Ingeniero Químico

The Mechanistic Studies of the Wacker Oxidation
Tyler W. Wilson SED Group Meeting 11.27.2007

Introduction
• Oxidation of ethene by Pd (II) chloride solutions (Phillips, 1894) -First used as a test for alkenes (Pd black was the indicator) C2H4 + Pd(II)Cl2 + H2O → C2H4O + Pd(0) + 2HCl

• Later Smidt and co-workers employed cupric chloride to regenerate the Pd (0) catalyst (Smidt, 1962)C2H4 + Pd(II)Cl2 + H2O Pd(0) + 2CuCl2 → C2H4O + Pd(0) + 2HCl → Pd(II)Cl2 + 2CuCl

• What made Smidt’s process applicable to large scale production was the final recycling of CuCl back to Cu(II)Cl2 by air
2 CuCl + 1/2 O2 + 2 HCl → 2CuCl2 + H2O

Introduction
• Adding up the three reactions gives the Wacker Oxidation Reaction :
PdCl42- + C2H4 + H2O Pd(0) + 2 CuCl2 + 2 Cl2 Cu(I)Cl + 2 HCl + 1/2 O2Net: → Pd(0) + CH3CHO +2 HCl + 2 Cl→ PdCl42- + 2 Cu(I)Cl → 2 Cu(II)Cl2 + H2O → CH3CHO (1) (2) (3) (4)

C2H4 + 1/2 O2

Wacker Oxidation Reaction

• Why is it called the Wacker Oxidation?
-Smidt and co-workers discovered the reaction at Consortium Fur Elecktrochemie Industrie G.M.BH., Munich, Germany (a subsidiary of Wacker Chemie)

• Industrial Importance:
- Plants with productioncapacity of 15,000 tons of acetaldehyde per year have been developed

Introduction
Rate Equation:
-d[olefin] = Rate = dt k [ PdCl4-2 ] [olefin] [ H+ ] [ Cl
- ]2

-First order in Pd and alkene -Chloride squared inhibition term -Proton inhibition term
PdCl3(C2H4)1- + Cl1PdCl2(C2H4)(H2O) + Cl11/[Cl-] term 1/[Cl-] term

Chloride Inhibition:
PdCl42+ C2H4 H 2O K1 K2 PdCl3(C2H4)1- +

ProtonInhibition:
Pd CH2 Cl OH2 Ka H 2C Pd CH2 OH H Slow H 2O Cl H 2C H2O K OH Cl Pd OH2 Cl outer sphere (trans) H

Slow CH3CHO

Cl Cl

Cl OH Pd OH2 Cl inner sphere (cis)

Fast

Early Evidence for Inner-Sphere Mechanism: KIEs
• Observed Kinetic Isotope Effects
C2D4 C2H4 + + PdCl42- + PdCl42- + H2O H 2O CD3CDO + Pd(0) + 2 HCl + 2 ClCH3CHO + Pd(0) + 2 HCl + 2 ClkD kH

Observed KIE = kH / kD =1.07

• Competitive isotope effect:
D H D + PdCl4 H
2-

+

H2O

H+ H2O

D D Cl2(OH2)Pd C C OH H H

CH2DCDO

Competitive isotope effect = kH / kD = 1.9

CHD2CHO

• No deuterium incorporation is observed when reaction is run in D2O

GROUP EXERCISE

Early Evidence for Inner-Sphere Mechanism: KIEs
• Inner-sphere was initially proposed based on observed kinetics in combinationwith kinetic isotope effect (circa 1964)
H2C H2O K Proton Inhibition OH Cl Pd OH2 Cl

Pd CH2 Cl OH2 Proton Inhibition Cl Cl Ka H 2C Pd CH2 OH

Cl

H RDS CH3CHO

RDS H H2O

Cl OH Pd OH2 Cl inner sphere (cis)

Fast

• Outer-sphere was disfavored because RDS is after hydroxypalladation reasonable to assume a KIE would be observed • Stereochemical probe of the reaction is neededEvidence For Outer-Spere Addition: Stille Study
• Trapping hydropalladated intermediate with CO
O H2O Pd Cl Cl Na2CO3 HO Pd Cl 2 CO C O

Problem: Sterically, syn addition would be disfavored with a chelating diolefin
D H D PdCl2 H 2 H2O Cl Pd H D CO retentive insertion D

NaOAc CH3CN

H

D

2

H

COPdL3 HD

HO

HO

Problems: -Solvent is CH3CN not water -Previous studies haveshown that dimeric Pd(II) complexes were prone to anti addition -Strong CO binds Pd strongly and could prevent coordination of H2O
H D H O O Trans stereochemistry confirmed by1H-NMR coupling constant D

Evidence for Outer-Spere: Backvall Study
• Product distribution depends on chloride and CuCl2 concentrations:
[Cl-] < 1M [CuCl2] < 1M H 2O Cl Cl H 2C Pd CH2 OH2 [Cl-] > 3 M [CuCl2] > 2.5 M O2 HOO2

H2C CH2 +

PdCl42-

CH3CO Stereochemistry is lost

CH3CO

Cl

• Major Assumptions:
-Chlorohydrin and ethanal are formed from the same intermediate: (PdCl2(H2O)(C2H4))

Stereochemistry is retained (potentially)

- Steric course of the reaction is not altered by running at higher chloride and cupric chloride concentrations

Evidence for Outer-Spere: Backvall Study
•...