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Conventional Catalytic Cycle for Hydrogenation with Wilkinson's Catalyst

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Conventional Catalytic Cycle for Hydrogenation with Wilkinson's Catalyst Conventional Catalytic cycle for hydrogenation with Wilkinson’s catalyst P P Rh P Cl PP The first step of this P catalytic cycle is the Cl Rh reductive H2 oxidative cleavage of a PPh3 to elimination 14e P addition generate the active form of the catalyst RCH2CH3 followed by oxidative addition of dihydrogen. R H H CH 2 P P H Rh H2C Rh P P Cl Cl R alkene 1, 2 -migratory P insertion coordination H H Rh P = PPh P Cl 3 R AJELIAS L7-S18 catalytic cycle for hydrogenation H H2 oxidative P P addition H P Kinetic studies have Rh Rh shown that the P Cl P Cl dissociation of PPh3 P from the distorted P square planar complex PP (due to trans RhCl(PPh3)3 in effect of H ) H benzene occurs only to a very small extent H2 oxidative P (k = 2.3 × 10–7 M at P addition HRh Cl Rh 25°C), and P P under an atmosphere Cl of H2, a solution of RCH2CH3 RhCl(PPh3)3 becomes reductive alkene yellow as a result of elimination the oxidative addition of H2 to give cis- R H P CH2 H2RhCl(PPh3)3. P H H H2CRh Rh P 1, 2 -migratory P Cl insertion Cl R The trans effect is the labilization (making unstable) of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence: H2O, OH− < NH3 < py < Cl− < Br− < I−, < PR3, CH3− < H−, NO, CO AJELIAS L7-S19 Relative reactivity of alkenes for homogenous catalytic hydrogenation • Cis alkenes undergo hydrogenation more readily than trans alkenes •Internal and branched alkenes undergo hydrogenation more slowly than terminal ones, and R R R R > > > > R R R R R > > R R R R R AJELIAS L7-S20 Fine tuning of a catalyst: hydrogenation catalysts which are more efficient than Wilkinsons catalyst + + Ph P PPh PCy3 3 PPh3 3 Ir Rh Rh PF6 PF6 Ph3P Cl PPh3 N Wilkinson's catalyst Schrock-Osborn's catalyst Crabtree's catalyst Catalyst Turnover frequency (TOF) in h–1 for hydrogenation of 25°C, 1 atm H2 alkenes Wilkinson’s catalyst 650 700 13 NA Schrock–Osborn 4000 10 NA NA catalyst Crabtree’s catalyst 6400 4500 3800 4000 The cationic metal center is relatively more electrophilic than neutral metal center and thus favours alkene coordination. AJELIAS L7-S21 Hydrogenation with Crabtree’s catalyst H PCy3 PF PCy3 Ir 6 H2 Ir PF6 N oxidative H addition π 16e N 18e migratory insertion PCy3 PCy reductive Ir PF6 Ir 3 repeat of PF6 elimination S H cycle with N solvent cyclooctene σ N 16e 16e coordination S PCy3 Ir PF6 S N 16e di-solvated active form of catalyst The di-solvated form of the active catalyst generated by the removal of COD [after it gets hydrogenated and leaves] favors coordination of sterically bulky alkenes as well. This mechanism is only for understanding not for the exam AJELIAS L7-S22 Factors which have been found to improve the efficiency (better TOF) of transition metal catalysts for hydrogenation • Making a cationic metal center : makes catalyst electrophillic for alkene coordination • Use of ligands (eg. Cyclooctadiene) which will leave at the initial stages of the cycle generating a di-solvated active catalyst : facilitates binding of even sterically hindered alkenes • Use of chelating biphosphines: Cis enforcing: reduces steric hindrance at the metal centre + P S PCy3 Rh PF Ir PF6 6 PCy3 P Ir PF6 S N N 16e 16e di-solvated active form Cis enforcing of catalyst Problem solving- fill in the blanks Oxidative addition 1,2 Migr. Insertion 1,1 Migr. Insertion Bio Inorganic chemistry Study of Inorganic elements in the living systems 11 20 Na Ca 22.98 40.08 19 12 K Mg 39.09 24.31 Sodium potassium pump (1/5th of all the ATP used) 26 27 29 30 Fe Co Cu Zn 55.85 58.94 63.55 65.38 Hemoglobin Vit B12 Hemocyanin Carbonic anhydrase Myoglobin Carboxypeptidase Cytochromes Ferredoxin Important roles metals play in biochemistry 1. Regulatory Action Sodium potassium channels and pump Na, K Nerve signals and impulses, action potential muscle contraction 2. Structural Role Calcium in bones, teeth Ca, Mg provide strength and rigidity 3. Electron transfer agents Cytochromes: redox intermediates Fe2+/Fe3+ membrane-bound proteins that contain heme groups and carry out electron transport in Oxidative phosphorylation 4. Metalloenzymes Carbonic anhydrase, Carboxypeptidase − Zn biocatalysts, CO2 to HCO3 , protein digestion 5. Oxygen carriers and storage Hemoglobin, Myoglobin, Hemocyanin Fe, Cu 18 times more energy from glucose in presence of O2 6. Metallo coenzymes Vitamin B 12 Co biomethylation Structure of a metallo-protein : A metal complex perspective Spiral - α helix form of protein Tape - β Pleated sheet form of protein Prosthetic groups – A metal complex positioned in a crevice. Some of the ligands for this complex or some times all of the ligands are provided by the side groups of the amino acid units. The geometry around the metal and bond distances and angles are decided by the protein unit Myoglobin Carbonic anhydrase Metalloenzymes and Oxygen carriers = Protein + Cofactor A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes. Cofactors are either organic or inorganic. They can also be classified depending on how tightly they bind to an enzyme, with loosely-bound or protein-free cofactors termed coenzymes and tightly-bound cofactors termed prosthetic groups. Porphyrins with different metals at its centre are a common prosthetic group in bioinorganic chemistry Cytochrome C Coenzyme B12 Hemocyanin Myoglobin Chlorophyll Protoporphyrin IX and Heme 15 different ways to arrange the substituents around the porphyrin. Only one isomer protopophyrin IX is found in the living system. Porphyrins are planar and aromatic Proteins –consists of different amino acids in a specific sequence connected by the peptide bond – A few important amino acids relevant to the present course HISTDINE This amino VALINE is a branched- GLUTAMIC ACID has carboxylic acid acid has a pKa of 6.5. chain amino acid having functional group which is hydrophilic, This means that, at a hydrophobic isopropyl has pKa of 4.1 and exists in its physiologically relevant R group. In sickle-cell negatively charged deprotonated pH values, relatively disease, valine carboxylate form at physiological pH small shifts in pH will substitutes for the ranging from 7.35 to 7.45. change its average hydrophilic amino acid charge. Below a pH of 6, glutamic acid in the imidazole ring is hemoglobin.Valine is mostly protonated. hydrophobic SERINE Serine is an amino acid having a CH2OH side group. By virtue of the hydroxyl group, serine is classified as a polar amino acid. Serine was first obtained from silk protein, a particularly rich source, in 1865. The primary structure of a protein The four levels of protein structure H bond between side chains, hydrophobic interactions, disulfur linkages, electrostatic interactions See youtube video “protein structure” Univ of Surrey ’ Hemoglobin- a quaternary structure of a protein 4 units Each unit has a prosthetic group (heme) embedded in a crevice and partly coordinated by histidine units Inorganic Active site / Prosthetic group In molecular biology the active site (prosthetic group) ispartofan enzyme where substrates bind and undergo a chemical reaction. It can perform its function only when it is associated with the protein unit Ferredoxin (e transfer) Heme in Myoglobin (O2 storage) Carbonic anhydrase Nitrogen Fixation Enzyme) Inorganic Prosthetic group of three well known oxygen carriers Present in Vertebrates Present in molluscs Present in some sea worms Can the prosthetic unit part of a metalloprotein perform its normal function without the protein unit around it ? 2+ 2+ Fe + O2 Fe O O Free Heme 4+ Fe2+ O + Fe2+ 2 Fe O O 2+ 3+ Fe4+ O + Fe Fe O Fe3+ Reversible binding of O2 is possible on when protein unit is present around the heme unit Oxygen : A few Questions Why do we need oxygen or why do we breathe? What happens to oxygen in our body and where does it happen? How exactly does oxygen change to water ? What does this reaction produce and how? How exactly is oxygen carried around and stored in the body? How exactly is CO2 removed from the body? Electron transfer agents Cytochromes: redox intermediates Fe2+/Fe3+ membrane-bound proteins that contain heme groups and carry out electron transport in Oxidative phosphorylation Cytochromes are, in general, membrane-bound (i.e. inner mitochondrial membrane) heme proteins containing heme groups and are primarily responsible for the generation of ATP via electron transport. They are found bound on the inner mitochondrial membrane either as monomeric proteins (e.g., cytochrome c) or as subunits of bigger enzymatic complexes that catalyze redox reactions. These heme proteins are classified on the basis of the position of their lowest energy absorption band in the reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm). Electron transfer agents; e.g. Cytochrome C S(Cys) Protein protein S(Cys) Protein N N N N CH3 H Fe S methionine N N residue of protein OH O HO O Mitochondria: The powerhouse of the Animal Cell Bio-units of the electron transport chain are present on the inner walls of the mitochondrion. Analogous powerhouses on the plant cells are chloroplasts Glycolysis + Oxidative phosphorylation: How food is converted into energy + Glucose + 36 ADP + 36 Pi + 36 H + 6 O2 6 CO2 + 36 ATP + 42 H2O Glucose gives 18 times more energy when oxidized + Δ 0 ATP + H2O ADP + Pi + H + energy G = - 7.3 kCal/mole ATP : Universal currency for energy Different forms of Cytochromes (except Cytochrome P-450) are involved in the in living systems electron transfer process leading to ATP synthesis and conversion of O2 to H2O See youtube video ‘cellular respiration ( electron transfer chain)’ See youtube video ‘gotta get that ATP’ for fun and learning! Actual structure of ATP synthase unit (a molecular machine!) Cytochrome c oxidase with electrons Cytochromes a and a3 delivered to complex by soluble cytochrome c (hence the name) Cytochromes b and c1 Cytochrome c reductase.
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