Nucleophilic attack on ligand

Transcription

1 Nucleophilic attack on ligand Nucleophile "substitutes" metal hapticity usually decreases xidation state mostly unchanged Competition: nucleophilic attack on metal usually leads to ligand substitution 1 Ligand Substitution

2 Nucleophilic abstraction ostly ligand deprotonation - Na + NaH C C Cr C C Cr Cp 2 WH 2 BuLi Cp 2 WH Li 2 Ligand Substitution

3 Nucleophilic addition (H 2 ) 2 + H - (H 2 ) 2 H - Key reaction of the Wacker process: C 2 H 4 + ½ 2 2, H 2 CH3 CH Cu 2 3 Ligand Substitution

4 The Wacker process 2- C 2 H 4 - H 2 H 2 (0) + H + + H e (Cu 2 Cu) H 2 H - H - H 2 H CH 3 CH - H H H 2 - H "β-h elim" H 2 H ins H 2 β-h elim H H 4 Ligand Substitution

5 The Wacker process Characteristics of the Wacker process: The oxygen in the product derives from water, not directly from the oxygen used as oxidant higher olefins yield ketones, not aldehydes large amounts of halides required: corrosive side products resulting from nucleophilic attack of halide on olefin No longer important for acetic acid synthesis Several variations (with more complicated nucleophiles) used in organic synthesis 5 Ligand Substitution

6 Nucleophilic attack on the ligand How can you distinguish between internal and external attack of H -? Use trans-ch=ch and trap the intermediate -C-C-H with : H 2 H 2?? H H 2 H - H H - H 2 - ins - - H 2 H H - H - ins nucl attack H 6 Ligand Substitution

7 Using isotopic labelling to study mechanisms H - H H H H 7 Ligand Substitution

8 Using isotopic labelling to study mechanisms Could acetaldehyde be formed directly as vinyl alcohol? Perform reaction in 2 : H H H H H diss H 2 CH 3 ins H H "β-h elim" diss CH H 3 H diss 2 CH 2 ins H "β-h elim" diss CH 3 8 Ligand Substitution

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10 Insertion reactions If at a metal centre you have a) a σ-bound group (hydride, alkyl, aryl) b) a ligand containing a π-system (olefin, alkyne, ) the σ-bound group can migrate to the π-system. R R R R 10

11 Insertion in en() 5 insertion H TS LU agostic adduct η 2 -acyl 11

12 Insertion reactions The σ-bound group migrates to the π-system. But 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. 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. 12

13 1,1 insertions 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., isonitriles (RNC) and S 2 often undergo 1,1-insertion. e e e S 2 S e 13

14 Insertion of and isonitriles insertion is hardly exothermic. An additional ligand may be needed to trap the acyl and so drive the reaction to completion. In the absence of added ligands often fast equilibrium. insertion in -H, -CF 3, -R endothermic. no polymerization. but isonitriles do polymerize! 14

15 ouble insertion? eriving a mechanism from a reaction stoichiometry is not always straightforward. The following catalytic reaction was reported a few years ago: 2 R 2 NH ArI "" R 2 NAr + R 2 NH 2 + I - This looks like it might involve double insertion. But the actual mechanism is more complicated. 15

16 No double insertion! R 2 NAr L 2 () n ArX - n red elim ox add L 2 Ar NR 2 L 2 X Ar HNR 2 - H + nucl attack Ar L 2 + subst - X - L 2 X ins Ar 16

17 Promoting insertion "Bulky" ligands R requires more space than R Lewis acids Coordinate to, stabilize product Al 3 Al 3 C R vs R rawback: usually stoichiometric 17

18 Sometimes it only looks like insertion Nucleophilic attack at coordinated can lead to the same products as standard insertion: Ir e Ir e Ir e Ir e ain difference: nucleophilic attack does not require an empty site. 18

19 1,2-insertion of 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. e 19

20 1,2-insertion of 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. Best known polyolefins: polyethene (polythene) polypropene In addition, there are many specialty polyolefins. Polyolefins are among the largest-scale chemical products made. They are chemically inert. Their properties can be tuned by the choice of catalyst and comonomer. 20

21 Why do olefins polymerize? riving force: conversion of a π-bond into a σ-bond ne 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 Radical (ethene, dienes, styrene, acrylates) Cationic (styrene, isobutene) Anionic (styrene, dienes, acrylates) Transition-metal catalyzed (α-olefins, dienes, styrene) Transition-metal catalysis provides the best opportunities for tuning of reactivity and selectivity 21

22 echanism of olefin insertion Standard Cossee mechanism R R R R Green-Rooney variation (α-agostic assistance): CH 2 P CH 2 P CH 2 P H P H Interaction with an α C-H bond could facilitate tilting of the migrating alkyl group The "fixed" orientation suggested by this picture is probably incorrect 22

23 Insertion in -H bonds Insertion in -H bonds is nearly always fast and reversible. Hydrides catalyze olefin isomerization Regiochemistry corresponds to arkovnikov rule (with δ+ -H δ- ) To shift the equilibrium to the insertion product: Electron-withdrawing groups at metal alkyl more electron-donating than H Early transition metals -C stronger (relative to -H) Alkynes instead of olefins more energy gain per monomer, both for -H and -C insertion 23

24 Catalyzed olefin isomerization etals have a preference for primary alkyls. But substituted olefins are more stable! Cp 2 Zr Cp 2 ZrH dominant alkyl dominant olefin Cp 2 Zr In isomerization catalysis, the dominant products and the dominant catalytic species often do not correspond to each other. For each separately, concentrations at equilibrium reflect thermodynamic stabilities via the Boltzmann distribution. 24

25 Catalyzed olefin isomerization Cp 2 ZrH xs or Cp 2 Zr ost stable alkyl or + + ost stable olefin + little 25