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Transition metal reagents and catalysts 2000 tsuji

Transition Metal Reagents and Catalysts: Innovations in Organic Synthesis.
Jiro Tsuji
Copyright  2000 John Wiley & Sons, Ltd.
ISBNs: 0-471-63498-0 (HB); 0-471-56027-8 (PB)

Transition Metal Reagents and Catalysts


Transition Metal Reagents
and Catalysts
Innovations in Organic Synthesis

Jiro Tsuji
Emeritus Professor,
Tokyo Institute of Technology
Tokyo, Japan

JOHN WILEY & SONS, LTD

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CONTENTS

Preface
Abbreviations
1

Pioneering Industrial Processes Using
Homogeneous Transition Metal Catalysts
1.1 Carbonylation of Alkenes and Alkynes Catalysed by Metal
Carbonyls to Produce Aldehydes, Carboxylic Acids, Esters, and
Alcohols
1.2 Production of Polyethylene and Polypropylene by Ziegler±Natta
Catalysts
1.3 Production of Acetaldehyde from Ethylene by the Wacker Process
1.4 Preparation of Organotransition Metal Complexes

2

Basic Chemistry of Transition Metal Complexes and Their
Reaction Patterns
2.1 Formation of Transition Metal Complexes
2.2 Fundamental Reactions of Transition Metal Complexes;
Comparisons of Transition Metal-catalysed Reactions with
Grignard Reactions
2.2.1 Oxidative Addition
2.2.2 Insertion
2.2.3 Transmetallation
2.2.4 Reductive Elimination
2.2.5 Elimination of -Hydrogen and -Hydrogen
(Dehydrometallation)
2.2.6 Nucleophilic Attack on Ligands Coordinated to Transition
Metals
2.2.7 Termination of the Metal-promoted or catalysed Reactions
and a Catalytic Cycle
2.3 Effects of Ligands

xi
xiii
1

2
3
4
5

7
7

10
10
14
17
18
19
20
23
25


vi

3

Contents

Reactions of Organic Halides and Pseudohalides
3.1 Reaction Patterns of Aryl, Alkenyl and Benzyl Halides, and
Pseudohalides
3.2 Cross-coupling with Alkenes (Carbometallation of Alkenes)
3.2.1 Intermolecular Reactions
3.2.2 Intramolecular Reactions
3.3 Reactions with Alkynes
3.3.1 Cross-coupling with Terminal Alkynes to Form Alkenyland Arylalkynes
3.3.2 Reactions of Internal and Terminal Alkynes via Insertion
3.4 Cross-coupling via Transmetallation
3.4.1 Magnesium Compounds
3.4.2 Zinc Compounds
3.4.3 Boron Compounds
3.4.4 Aluminium and Zirconium Compounds
3.4.5 Tin Compounds
3.4.6 Silicon Compounds
3.4.7 Chromium Compounds
3.4.8 Reactions with Dimetallic Compounds
3.5 Reactions with C, N, O, S and P Nucleophiles
3.6 Carbonylation and Reactions of Acyl Chlorides
3.6.1 Preparation of Carboxylic Acids and Their Derivatives
3.6.2 Preparation of Aldehydes and Ketones
3.6.3 Decarbonylation of Acyl Halides and Aldehydes
3.7 Preparation of Biaryls by the Coupling of Arenes with Aryl Halides
3.8 Hydrogenolysis with Hydrides

4

Reactions of Allylic Compounds
4.1 Catalytic and Stoichiometric Reactions of Allylic Compounds
4.2 Stoichiometric Reactions of %-Allyl Complexes
4.2.1 Reactions of Electrophilic %-Allyl Complexes
4.2.2 Reactions of Nucleophilic %-Allyl Complexes
4.3 Catalytic Reactions of Allylic Compounds
4.3.1 Allylation of Nucleophiles
4.3.2 Allylation of C, N and O Nucleophiles
4.3.3 Amphiphilic Bis-%-allylpalladium
4.3.4 Reactions via Transmetallation
4.3.5 Carbonylation
4.3.6 Insertion of Alkenes and Alkynes
4.3.7 Hydrogenolysis
4.3.8 Allyl as a Protecting Group and its Deprotection
4.3.9 Preparation of Conjugated Dienes by 1,4-Elimination
4.3.10 Pd-catalysed Reactions of Allyl -Keto Carboxylates,
Malonates, and Enol Carbonates
4.3.11 Allylic Rearrangement and Isomerization

27
27
33
33
40
46
46
50
56
57
59
62
67
68
72
73
76
79
85
86
90
92
94
96

109
109
111
111
113
116
116
119
127
128
134
137
139
142
146
151
157


Contents

5

Reactions of Conjugated Dienes
5.1 Formation of Cyclic Oligomers by Cycloaddition
5.2 Formation of Linear Oligomers and Telomers
5.3 Bis-metallation, Carbometallation and Hydrometallation

6

Reactions of Propargylic Compounds
6.1 Classification of Catalytic Reactions Based on Mechanistic
Consideration
6.2 Reactions via Insertion to the Palladium±sp2 Carbon Bond (Type I)
6.2.1 Reactions of Alkenes and Alkynes
6.2.2 Carbonylation
6.3 Reactions via Transmetallation and Related Reactions (Type II)
6.4 Reactions with C, O and N Nucleophiles (Type III)
6.5 Hydrogenolysis with Formate and Other Hydrides to Give Allenes
and Internal Alkynes (Types II and IV)
6.6 Elimination Reactions via Propargylpalladium Intermediates (Type IV)
6.7 Miscellaneous Reactions

7

8

Reactions of Alkenes and Alkynes

vii

169
169
179
189

199
199
202
202
203
211
213
219
223
223

227

7.1 Carbonylation
7.1.1 Preparation of Carboxylic Acids, Esters and Ketones
7.1.2 Preparation of Aldehydes (Hydroformylation) and Alcohols
7.1.3 Decarbonylation of Aldehydes
7.2 Cycloaddition Reactions
7.2.1 Cyclotrimerization and Cyclotetramerization of Alkynes
7.2.2 Formation of Five- and Six-membered Rings by the
Cocyclization of Two Molecules of Alkynes with Other
Unsaturated Compounds
7.2.3 Synthesis of Cyclopentenones by the Reaction of Alkyne,
Alkene and Carbon Monoxide (Pauson±Khand Reaction)
7.2.4 Reductive Cyclization of 1,6- and 1,7-Dienes, Diynes,
Enynes and Arenes via Zirconacycles and Titanacycles
7.2.5 Pd-catalysed Intramolecular Alder-ene Reaction of
1,6- and 1,7-Diynes and Enynes
7.2.6 Skeletal Reorganization of 1,6- and 1,7-Enynes Catalysed
by Pt, Ru and Pd catalysts
7.3 Coupling Reactions
7.4 Addition of Main Group Metal Compounds
7.4.1 Carbometallation
7.4.2 Metalametallation (Bis-metallation)
7.4.3 Hydrometallation
7.5 Hydroacylation of Alkenes and Alkynes

227
227
231
237
238
239

Synthetic Reactions via Transition Metal Carbene Complexes

305

8.1 Chemistry of Transition Metal Carbene Complexes
8.2 Catalytic Metatheses of Alkenes and Alkynes, and Their Synthetic

305

244
250
254
263
267
271
277
277
281
284
294


viii

Contents

8.3
8.4

8.5
8.6

9

Applications
8.2.1 Historical Background and Mechanism of Alkene Metathesis
8.2.2 Development of Catalysts for Metathesis
8.2.3 Classification of Alkene Metathesis
8.2.4 Synthetic Applications of Alkene Metathesis
8.2.5 Metathesis of Alkynes and Enynes
Carbonyl Alkenation Reactions via Carbene Complexes
Synthetic Reactions Using Carbene Complexes of Metal
Carbonyls as Stoichiometric Reagents
8.4.1 Reactions of Electrophilic Carbene Complexes of Cr, Mo,
W, Fe and Co
Rh and Pd-catalysed Reactions of Diazo Compounds via
Electrophilic Carbene Complexes
Other Reactions

Protection and Activation by Coordination
9.1 Protection and Activation of Alkenes by the Coordination of Iron
Carbonyls
9.2 Protection and Activation of 1,3-Dienes by the Coordination of Iron
Carbonyls
9.3 Protection and Activation of Alkynes by the Coordination of Cobalt
Carbonyl
9.4 Activation of Arenes and Cycloheptatrienes by Coordination of
Chromium Carbonyl and Other Metal Complexes
9.4.1 Reactions of Carbanions
9.4.2 Nucleophilic Substitution of Aromatic Chlorides
9.4.3 Lithiation of Aromatic Rings
9.4.4 Activation of Benzylic Carbons by Coordination
9.4.5 Steric Effect of Coordination
9.4.6 Asymmetric Synthesis using Chiral Cr(CO)3±arene
Complexes
9.4.7 Reactions of Cycloheptatriene Complexes
9.4.8 Activation of Arenes by the Coordination of an Osmium
Complex

10

11

Catalytic Hydrogenation, Transfer Hydrogenation
and Hyrosilylation

306
306
308
310
313
322
326
331
332
340
348

355
355
356
366
371
372
376
377
379
381
384
384
388

393

10.1 Homogeneous Hydrogenation of Alkenes
10.2 Asymmetric Reduction of Carbonyl and Imino Groups by
Homogeneous Hydrogenation, Transfer Hydrogenation and
Hydrosilylation

393

404

Reactions Promoted and Catalysed by Pd(II) Compounds

419

11.1 Oxidative Reactions of Alkenes
11.1.1 Reactions with Water

420
420


Contents

11.2
11.3
11.4

11.5
11.6
11.7

11.1.2 Reactions with Oxygen Nucleophiles
11.1.3 Reactions with Amines
11.1.4 Reactions with Carbon Nucleophiles
11.1.5 Oxidative Carbonylation
Difunctionalization of Conjugated Dienes
Reactions of Aromatic Compounds
Synthetic Reactions Based on the Chelation of Heteroatoms
11.4.1 ortho-Palladation of Aromatic Compounds
11.4.2 Reactions of Allylic Amines
Reactions of Alkynes
Oxidative Carbonylation Reactions
Reactions via Pd(II) Enolates

Index

ix
423
430
431
432
436
439
441
441
443
444
446
448

457


PREFACE

Use of transition metal compounds or complexes as catalysts or reagents in organic
synthesis is an exciting ®eld of research, and numerous novel reactions which are
impossible to achieve by conventional synthetic methods have already been
discovered. They are extensively employed in a wide range of areas of preparative
organic chemistry. Total syntheses of many complex molecules have been achieved
ef®ciently in much shorter steps, which was unbelievable ten years ago. Applications
of transition metal catalysts and reagents to organic synthesis are still being actively
investigated, and these days we can hardly open an organic chemistry journal that does
not contain examples of these reactions. Now the research on the application of
transition metal complexes to organic synthesis is in its golden age. Without doubt, in
the last decade, the introduction of transition metal catalysts and reagents has caused
revolutionary change in organic synthesis.
Today, the knowledge of organotransition metal chemistry is indispensible for
synthetic organic chemists. However, the organotransition metal chemistry is clearly
different mechanistically from traditional organic chemistry. I undertook to write this
book in order to give a birds-eye view of the broad ®eld of organotransition metal
chemistry applied to organic synthesis. I intended to give a better understanding of the
present arts of this chemistry to many synthetic organic chemists, who are not very
familiar with organotransition metal chemistry, but eagerly wish to apply transition
metal-catalyzed reactions to their synthetic works. I have tried to accomplish this task
®rst by giving a simple mechanistic explanation in chapter 2. Then a number of
important types of reactions classi®ed mainly by representative substrates such as
organic halides and allylic derivatives are surveyed with pertinent examples. For this
purpose, I cited many references; these were selected from a much larger number
which I have collected over the years. I wanted to make the book as comprehensive as
possible by selecting those references which reported original ideas and new reactions,
or evident synthetic utility. Synthetic utility is clearly biased towards catalytic rather
than stoichiometric reactions. The overall task of selecting good examples to include
was very dif®cult. It was done based on my own knowledge and understanding of the
chemistry, and hence there must be many signi®cant omissions. I apologize for the


xii

Preface

errors and incorrect citations which must inevitably be present in a book written by a
single author.
In 1997, I wrote a book with a similar title in Japanese, and the present book is an
expanded English edition. However, I replaced many old examples in the Japanese
edition with new ones and added many more in order to make the book up-to-date.
I wish to acknowledge valuable suggestions and corrections given by Professor H.
Nozaki who read the whole manuscript. I also thank my wife Yoshiko for her help
during the preparation of the manuscript.
Jiro Tsuji
Kamakura, Japan


ABBREVIATIONS

acac
Ar
atm
BBEDA
9-BBN
9-BBN-H
BINAP
Bn
Boc
BPPFA
BPPM
bpy
BQ
BSA
BTMSA
Bz
CAN
cat.
CDT
COD
COT
CTAB
Cp
Cy
DBA
DABCO
DBU
DCHPE
de
DEAD
DIPPP
DIOP

acetylacetonate
aryl
atmospheric pressure
N, NH -bis(benzylidene)ethylenediamine
9-borabicyclo[3.3.1]nonyl
9-hydroborabicyclo[3.3.1]nonane
2,2H -bis(diphenylphosphino)-1,1H -binaphthyl
benzyl
t-butoxycarbonyl
1-[1,2-bis(diphenylphosphino)ferrocenyl]ethyldimethylamine
(2S,4S)-N-t-butoxycarbonyl-4-(diphenylphosphino)-a-(diphenylphosphinomethyl)pyrrolidine
2,2H -bipyridyl; 2,2H -bipyridine
1,4-benzoquinone
N,O-bis(trimethylsilyl)acetamide
bis(trimethylsilyl)acetylene
benzoyl
ceric ammonium nitrate
catalyst
1,5,9-cyclododecatriene
1,5-cyclooctadiene
cyclooctatetraene
cetyltrimethylammonium bromide
cyclopentadienyl
cyclohexyl
dibenzylideneacetone
1,4-diazabicylo[2.2.2]octane
1,8-diazabicyclo[5.4.0]undec-7-ene
bis(dicyclohexylphosphino)ethane
disatereomeric excess
diethyl azodicarboxylate
bis(diisopropylphosphino)propane
2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane


xiv

Abbreviations

DMAD
DME
DMF
DMSO
DPEN
DPMSPP (TPPMS)
DPPB
DPPE
DPPF
DPPP
EBTHI
ee
EWG
HMPA
L
LDA
MA
Mes
NMP
MOM
MOP
Ms
MTO
NBD
Nf
Nu
OAc
phen
PhH
PhMe
PHMS
PMB
PMHS
PPFA
i-Pr-BPE
py
R
RCM
ROMP
Sia
TASF
TBAC
TBAF
TBDMS (TBS)
TCPC
TDMPP
Tf

dimethyl acetylenedicarboxylate
1,2-dimethoxyethane
dimethylformamine
dimethylsulfoxide
1,2-diphenylethylenediamine
diphenyl(m-sulfophenyl)phosphine
bis(diphenylphosphino)butane
bis(diphenylphosphino)ethane
1,1H -bis(diphenylphosphino)ferrocene
bis(diphenylphosphino)propane
ethylenebis(tetrahydroindene)
enantiomeric excess
electron-withdrawing group
hexamethylphosphoric triamide
any unidentate ligand, often Ph3P
lithium diisopropylamide
maleic anhydride
mesityl
N-methylpyrrolidone
methoxymethyl
monodentate optically active phosphine
methanesulfonyl (mesyl)
methyltrioxorhenium
norbornadiene
nonaflate
nucleophile
acetate anion
1,10-phenanthroline
benzene
toluene
poly(hydromethylsiloxane)
p-methoxybenzyl
polymethylhydrosiloxane
N,N-dimethyl-1,2-(diphenylphosphino)ferrocenylethylamine
1,2-bis(trans-2,5-diisopropylphospholano)ethane
pyridine
alkyl group
ring-closing metathesis
ring-opening metathesis polymerization
siamyl; sec-isoamyl, or 1,2-dimethylpropyl
tris(diethylamino)sulfonium difluoro(trimethyl)silicate
tetrabutylammonium chloride
tetrabutylammonium fluoride
tert-butyldimethylsilyl
2,3,4,5-tetrakis(methoxycarbonyl)palladacyclopentadiene
tri(2,6-dimethoxyphenyl)phosphine
trifluoromethanesulfonyl (triflyl)


Abbreviations
TFP
THP
TMPP
TMSPP (TPPTS)
TMEDA
TMM
TMS
o-Tol
Ts
TsOH
TTMPP
tu

xv

tri(2-furyl)phosphine
tetrahydropyran
trimethylolpropane phosphite, or 4-ethyl-2,6,7-trioxa-1-phosphobicyclo[2,2,2]octane,
tri(m-sulfophenyl)phosphine
N,N,N,N-tetramethyl-1,2-ethylenediamine
trimethylenemethane
trimethylsilyl
o-tolyl
tosyl, p-toluenesulfonyl
p-toluenesulfonic acid
tri(2,4,6-trimethoxyphenyl)phosphine
thiourea


Transition Metal Reagents and Catalysts: Innovations in Organic Synthesis.
Jiro Tsuji
Copyright  2000 John Wiley & Sons, Ltd.
ISBNs: 0-471-63498-0 (HB); 0-471-56027-8 (PB)

1

PIONEERING INDUSTRIAL
PROCESSES USING
HOMOGENEOUS
TRANSITION METAL
CATALYSTS

Some main group metals have long been used for organic synthesis. Most importantly
Grignard reagents, introduced in the beginning of the twentieth century, have
widespread application in organic synthesis. Compared with the extensive use of
Grigrard reagents and other main group metal reagents, transition metal compounds
have received little attention from synthetic organic chemists. Their application as
catalysts to organic syntheses started much later. The application of transition metal
compounds, particularly as homogeneous catalysts, was initiated by the following
three industrial processes and related reactions, developed from the 1930s to 1950s
without an understanding of their mechanisms:
1. Carbonylation of alkenes and alkynes catalysed by metal carbonyls, typically
Co2 …CO†8 Y Ni…CO†4 and Fe…CO†5 to produce aldehydes, carboxylic acids, esters,
and alcohols.
2. Production of polyethylene and polypropylene by the Ziegler±Natta catalysts,
prepared from Ti chlorides and organoalanes.
3. Production of acetaldehyde from ethylene by the Wacker process using PdCl2 and
CuCl2 as catalysts.
These commercial processes led to the development of other synthetic reactions
catalyzed by transition metal complexes. The impact and effect of these processes on
organic synthesis are surveyed brie¯y.


2

Pioneering industrial processes

1.1 Carbonylation of Alkenes and Alkynes Catalysed by
Metal Carbonyls to Produce Aldehydes, Carboxylic
Acids, Esters and Alcohols
In 1925, Fischer and Tropsch developed a process for producing a mixture of saturated
and unsaturated hydrocarbons, and oxygenated products such as alcohols and esters by
the reaction of synthesis gas (a mixture of CO and H2 ) using heterogeneous catalysts
of Fe and Co (eq. 1.1) [1].

This process is called the Fischer±Tropsch process and attracted attention as an
important method of commercial production of synthetic oil. In 1938 RoÈlen, one of
Fischer's co-workers, tried the reaction of alkene with synthesis gas using a Co catalyst
and found the formation of aldehydes. This reaction is now called the oxo reaction or
hydroformylation, because hydrogen and a formyl group add to an alkene bond [2,3].
At present, butyraldehyde is produced by the hydroformylation of propylene on a large
scale. 2-Ethyl-1-hexanol is produced by aldol condensation of butyraldehyde and
subsequent hydrogenation of the resulting enal. Bis(2-ethylhexyl) phthalate is utilized
as a plasticizer for poly(vinyl chloride). At ®rst, Co2 …CO†8 was used as the catalyst
under homogeneous conditions. Then an Rh complex was found to be a more ef®cient
catalyst. Rh is much more active, and hence its high cost is easily offset.
HRh…CO†…Ph3 P†3 or RhCl…Ph3 P†3 is the catalyst precursor (eq. 1.2).

In the 1930s, the Reppe group developed commercial processes for the production
of carboxylic acids and esters by the carbonylation of alkynes and alkenes using metal
carbonyls [4]. In particular, an industrial process for producing acrylic acid or ester by
the carbonylation of highly explosive acetylene, catalysed by extremely toxic Ni…CO†4 ,
was established (eq. 1.3).

Another commercial process, for 1-butanol production by reductive carbonylation
of propylene with water, catalysed by Fe…CO†5, was developed by the Reppe group (eq.
1.4).

The discovery of these carbonylation processes enabled the industrial production of
aldehydes, carboxylic acids or esters, and alcohols from alkenes and alkynes using Fe,


Polyethylene and polypropylene by Ziegler±Natta catalysts

3

Co and Ni carbonyl catalysts. These processes have stimulated development of
carbonylation as an important unit reaction, catalysed by transition metal compounds.
Later, in the 1970s, a new commerial process for AcOH by the Rh-catalyzed
carbonylation of MeOH in the presence of HI, the Monsanto process, was developed
(eq. 1.5) [5]. It is signi®cant that MeOH, which is a saturated compound, was shown to
be carbonylated via MeI in this process, rather than unsaturated substrates such as
alkenes and alkynes.

1.2 Production of Polyethylene and Polypropylene by
Ziegler±Natta Catalysts
Ziegler started his research on organolithium compounds before the second world war
and synthesized n-BuLi for the ®rst time. After the war, his research extended to
organoaluminium compounds and he synthesized Et3 Al. Then a commercial process
for producing higher alcohols by the oligomerization of ethylene using Et3 Al was
developed. During this research the famous Ni effect on the reaction of Et3 Al with
ethylene was discovered [6], namely that 1-butene is formed selectively and no
oligomers are produced by the reaction of ethylene with Et3 Al, when a small amount
of a Ni compound is present in the reaction system (eq. 1.6).

The discovery of the Ni effect led to the invention of polyethylene production
catalysed by TiCl4 combined with Et3 Al, the so-called the Ziegler catalyst, in 1953.
Soon after, the process for isotactic polypropylene was invented by Natta using a
slightly modi®ed catalyst prepared from TiCl3 and Et3 Al, which is called the Natta
catalyst (eq. 1.7) [7].

The impact of Ziegler±Natta catalysis was enormous. The combination of TiCl4 as
a transition metal compound with Et3 Al as a main group metal compound opened the
possibility for transmetallation. This is one of the most important unit reactions in
transition metal-catalysed reactions. Also, production of isotactic polypropylene is a
harbinger of stereocontrolled reactions catalysed by transition metal complexes,
leading ®nally to asymmetric catalysis.
The Ziegler±Natta chemistry was extended to the polymerization of butadiene to
produce polybutadiene using similar catalysts. However, Wilke found that cyclic


4

Pioneering industrial processes

oligomers, such as COD and CDT, rather than linear polybutadiene, are formed by
changing ratios of TiCl4 and Et3 Al [8]. Several years before Wilke's discovery, Reed
gave the ®rst report on the formation of COD from butadiene using a catalyst derived
from Ni…CO†4 [9]. However, its catalytic activity was lower due to the strong
coordination of CO. Wilke subsequently found that naked Ni(0) or Ni(0) complexes of
phosphines are active catalysts for cyclodimerization and trimerization. Based on these
discoveries, the chemistry of p±allylnickel was developed.
The Reppe group reported in 1948 the formation of benzene and cyclooctatetraene
by Ni-catalysed cyclotrimerization and cyclotetramerization of acetylene [10].
The Ni-catalyzed cyclizations of butadiene and acetylene opened a fruitful ®eld of
cycloaddition of various unsaturated compounds to afford various cyclic compounds.
These cyclizations are now understood by the formation of metallacycles as
intermediates (eq. 1.8).

Alkene metathesis, a remarkable reaction catalyzed by transition metal catalysts,
can be traced back to Ziegler±Natta chemistry as its origin [11]. In 1964, Natta et al.
reported a new type polymerization of cyclopentene using Mo- or W-based catalyst,
without knowing the mechanism. This was the ®rst example of ring-opening
metathesis polymerization (ROMP; eq. 1.9) [12].

1.3 Production of Acetaldehyde from Ethylene by the
Wacker Process
For long time, Pd has been used mainly as a heterogeneous catalyst for the
hydrogenation of unsaturated bonds. A revolution in Pd chemistry occurred with the
development of homogeneous Pd catalysts. The ®rst example was the invention of the
Wacker process in 1959, by which ethylene is oxidized to acetaldehyde using PdCl2
and CuCl2 as catalysts in aqueous solution (eq. 1.10) [13].

In 1894 Philips found that, when ethylene is passed into an aqueous solution of
PdCl2 , Pd(II) is reduced to Pd(0), which precipitates as black powder [14] and this


References

5

reaction was used for quantitative analysis of Pd(II) [15]. Ethylene is oxidized to
acetaldehyde at the same time. This reaction is the basis of the Wacker process.
Soon after the invention of the Wacker process the formation of vinyl acetate by the
oxidative acetoxylation of ethylene using Pd…OAc†2 was discovered by Moiseev [16],
and the industrial production of vinyl acetate based on this reation was developed. At
present, vinyl acetate is produced commercially by a gas-phase reaction of ethylene,
acetic acid and O2 using Pd catalyst supported on alumina or silica (eq. 1.11).

1.4 Preparation of Organotransition Metal Complexes
In 1951, ferrocene was synthesized by Pauson [17] and Miller [18]. Soon after this
synthesis, two groups led by Wilkinson and Fischer, independently reported that
ferrocene has a stable carbon±iron p-bond [19]. This was the ®rst example of a true
organotransition metal complex containing a carbon±metal bond. Since then,
numerous organotransition metal complexes have been prepared. The importance of
these complexes as intermediates of many synthetic reactions has been discovered.
More importantly, some transition metal complexes were found to behave as
precursors of active catalysts.
The industrial processes and related reactions described above, combined with the
progress of organometallic chemistry, have stimulated further remarkable development
in applying transition metal complexes to organic synthesis. Various novel synthetic
methods, which are impossible by conventional means, have been discovered, bringing
revolution in organic synthesis.

References
1. C. Masters, Adv. Organometal. Chem., 17, 61 (1979).
2. R. L. Pruett, Adv. Organometal. Chem., 17, 1 (1979).
3. B. Cornils, W. A. Hermann and M. Basch, Angew. Chem., Int. Ed. Engl., 33, 2144 (1994);
J. T. Morris, Chem. in Britain, 38 (1993).
4. W. Reppe, Liebigs Ann. Chem., 582, 1 (1953); W. Reppe and H. KroÈper, Liebigs Ann.
Chem., 582, 38 (1953); Review of a historical account of chemistry of metal carbonyls, L.
Mond, C. Langer and F. Quincke, J. Organometal. Chem., 383, 1 (1990).
5. J. F. Roth, J. H. Caddock, A. Hershman and F. E. Paulik, Chem Tech., 347 (1971).
6. K. Ziegler, H. G. Gellert, E. Holzkamp, G. Wilke, E. W. Duck and W. R. Kroll, Liebigs
Ann. Chem., 629, 172 (1960); K. Fischer, K. Jonas, P. Misbach, R. Stabba and G. Wilke,
Angew. Chem., Int. Ed. Engl., 12, 943 (1973).
7. K. Ziegler and G. Natta, Angew. Chem. Int. Ed. Engl., 76, 545 (1964).
8. G. Wilke, J. Organometal. Chem., 200, 349 (1980). Angew. Chem., Int. Ed. Engl., 27, 186
(1988).
9. H. W. B. Reed, J. Chem. Soc., 1931 (1954).
10. W. Reppe, O. Schlichting, K. Klager and T. Toepel, Liebigs Ann. Chem., 560, 1 (1948).


6

Pioneering industrial processes

11. N. Calderon, J. P. Lawrence and E. A. Ofstead, Adv. Organometal. Chem., 17, 449 (1979);
N. Calderon, H. Y. Chen and K. W. Scott, Tetrahedron Lett., 3327 (1967).
12. G. Natta, G. Dall'Asta and G. Mazzanti, Angew, Chem., Int. Ed. Engl., 3, 723 (1964).
13. J. Smidt, W. Hafner, R. Jira, R. Sieber, J. Sedlmeier and A. Sabel, Angew. Chem., Int. Ed.
Engl., 1, 80 (1962).
14. F. C. Philips, Am. Chem. J., 16, 255 (1894).
15. S. C. Ogburn and W. C. Brastow, J. Am. Chem. Soc., 55, 1307 (1933).
16. I. I. Moiseev, M. N. Vargaftik and Ya. K. Syrkin, Dokl. Akad. Nauk SSSR, 133, 377 (1960).
17. T. J. Kealy and P. L. Pauson, Nature, 168, 1039 (1951).
18. S. A. Miller, J. A. Tebboth and J. F. Tremaine, J. Chem. Soc., 632 (1952).
19. G. Wilkinson, J. Organometal. Chem., 100, 273 (1975).


Transition Metal Reagents and Catalysts: Innovations in Organic Synthesis.
Jiro Tsuji
Copyright  2000 John Wiley & Sons, Ltd.
ISBNs: 0-471-63498-0 (HB); 0-471-56027-8 (PB)

2

BASIC CHEMISTRY OF
TRANSITION METAL
COMPLEXES AND THEIR
REACTION PATTERNS

Organic reactions involving transition metal compounds proceed via complex
formation; that is, coordination of a reactant molecule to a low-valent transition
metal is essential for the reaction to occur. In order to explain how synthetic reactions
involving transition metal complexes proceed, it is important to understand the
fundamental behaviour of complexes and their reaction patterns.

2.1 Formation of Transition Metal Complexes
Metallic Mg is used for the preparation of Grignard reagents. However, transition
metal complexes formed by the coordination of ligands (L) to metals are used for
synthetic reactions. The transition metal itself is used rarely. The change of properties
of transition metals, brought about by complex formation, is considerable. For
example, metallic Ni has a very high melting point and is insoluble in organic solvents,
whereas Ni(CO)4 is a volatile, extremely toxic liquid (b.p. 43  C) and is soluble in
organic solvents [1]. Although Pd and Pt are stable noble metals, their complexes
Pd…Ph3 P†4 and Pt…Ph3 P†4 are greenish-yellow crystals and soluble in organic solvents.
Four molecules of CO coordinate to Ni to form Ni(CO)4 , but Ni(CO)5 is never
formed. The stoichiometry of complex formation can be understood by the 18-electron
rule. According to this rule, a stable complex with an electron con®guration of the next
highest noble gas is obtained when the sum of d electrons of metals and electrons
donated from ligands equals 18. Complexes that obey the 18-electron rule are said to


8

Basic chemistry of transition metal complexes

be coordinatively saturated. Ni(0) has the following ground state electronic structure,
and forms complexes using 3d8 and 4s2 electrons equally (10 d electrons, or d10 ):
1s2 2s2 2p6 3s2 3p6 3d 8 4s2
Similarly, Pd(0) and Pt(0) form complexes using their d10 electrons. The numbers of
d electrons of major transition metals used for the complex formation are shown in
Table 2.1. Coordinatively saturated complexes are formed by the donation of electrons
from the ligands until total numbers of the electrons reach 18.
Well-known complexes that obey the 18-electron rule are shown below. Typical
ligands, such as CO, phosphine and alkenes, donate two electrons each. The total
number of d electrons of Ni(CO)4 can be calculated as 10 ‡ …2  4† ˆ 18, and hence
Ni(CO)5 cannot be formed. In Co2 …CO†8 , the number of d electrons from Co(0) is nine
and four CO molecules donate eight electrons. Furthermore, a Co±Co bond is formed
by donating one electron each. Therefore, the total is 9 ‡ 8 ‡ 1 ˆ 18 electrons, to
satisfy the 18-electron rule. The relationship between the coordination numbers and
numbers of d electrons of metal carbonyls is shown in Tables 2.2 and 2.3.
Number of electrons in ferrocene 1 can be counted in the following way. In
ferrocene, Fe is Fe(II) and has six d-electrons. The cyclopentadienyl anion donates six
electrons (2 Â 2 from two double bonds and two electrons from the anion), and
6 ‡ …4  2† ‡ …2  2† ˆ 18 electrons satisfy the rule. In another calculation Fe,
regarded as Fe(0), offers eight electrons and the cycloptendienyl radical supplies one
electron. Therefore, total electron count is 8 ‡ …4  2† ‡ …1  2† ˆ 18.

In bis-p-allylnickel 2, Ni(II) has 8e and the two allyl anions supply four electons
each: 8 ‡ …4  2† ˆ 16. The following calculation is also possible: if Ni(0) supplies
10e and the two allyl radicals supply three electrons each, the total number is
10 ‡ …3  2† ˆ 16. Therefore, this complex is coordinatively unsaturated.

Table 2.1

Numbers of d electrons of transition metals

Valency

Cr
Mo
W

Mn
Tc
Re

Fe
Ru
Os

Co
Rh
Ir

Ni
Pd
Pt

0
1
2
3

6
5
4
3

7
6
5
4

8
7
6
5

9
8
7
6

10
9
8
7


Formation of transition metal complexes
Table 2.2

Complexes that obey the 18-electron rule

Complex

Coordination
number

Number of
electrons

Pd…PPh3 †4

Pd(0)
4PPh3

Fe…CO†5

Fe(0)
4 CO

Mo…CO†6

Mo(0)
6 CO

Ni…CO†4

Ni(0)
4 CO

Co2 …CO†8

Co(0)
4 CO
Co±Co

10e
8e
18e
10e
8e
18e
6e
10e
18e
10e
8e
18e
9e
8e
1e
18e

Table 2.3

Number of
d electrons
6
8
10

9

Numbers of d electrons and
cordination numbers of metal
carbonyls
Coordination
number

Examples

6
5
4

Cr…CO†6
Fe…CO†5
Ni…CO†4

When a reaction of an organic compound (either promoted or catalysed by a
transition metal complex) occurs, the reactant must coordinate to the metal. For
coordination of the reactant to the metal prior to its reaction, the complex must be
coordinatively unsaturated so as to offer a vacant site in order to make the coordination
of the reactants possible. Therefore, Ni(CO)4 or Fe(CO)5 should be made
coordinatively unsaturated by liberating some of the coordinated CO by heating or
irradiation. When Pd…Ph3 P†4 is used as a catalyst, it becomes an unsaturated complex
by liberating two molecules of Ph3 P in solution. Furthermore, in this complex, Ph3 P is
a kind of `innocent' or `spectator' ligand, which does not take part in synthetic
reactions directly, but which modi®es the reactivity of the metal. Various phosphines,
namely arylphosphines, alkylphosphines, and bidentate phosphines, acting as innocent
ligands, have different steric effects and electron-donating abilities, and electron
density of the central metal changes depending on the kind of the ligand involved.
Thus in many cases different catalytic activity is observed by the same metal
depending on the innocent ligands. From this consideration, clearly subtle design of


10

Basic chemistry of transition metal complexes

the best complexes suitable for a desired reaction becomes important, although it is not
always easy to do this.

2.2 Fundamental Reactions of Transition Metal
Complexes; Comparison of Transition Metalcatalysed Reactions with Grignard Reactions
Six fundamental reactions of transition metal complexes are brie¯y explained in order
to demonstrate how reactions either promoted or catalysed by transition metal
complexes proceed. In the reaction schemes throughout this book, some of the
spectator ligands that do not participate in the reactions are omitted for simplicity.

2.2.1

Oxidative Addition

The term `oxidative' may sound strange to organic chemists who are not familiar with
organometallic chemistry. The use of this term in organometallic chemistry has a
meaning different from the `oxidation' used in organic chemistry, such as the oxidation
of secondary alcohols to ketones. Thus, oxidative addition means the reaction of a
molecule X±Y with a low-valent coordinatively unsaturated metal complex M…n†Lm
under bond cleavage, forming two new bonds 3. As two previously nonbonding
electrons of the metal are involved in the new bonding, the metal increases its formal
oxidation state by two units, namely, M…n† is oxidized to M…n ‡ 2†, and increases the
coordination number of the metal centre by two. In oxidative addition, it is de®ned that
the electrons in the two new bonds belong to the two ligands, and not to the metal.
This process is similar to the formation of Grignard reagents 4 from alkyl halides
and Mg(0). In the preparation of Grignard reagents, Mg(0) is oxidized to Mg(II) by the
oxidative addition of alkyl halides to form two covalent bonds.

Another example, which shows clear difference between oxidation in organic
chemistry and oxidative addition in organometallic chemistry, is the oxidative addition
of a hydrogen molecule to M(n) to form M(n ‡ 2) dihydride 5. In other words, M(n) is
oxidized to M(n ‡ 2) with hydrogen. This sounds strange to organic chemists, because
hydrogen is a reducing agent in organic chemistry.


Fundamental reactions of transition metal complexes

11

Oxidative addition is facilitated by higher electron density of the metals and, in
general, s-donor ligands such as R3 P and HÀ attached to M facilitate oxidative
addition. On the other hand, p-accepter ligands such as CO and alkenes tend to
suppress oxidative addition.
A number of different polar and nonpolar covalent bonds are capable of undergoing
the oxidative addition to M(n). The widely known substrates are CÀX (X ˆ halogen
and pseudohalogen). Most frequently observed is the oxidative addition of organic
halides of sp2 carbons, and the rate of addition decreases in the order
CÀI b CÀBr bb CÀCl bbb CÀF. Alkenyl halides, aryl halides, pseudohalides,
acyl halides and sulfonyl halides undergo oxidative addition (eq. 2.1).

The following compounds with HÀC and HÀMH bonds undergo oxidative addition
to form metal hydrides. This is exampli®ed by the reaction of 6, which is often called
ortho-metallation, and occurs on the aromatic CÀH bond at the ortho position of such
donar atoms as N, S, O and P. Reactions of terminal alkynes and aldehydes are known
to start by the oxidative addition of their CÀH bonds. Some reactions of carboxylic
acids and active methylene compounds are explained as starting with oxidative
addition of their OÀH and CÀH bonds.

Metal±metal bonds MH ÀMH such as R2 BÀBR2 and R3 SiÀSiR3 undergo oxidative
addition, (where MH represents main group metals; eq. 2.2).


12

Basic chemistry of transition metal complexes

Oxidative addition involves cleavage of the covalent bonds as described above. In
addition, oxidative addition of a wider sense occurs without bond cleavage. For
example, p-complexes of alkenes 7 and alkynes 9 are considered to form Z2 complexes
8 and 10 by oxidative addition. Note that to specify the numbers of carbon atoms that
interact with the metal center, the pre®x Zn is used before the ligand formula to imply
bonding to n carbons. Two distinct metal±carbon bonds are formed, and the resulting
alkene and alkyne complexes are more appropriately described as the metallacyclopropane 8, and the alkyne complex 9 may be regarded as the matallacyclopropene 10.
Thus the coordination of the alkene and alkyne results in the oxidation of the metal.
The metallacyclobutane 11 is formed by the oxidative addition of cyclopropane with
bond cleavage.

Oxidative cyclization is another type of oxidative addition without bond cleavage.
Two molecules of ethylene undergo transition metal-catalysed addition. The
intermolecular reaction is initiated by p-complexation of the two double bonds,
followed by cyclization to form the metallacyclopentane 12. This is called oxidative
cyclization. The oxidative cyclization of the a,o-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative
cyclization of the a,o-enyne 15 affords the metallacyclopentene 16. Formation of
the ®ve-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be
understood by the formation of the metallacyclopropene or metallacyclopropane 17.
Then the insertion of alkyne or alkene to the three-membered ring 17 produces the
metallacyclopentadiene or metallacyclopentane 18.
The term oxidative cyclization is based on the fact that two-electron oxidation of the
central metal occurs by the cyclization. The same reaction is sometimes called
`reductive cyclization'. This term is based on alkene or alkyne bonds, because the
alkene double bond in 13 is reduced to the alkane bond 14, and the alkyne 15 bond is
reduced to the alkene bond 16 by the cyclization. Cyclizations of alkynes and alkenes
catalyzed by transition metal complexes proceed by oxidative cyclization. In particular,
low-valent complexes of early transition metals have a high tendency to obtain the
highest possible oxidation state, and hence they react with alkynes and alkenes
forming rather stable metallacycles by oxidative addition or oxidative cyclization.


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