Homogeneous Catalysis (eBook)

Table of contents 5
Preface 11
Acknowledgments 13
INTRODUCTION 14
1.1 Catalysis 14
1.2 Homogeneous catalysis 19
1.3 Historical notes on homogeneous catalysis 20
1.4 Characterisation of the catalyst 21
1.6 Ligands according to donor atoms 33
ELEMENTARY STEPS 42
2.1 Creation of a “vacant” site and co-ordination of the substrate 42
2.2 Insertion versus migration 43
2.3 ß-Elimination and de-insertion 48
2.4 Oxidative addition 49
2.5 Reductive elimination 52
2.6 a-Elimination reactions 54
2.7 Cycloaddition reactions involving a metal 55
2.8 Activation of a substrate toward nucleophilic attack 57
2.9 s-Bond metathesis 61
2.10 Dihydrogen activation 61
2.11 Activation by Lewis acids 63
2.12 Carbon-to-phosphorus bond breaking 65
2.13 Carbon-to-sulfur bond breaking 68
2.14 Radical reactions 70
KINETICS 75
3.1 Introduction 75
3.2 Two-step reaction scheme 75
3.3 Simplifications of the rate equation and the ratedetermining step 76
3.4 Determining the selectivity 80
3.5 Collection of rate data 83
3.6 Irregularities in catalysis 84
HYDROGENATION 86
4.1 Wilkinson's catalyst 86
4.2 Asymmetric hydrogenation 88
4.3 Overview of chiral bidentate ligands 97
4.4 Monodentate ligands 101
4.5 Non-linear effects 104
4.6 Hydrogen transfer 105
ISOMERISATION 112
5.1 Hydrogen shifts 112
5.2 Asymmetric Isomerisation 114
5.3 Oxygen shifts 116
CARBONYLATION OF METHANOL AND METHYL ACETATE 119
6.1 Acetic acid 119
6.2 Process scheme Monsanto process 124
6.3 Acetic anhydride 126
6.4 Other systems 128
COBALT CATALYSED HYDROFORMYLATION 135
7.1 Introduction 135
7.2 Thermodynamics 136
7.3 Cobalt catalysed processes 136
7.4 Cobalt catalysed processes for higher alkenes 138
7.5 Kuhlmann cobalt hydroformylation process 140
7.6 Phosphine modified cobalt catalysts: the Shell process 141
7.7 Cobalt carbonyl phosphine complexes 142
RHODIUM CATALYSED HYDROFORMYLATION 149
8.1 Introduction 149
8.2 Triphenylphosphine as the ligand 151
8.3 Diphosphines as ligands 163
8.4 Phosphites as ligands 171
8.5 Diphosphites 173
8.6 Asymmetric Hydroformylation 176
ALKENE OLIGOMERISATION 185
9.1 Introduction 185
9.2 Shell-Higher-Olefins-Process 186
9.3 Ethene trimerisation 194
9.4 Other alkene oligomerisation reactions 197
ALKENE POLYMERISATION 201
10.1 Introduction to polymer chemistry 201
10.2 Mechanistic investigations 209
10.3 Analysis by 13C NMR spectroscopy 212
10.4 The development of metallocene catalysts 216
10.5 Agostic interactions 222
10.6 The effect of dihydrogen 224
10.7 Further work using propene and other alkenes 225
10.8 Non-metallocene ETM catalysts 230
10.9 Late transition metal catalysts 232
HYDROCYANATION OF ALKENES 239
11.1 The adiponitrile process 239
11.2 Ligand effects 243
PALLADIUM CATALYSED CARBONYLATIONS OF ALKENES 248
12.1 Introduction 248
12.2 Polyketone 248
12.3 Ligand effects on chain length 265
12.4 Ethene/propene/CO terpolymers 271
12.5 Stereoselective styrene/CO copolymers 272
PALLADIUM CATALYSED CROSS-COUPLING REACTIONS 280
13.1 Introduction 280
13.2 Allylic alkylation 282
13.3 Heck reaction 290
13.4 Cross-coupling reaction 295
13.5 Heteroatom-carbon bond formation 299
13.6 Suzuki reaction 303
EPOXIDATION 308
14.1 Ethene and Propene oxide 308
14.2 Asymmetric epoxidation 310
14.3 Asymmetric hydroxylation of alkenes with osmium tetroxide 317
14.4 Jacobsen asymmetric ring-opening of epoxides 323
14.5 Epoxidations with dioxygen 325
OXIDATION WITH DIOXYGEN 328
15.1 Introduction 328
15.2 The Wacker reaction 329
15.3 Wacker type reactions 333
15.4 Terephthalic acid 336
15.5 PPO 341
ALKENE METATHESIS 346
16.1 Introduction 346
16.2 The mechanism 348
16.3 Reaction Overview 352
16.4 Well-characterised tungsten and molybdenum catalysts 353
16.5 Ruthenium catalysts 355
16.6 Stereochemistry 358
16.7 Catalyst decomposition 359
16.8 Alkynes 361
16.9 Industrial applications 363
ENANTIOSELECTIVE CYCLOPROPANATION 367
17.1 Introduction 367
17.2 Copper catalysts 368
17.3 Rhodium catalysts 372
HYDROSILYLATION 378
18.1 Introduction 378
18.2 Platinum catalysts 380
18.3 Asymmetric palladium catalysts 385
18.4 Rhodium catalysts for asymmetric ketone reduction 387
C–H FUNCTIONALISATION 393
19.1 Introduction 393
19.2 Electron-rich metals 395
19.3 Hydrogen transfer reactions of alkanes 400
19.4 Borylation of alkanes 401
19.5 The Murai reaction 402
19.6 Catalytic s-bond metathesis 403
19.7 Electrophilic catalysts 403
Subject index 408

Chapter 13
PALLADIUM CATALYSED CROSS-COUPLING REACTIONS (p. 271-272)

The new workhorse for organic synthesis

13. PALLADIUM CATALYSED CROSS-COUPLING REACTIONS

13.1 Introduction

The making of carbon-to-carbon bonds from carbocations and carbanions is a straightforward and simple reaction. Easily accessible carbanions are Grignard reagents RMgBr and lithium reagents RLi. They can be conveniently obtained from the halides RBr or RCl and the metals Mg and Li. They are both highly reactive materials, for instance with respect to water. The thermodynamic driving force for the formation of such reactive materials and their subsequent reactions is the formation of metal halides. The reactions of these carbon centred anions with polar compounds such as esters, ketones, and metal chlorides are indeed very specific and give high yields. The reaction of Grignard reagents with alkyl or aryl halides, however, is extremely slow giving many side-products, if anything happens at all. Note that this is also the key to the success of preparing Grignard type reagents(!), otherwise the partially formed RMgBr would react with the starting material RBr still present to give the "homocoupled" R-R. Exceptions are allylic and benzylic halides which react very fast amongst themselves during their synthesis. The Grignard reagents of this structure require specific practical procedures otherwise the homocoupled species are formed.

Reactions that can be expected for the reaction of an alkyl halide and a metal alkyl are depicted in Figure 13.1. The reaction may require several days at room temperature or may proceed in a few minutes, depending on the nature of the species. Many by-products may be formed. First a metal-halide exchange may occur. The resulting exchange products can give coupling products as well. Secondly, elimination reactions instead of C-C coupling can occur. Also, a radical reaction may take place. In summary, the yield and selectivity of this simple reaction will be surprisingly low. Only if a Grignard reagent is used in a coupling reaction with compounds that contain electrophilic carbon atoms, such as esters, ketones, and hetero-atom halides, the direct use of Grignard reagents (and related reagents) leads to high coupling efficiencies.

Figure 13.1. Products formed in a coupling reaction of a Grignard reagent and an alkyl halide

Thus, this reaction was of limited practical value until the transition metal catalysed cross-coupling reaction became known. Ever since, the "cross- coupling" reaction has found wide application in organic synthesis both in the laboratory and in industry. One might state that in any multi-step sequence for making an organic chemical, one of the steps involves a transition metal catalysed coupling reaction! The transition metal catalysts are usually based on palladium and sometimes nickel. In addition to organomagnesium and organolithium a great variety of organometallic precursors can be used. Also, many precursors can serve as starting materials for the carbocation. Last but not least, the ligand on the transition metal plays an important role in determining the rate and selectivity of the reaction. Here we will present only the main scheme and take palladium as the catalyst example, although many more metals have been found to be very useful. The reactions to be discussed are: allylic alkylation, Heck reaction, cross-coupling, and Suzuki reaction, a variant of the latter. Initially the cross-coupling chemistry focussed on carbon-tocarbon bond formation but in the last decade it has become also extremely useful for making carbon-to-heteroatom bonds. The organyl halide (or other anion used) involves in general an aryl, vinyl, or allylic species.