminform, 2010
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Journal of Organometallic Chemistry, 1997
Journal of Catalysis, 1996
The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient ... more The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient behavior at the beginning of the reaction, and a period of rising rate and enantioselectivity was linked to conversion of substrate for a wide range of reaction temperatures and initial substrate concentrations. A "reaction-driven equilibration" of the surface environment for optimal enantioselective catalysis was proposed for these reactions carried out under conditions where moderate enantioselectivities are observed. c 1996 Academic Press, Inc. We were remiss in Ref. in neglecting to note that Wells' group had made this observation prior to that of our work.
Journal of Molecular Catalysis A-chemical, 1997
Journal of Catalysis, 1995
Journal of Catalysis, 1996
Marked shifts in enantioselectivity from ca. 20 to 60 ee% as a function of reactor agitation spee... more Marked shifts in enantioselectivity from ca. 20 to 60 ee% as a function of reactor agitation speed were observed in the hydrogenation of ethyl pyruvate over cinchona-alkaloid-modified Pt. Concomitant with this shift in enantioselectivity was a change in the observed kinetics from the zero-order substrate dependence typical of gas-liquid diffusion control to a positive dependence on substrate concentration, even for a series of reactions carried out at constant pressure. A strong relationship between the solution hydrogen concentration and this rate and enantioselectivity behavior was confirmed. For systems where enantioselectivity exhibits a positive dependence on hydrogen concentration, the intrinsic ability of a catalyst to effect asymmetric hydrogenation may be masked in a reaction carried out under conditions where gas-liquid diffusion is the rate-limiting step.
Journal of The American Chemical Society, 1995
... Yongkui Sun, Carl LeBlond, Jian Wang, and Donna G. Blackmond* ... Joseph Laquidara and John R... more ... Yongkui Sun, Carl LeBlond, Jian Wang, and Donna G. Blackmond* ... Joseph Laquidara and John R. Sowa, Jr. ... us to observe a striking inversion in enanti-oselectivity in the hydrogenation of geraniol (I) with [RuClz-((S)-(-)tol-binap)]2*N(C2H~)3 (11) [tol-binap = (di-p-tolylphos ...
Journal of The American Chemical Society, 1999
Tetrahedron Letters, 2001
The evasive hypoiodous acid is generated in situ from NaOCl and NaI and used efficiently for clea... more The evasive hypoiodous acid is generated in situ from NaOCl and NaI and used efficiently for clean iodohydroxylation of 1, producing the Crixivan® intermediate 2 in high yield with highly efficient 1,3-asymmetric induction. This pH-tunable process allows HOI generation at a pH optimal for supressing byproduct formation in pH-sensitive iodohydroxylation reactions.Graphic
Journal of Organic Chemistry, 2004
The concise synthesis of a potent thrombin inhibitor was accomplished by a mild lactone aminolysi... more The concise synthesis of a potent thrombin inhibitor was accomplished by a mild lactone aminolysis between an orthogonally protected bis-benzylic amine and a diastereomerically pure lactone. The lactone was synthesized by the condensation of l-proline methyl ester with an enantiomerically pure hydroxy acid, which in turn was synthesized by a highly stereoselective (>500:1 er) and productive (100,000:1, S/C) enzymatic reduction of an alpha-ketoester. In addition, a second route to the enantiomerically pure lactone was accomplished by a diastereoselective ketoamide reduction.
Thermochimica Acta, 1996
ABSTRACT Due to its ability to provide directly reaction rate data and its in-situ nature, reacti... more ABSTRACT Due to its ability to provide directly reaction rate data and its in-situ nature, reaction calorimetry has become one of the most powerful probes of reaction pathways and mechanisms of chemical reactions by virtue of providing high-quality kinetic data. In this paper, examples of enantioselective hydrogenation and selective consecutive hydrogenation reactions are presented to demonstrate the high quality of kinetic data obtainable from the reaction calorimetry. They are also used to illustrate the use of reaction calorimetry for elucidating reaction pathways and mechanisms from detailed kinetic and thermodynamic information about individual step involved in multi-step reactions that is otherwise difficult to obtain without calorimetry.
Journal of The American Chemical Society, 1996
Marked shifts in enantioselectivity in the asymmetric hydrogenation of several prochiral substrat... more Marked shifts in enantioselectivity in the asymmetric hydrogenation of several prochiral substrates were observed as a function of the availability of hydrogen to the catalyst in both heterogeneous and homogeneous catalytic reactions. The key kinetic parameter affecting ...
Studies in Surface Science and Catalysis, 1997
ABSTRACT A two-site, two-step kinetic model is proposed to rationalize the observed effects of so... more ABSTRACT A two-site, two-step kinetic model is proposed to rationalize the observed effects of solution hydrogen concentration on enantioselectivity in the asymmetric hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified heterogeneous Pt catalyst. The model successfully predicted enantioselectivity at a hydrogen concentration outside the range used in the kinetic fit. This work demonstrates how the perturbation from equilibrium adsorption of the organic substrate on a heterogeneous catalyst may account for the observed effects of pressure on enant ioselectivity. Both positive and negative hydrogen dependences on enantioselectivity may be rationalized using the same model.
Topics in Catalysis, 1998
Several in situ probes for continuously monitoring rate of catalytic reactions under reaction con... more Several in situ probes for continuously monitoring rate of catalytic reactions under reaction conditions are described. They are reaction calorimetry, measurements of hydrogen uptake in the case of hydrogenation, and infrared spectroscopy. In studying catalytic hydrogenation reactions, for example, these in situ probes provide kinetic details of the reactions from different perspectives over the entire course of the reaction. The reaction calorimetry and the hydrogen uptake measure directly, continuously, and in a non-invasive manner the rate of reaction, while the in situ infrared spectroscopy provides time-resolved compositional information in the liquid phase. A combination of the information thus obtained leads to a clear and coherent kinetic picture of the reaction under study which can greatly facilitate pathway analysis and mechanistic description of the catalytic reaction. In this report, the usefulness of the combination of these in situ probes is illustrated with two examples of heterogeneously-catalyzed hydrogenation reactions.
Studies in Surface Science and Catalysis, 1997
Cheminform, 2010
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Journal of Organometallic Chemistry, 1997
Journal of Catalysis, 1996
The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient ... more The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient behavior at the beginning of the reaction, and a period of rising rate and enantioselectivity was linked to conversion of substrate for a wide range of reaction temperatures and initial substrate concentrations. A "reaction-driven equilibration" of the surface environment for optimal enantioselective catalysis was proposed for these reactions carried out under conditions where moderate enantioselectivities are observed. c 1996 Academic Press, Inc. We were remiss in Ref. in neglecting to note that Wells' group had made this observation prior to that of our work.
Journal of Molecular Catalysis A-chemical, 1997
Journal of Catalysis, 1995
Journal of Catalysis, 1996
Marked shifts in enantioselectivity from ca. 20 to 60 ee% as a function of reactor agitation spee... more Marked shifts in enantioselectivity from ca. 20 to 60 ee% as a function of reactor agitation speed were observed in the hydrogenation of ethyl pyruvate over cinchona-alkaloid-modified Pt. Concomitant with this shift in enantioselectivity was a change in the observed kinetics from the zero-order substrate dependence typical of gas-liquid diffusion control to a positive dependence on substrate concentration, even for a series of reactions carried out at constant pressure. A strong relationship between the solution hydrogen concentration and this rate and enantioselectivity behavior was confirmed. For systems where enantioselectivity exhibits a positive dependence on hydrogen concentration, the intrinsic ability of a catalyst to effect asymmetric hydrogenation may be masked in a reaction carried out under conditions where gas-liquid diffusion is the rate-limiting step.
Journal of The American Chemical Society, 1995
... Yongkui Sun, Carl LeBlond, Jian Wang, and Donna G. Blackmond* ... Joseph Laquidara and John R... more ... Yongkui Sun, Carl LeBlond, Jian Wang, and Donna G. Blackmond* ... Joseph Laquidara and John R. Sowa, Jr. ... us to observe a striking inversion in enanti-oselectivity in the hydrogenation of geraniol (I) with [RuClz-((S)-(-)tol-binap)]2*N(C2H~)3 (11) [tol-binap = (di-p-tolylphos ...
Journal of The American Chemical Society, 1999
Tetrahedron Letters, 2001
The evasive hypoiodous acid is generated in situ from NaOCl and NaI and used efficiently for clea... more The evasive hypoiodous acid is generated in situ from NaOCl and NaI and used efficiently for clean iodohydroxylation of 1, producing the Crixivan® intermediate 2 in high yield with highly efficient 1,3-asymmetric induction. This pH-tunable process allows HOI generation at a pH optimal for supressing byproduct formation in pH-sensitive iodohydroxylation reactions.Graphic
Journal of Organic Chemistry, 2004
The concise synthesis of a potent thrombin inhibitor was accomplished by a mild lactone aminolysi... more The concise synthesis of a potent thrombin inhibitor was accomplished by a mild lactone aminolysis between an orthogonally protected bis-benzylic amine and a diastereomerically pure lactone. The lactone was synthesized by the condensation of l-proline methyl ester with an enantiomerically pure hydroxy acid, which in turn was synthesized by a highly stereoselective (>500:1 er) and productive (100,000:1, S/C) enzymatic reduction of an alpha-ketoester. In addition, a second route to the enantiomerically pure lactone was accomplished by a diastereoselective ketoamide reduction.