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OC VI (HS 2015) Bode Research Group T OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ Chapter 2: Key concepts in catalysis 1 Reaction coordinate - Free energy, enthalpy and entropy are thermodynamic phenomena. License International 4.0 ShareAlike - Boger’s Modern Organic Synthesis C.2 2 Transition state theory 2.1 Energy of activation NonCommercial - - Energy, enthalpy and entropy of activation are kinetic phenomena. - 20 kcal/mol energy available at 25°C for free energy of activation (∆G‡). - Increasing reaction temperature increases the rate of reaction but may decrease selectivity. - R = the universal gas constant; kB = Boltzmann constant; and h = Planck's constant. Attribution Commons Creative Boger’s Modern Organic Synthesis C.2 2.2 Rate determining step (rds) - In a reaction involving more than one elementary step – that is where one or more intermediates are formed – there is more than one energy barrier (more than one TS). - The elementary step involving the highest energy barrier going to the TS is the rate-determining step (a). - Note that the pathway involving the highest energy TS is not necessarily the rate-determining step (b & c). This work is licensed under a Anslyn and Dougherty, Modern Physical Organic Chemistry, C.7 Page 1 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 2.3 Kinetic and thermodynamic control - In a reversible reaction, the majority of the product will be the thermodynamic product. - In an irreversible reaction, the majority of the product may be the kinetic product. License Trost JOC 1965, 30, 1341 International 4.0 3 Catalysis 3.1 Catalyst definition and energy diagram ShareAlike - NonCommercial - Attribution Boger’s Modern Organic Synthesis C. 2 4 Enantioselective catalysis Commons - Enantiomeric ratio is directly proportional to the relative rates of formation of the enantiomeric products. - Enantiomeric ratio is governed by differential activation parameters (∆∆G‡, ∆∆H‡ and ∆∆S‡). - R and S are chosen below arbitrarily. Creative This work is licensed under a Walsh and Kozlowski, Fundamentals of Asymmetric Catalysis, C.1 Some useful number to think about in enantioselective catalysis: - ∆∆G‡ of 1.38 kcal/mol is needed to achieve 80% ee at room temp - ∆∆G‡ of ~2.0 kcal/mol is needed to achieve 90% ee at room temp - ∆∆G‡ of 2.60 kcal/mol is needed to achieve 98% ee at room temp - ∆∆G‡ of 2.73 kcal/mol is needed to achieve 99% ee at room temp - ∆∆G‡ of 1.80 kcal/mol is needed to achieve 98% ee at -78oC Hartwig (Walsh) Organotransition Metal Chemistry, C.14 Page 2 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 4.1 Diastereomeric transition states - Case 1: simple complex with a diastereomeric transition state . License Shibasaki Adv. Synth. Catal 2004, 346, 1533 - Case 2: a more complicated TS involving a complex with multiple catalysts International 4.0 ShareAlike - NonCommercial - Blackmond and Jacobsen JACS 2004, 126, 1360 4.2 Transition state stabilization Attribution Commons Creative Hiersemann & Strassner JOC 2007, 72, 4001 4.3 Microscopic reversibility - The conversion of the product back to the reactant has to proceed through the same pathway with the This work is licensed under a forward reaction, encountering exactly the same intermediate(s) and transition state(s). Blackmond ACIE 2009, 48, 2648 4.4 The Hammond postulate - Activated complex (TS) most resembles the structure of adjacent reactant, intermediate, or product that is closest in energy (thermodynamic factor). - For example, in a highly exothermic reaction, the TS is closer in energy and in structure to the reactant than the product (early transition state e.g. Grignard reagent addition to carbonyl compounds). Page 3 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ + R R Nu- H C R3COH R3CNu R E E TS1 TS 2 CH2R The transition states The ralative stability of resemble the geometry carbocation: R R CHR C of the carbocation 2 the TS becomes more intermedate, not the stable as the reaction . R reactant nor the product. becomes less endothermic. R3COH CR3 R3CNu License reaction coordinate reaction coordinate Hammond JACS 1955, 77, 334 4.5 The Curtin-Hammett Principle International 4.0 - In multistep reactions, there may exist an equilibrium between two diastereomeric intermediates. - The overall enantioselectivity is determined by the difference in the relative heights of the turnover-limiting barrier (∆∆G‡). - From the graph below, I1 is more stable than I2 (from ∆∆G). But formation of I2 is more favorable because of the lower relative activation energy (∆∆G‡). ShareAlike - I1 gradually reverses back to the starting material (SM) then to I2 (SM, I1 and I2 are in equilibrium). NonCommercial - Attribution Halpern Science 1982, 217, 401 Commons 4.6 Catalyst turnover - Catalyst productivity: Turn Over Number (TON) = mol product/mol catalyst -1 Creative - Catalyst reactivity: Turn Over Frequency (TOF) = (mol product/mol catalyst)/hour = TON/hour (unit of h ) - For example, hydrogenation should have TON > 1000 for high value product and >50,000 for large-scale. - For hydrogenation, TOF > 500 h-1 for small scale and TOF>10,000 h-1 for large scale This work is licensed under a Blaser Appl. Catal. A 2001, 221, 119 Page 4 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 4.7 Catalyst resting state . License Stoltz ACIE 2009, 48, 6840 4.8 Product inhibition International - Product inhibition occurs when the product binds better to the catalyst than the starting material. This is a 4.0 common problem in the catalysis of the Claisen reaction. ShareAlike - NonCommercial - Yamamoto JACS 1990, 112, 316 4.9 Background rate Attribution - The starting materials may react to form the product without the aid of the catalyst. If the background rate of reaction is comparable to or faster than the catalyzed reaction, lower selectivity is obtained. (The background reaction is normally unfavorable and has to be avoided). Commons Creative Evans JACS 1999, 121, 7582 This work is licensed under a 5 Modes of binding 5.1 Single point binding 10 mol% (R)-BINOL O OH 10 mol% TiCl2(Oi-Pr) + Me TiL Me CF3 H CF3 O Me H CF3 OH L = 98% syn OH 96% ee H Mikami Tetrahedron 1996, 52, 85 Page 5 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 5.2 Multiple points binding (tends to give higher selectivity because of a highly organized TS) . License Hiersemann ACIE 2001, 40, 4700 6 Types of catalysis International BrØnsted acid catalysis 6.1 4.0 NMe2 NMe2 NMe2 NMe2 B ShareAlike H - 2 mol% B-H O N Toluene, 1 d N O HN O H O N H Ar t-BuO C t-BuO2C H t-BuO2H2C Ar B 2 t-BuO2C H Ar H H Ar N N N N N N N N 9-anthryl NonCommercial B-H = - Ar = 4-FC H , 74% yield, 97% ee O O 6 4 P 4-PhC H , 71% yield, 97% ee O OH 6 4 4-MeOC6H4, 62% yield, 97% ee 9-anthryl Attribution 9-anthryl Terada JACS 2005, 127, 9360 6.2 Lewis acid-base catalysis Commons Creative Walsh and Kozlowski, Fundamentals of Asymmetric Catalysis, C.2 This work is licensed under a Denmark JACS 1999, 121, 4982 Page 6 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 6.3 Transition metal catalysis . License Trost Acc. Chem. Res 1996, 29, 355 6.4 Organocatalysis International 4.0 ShareAlike - Barbas JACS 2000, 122, 2395 6.5 Hydrogen bonding catalysis O Ph NH HN CN NonCommercial N - O HN Ph HN O OH H 2 mol% Ph NH O O H H CN N HCN H H N toluene, -20 oC H O Attribution 97% conv., 97% ee Inoue JOC 1990, 55, 181 Commons 6.6 Ion-pair catalysis Phase Transfer Catalysis (PTC) – (convenient for process chemists because of the ease of product isolation) Creative O’Donnell Acc. Chem. Res. 2004, 37, 506 7 Modes of activation This work is licensed under a 7.1 Electrophile activation Fu Acc. Chem. Res. 2000, 33, 412 Page 7 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 7.2 Nucleophile activation Ph Ph Ph N H Ph O NO2 TMSO N + 10 20 mol% TMSO Yield %ee H R1 R2 syn:anti Hexanes (%) (syn) 1 2 1 R R 1 48 h R Me Ph 85 94:6 99 activated nucleophile . Me n-Bu 52 84:16 99 Me Cy 56 96:4 99 Ph Et Ph 66 93:7 99 2 Ph 2 i-Pr Ph 72 93:7 99 License O R N R O NO TMSO N H 2 O R1 R1 Hayashi ACIE 2005, 44, 4212 International 8 Ligand effect on catalysis 8.1 Ligand decelerated reaction 4.0 - A chiral reagent adds more quickly than the ligated adduct (faster background reaction). - For example, ligand decelerated catalysis is a common problem in asymmetric catalytic Grignard addition. This is usually overcome by using chiral reagents in stoichiometric fashion. ShareAlike - NonCommercial - Attribution Cram JACS 1981, 103, 4585 8.2 Ligand accelerated catalysis - This is a case where there is almost no background rate (the two starting materials do not react at 0 oC). Commons - The binding of Et2Zn to the ligand DAIB increases the Lewis acidity of the central Zn and accelerates the reaction rate. The product enantiomeric outcome is governed by the catalyzed pathway. Creative This work is licensed under a Noyori JACS 1986, 108, 6071 Page 8 OC VI (HS 2015) Bode Research Group http://www.bode.ethz.ch/ 8.3 Non-linear effect - (product enantiopurity does not correlate with catalyst enantiopurity) Me Me Me Me N R N R Me Me HO Ph PhCHO Zn Zn O O O O R H homodimer formation Me Zn Me Zn is reversible R = Et R N R N cat %ee = 15 Me Me Me Me .
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