Platinum Group Metals in Phase Transfer Catalysis


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Platinum Group Metals in Phase Transfer Catalysis
By Ernest S. Gore
Johnson Matthey, West Deptford, New Jersey, U.S.A.

Phase transfer catalysis is an elegant method for ameliorating the reaction conditions of many organic reactions and improving yields and selectivities. A review is presented of the role and scope of the compounds of the platinum group metals as cocatalysts under phase transfer conditions. The most useful improvements have beenfound in reactions involving theformation of carbon-carbon bonds such as in carbonylations and uinylations, and in reductions with formate.

The concept of phase transfer catalysis arose about 30 years ago as a solution to the problem of facilitating reaction between organic substrates immiscible in water and inorganic
speciesinsolublein non-aqueoussolvents(I ,2).
Since then the field has burgeoned with over 2700 citations in Chemical Abstracts as of the end of 1988. Present day commercial usage is estimated to involve 50 to 75 applications and over 50 million pounds per year of products (2, 3). Such an impressivegrowth has been due to the manifold advantages of phase transfer catalysis:
improved selectivity 0 increased reaction rates
milder reaction conditions reduced energy requirements simplifiedisolationand purification methods reduced consumption of organic solvents elimination of need for expensive solvents 0 allows the use of cheaper raw materials, especially oxidants elimination of need for rigorous drying of organic solvents very broad scope of action-numerous reaction types possible.
Although general reviews of phase transfer catalysis (2-23) and some reviews of phase transfer catalysis with transition metal compounds (24-29)have been published, it is the purpose of this article to review the role played

by compounds of the platinum group metals as cocatalysts in phase transfer catalysis with emphasis on the scope of the reactions rather than on mechanisms.
Carbonylation
Carbonylations by palladium are very sensitive to reaction conditions, the product distribution being particularly dependent on the nature of the palladium complex and the organic solvent. This is illustrated in the Figure for the carbonylation of benzylic halides. Carbonylation gives the expected carboxylic acid when either Pd(PPh,), or Pd(diphos),, where diphos is 1,2-bis(diphenylphosphino)ethane, is used in the absence of a phase transfer catalyst (30); however, when Pd(diphos), is used in the presence of a phase transfer catalyst (31)t~he course of the reaction changes dramatically, as shown in the Figure, esters being formed.
An attempt was made to control the stereospecificity of the carbonylation of PhCH(Me)Br using a palladium catalyst form-
, ed in situ by the addition of an optically active
phosphorus compound to Pd(dba) ,where dba
is dibenzylideneacetone. Yields were poor and in only one case was there a significant enantiomeric excess of the product (32). In the absenceof any phosphineligand only the coupled product, PhCH(Me)CH(Me)Ph,was obtained with Pd(dba), (33). When Pd(PPh,), was used, a mixture of the acid (36 per cent) and the

Platinum Metals Rev., 1990, 34, (l), 2-9

L

Pd(PPh&C12
ArCH2COOH-

I I0
Pd(di p h o s ) ~
ArCHzX + CO -ArCH2COCH2Ar

ArCH2CH2Ar + ArCH3
This schematic diagram of the carbonylationof benzylic halides catalysed by palladium complexes shows typical products formed during phase transfer catalysis, with (a) 5 N NaOHll atm CO/F'hzO/BzEt,NC1180-840C/7-9hours (31); 30 per cent NaOH15 atm COlxylene/Bu,NI/95 OC15 hourelexcess PPh, (34) (b) 5N NaOHll atm CO/CH,CI. I K , H ,,),NHSO./room temperaturel6-10 hours (31)

ester PhCH(Me)COOCH(Me)Ph (19 per cent) was obtained (30).
The catalyst Pd(PPh,),Cl, has been used to selectively carbonylate phenyl halides having more than one halide (24,34). I n each case only one halide was displaced. Thus pdibromobenzene gave p-bromobenzoic acid (conversion90 per cent, selectivity95 per cent), and 1,3,5-trichlorobenzene gave 3,s-dichlorobenzoic acid (selectivity 97 per cent).
The organic solvent markedly affects the course of reaction in the palladium catalysed carbonylation of vinyllic dibromides, RCH=CBr,. When R is phenyl coupling occurs in benzene to give PhC=CC=CPh in 54
, per cent yield, while in t-amyl alcohol car-
bonylation occurs giving PhCH =C(CO0H) in 93 per cent yield (35). Coupling does not occur when R is aliphatic. Thus for R=EtCHMe a mixture of the monoacid and diacid is obtained, in benzene giving 64 per cent RCH =CHCOOH and in t-amyl alcohol giving 82 per cent RCH=C(COOH),. Vinyllic bromides, RCH=CHBr, are carbonylated in good yields to the expected acid using Pd(diphos), or Pd(PPh,), (36), although in the latter case the reaction can also proceed without a phase transfer catalyst.
Esters can be prepared via solid-liquid phase

transfer catalysis from organic halides and alcohols (37). Thus Pd(PPh,),Cl,/1-7 atm
CO/NaHCO /ethyl alcohol/Bu, NI was used to
facilitate the preparation of ethyl esters from a variety of aliphatic and aryl halides. Yields were 56-95 per cent, but dropped to less than 30 per cent in the absence of a phase transfer catalyst. Phase transfer catalysis with Pd(OAc), provides a modest enhancement in the yields of anhydrides from carbon monoxide, aryl halides, and carboxylic acid salts (38).
The system PdBr, /Mn(acac) /NaOH/ Bu, NBr, where acac is 2,5-pentanedioneYwas used to synthesisediphenyl carbonatein 60 per cent yield from phenol, carbon monoxide and oxygen (39). Many phase transfer catalysts were examined and the wide range of yields obtained illustrates the necessity of optimising such reactions with respect to the phase transfer catalyst. Under similar conditions, alkyl carbonates can be produced from aliphatic alcohols (40).
Efforts have been made (41-44) to improve the recovery of rhodium from commercial hydroformylation processes which use rhodium
catalysts, Equation [il, by employing phase
transfer conditions. Both phase transfer catalysis and rhodium-phosphine complexes with amphiphilic phosphine ligands, such as

Platinum Metals Rev., 1990, 34,(1)

3

, , Ph, P(CH,) N +Me (40, (3-H03sc6H 4 ) P two isomers can be produced (46). Using , (42, 44)and Ph,PC,H,COOH (43), were in- Pd(0Ac) MaHCO /Bu4NCI/DMF, (a) was

vestigated.

produced with high selectivity and yield (82-94

-Rh

per cent). Very recently, it has been shown that

RCH=CH,+CO+H,

RCH’CHO [il high selectivity can be achieved under non

phase transfer conditions when the reaction is

Vinylation

properly optimised (49).

The Heck vinylation, Equation [ii], of The addition of organic halides to alkynes has

organichalides is an extremelyvaluablemethod also been investigated; thus reaction [ivl shows

of synthesising carbon-carbon bonds (45).

yields of 24-87 per cent with almost complete

I I
RX+H-C=C-

-mi0] I I R-C=C-

[iil

In the absence of phase transfer catalysis

stereospecificity using the system Pd(PPh3), /

- CuI/C,H,/NaOH/BzEt ,NCI (50).

R1CH =CR’ X+HC=CR3

RICH =CR’C =CR’

[ivl

these reactions can be slow, and require high
temperatures (>IwOC) and long reaction Oxidation

times, and the yields are not always good. Phase Olefine

transfer catalysis has been shown to improve Phase transfer catalysts have been used in an

the yields and stereospecificityand to allow the attempt to improve the yield of the Wacker

reactions to proceed at room temperature. A reaction, [vl.

series of aromatic iodides, RC,H41, was

vinylated with CH, =CHR1, in excellent
, , yields, 80-97 per cent, using the system
Pd(0Ac) MaHCO /Bu,NCl/DMF, where

H

0

I

Pd”, Cu’+ 11

R-C=CH, + 1/20’

*RCCH2 [vl

(a- DMF is N,N‘-dimethyl formamide, at room

temperature (46). The product was exclusively

the (I?) isomer. A similar system,

Pd(OAc),/K,CO,/Bu,NCI/DMwFas also us-

ed to study the vinylation of (I?)- and

C4H, CH =CHI

(47).

Excellent

stereospecificitiesfor the (E,E) isomer of the

product were achieved even when the substrate

was the (Z)isomer. Various alkynyl iodides

have been vinylated at room temperature in

40-60 per cent yield to Q-methylenoates and

methylenyones using the systems

Pd(OAc),/K,CO, or Na,CO,/Bu,NCl/DMF

(48). Under the same conditions, when ally1

alcohols are reacted, Equation [iiil below:

Although the yields are generally good, only in a few cases is there an improvement compared with the yields obtained without using phase transfer catalysts (some comparisons are given in Table I). The oxidation of styrene was unusual (51) as the product distribution was significantlydifferentunder phase transferconditions (TableI). Wacker type oxidationsunder phase transfer conditions using rhodium and ruthenium catalystswere inferior to those using palladium (52). However the fact that moderately good yields were obtained with ruthenium is significant, since oxidations with ruthenium generally lead to cleavage of the double bond (53, 54). Examples of cleavage under phase transfer conditions are given in

I

[PdOI

RX+ -C=CHCH(R’)OH

Table 11. Again, with a few exceptions, there is usually little improvement in yields compared

RO

HRO

H -CI -CI HICI R’+H -CI -CI -CII R1

I

II

H (a)

(b)

with non phase transfer conditions (53-55). Phase transfer catalysis in the Wacker reac-
[iiil tion with cyclodextrins is notable, as the olefin is transferred to the aqueous phase where oxidation takes place; on the other hand

Platinum Metals Rev., 1990, 34, (1)

4

quaternary ammonium salts transfer the olefin to the organic phase. Internal olefms are not oxidised using quaternary ammonium salts (56), but are oxidised using 8-cyclodextrin (51). Thus cis- and trans-MeCH =CHMe are oxidised to 2-butanone in 76 per cent and 70 per cent yields, respectively. The yield of methyl ketone from straight chain olefms peaks when the carbon chain of the substrate is nine carbon atoms
long (57). No doubt this is related to the hole size of the
cyclodextrin. Internal olefins are also oxidised using PEG-400 (58), where PEG is polyethyleneglycol, although a signifcant amount of double bond migration also occurs. Yields decreasewhen either shorter, PEG-200, or longer, PEG-1000, chain lengths are used.

Aromatic Hydrocarbons

In most cases subjectingaromatic compounds

to oxidative phase transfer conditions results in

oxidation of an aliphatic side chain.

, Methylbenzenes were oxidised with RuC1,/NaOCl/Bu NBr/CH, ClCH CI and the
products depended on the electronic properties

of the non methyl substituent (68). When the

substituent was H or electron withdrawing, for
example, NO, ,halide or NO, the methyl group

was oxidised to a carboxyl group in yields of

93-98 per cent. Electron donating groups

, resulted in ring chlorination.
The system RuCl /H, 0,IDDAB, where

DDAB is (C,,H,,),Me,NBr, was used to ox-

idise the side chains of aromatic rings (69) in

good yields, but the selectivitywas usually not

very good. Tetralin was oxidised to a-tetralone

with this system in 65 per cent yield (69), and

with RuO,/NaIO,/C,H,/Adogen-464in 74

per cent yield (70). Ring cleavage occurred,

resulting in a mixture of phthalic acid and

3-methyl-phthalic

acid,

when

1-methylnaphthalene was oxidised with

RuO, /NaIO, /C, H /Adogen-464 (70).

Alcohols There are many methods available for the
platinum group metals catalysed oxidation of alcohols (53, 54). Indeed, when properly op-

timised, some of these reactions have given results equal to those employing phase transfer conditions. Thus although high yields have been obtained under phase transfer conditions using RuO,/NaIO, (71) or RuCl,/NaBrO, (72), the same oxidations can be carried out in the absence of a phase transfer catalyst in equally high yield when the reaction conditions are finely tuned. For example, oxidations with RuO,/NaBrO, have been performed in the absence of phase transfer catalysts in nearly quantitative yields (73), and the addition of acetonitrile to the conventional RuO,/NaIO,/CCI, system dramatically improves yields in many cases (55).
, ,. Phase transfer catalysis provides a definite
benefit in the case of oxidationswith Ru/H 0 No oxidation of alcohols takes place in the absence of a phase transfer catalyst; instead ruthenium metal precipitates and the hydrogen peroxide is catalytically decomposed to oxygen
(74) * When a phase transfer catalyst is added,
yields of 60-100per cent are obtained, primary alcohols being converted into aldehydes and secondary alcohols into ketones.
Some oxidations involving transfer between a solid and a liquid phase are greatly enhanced by
phase transfer catalysis. Thus the oxidation of
primary and secondary alcohols to aldehydes and ketones, respectively, takes place in high
, yields with the phase transfer systems
Pd(0Ac) /NaHCO, /PhI/Bu, NCl/DMF (75) or RuCl,/Na,CO,/CCI,/DDAB (76).
Reduction
Reductions with Hydrogen
Rhodium complexes with , amphiphilic phosphine ligands have been used to reduce unsaturated organic compounds with hydrogen (77, 78). Benzene derivatives were reduced to cyclohexane derivatives under mild conditions using the system IRh(~,j-hexadiene)CIl,/1-2 atm H,/CH, ClCH,Cl/H, O/Aliquat-336/room temperature (79), or RhCI,/2 atm H I / CH, CICH,CI/H, O/Aliquat-336/room temperature (80).
The latter system was also employed in

Platinum Metak Rev., 1990, 34, (1)

5

Table I
Oxidation of Some Olefms to Methyl Ketones with Oxygen' A Comparison with non Phase Transfer Conditions

Substrate

Products

Conditions

Yield, per cent

Reference

1-dodecene 2-dodecanone

PdCI,/CuCI,/DMF

87

59

1-dodecene 2-dodecanone

PdCI,/CuCI,/CTAB/n-C,,H,I

98

60

1-dodecene 2-dodecanone

, , n-C, F,
PdCI /CuCI /CTA B

62

56

1-decene 1-decene 1-decene 1-decene 1-decene 1-decene

2-decanone 2-decanone 2-decanone 2-decanone 2-decanone 2-decanone

PdCI,/CuCI,IDMF PdCI,/CuCI, /CTAB PdCI,/CuCl,/&D RhCI,/CuCI,/CTAB RuCI,(PPh,),/CTAB PdCI, ICUCI, /PEG-400"

73

61

73

56

76

57

44

52

64

52

60

58

1-butene 1-butene 1-butene 1-butene

2-butanone 2-butanone 2-butanone 2-butanone

PdCI,/PMVb PdCI,/CuCI, /CTABd P~CI,/CUCI,/PEG-~OO" PdCI,/PMV/CTAB/CloH,'

14

62

65

56

71

58

85

62

styrene styrene

PhCH,CHO PhCOCH, PhCHO PhCH,CHO

a1 atm 0,.unless otherwise noted CTAB = cetyltrimethylammonium bromide PMV = phospho-6-molybdo-6-vanadicacid

PdCI, /CuCI,/CH, CN PdCI,/CUCI,//%CD

O400 psi 0, CD = cyclodextrin

Y200 psi 0,

44

63

80

51

10

10

a 180 psi O2

Table II
Oxidative Cleavage of Olefms

Substrate

Products

1-octene cis-4-octene 1-pentadecene styrene
styrene
styrene trans-stilbene cis-stilbene

CH,(CH, ),COOH CH,(CH, ),COOH CH, (CH, ,,COOH PhCHO PhCOOH styrene oxide PhCHO PhCOOH PhCOCH, PhCHO styrene oxide PhCHO styrene oxide PhCHO

I T C =BzMe,(C,,H,.)

Conditions
RuO,/H,IO,/hexane/(C,,H,,),N RuO,/NaOCI/NaOH/CH,CI,/Bu,NBr RuO,/NaOCI/NaOH/CH,CI,/Bu,NBr RuCI,/H,O,/CH,CICH,CI/DDAB
PdCI,/H,O,/CH,CICH,Cl/DDAB
1(bpy), ,+ (py)RuOl /NaOCI/
CH,CI,/BDTC [(bpy), (py)RuOlz+/NaOCI/
CH,CI,/BDTC [(bpy), (pylRu012+/NaOCI/
CH,CI,/BDTC

Yield, per cent Referenc

93

64

84

65

100

65

64

66

6

4

12

66

14

56

47

67

13

42

67

7

35

67

Platinum Merak Rev., 1990, 34,(1)

6

reducing acetylenes to olefms, and therefore is the phase transfer analog of the Lindlar catalyst (81) (palladium poisoned with lead supported on calcium carbonate). Like the Lindlar catalyst, which gives mainly cis products, the phase transfer conditions could be adjusted to give mainly the cis isomers, although the selectivity is not as high as that usually achieved with the Lindlar catalyst.
Under these conditions hydrogenolysis of halides tends to occur; thus 59 per cent of the fluorine in fluorobenzene and all the chlorine in chlorobenzene are removed (80).
The system Rhc1,/1 atm H 2 / CH2CICH2CI/H20/Aliquat-3h3a6s also been used to reduce a$ unsaturated esters, ketones, and carboxylic acids at the carbon-carbon double bond with yields of 87-96 per cent (82).
Imines have been reduced to the corresponding amine in the three phase system [Rh(PPh,), (~,~-cyclooctadiene)lPF,/~ atm
H /ether/H O/Triton X-lOO/solid imine/room
temperature (83). The choice of solvent was critical; no reaction occurred in PEG/water or toluene/water.

Reductions with Formate

Formate has been used to reduce a variety of

organic substrates under phase transfer condi-

tions. For example, PhCH=CHOPh was

a o/(c reduced to PhCH CH COPh by NaOOCH

with

RUa 2 (PPh3) 3 /O-c6 H 4 2/H2

6 -

H I3)4NHS04at 1 0 g O c in 99 per cent yield

(84). Other a,B unsaturated carbonyl compounds

were also reduced successfully at the carbon-

carbon double bond. The best phase transfer

catalyst was (C6Hl3),NHS0,, which gave

quantitative reduction in minutes; others re-

quired hours.

Saturated aldehydes and ketones are reduced

to alcohols at p ° C using Aliquat-336 (85).

The best catalyst for aldehyde reduction was

found to be RuCl,(PPh,),, while a I:IO mix-

ture of RhCI(PPh,), and triphenylphosphine

was the best for ketones. Nitrobenzenes could

not be reduced with formate under phase

transfer conditions (85).

Aryl bromides underwent hydrogenolysisusing PdCI,(PPh,),/PPh, and (C,H, ,),NHSO, (86, 87). This proved to be a useful technique for preparing monodeuterated aromatic compounds on substituting D 2 0 for H 2 0 (87). Hydrogenolysis of I-(chloromethy1)naphthalene was achieved using a palladium catalyst in which a phosphine functionalised with a crown ether was bound to the palladium (88). A mixture of Pd(PPh,),Cl, and benzo-[ 18-crown-61 was far less effective than the functionalised palladium complex.

Miscellaneous Reductions

Aniline derivatives are prepared in excellent

yields, 84-100 per cent, from the correspon-

ding nitrobenzenes by reduction with carbon

monoxide at I atmosphere pressure, at room

, temperature, using Ru, (CO)12/NaOH/
c6H6/MeO(CH,) OH/BzEt Na (89). The
method gives good selectivity. No

hydrogenolysis occurs in the reduction of p-

chloronitrobenzene to p-chloroaniline (100 per

cent yield), and no reduction of the aldehyde

group occurs in the reduction of p-

nitrobenzaldehyde to p-aminobenzaldehyde

(100 per cent yield). Similar results are obtain-

ed with the mixed catalyst

[Rh(COD)CIl,/Co,(CO),, where COD is

I,5-cyclooctadiene, (9). Unhindered

nitrobenzenesare reduced successfullyat room

temperature and I atmosphere pressure with

> , syngas (I :I carbon monoxide:hydrogen) using

RuCI, (PPh, ,/c6H,CH /gM

NaOH/

BzEt3NCI(91). Reduction is much less facile

when either carbon monoxide or hydrogen is

used alone. Olefm, halide, and carbonyl func-

tional groups are not reduced under the same

conditions.

The reduction of phenyl bromide in yields of

up to 93 per cent was achieved with sodium

hydride, under solid-liquid conditions, using a

palladium phosphine catalyst in which the

phosphine was functionalised with an aliphatic

polyether (92). The optimum yield was critical-

ly dependent on the chain length of the

polyether, and little reduction occurred without

a phase transfer catalyst or even with a mixture

Platinum Metals Rev.,1990, 34, (1)

7

of a phase transfer catalyst and a palladium catalyst.
Benzylic alcohols were dehydrogenated to ketones in fair to good yields using [Rh(CO),Cll,/8M NaOH/C,H,/BzEt,NCI (93). Secondary alcohols gave ketones, while the primary alcohol 2-naphthalenemethanol gave z-naphthaldehyde.
Conclusions
The most useful application of phase transfer catalysis with platinum group metal complexes appears to be the formation of carbon-carbon bonds, particularly in carbonylations and vinylations. Indeed, the scope of vinylation reactions has recently been extended to include the allylation of alkyl iodides with allylsilanes using Pd(OAc), (94) and the allylation of acrylate esters with arylsulphonyl chlorides (95). The only industrial process employing a platinum group metal catalyst under phase

transfer conditions is the hydroformylation of propylene to butanal by Ruhr Chemie using a rhodium compound with the ligand (3-H03SC,H,),P. The separation of the rhodium complex from the products is simplified as a result of the water solubility of the rhodium complex.
While there arc a few instances of phase transfer catalysed oxidationsin which the use of platinum group metal complexes have proven advantageous, in general there are many facile methods of oxidation using platinum group metal catalysts which do not require phase transfer conditions.
Similarly, reductions with hydrogen and platinum group metals complexes under phase transfer catalysis conditions are not generally used, since there is a wide variety of supported platinum group metal catalysts which are readily accessible to perform such reductions conveniently (96).

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Platinum Metals Rev., 1990, 34, (1)

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Platinum Group Metals in Phase Transfer Catalysis