Reaction chemistry of complexes Three general forms: 1. Reactions involving the gain and loss of ligands a. Ligand Dissoc. and Assoc. (Bala) b.

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1 eaction chemistry of complexes Three general forms: 1. eactions involving the gain and loss of ligands a. Ligand Dissoc. and Assoc. (Bala) b. Oxidative Addition c. eductive Elimination d. Nucleophillic displacement 2. eactions involving modifications of the ligand a. Insertion b. Carbonyl insertion (alkyl migration) c. Hydride elimination (equilibrium) 3. Catalytic processes by the complexes Wilkinson, onsanto Carbon-carbon bond formation (Heck etc.)

2 a) Ligand dissociation/association (Bala) Electron count changes by -/+ 2 No change in oxidation state Dissociation easiest if ligand stable on its own (CO, olefin, phosphine, Cl -,...) Steric factors important Br + Br -

3 b) Oxidative Addition Basic reaction: L n + X Y L n X Y Electron count changes by +/- 2 (assuming the reactant was not yet coordinated) Oxidation state changes by +/- 2 echanism may be complicated The new -X and -Y bonds are formed using: the electron pair of the X-Y bond one metal-centered lone pair

4 One reaction multiple mechanisms Concerted addition, mostly with non-polar X-Y bonds H 2, silanes, alkanes, O 2,... Arene C-H bonds more reactive than alkane C-H bonds (!) X L n + L n X Y Y A Intermediate A is a σ-complex L n X Y eaction may stop here if metal-centered lone pairs are not readily available Final product expected to have cis X,Y groups

5 Stepwise addition, with polar X-Y bonds HX, 3 SnX, acyl and allyl halides,... low-valent, electron-rich metal fragment (Ir I, Pd (0),...) X L n X Y L n X Y L n Y B etal initially acts as nucleophile Coordinative unsaturation less important Ionic intermediate (B) Final geometry (cis or trans) not easy to predict adical mechanism is also possible

6 Cis or trans products depends on the mechanism H 2 H PEt 3 OC Ir H Ir(III) Et 3 P Cl cis H PEt 3 OC Ir Cl HI PEt 3 OC Ir I Ir(III) Et 3 P Ir(I) Et 3 P Cl cis CH 3 PEt 3 CH 3 Br OC Ir Cl Ir(III) Et 3 P Br trans

7 c) eductive elimination This is the reverse of oxidative addition - Expect cis elimination ate depends strongly on types of groups to be eliminated. Usually easy for: H + alkyl / aryl / acyl H 1s orbital shape, c.f. insertion alkyl + acyl participation of acyl p-system Si 3 + alkyl etc Often slow for: alkoxide + alkyl halide + alkyl thermodynamic reasons? We will do a number of examples of this reaction

8 ost crowded is the fastest reaction elative rates of reductive elimination L CH 3 Pd + solv L CH 3 -L L Pd CH 3 E solv CH 3 LPd(solv) + CH 3 CH 3 Complex ate Constant (s -1 ) T( o C) Ph 3 P CH 3 Pd Ph 3 P CH x eph 2 P CH 3 Pd eph 2 P CH x Ph Ph P CH 3 Pd P CH 3 Ph Ph 4.78 x

9 Special case: Nucleophilic Attack on a Coordinated CO acyl anion Fischer carbenes are susceptible to nucleophilic attack at the carbon Fisher carbene This is Fischer carbene It has a metal carbon double bond Such species can be made for relatively electronegative metal centers N.B. mid to late Ts

10 Fischer carbenes act effectively as σ donors and π acceptors The empty antibonding =C π orbital is primarily on the carbon making it susceptible to attack by nucleophiles Other type is called a Shrock carbene (alkylidene) Characteristic Fischer-type Schrock-type Typical metal (Ox. State) Substituents attached to carbene carbon Typical other ligands iddle to late T.. Fe(0), o(0) Cr(0) At least one highly electronegative heteroatom Good p acceptors Early T.. Ti(IV), Ta(V) H or alkyl Electron count Good s and p donors

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12 Nucleophilic displacement Ligand displacement can be described as nucleophilic substitutions O.. complexes with negative charges can behave as nucleophiles in displacement reactions Iron tetracarbonyl (anion) is very useful O 'X ' O [Fe(CO) X 4 ] 2- [ Fe(CO) 4 ] - O 2 OH X O CO H + X 2 H O X O [ Fe(CO) 4 ] - H + H O

13 odifications of the ligand a) Insertion reactions igratory insertion! The ligands involved must be cis - Electron count changes by -/+ 2 No change in oxidation state If at a metal centre you have a σ-bound group (hydride, alkyl, aryl) a ligand containing a π-system (olefin, alkyne, CO) the σ-bound group can migrate to the π-system 1. CO, NC (isonitriles): 1,1-insertion 2. Olefins: 1,2-insertion, β-elimination CO O 1,1 1,2

14 1,1 Insertion The σ-bound group migrates to the π-system if you only see the result, it looks like the π-system has inserted into the -X bond, hence the name insertion To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial) The reverse of insertion is called elimination Insertion reduces the electron count, elimination increases it Neither insertion nor elimination causes a change in oxidation state α- elimination can release the new substrate or compound

15 In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate. The metal-bound substrate atom increases its valence e CO O e e SO 2 S O O e CO, isonitriles (NC) and SO 2 often undergo 1,1-insertion 1,2 insertion (olefins) Insertion of an olefin in a metal-alkyl bond produces a new alkyl Thus, the reaction leads to oligomers or polymers of the olefin polyethene (polythene) polypropene

16 Standard Cossee mechanism Why do olefins polymerise? Driving force: conversion of a π-bond into a σ-bond One C=C bond: 150 kcal/mol Two C-C bonds: 2 85 = 170 kcal/mol Energy release: about 20 kcal per mole of monomer (independent of mechanism) any polymerization mechanisms adical (ethene, dienes, styrene, acrylates) Cationic (styrene, isobutene) Anionic (styrene, dienes, acrylates) Transition-metal catalyzed (a-olefins, dienes, styrene)

17 β Hydride elimination (usually by β hydrogens) any transition metal alkyls are unstable (the reverse of insertion) the metal carbon bond is weak compared to a metal hydrogen Bond Alkyl groups with β hydrogen tend to undergo β elimination -CH 2 -CH 3 - H + CH 2 =CH 2 Two examples

18 Do not contain beta-hydrogens Are oriented so that the beta position can not access the metal center Would give an unstable alkene as the product A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product. To prevent beta-elimination from taking place, one can use alkyls that:

19 The onsanto acetic acid process ethanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid Insertion of carbon monoxide into the C-O bond of methanol The catalyst system - iodide and rhodium Iodide promotes the conversion of methanol to methyl iodide, ethyl iodide - the catalytic cycle begins: 1. Oxidative addition of methyl iodide to [h(co) 2 I 2 ] - 2. Coordination and insertion of CO - intermediate 18-electron acyl complex 3. Can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst

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21 Catalysis (homogeneous) eduction of alkenes etc.

22 Alternative starting material

23 The size of the substrate has an effect on the rate of reaction

24 Same reaction different catalyst

25 Homogeneous cross coupling reactions: Heck reaction HX Pd(II) Pd(0) -X Y -Pd(II)-X H Pd(II)-X H H Y -Pd(II)-X -Pd(II)-X Y Y Y = H,, Ph, CO 2, CN, Oe, OAc NHAc CH 2 =CH 2 > CH 2 =CH-OAc > CH 2 =CH-e > CH 2 =CH-Ph > CH 2 =C(e)Ph k rel : 14,

26 Wacker process (identify the steps)

27 Identify the steps