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• Oxidative Addition • Reductive Elimination • Migratory Insertion • Β - Hydrogen Elimination AJELIAS L7-S2 Oxidative Addition

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AJELIAS L7-S1 Unique reactions in organometallic chemistry • Oxidative Addition • Reductive Elimination • Migratory Insertion • β - Hydrogen Elimination AJELIAS L7-S2 Oxidative addition When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction LnM+XY Ln(X)(Y)M dn dn-2 OX state of metal increases by 2 units H H2 oxidative Ph3P PPh3 addition H PPh3 Coordination number increases by 2 units Rh Rh Ph3P Cl Ph3P Cl PPh3 2 new anionic ligands are added to the metal Rh+1 Rh+3 Requirements for oxidative addition • availability of nonbonded electron density on the metal, • two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated, • a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. AJELIAS L7-S3 Examples of Oxidative addition : Cis or trans ? Cl Cl PPh3 Ir 18E Cl2 Ph3P CO Cl O Cl PPh3 O2 O PPh3 Ir Ir Ph3P CO Ph3P CO 16E Cl Me Me MeI Cl PPh 3 I PPh3 Ir Ir Ph P CO 3 Ph3P CO I Cl Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans AJELIAS L7-S4 An important step in many homogeneous catalytic cycles Hydrogenation of alkenes- Wilkinson catalyst H H2 oxidative Ph3P PPh3 addition H PPh3 Rh Rh Often thefirststepofmechanism Ph3P Cl Ph3P Cl PPh3 Rh+1 Rh+3 Methanol to acetic acid conversion- Cativa process CH3 I CO CH3I I CO Ir Ir I CO I CO Ir+1 Ir+3 I Pd catalyzed Cross coupling of Ar-B(OH)2 and Ar-X – Suzuki Coupling Br Br Ph3P Pd PPh3 Pd Ph3P PPh3 Pd0 Pd+2 The more electron rich the metal, more easy is the oxidative addition AJELIAS L7-S5 Oxidative addition involving C-H bonds and cyclo/ortho metallation Agostic interaction This type of reactions help to activate unreactive hydrocarbons such as methane – known as C-H activation AJELIAS L7-S6 Reductive elimination Almost the exact reverse of Oxidative Addition CH3 Ph2 Ph2 P CH P CH3 reductive elimination 3 Pt Pt + H3CCH3 165 °C, days P CH P CH3 3 Ph2 Ph2 CH3 2+ Pt4+ Pt Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable σ bond and leave the metal Factors which facilitate reductive elimination • a high formal positive charge on the metal, • the presence of bulky groups on the metal, and • an electronically stable organic product. Cis orientation of the groups taking part in reductive elimination is a MUST AJELIAS L7-S7 Final step in many catalytic cycles Hydroformylation ( conversion of an alkene to an aldehyde) Sonogashira Coupling (coupling of a terminal alkyne to an aryl group Cativa Process (Methanol to Acetic acid) AJELIAS L7-S8 Migratory Insertion X L + L M Y [M-Y-X] M Y X d n d n No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another CH 3 Ph3P O OC OC CO C CH Mn + PPh3 Mn 3 OC CO OC CO OC OC These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates AJELIAS L7-S9 Types of Migratory Insertion X X M A B M A 1, 1 - migratory insertion B X X A B 1, 2 - migratory insertion M M A B CO O 1, 1-migratory CH2CH2R CCH2CH2R Ph3P insertion Ph3P Rh Rh OC PPh3 OC PPh3 H R 1, 2-migratory CH CH R Ph3P 2 2 Ph3P insertion Rh Rh OC PPh Ph3P 3 CO AJELIAS L7-S11 Stability of σ Bonded alkyl groups as ligands Joseph Chatt 1962 - 68 Br PEt H PEt CH2 3 EtMgBr H3CH2C PEt3 Heat 3 Pt Pt Pt + Et P Br Et3P Br Et3P Br 3 Pressure CH2 poor yields unstable Ph3P n - BuLi Pt cis PtCl2(PPh3)2 Ph3P Ph P Ph3P + 3 Pt + Pt 60 °C, Ph P Ph3P 3 sealed tube Ph3P CH3 Pt No decomposition 60 °C Ph3P CH3 sealed tube Why does some σ bonded alkyl complexes decompose readily? AJELIAS L7-S12 β-Hydride elimination Beta-hydride elimination is a reaction in which an alkyl group having a β hydrogen, σ bonded to a metal centre is converted into the corresponding metal-bonded hydride and a π bonded alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. No change in the formal oxidation state of the metal mechanism H H H H H H H H C H C C M M C M C C H H H H H H Can either be a vital step in a reaction or an unwanted side reaction AJELIAS L7-S13 β-hydrogen elimination does not happen when • the alkyl has no β-hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2) • (ii) the β-hydrogen on the alkyl is unable to approach the metal (as in C≡CH) • the M–C–C–H unit cannot become coplanar Select the most unstable platinum σ complex from the given list. Justify your answer H C Ph P Ph P Ph P 3 Et3P C 3 3 SiMe3 Pt Pt Pt Pt SiMe Ph P 3 Ph P Ph P 3 Et3P C 3 3 C H A B C D No β-H β-H unable to MCCH unit will not be approach M coplanar Problem solving Classify the following reactions as oxidative addition, reductive elimination, (1,1 / 1,2)migratory insertion, β-H elimination, ligand coordination change or simple addition − − (a) [RhI3(CO)2CH3] → {RhI3(CO)( solvent)[C(O)CH3]} (b) Ir(PPh2Me)2(CO)Cl + CF3I → Ir(I)(CF3)(PPh2Me)2(CO)Cl (c) TiCl4 +2 Et3N → TiCl4 (NEt3)2 (d) HCo(CO)3(CH2=CHCH3)+CO→ CH3CH2CH2Co(CO)4 Step 1. determine the oxidation state of the metal in reactant and product Step 2. count the electrons for reactant and product Step 3. see if any ligand in the reactant has undergone change AJELIAS L7-S14 Homogeneous catalysis using organometallic Catalysts A catalyst typically increases the reaction rates by lowering the activation energy by opening up pathways with lower Gibbs free energies of activation (G). Gibbs energy of activation uncatalyzed gy r catalyzed Gibbs Ene stable intermediate Reactants Products Heterogeneous Homogeneous Homogeneous versus Heterogeneous Catalysis Parameter Heterogeneous Homogeneous Phase Gas/solid Usually liquid/ or solid soluble in the reactants Required temperature High Low ( less than 250°C) Catalyst Activity Low High Product selectivity Less (often mixtures) More Catalyst recycling Simple and cost effective Expensive and complex Reaction mechanism Poorly understood Reasonably well understood Product separation Easy Elaborate and sometimes from catalyst problematic Fine tuning of catalyst Difficult Easy AJELIAS L7-S15 Heterogeneous Catalyst- Catalytic Converter of a Car Platinum and Palladium Platinum and Rhodium Chemistry at the molecular level – Poorly understood Home assignment : See Youtube video ‘Catalysis’ AJELIAS L7-S16 Comparing different catalysts; Catalyst life and Catalyst efficiency Turnover Number (TON) TON is defined as the amount of reactant (in moles) divided by the amount of catalyst (in moles) times the percentage yield of product. A large TON indicates a stable catalyst with a long life. Turnover Frequency (TOF) It is the number of passes through the catalytic cycle per unit time (often per hour). Effectively this is dividing the TON by the time taken for the reaction. The units are just time–1 . A higher TOF indicates better efficiency for the catalyst AJELIAS L7-S17 Wilkinson’s Catalyst for alkene hydrogenation RhCl3 (H2O)3 + RhCl(PPh3)3 + CH3CH2OH + CH3CHO + 3 PPh3 2HCl + 3H2O Wilkinson’s catalyst: The first example of an effective and rapid homogeneous catalyst for hydrogenation of alkenes, active at room temperature and atmospheric pressure. 8 Square planar 16 electron d complex (Ph3P)3RhCl Discovered by G Wilkinson as well as by R Coffey almost at the same time (1964–65) 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.
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