The concerted mechanism of ligand migration requires a cis configuration of ..... 3 ,3 ,3 -trifluoropropylene (1.20) catalysed by dichlorobis(triphenylphosphine) ...
Palladium catalysed hydroformylation of alpha-olefins by Alta Ferreira
Dissertation in fulfilment of the requirements for the degree Magister Scientiae
in Chemistry
in the Faculty of Science
at the Rand Afrikaans University /
Supervisor: Prof. C.W. Holzapfel
March 2001
Acknowledgements I would like to extend my sincere gratitude to the following people:
My supervisor: Prof C. W. Holzapfel for sharing is knowledge about chemistry, but also for teaching me more about life. SASOL for their financial assistance. Dr. J.P. Steynberg for his continued interest in the project and Dr. P.J. Steynberg for sharing his knowledge. Linda van der Merwe and Ian Vorster for performing IVMR and mass spectra. Mr. H Bezuidenhout, Mr. C. van der Merwe and Mr R. Buker for their technical help. My fellow students for their friendship, a gift no one can do without. Elmien Marais for all her help and support in the lab. My parents for giving me the best in life and for their support and love. Paul and Lize for their support and love. Anneline, for being the best friend anyone can hope for. My Creator for leading me on the path which He has planned for me and for giving me the opportunity to live life to it's fullest.
Psalm 145:3: " Great is the LORD! He is most worthy of praise! His greatness is beyond discovery!"
Table of Contents
Abbreviations Synopsis
II
Samevatting
IV
Chapter 1: Palladium catalysed carbonylation and other related reactions 1.1 Introduction
1
1.2 Palladium as catalyst for carbonylation
2
1.3 Some mechanistic considerations
3
1.3.1 The carbon monoxide molecule
3
1.3.2 The carbonylation reaction
4
1.3.2.1 Oxidative addition
4
1.3.2.2 Insertion
5
1.3.2.3 Product formation step
5
1.3.2.3.1 Reductive elimination
6
1.3.2.3.2 P-Hydride elimination
6
1.3.2.3.3 Reductive displacement
6
1.4 Palladium catalysed carbonylation reactions
7
1.4.1 Olefins
7
1.4.1.1 Oxidative carbonylation
9
1.4.1.2 Copolymerisation
11
1.4.1.3 Hydroformylation
12
1.4.1.4 Alkoxycarbonylation
13
1.4.2
Reactions of conjugated dienes via n-allylpalladium complexes
14
1.4.3 Cyclic alkenes and aromatic compounds
15
1.4.4 Other compounds
16
1.4.4.1 Alcohols
16
1.4.4.2 Carbonylation reactions of alkenyl and aryl halides
18
1.4.4.3 Ketones
19
1.4.5 Palladium catalysed carbonylation reactions in biphasic media
20
1.4.6 Promising carbonylation reactions for the chemical industry
21
1.5 Conclusion
24
1.6 References
24
Chapter 2: Palladium catalysed hydroformylation of alpha-olefins 2.1 Background
28
2.2 Results and discussion
31
2.2.1 The effect of various anions on hydroformylation
32
2.2.2 The effect of solvents on hydroformylation
34
2.2.3 The effect of change in palladium source on hydroformylation
36
2.2.4 Effect of ligands on hydroformylation
38
2.2.4.1 Arylphosphine ligands
39
2.2.4.2 Alkylphosphine ligands
43
2.2.4.3 The effect of acid on the ligand and hydroformylation
46
2.2.5 Additives
49
2.2.6 Isomerisation
57
2.2.7 Ketones
60
2.2.8 Reinvestigation of the effect of acid on the rate of hydroformylation
63
2.2.9 A comparison between cobalt, rhodium and palladium
64
2.3 References
69
Chapter 3: Some mechanistic considerations with respect to hydroformylation 3.1 On the mechanism of the palladium-catalysed hydroformylation of a-olefins
70
3.1.1 Methoxycarbonylation of a-olefins
70
3.1.2 Ethene (alkene) — carbon monoxide copolymers
74
3.1.3 Polyketone and "dimeric ketone" formation
77
3.1.4 Conversion of alkenes to aldehydes by palladium catalysed hydroformylation 3.1.5 Alkene isomerisation during hydroformylation
81 83
3.1.6 Effect of solvents and acids on the rate of hydroformylation
84
3.1.7 Effects of ligands and additives on the hydroformylation process 3.2 Cobalt catalysed hydroformylation
85 90
3.2.1 Mechanism for the unmodified cobalt catalysed hydroformylation
90
3.2.2 Differences between unmodified and phosphine-modified cobalt catalysts 3.3 Rhodium catalysed hydroformylation
94 96
3.3.1 Mechanistic considerations for unmodified rhodium catalysed hydroformylation
96
3.3.2 Mechanistic considerations for phosphine-modified rhodium catalysed hydroformylation
97
3.4 A Mechanistic comparison between cobalt, rhodium and palladium catalysed hydroformylation
100
3.5 Conclusion
103
3.6 References
103
Chapter 4: Experimental
4.1 Introduction
106
4.2 Synthesis gas
106
4.2.1 Handling of synthesis gas
106
4.2.2 Impurities and purification
106
4.2.3 Hazards
107
4.3 Equipment
107
4.3.1 Reactors
107
4.3.2 Special connectors and valves
107
4.4 Gas Chromatography
108
4.5 Internal Standard method
111
4.6 GC-MS
112
4.7 Flowmeter
113
4.8 Experimental procedure
114
4.8.1 Materials
114
4.8.2 Batch autoclave procedure
115
A Palladium as catalyst
115
B Rhodium as catalyst
115
C Cobalt as catalyst
115
4.8.3 GC Analysis of hydroformylation products
116
4.9 Helpful suggestions in setting up a new high-pressure laboratory
117
4.10 References
118
Appendix
119
Abbreviations NMR
nuclear magnetic resonance
MO
molecular orbital
HOMO
highest occupied molecular orbital
LUMO
lowest unoccupied molecular orbital
Ar
aryl
DPPP
1,3,-bis(diphenylphosphino)propane
dba
dibenzylideneacetone
dppb
1,4-bis(diphenylphosphino)butane
dppe
1,2-bis(diphenylphosphino)ethane
PAA
phenyl acetic acid
ca.
approximately
TPP
triphenylphosphine
TFA
trifluoroacetic acid
HOTf
trifluoromethanesulfonic acid
GC
gas chromatograph
MS
mass spectrum
THE
tetrahydrofuran
EP
9-eicosy1-9-phosphabicyclo(3,3,1) nonane
LIM-1
2-phospha-2-octadecy1-4,8-dimethylbicyclo(3,3,1) nonane.
psi
pounds per square inch
TCD
thermal conductivity detector
FM
flame-ionisation detector
AFID
alkali flame-ionisation detector
TID
termionic detector
FPD
flame photometric detector
EI
electron ionisation
CI
chemical ionisation
I
Synopsis
The main objective of the research described in this dissertation, was the optimisation of the palladium catalysed hydroformylation of a-olefms. An evaluation of the efficiency of the palladium catalysed hydroformylation process required a comparison with the hydroformylation processes based on cobalt and rhodium. Variation of ligands (diphosphines of the size R2P(CH2)nPR2), solvents, acids, etc. had a dramatic effect on the products and the rate of the reaction. Trifluoroacetic acid was used to yield C-6 aldehydes from 1-pentene while trifluoromethanesulfonic acid yields C-11 ketones. Corresponding results were obtained with 1-octene as substrate. The length of the carbon bridge between the two phosphorous atoms has an optimum length of two in the case of alkylphosphine ligands, while an optimum length of three was found in the case of arylphosphine ligands. One disadvantage of the palladium catalysed hydroformylation reaction is that this reaction requires the use of bidentate phosphine ligands. These ligands are relatively expensive and also difficult to synthesise.
The instability of the palladium complex and thus the precipitation of palladium were one of the major obstacles that had to be overcome. The use of additives not only increased the rate of hydroformylation but also increased catalyst stability, which in turn allowed an increase in the reaction temperature. This further increased the rate of the palladium catalysed hydroformylation reaction.
These palladium catalysts were found to affect isomerisation of the a-olefin, but isomerisation was not a rate limiting process with respect to the hydroformylation reaction. Palladium catalysed isomerisation reactions occurred at a slower rate than the corresponding cobalt catalysed isomerisation process. However, with rhodium no isomerisation occurred.
The comparison between cobalt, rhodium and palladium showed that rhodium is the best catalyst for the hydroformylation of a-olefins. The pressures and temperatures required for this process are much slower than that required for palladium and cobalt.
II
The ligand used is triphenylphosphine, which is relatively inexpensive and non-toxic, in contrast with the more expensive ligands required for the cobalt and palladium hydroformylation processes.
The use of palladium opens up the unique possibility of converting a-olefins into "dimeric" ketones, which show promise as precursors for the new class of geminidetergents.
III
Opsomming
Die hoofdoel van die navorsing wat in hierdie verhandeling beskryf word, was die optimisering van die palladium gekataliseerde hidroformilering van a-olefiene. In die evaluering van die effektiwiteit van die palladium gekataliseerde hidroformilering proses was dit nodig om die proses to vergelyk met die prosesse gebasseer op kobalt en rodium. Verandering van ligande (difosfiene van die soort R2P(CH2) nPR2), oplosmiddels, sure ens. het 'n dramatiese effek getoon op die produkte wat gevorm word in die reaksie, asook op die tempo van die reaksie. Trifluoorasynsuur gee C-6 aldehiede as die hoofproduk met 1-penteen as reagens, terwyl trifluoormetaan-sulfoonsuur C-11 ketone as die hoofproduk gee. Soortgelyke resultate was met 1-okteen as substraat verkry. Die lengte van die koolstofbrug gaan•deur 'n optimum, afhangende van die ligand wat gebruik word. In die geval van alkielfosfien ligande is die optimum lengte van die brug twee koolstowwe, terwyl die optimum lengte in die geval van arielfosfien ligande drie koolstowwe is. 'n Nadeel van palladium gekataliseerde hidroformilering is dat bidentate ligande benodig word. Hierdie ligande is egter duur en die sintese daarvan is moeilik.
`n Groot probleem wat aangespreek is tydens hierdie studie is die presipitasie van palladium as gevolg van die onstabiliteit van die palladium komplekse. Die byvoeging van geselekteerde bymiddels het die stabiliteit van die palladium kompleks verbeter en na aanleiding hiervan kon die temperatuur van die reaksie ook verhoog word. Hierdie verhoging in temperatuur het gelei tot 'n verdere verhoging in die tempo van die reaksie.
Isomerisasie van a-olefiene word gekataliseer deur die palladium katalisators wat in hierdie studie ondersoek is. Hierdie isomerisasie het egter nie 'n inhiberende effek op die tempo van hidroformilering nie. Die isomerisasie van a-olefiene in die kobalt gekataliseerde hidroformilerings reaksie vind teen 'n vinniger tempo plaas as in die geval van palladium. Geen isomerisasie van a-olefiene is in die geval van rodium gevind nie.
IV
Die vergelyking van kobalt, rodium en palladium gekataliseerde hidroformilering prosesse het getoon dat rodium die beste katalisator vir die hidroformilering van aolefiene is. Beide temperatuur en druk vir laasgenoemde proses is laer as in die geval van kobalt en palladium. Trifenielfosfien is die voorkeur ligand en is goedkoop en nie-giftig. Die gebruik van palladium skep egter die unieke moontlikheid van a-olefiene om to sit na "dimeriese" ketone, wat besonderse belofte inhou as uitgangstowwe in die bereiding van 'n nuwe klas van sogenaamde "tweeling" detergente.
v
Chapter 1
Palladium catalysed carbonylation and other related reactions
1.1 Introduction The heterogeneously catalysed conversion of carbon monoxide and hydrogen to hydrocarbons and oxygenated products was first described by Fischer and Tropsch in 1922. 1,2 Since then, innumerable homogeneous and heterogeneous catalysed reactions were developed with carbon monoxide and organic compounds. The importance of carbon monoxide derives not only from its own inherent reactivity (it is susceptible to both nucleophilic attack at the carbon and electrophilic attack at the oxygen) but also from the polarising effect it has on neighbouring atoms and functional groups once incorporated into a molecule.
In particular, carbonyl groups have the ability to stabilise an adjacent carbanion by charge delocalisation into the C=0 double bond. 3 In addition, carbon monoxide, which can easily be purified, has a big potential as a one-carbon feedstock due to its ready availability (from coal and natural gas). Carbon monoxide presents an excellent method for the introduction of additional/alternative functionality into alkenes, specifically cc-olefins.
Otto Roelen4 discovered the first well-defined carbonylation reaction while working on the high-pressure cobalt catalysed Fischer-Tropsch synthesis. While investigating the mechanism of the Fischer Tropsch synthesis, Roelen observed that addition of ethene to synthesis gas (a mixture of carbon monoxide and hydrogen) led to the formation of propanal in high yield. This carbonylation process relies on heterogeneous catalysis. However, increasing emphasis was devoted to the development of carbonylation reactions based on homogeneous catalysis.
1
Many industrial and academic teams are pursuing studies on carbonylation reactions. Important aspects in these studies are to improve the separation of the catalyst from the reaction mixture and to improve both selectivity and rate of the reaction, through the fine-tuning of the coordination sphere of the transition metal. Understanding of the reaction, in each step of the catalytic cycle, is also an important goal. By using homogeneous catalysis the catalytic cycle of the reaction can easily be studied with the aid of high-pressure nuclear magnetic resonance (NMIt) and infrared studies.
Today, environmental factors in addition to economic motives are more and more taken into consideration as a driving force for technological innovations in the chemical industry. In this regard, homogeneous catalysis can be regarded as particularly attractive. The main advantages are the high activity and selectivity under mild conditions. 5 Carbonylation (particularly hydroformylation) remains an important process for the manufacturing of commodities and specialities. 6
Although the catalyst systems described in most carbonylation reactions are not specifically aimed at palladium, this short overview will focus on palladium catalysed carbonylation reactions and more specifically those reactions under homogeneous catalysis used in the chemical industry.
1.2 Palladium as catalyst for carbonylation
Palladium was originally used in laboratories and industrial processes as support catalyst for the hydrogenation of unsaturated compounds in a heterogeneous phase. 7 Amongst the many transition metals used in organic synthesis as catalysts, palladium is one of the most versatile and catalyses many types of organic reactions, and is particularly useful for carbon-carbon bond-forming reactions. 8 Another advantage of palladium is its tolerance of many functional groups such as carbonyl and hydroxyl groups because reactions can be carried out without protection of these groups. 9 Most palladium reagents and catalysts are also not overly sensitive to oxygen, moisture or acid. Though palladium is somewhat expensive, it can be economically tolerated if it is an efficient catalyst. i° To date toxicity of palladium has posed no problem.
2
1.3 Some mechanistic considerations 1.3.1 The carbon monoxide molecule If one views the conventional valence bond description of carbon monoxide, two canonical forms are involved: a " carbene-like" form and a "dinitrogen-like" form. In the "carbene-like" form divalent carbon is linked to oxygen by a double bond while in the "dinitrogen-like" form both atoms carry a lone pair and are linked by a triple bond. The latter canonical form is the more important one and leads to the assignment of formal charges O(+) and C(-) (Figure 1.1). - +
:C=O Z-••3 -4( minor contribution
:C-O: major contribution
Figure 1.1: Canonical forms of CO. The coordination chemistry of carbon monoxide is more often rationalised on a qaulitative molecular orbital (MO) basis (Figure 1.2) than in valence bond terms. Coordination of carbon monoxide to a transition metal involves removal of electron 11 density from the HOMO (5a*) and addition of electron density to its LUMO (2e). 6a * s s, sss,
2p
,. -.. —0-5a* ' . . . 4, , -,', - e/ ,, • , , , '
,
e
i I
I
2s
,
2p
17Z' '
,
•'
,
e
it
, ee
4'a,/
36 2s C
CO
Figure 1.2: Molecular orbital description of carbon monoxide.
3
1.3.2 The carbonylation reaction 1.3.2.1 Oxidative addition The first step in palladium catalysed carbonylation reactions is the oxidative addition of the reagent, e.g. alkylhalide, etc., to the palladium (Scheme 1.1). Oxidative addition of Ar-X to Pd(0) and H2 to Pd(II) are both implicated in the catalytic carbonylation of aryl and heteroaryl halides to aldehydes. 12 Since the coordination number of the metal increases with the oxidative addition reaction it is clearly forbidden for electronically and/or coordinatively saturated complexes. However, Pd(PPh3)4 is a "saturated" complex [4-coordinate, 18-electron species] and undergoes reversible dissociation in solution to give a two coordinate, 14electron complex Pd(PPh3)2 (Scheme 1.1). 13 This complex can thus undergo oxidative addition. This oxidative addition takes place via the mechanism shown in Scheme 1.2.
Ph— I PPh3\ , Ph Pd(PPh3)4
la-
Pd
Pd(PPh3)2 PPh3 "
2 PPh 3
Scheme 1.1: Oxidative addition of aryl halide during carbonylation.
X
X M
.Y
Scheme 1.2: Mechanism of oxidative addition.
4
Y
I
▪▪
Palladium has the ability to take on a number of different oxidation states (0, +2, +4). It can either acquire electrons from or supply electrons to its environment. If this "environment" is a a-antibonding orbital of an approaching molecule (X-Y), the progressive transfer of electron density to this orbital from the metal orbital of matching symmetry occurs. This together with a degree of electron transfer to the metal from the corresponding a-bonding orbital can result in the formation of a threecentre metal-X-Y bond (Scheme 1.2). The "metal" electrons are then used to break the a-bond of the incoming X-Y molecule with the formation of two new bonds (M-X and M-Y).
1.3.2.2 Insertion The concerted mechanism of ligand migration requires a cis configuration of the combining ligands. Insertion is normally stereospecific and the insertion of carbon monoxide has been shown to proceed with complete retention of configuration at the migrating carbon atom (Scheme 1.3). 14
R 1
R
-Es
C' L— Pd— 0+
L—
C. 0 L
R y RI C' L / Pd— C / \\ 0 L, .1.1„:.
Scheme 1.3: Mechanism of carbon monoxide insertion.
1.3.2.3 Product formation step Product formation can take place via three processes, namely reductive elimination, 13hydride elimination and reductive displacement.
5
1.3.2.3.1 Reductive elimination This is the reverse of the oxidative addition step and requires the loss of two "oneelectron" ligands from the palladium centre. In many carbonylation reactions, it is this reductive elimination step that is the product-releasing step of the catalytic cycle (Scheme 1.4). 0
0
PPh3 _Pd(,PPh3 ), 2
H
Pp. h—3/ \ H
Scheme 1.4: Reductive elimination step. 1.3.2.3.2 13-Hydride elimination 13-Elimination needs to be preceded by a strong agostic effect that on its turn require a vacant coordination site (Scheme 1.5). Thus, coordinatively saturated, kinetically stable complexes are not susceptible to this type of process. The use of strongly bound ligands such as chelating phosphines and carbon monoxide in coordinatively saturated complexes can inhibit 13-elimination of the hydride from alkyl ligands.
H
C
/
R \ /H Pd / \ L
k)H
Scheme 1.5: Example of (3-hydride elimination. 1.3.2.3.3 Reductive displacement This process involves the reductive cleavage of a metal-ligand bond and is a characteristic reaction of metal-acyl complexes under basic conditions (Scheme 1.6). 15
6
PPh3 ,/ Br—Pd—C
Pd(PPh3)2 +
+ NuH
+ FIBr Ph'
P PPh3 \ Ph
Nu
Scheme 1.6: Reductive displacement reaction.
1.4 Palladium catalysed carbonylation reactions There are many useful reactions catalysed by Pd(0) complexes, in particular palladium complexes with phosphines. Tetrakis(triphenylphosphine)palladium [Pd(PPH3)4] is commercially available but is air sensitive. For this reason it is more convenient to prepare Pd(PPh 3)„ as the active catalyst by forming it in situ by mixing palladium(II) acetate and triphenylphosphine in solution. Pd(II) salts are also used to catalyse a number of carbonylation reactions. Palladium catalysed carbonylation reactions of olefms, dienes, alkenyl halides, aryl halides and aromatic as well as cyclic alkenes are described in this discussion as well as their use in industrial processes.
1.4.1 Olefins The carbonylation of olefins catalysed by palladium complexes affords saturated esters, aldehydes or alcohols depending on whether the reactions are carried out in the presence of an alcohol or in the presence of hydrogen. 16 Regioselectivity between 17
'
linear and branched esters is a problem in the carbonylation of terminal olefins and a mixture is usually obtained.
The carbonylation of 4-isobutyl-1-vinylbenzene
(1.1)
in ethanol proceeds
regioselectively to give ethyl 2-(4-isobutylphenyl)propionate (1.2a) in high yield. 18 When the same reaction is carried out in a mixture of benzene and water, 2-(4isobutylphynyl)propionic acid (1.2b) is obtained. This reaction is useful for the synthesis of the important drug Ibuprofen (Scheme 1.7).
7
OH PdC12
C O 2R
PPh3
CO
1.1
1.2a R = Et 1.2b R = H
Scheme 1.7: Carbonylation of 4-isobutyl-1-vinylbenzene.
The carbonylation of ethylene in alcohols is another important reaction. Propionate esters are the main products and 4-oxohexanoate ester (1.3) the by-product in the reaction in the presence of a monodentate phosphine ligand. I9 When the same reaction is carried out in the presence of methanol using palladium(II) acetate as a catalyst and 1,3-bis(diphenylphosphino)propane (DPPP) as a ligand in the presence of acids such as hydrochloric acid and p-toluenesulfonic acid, the polyketone (1.4) is obtained rather than a simple ester (Scheme 1.8). 20 This process is described in greater detail in section 4.1.2. ROH PdC12 PPh3
CO2R
II
CO2R
0 1.3
n CHI CH2 + n CO 0 Pd(OAc)2/dppp Ts0H/Me0H 1.4 Scheme 1.8: Carbonylation of ethylene in alcohols.
8
1.4.1.1 Oxidative carbonylation Palladium(11) is used as catalyst for the oxidative carbonylation of olefins. The first example of this type of reaction of olefins is that of ethylene to give 2-chloropropionyl chloride (13) (Scheme 1.9). 21
C1
CH2 = CH2 + C O + P d C12
COC1 + N(0) 1.5
Scheme 1.9: Oxidative carbonylation of ethene. This reaction promoted the development of a number of processes for industrial applications. Union Oil developed the production of acrylic acid (1.6) via the oxidative carbonylation of ethylene in acetic acid (Scheme 1.1 0). 22
CH2= CH2
+
CO
+
AcO
02
PdC12/CuC12
E kc0
CO2Ac ---y■
CO2A1
CO2H + Ac20 1.6
Scheme 1.10: Production of acrylic acid. This method has, however, no advantage over the oxidation of propylene. The main problem of this industrial application of palladium is related to the problem of efficient reoxidisation of Pd(0). 23 For this purpose the use of nitrites has been studied for the oxidative carbonylation of ethylene and propylene. 24 The industry also paid considerable attention to the oxidative carbonylation of styrene in methanol to produce methyl cinnamate. 25 The catalyst system described consists of ordinary palladium and copper salts as well as other metals as co-catalysts. It was later
9
found that the introduction of Mn(OAc) 2 as additive substantially increased the activity of the catalyst system. 26 Mitsubishi Kasei Corp.27'28 developed the process for the production of phenylalanine
(1.9) by the selective oxidative carbonylation of styrene to give cinnamate (1.7), followed by hydrolysis and enzymatic amination (Scheme 1.11).
Ph" + CO + Me0H +
C O 2Me Ph/
02
ph/
Pd—C Cu(OAc)2 BaC12
CO2H
/
enzyme NH3
CO2Me Ph/
/ 1.7
//CO2H Ph ; NH2
1.8
1.9
Scheme 1.11: Process for the production of phenylalanine. The catalyst system in this process is palladium on carbon/copper(111) acetate/barium chloride in acetic acid. An advantage is that these catalysts are robust and can be used repeatedly. Conversion of styrene is 98% and selectivity for methyl cinnamate is 89%. This method is thus a very attractive process for the industry with little loss of the expensive palladium catalyst.
Olefins react with formate esters, carbon monoxide, oxygen and hydrochloric acid at room temperature and atmospheric pressure to give mainly branched chain carboxylic esters in good yields (Scheme 1.12). The catalyst system in this reaction is copper(II) chloride and palladium(II) chloride. 29
RCH=CH2 + CO + 02 + HCOOR'
PdC12, CuC12 HCl, rt , 1 atm
Scheme 1.12.
10
RCHCH3 + RCH2CH2COOR' COOR'
In these reactions Pd(II) is reduced to Pd(0) in the product-releasing reductive step, and it is therefore it is necessary to add a reoxidant. Although Cu(II) is usually and conveniently used as reoxidant of Pd(0), an unique reoxidant of Pd(0) is alkyl nitrites. This method is used by Ube Industries in the production of succinate (1.10) by the palladium catalysed oxidative carbonylation reaction (Scheme 1.13). 30
PdC12
H2 C= CH2 + 2 CO + 2 RONO
RO2C\ /\ CO2R + 2 NO 1.10
Scheme: 1.13.
1.4.1.2 Copolymerisation Alternating copolymerisation of ethene and carbon monoxide has attracted considerable interest from both academia and industry. The product "polyketone" (poly-3oxotrimethylene) (Figure 1.3) is predicted to have interesting engineering plastic type properties, typical for polymers with a high density of heteroatom functionalities. 0
0
Figure 1.3: Polyketone.
The starting materials for this copolymerisation are also readily available. 31 Complete alternating copolymerisation of carbon monoxide with ethene was achieved by a group from Shell International in the early 1950's. Several metals (NO), Pd(II) and Rh(I)) were examined, but Pd(II) became the most promising candidate. From this originated the development of highly efficient homogeneous palladium catalyst systems for the production of perfectly alternating copolymers of carbon monoxide with ethylene. The first palladium catalysts for alternating polyketone formation were first disclosed by Gough at ICI in 1969. 32 Bis(tertiaryphosphine)palladium dichloride was the catalyst
11
system described and the process yielded polyketone at a rate of 300 g(g of
Pa)-'
h" 1 .
The big disadvantage of this process was that severe conditions were required (250 °C, 2000 bar) with low turnover numbers. The discovery of catalyst systems comprising a palladium(11) species with a weakly coordinating anion together with an equimolar quantity of a suitable bidentate phosphine ligand enabled Shell 33'34 to move these polyketones to commercial reality. When 1-alkenes such as propene or styrene are used in the place of ethene, three more factors need to be controlled in order to obtain a stereoregular alternating copolymer. These factors are regioselectivity, tacticity selectivity and enantioselectivity. 35
1.4.1.3 Hydroformylation A rhodium-phosphine hydroformylation process to produce n-butanal from propene was jointly developed by Union Carbide, Johnson and Matthew and Davy Powergas. This process was first commercialised in 1976. Since then numerous hydroformylation catalysts predominantly based on Co and Rh, combined with a wide variety of ligands, were developed. However, Shell International developed a process for the preparation of aldehydes by hydroformylation in the presence of a palladium based catalyst 36 with a bidentate phosphine ligand (L2) (Scheme 1.14).
R
• CO/H2 (1:1) Pd(OAc)2
0
L2
0
Acid H
Scheme 1.14: Palladium catalysed hydroformylation of a-olefins.
Under these same hydroformylation conditions an o-allylic phenol can be converted into five- (1.11), six- (1.12) and seven-membered lactones (1.13) (Scheme 1.15). 37
12
R
CO/H2 Pd(H), dppb 42 bar, 100-150 ?
1.12
1.13 Scheme 1.15: Hydroformylation of o-allylic phenol. 1.4.1.4 Alkoxycarbonylation The alkoxycarbonylation of alkenes is of growing importance. A range of products are accessible, from high molecular weight `polyketones', which have useful properties as engineering thermoplastics, 38 to methyl propionate, a potential intermediate in the manufacturing of methyl methacrylate (n=1 in Scheme 1.16). 39
0 II H--(CH2CH2C )11 OCH3
CH3 OH + CO + CH2= CH2
Scheme 1.16: Production of methyl methacrylate. Palladium catalysts for this transformation show a degree of selectivity, dependent on the nature of the phosphine ligand. It was found that monodentate phosphines generally gave methyl propanoate and bidentate phosphines gave high molecular weight co-polymers. 4° A highly active and selective catalyst for the production of methyl propanoate was described based on the new zero valent palladium complex L2Pd(dba)2, (dba = trans, trans-dibenzylideneacetone) where the bidentate ligand L2 is 1,2-bis(di-tert-butylphosphino-methypbenzene (Figure 1.4). 41
13
p(tBu)2 p (tBu)2 Figure 1.4: 1,2-Bis(di-tert-butylphosphinomethyl)benzene.
Bisalkoxycarbonylation of olefins to succinate derivatives has been carried out with a number of catalysts, among them PdC12 and butylnitrite, 42 Pd(OAc)2, 02 and benzoquinone43 as well as Pd(acac)2 and di-tert-butylperoxide. 44 Consigilio et a/. 45 developed the first stereoselective version of this reaction. Bisalkoxycarbonylation of styrene using chiral PdL2X2 complexes and benzoquinone proceeds with enantioselectivities of up to 93% in the presence of chiral biphenylphosphines (Scheme 1.17).
+ CO + CH3OH
PdL2X2 benzoquinone 50 °C 50-350 bar
Scheme 1.17: Bisalkoxycarbonylation of styrene.
1.4.2 Reactions of conjugated dienes via 7c-allylpalladium complexes
Depending on the catalytic species, the carbonylation of butadiene affords 3-pentenoate (1.14) or 3,8-nonadienoate (1.15).
46,47 A new method for the production of adipate
(1.17) from butadiene was developed by Shell (scheme 1.18). 48
14
P dC 12
CO2R
st"
1.14
+ CO + ROH Pd(OAc)2/PPh3 i.
C O2R
1.15 + CO + Et0H
Pd(OAc)2 dppb/H+
CO 2Et 1.16
Co2(CO)8 CO2Et
CO/EtOH
EtO2C,, „CO2Et
1.17
1.16
Scheme 1.18: Production of adipate from butadiene. The carbonylation of butadiene catalysed by palladium(11) acetate in ethanol in the presence of 1,4-bis(diphenylphosphino)butane (dppb) and 2,4,6-trimethylbenzoic acid affords ethyl 3-pentenoate (1.16) in high yield. This product is carbonylated to ethyl adipate (1.17) by using dicobalt octacarbonyl as catalyst. 1.4.3 Cyclic alkenes and aromatic compounds
The hydrocarbonylation of cyclopentene with PdX2(dppe) (X= TFA, OTs) catalyst proceeds to form saturated and unsaturated ketones influenced by the hydrogen bonding of urea derivatives to the anionic ligands X of the catalyst precursor (Scheme 1.19).49
15
CO/H2 (1:1) PdX2(dppe) 80 bar, 110 °C
Scheme 1.19: Hydrocarbonylation of cyclopentene.
The one-step carboxylation of aromatic compounds such as benzene, anisole and naphthalene with carbon monoxide giving the corresponding aromatic acids, is a process with high value to the industry. The active catalyst system is Pd(OAc)2/tBuO0H/CH2=CHCH2C1. 5°
1.4.4 Other compounds
1.4.4.1 Alcohols Oxidative carbonylation of alcohols with Pd(II) produces the corresponding dicarbonate or oxalate, depending on the CO pressure. In order to make the reaction catalytic, Cu(II) or Fe(III) salts are generally used as reoxidants. However, Ube Industries used butyl nitrite as reoxidant in a unique process in the commercial production of butyl oxalate (1.19) in 1978 (Scheme 1.20). 51 In this process the oxidation of CO to CO2, a problem with the use of Cu(II) as reoxidant, is suppressed. The butyl nitrite (1.18) is used as the reoxidant in this liquid-phase process. The nitrogen monoxide formed in the reaction (Scheme 1.20) is used to regenerate the butyl nitrite. The nitrogen monoxide reacts with oxygen and butanol to give butyl nitrite. One of the advantages of using the nitrite method is the facile removal of water formed by the oxidation by azeotropic distillation with butyl nitrite. Alkyl nitrite is also used as reoxidant of Pd(0) in the production of oxalate from CO, alcohol and oxygen. 52
16
Pd—C/PdC12
2 CO + 2 BuONO
BuO
1.18
OBu + 2 NO 1.19
2 BuOH + 2 NO +
2
02 --31. 2 BuONO + H2O
Scheme 1.20: The production of butyl oxalate. Treatment of allylic alcohols with carbon monoxide, oxygen, copper(II) chloride and hydrochloric acid in tetrahydrofuran containing palladium chloride affords furanones (Scheme 1.21). 53'54
OH
PdC12 CuC12, HC1 CO/02, r.t 15-70%
R = H, Me, Pr
Scheme 1.21: Formation of furanes. Another promising reaction is the preparation of commercially important diphenylcarbonate by the oxidative carbonylation of phenol (Scheme 1.22). 55
0
OH
0— C— 0—
+ CO + PdC12
Scheme 1.22: Oxidative carbonylation of phenol.
17
+ Pd(0) + 2 HC1
1.4.4.2 Carbonylation reactions of alkenyl and aryl halides From an industrial point of view aryl halides are generally a less attractive feedstock for the synthesis of benzoic acids using CO compared to compounds such as toluene which can lead to benzoic acids via air oxidation. The use of organic halides in general is technically restricted to the synthesis of higher value fine chemicals and complex synthetic intermediates. 56
A good preparative method for aromatic or a,(3-unsaturated esters is the carbonylation reaction of aryl and alkenyl halides under low pressure and this reaction were reported by several laboratories in Japan and America. 57 One practical example is the production of methyl trifluoromethylacrylate (1.21) by the carbonylation of 2-bromo3 ,3 ,3 -trifluoropropylene (1.20) catalysed by dichlorobis(triphenylphosphine)palladium [PdC12(PPh3)2] in the presence of triethylamine (Scheme 1.23). 58 CF3 /
+ Br2 ---o-
/"\\/CF3 _____).. ,,,,,,..// CF3 Br Br
Br
1.20 CO/ROH/Pd/PPh3 \ CF3 CO2R CF3
RNHCONHR CO/Pd/PPh 3
Br
1.21 0
0 R\ .....„...........,/..„.CF3
R\ N
0N
0N
CF3
N
CF3
RNHCONHR CO2R
I R
I R
1.22
1.23
Scheme 1.23.
18
/
When the reaction is carried out in dimethylformamide in the presence of urea derivatives, 5-trifluoromethyldihydoruracils (1.22) were obtained. These compounds were converted to trifluoromethyluracil (1.23), which has anti-tumour activity (Scheme 1.23). The oxidative addition of an unsaturated metal complex to aryl chlorides is thought to be the rate-determining step in the substitutive carbonylation reactions of arylchlorides. One strategy to overcome this lack of reactivity for oxidative addition is to use very active palladium complexes based on the electron rich and bulky chelating diphosphine ligands e.g. 1,3-bis(diisopropylphosphino)propane (Scheme 1.24). 59
Cl
/
CO2 R
1)/\/\ p /\
Pd(OAc)2 + 2 + CO + ROH
•
150 °C, 5 bar CO
Scheme 1.24. Other phosphines which can be employed are those which posses both high basicity (pKa > 6.5) and defined steric hindrance 60'61 and palladium complexes with triisopropylphosphine or tricyclohexylphosphine.
1.4.4.3 Ketones
As already mentioned, the palladium catalysed oxidative carbonylation transformations of olefins are one of the most synthetically useful transition metal catalysed reactions. However, the oxidative carbonylation reaction of ketones (in their enol form) are rare. One example of this type of reaction is shown in Scheme 1.25. Although little work has been done, it appears to be an excellent alternative to olefins. It is also an useful reaction with which to introduce two functional groups (Scheme 1.25). 62
19
CI
Ho C
O—H
Pd PdC12, }1± CO, CH3OH
CO2CH3
-
CH3OH
HO PdC12(CH3OH) CO2CH3 C H 3OCO ( CH 2 )5 C O 2 CH3
CH3OH
2 CuC12
CO2CH3
Cl"
Scheme 1.25: Oxidative carbonylation of ketones.
1.4.5 Palladium catalysed carbonylation reactions in biphasic media The major drawback of homogeneous catalysis over heterogeneous catalysis is the separation of the catalyst from the reaction mixture. This reason has dictated the development of biphasic catalyst systems advantageous for the industry. The first example of a carbonylation reaction which uses a catalyst in the aqueous phase was the palladium catalysed carbonylation of benzyl chloride (Scheme 1.26). 63
PhCH2C1 + CO + 2011"
PdC12(TPPMS)2 H2O -n-heptane T = 50 , Pco = 1 bar time = 5 hr
PhCH2C00 - + Cl" + H20
Scheme 1.26: Carbonylation of benzyl chloride.
The homogeneous, aqueous biphasic catalysis has also acquired industrial significance for the production of phenyl acetic acid (PAA) (1.24). The classical process i.e. benzyl chloride-+benzyl cyanide-*PAA, suffered from the formation of large amounts of salts (1400kg/kg of PAA). 64 The new biphasic carbonylation method reduces the amount of
20
salt forming by 60% and benefits from the great cost difference between HCN (ca. $14/kg) and CO (
