Homogeneous Transition Metal Catalyzed Reactions - American

28

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch028

Influence of Organophosphines on the Hydroformylation of Olefins Catalyzed by Anionic Ruthenium Clusters Georg Süss-Fink Institut de Chimie, Université de Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland

Anionic ruthenium clusters catalyze the hydroformylation of olefins with excellent chemo- and regioselectivity. Isotope-labeling studies and trapping of intermediates suggest that the catalytic reaction proceeds through the intermediacy of intact clusters. Organophosphines have a remarkable influence on the catalytic reaction. Triphenylphosphine completely blocks the catalytic activity of the ruthenium cluster. Diphenylphosphine, by contrast, enhances the catalytic activity but modifies the selectivity of the catalyst. The chemistry underlying these influences is discussed.

DISCUSSION OF TRANSITION METAL CLUSTERS as catalysts has led to controversy over whether such molecules can really be useful in catalysis. In the 1970s the goal in chemistry of transition metal clusters was to contribute significantly to the development of systematic catalysis. This movement was stimulated mainly by Johnson and Lewis (I) and the late Earl Muetterties (2). As oligonuclear species with intermetallic bonds, metal clusters occupy the "no man's land" between mononuclear metal complexes and polynuclear metal surfaces (Chart I). Because of this intermediary position between typical homogeneous catalysts and typical heterogeneous catalysts, transition metal clusters may be a new generation of catalysts (3). The aim of research in this area is twofold: to find transition metal clusters that provide a unique 0065-2393/92/0230-0419$06.00/0 © 1992 American Chemical Society

Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

420

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

\

\ Ml /

—M

/ j*'

1/ \ —-M-——M—

/ | \

/,

mononuclear

—M / l \

—M

i \ 1

M

M—

/ « \ / ' \

/I

1

oligonuclear

/ '

M M— / l \ / l \ M -M—

/ ! \

/ l \

potynuclear


Chart I. Intermediary position of metal clusters.

catalytic potential and to find transition metal clusters that are highly selec tive in their catalytic applications.

Trinuclear Ruthenium Cluster Anions as Catalysts The trinuclear cluster anion [ H R u ( C O ) ] " ( l ) , which we discovered in 1979, proved to be particularly interesting in this twofold respect. It is easily accessible from carbonylruthenium and base reagents (4, 5). In recent years we and others have found a considerable number of reactions catalyzed by 1, only two of which will be mentioned here. The unique catalytic potential of 1 expresses itself by the spirocyclization of alkyl isocyanates, which gives a surprisingly simple access to a new series of spiroheterocycles (6) (Scheme I). The high selectivity of 1 is demonstrated by the hydroformylation of propylene, which leads exclusively to C alde hydes with very high regioselectivity (n:i) ratio (7) (Table I). 3

u

4

This chapter describes the influence of phosphines (as cocatalysts) on the catalytic activity and the selectivity of 1 for the hydroformylation of olefins.

R \

0 5 R

-N=C=0

X

R-< Et SiH 3

Et SiOH 3

Yields :

η

R

I

R ^

0

R

R

Me Et

°Pr

*pτ "Bu

%

L\

69

22 51

43

Scheme I. Spirocyclization of alkyl isocyanates in tetrahydrofuran at 120 °C. The molar ratio of isocyanate to silane to cluster was 5000:1000:1.


28.

SUSS-FINK

421

Organophosphines and Hydroformylation of Olefins

Table I. Chemo- and Regioselectivity of Compound 1

H |— Chb-CHj-OV-CHO—^-CH3-CH CH -CH OH (n| 2

CHrCH=CH

CO + H

r

2 1


?

2

2

H

— CH —CH —CHO I

Solvent

Temperature CC)

DM F DM F DM F Clyme Diglyme

70 80 90 80 80

2

3

CH -CH-CH OH I 3

2

(i|

CH.

CH

(al)Chemoselectivity

(ol) Regioselectivity

3

(Aldehyde Alcohol)

(Normal:!sopropyl)

100.0:0.0 100.0:0.0 100.0:0.0 100.0:0.0 100.0:0.0

94.1:5.9 94.0:6.0 93.6:6.4 98.0:2.0 98.6:1.4

N O T E : The reaction took place with 0.17 mmol of [NEt ][HRu (CO)nl in 10 mL of solution at 10 bars of total pressure. 4

Organophosphines as Hydroformytotion

3

Cocatalysts

The high selectivity of 1 as a hydroformylation catalyst can be best understood on the basis of a catalytic cycle involving exclusive intact trinuclear clusters as intermediates by the steric demands of a trinuclear metal framework. The catalytic cycle as proposed in Scheme II is based on the trapping of the intermediate [Ru (CO) (OCEt)]~ by acidification of the reaction mixture with C F 3 C O O H and C H C O O D to give the neutral clusters H R u ( C O ) ( O C E t ) (8) and D R u ( C O ) ( O C E t ) , and on isotope-labeling studies with molecular deuterium as the hydroformylation component (9). 3

I()

3

3

1()

3

l0

For a large number of hydroformylation catalysts, both catalytic activity and selectivity can be improved by using organophosphines as cocatalysts (20). Accordingly, we attempted to make the hydroformylation of propene, which is already highly selective in the presence of 1, chemo- and regiospecific by adding triphenylphosphine or derivatives thereof. However, the catalytic activity of 1 collapsed completely in the presence of excess P P h . In contrast, catalytic activity increased in the presence of P P h H , but this increase was accompanied by a complete change of selectivity. 3

2

These effects cannot be explained on the basis of simple phosphine substitution products of 1. Therefore, we undertook to study the rather complex reactions of 1 with P P h and P P h H . Our goal was to elucidate the chemistry underlying the strange influences of organophosphines on the hydroformylation catalyzed by the cluster anion 1. 3

2

The reaction of 1 with triphenylphosphine was reported to give the monosubstitution product [HRu (CO) (PPh )] (2) (22). This anionic species was characterized in a careful kinetic study (22, 22); however, it has never 3

1()

3


422


CH =CH 2

+

2

CO

+

H

•>

2

CH ~CH -CH=0 3

2

0


c

CO Scheme II. Proposed mechanism for hydroformylation catalyzed by [NEt ][HRu3(CO)ii] in tetrahydrofuran at 100 ° C and 50 bar of pressure for 4 h. The catalytic turnover (ratio of products to catalyst) was 345 ± 5, and the catalyst recovery was 98%. 4

been isolated. As a catalyst, 2 should be at least as active as 1. Therefore, the breakdown of the catalytic activity of 1 in the presence of P P h

3

cannot

be caused by the formation of 2. Rather, it must originate in transformations of the cluster that are much more complicated than the substitution of a carbonyl by a phosphine ligand. We therefore undertook a careful Η N M R study of the reaction system !

l-PPh

3

(Figure 1). The monosubstitution product [ H R u ( C O ) ( P P h ) ] - (2) 3

10

3

is formed and gives rise to a doublet hydride signal at-12.01 ppm. However, even before 1 has completely disappeared, the formation of the disubstitution product [ H R u ( C O ) ( P P h ) ] - (3) (Scheme III) is indicated by a triplet hy 3

9

3

2

dride signal at -11.20 ppm. Anions 2 and 3 are not very stable. Even at 20 °C, 3 undergoes elimi nation of benzene and converts into the phosphine-phosphido derivative [Ru (CO) (PPh )(PPh )]~ 3

9

3

2

(4), which can be isolated and characterized as


Suss-FiNK

I

6

• -11

423



28.

.

-

-12

-13

-1H

-15

-16

20°C

0

min

20°C

10

min

20°C

120 min

20°C

900 min

66°C

120 min

ppm

Figure 1. H NMR spectra of the reaction of 0.5 mmol of [N(PPh) ] [HRue(CO)u] with 0.5 mmol of PPh in 50 mL of tetrahydrofuran at various temperatures. l

3

3


424


Ί

0 IHRU (CO) ]3

U

(1)

r

P

P

h

-

R

"

3

20 C E


CO

(HRuj(CO)

1 0

f

(PPh,)l-

P

P

h

( 2

)

°

^ - RJu

S

Ph,P

ao c e

1

A c ./ -/-Ru'

1

x

CO

(HRu (CO) (PPh ) ]' 3

t

3

2

(3)

/ — Ru

— Ruι I

— ΒRu11

10«C

k

Ph P 3

PhH

,

[Ruj(CO) (PPh,)(PPh )]9

2

(4)

C

'

^

i/\

\

^ R u ^ ^ O

CO Κ

1

Ο ^ x - — / ^ „ — Ru /—Ru

CO

P

h

3P

/

X

[HRuj(CO) (PPh,)(PPhC,e ) ] 4

t

Ru isolated aa t e t r a e t h y l ammonium salt

x

/ —

\ / / H - i i . R

' ι Ph P

R

u

—

\

u

\

3

Scheme III. The reaction system l-PPh 3


28.

SCSS-FINK


425

bis(triphenylphosphine)iminium salt. Heating at conditions similar to those of the catalytic process converts 4 with orthometalation of one of the aromatic rings and carbonyl substitution into the anion [HRu (CO) (PPh )(PPhC H )]~ (5), which was isolated as tetraethylammonium salt. The singlecrystal X-ray structural analysis of 5 was performed. 3

8

3

4


6

Molecular structure of 5. The isolation and characterization of the cluster anion 5 as the species formed from 1 and P P h under catalytic conditions explains why the catalytic cycle according to Scheme II is suppressed. One of the two P P h ligands coordinated initially on the metal framework undergoes benzene elimination and orthometalation to give a tripodal phosphorus-carbon handle over the triangular metal face blocking the catalytic activity of the cluster. The variable-temperature N M R spectra in fact show that 5 is a rigid cluster, the ligands of which are not fluxional. 3

3

In contrast to P P h , diphenylphosphine enhances the catalytic activity of the cluster anion 1. We therefore studied the stoichiometric reaction of 1 with P P h H , from which we isolated the bisphosphido derivative 6 as bis(triphenylphosphine)iminium salt. 3

2

[HRu (CO)i,]- + 2 P P h H 1 3

2

[HRu (CO) (PPh ) ] 6 3

8

2

2

+ H

2

+ 3CO


426

HOMOGENEOUS

TRANSITION

METAL CATALYZED

REACTIONS

The crystal structure analysis reveals that anion 6 contains a triangular metal framework with two different R u - R u bonds bridged by P P h ligands, which occupy different sides with respect to the R u plane. One of the phosphido-bridged R u - R u edges also carries the hydride bridge. The solidstate structure of compound 6 is found in solution only at low temperature. Variable-temperature N M R spectra (Figure 2) prove that 6 is fluxional in solution. These spectra are best interpreted in terms of dynamic site exchange of the hydride ligand between the two phosphido-bridged R u - R u bonds. 2


3

In contrast to the rigid orthometalated cluster anion 5, the fluxional cluster anion 6 catalyzes the hydroformylation of olefins. However, the selectivity is completely changed with respect to that of 1. Whereas 1 is

Molecular structure of 6.



28.

6

190

170 31

427


SCSS-FINK

150 ppm

P-( H}-NMR 1

+60° C

•30°C

-30°C

-30°C

6

-16 -17

-18 ppm

*H-NMR

Ί-

Figure

2. Variable-temperature

Τ

NMR spectra of 6 and dynamic of hydride ligand.

site

exchange

extremely chemoselective for aldehydes (even chemospecific under optimum conditions), 6 provides a mixture of aldehydes, alcohols, and ketones. For ethylene, the ratio of the aldehyde to ketone to alcohol products can be varied between 90:2:8 and 29:70:1, depending upon the ethylene partial pressure. This loss of selectivity of 1 in the presence of P P h H , accompanied by an enhancement of catalytic activity (Table II) can be ex plained by the formation of 6. This cluster anion, fluxional as the parent 2



s

2

2

3

s

2

3

2

2

2

2

2CH =CH 2

3

2

2

+ C O + H - » (CH -CH ) C=0

100 140 140 140

Temperature (°C)

3

2

4

15, 15, 25, 40,

2

30, 20 15, 10 15, 10 15, 10

2

Partial Pressures" (C H , CO, H )

2

2

345 1210 1240 1260

Catalytic Turnover ( Products 1C atalyst )

2

CH -CH -CH=0 + H -» CH3-CH -CH -OH

2

2

+ CO + H -> CH3-CH2-CH=0

NOTE: All reactions took place in tetrahydrofuran for 18 h. "Partial pressures are in bars. ^Aldehyde to ketone to alcohol ratio.

3

[HRu (CO)„][HRu,(CO) (PPh ) ][HRu (CO),(PPh ) ][HRu (CO) (PPh ) ]-

Catalyst [NEU]*

2

2

CH =CH

Table II. Hydroformylation versus Hydrocarbonylation


99:0:1 90:2:8 65:30:5 29:70:1

1

Selectivity "

28.

SUSS-FINK


429

anion 1, also catalyzed the hydroformylation of olefins. However, it proceeded by a different mechanism than that determined for 1, which was depicted in Scheme II. Studies to elucidate the catalytic cycle of the ethylene hydroformylation, catalyzed by phosphido derivative 6, are under way.


Acknowledgments We gratefully acknowledge financial support by the Fonds National Suisse de la Recherche Scientifique and by the Stiftung Volkswagenwerk. We thank the Johnson Matthey Technology Centre for a generous loan of ruthenium(III) chloride hydrate.

References 1. Johnson, B. F. G.; Lewis, J. Colloq. Int. C. N. R. S. 1977, 281, 101; Pure Appl. Chem. 1975, 44, 43. 2. Muetterties, E. L. Science (Washington, D.C.) 1977, 196, 839; Bull. Soc. Chim. Belg. 1975, 84, 959. 3. Muetterties, E. L.; Krause, M. J. Angew. Chem. 1983, 95, 135; Angew. Chem., Int. Ed. Engl. 1983, 22, 135. 4. Suss-Fink, G . Inorg. Synth. 1986, 24, 186. 5. Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Süss(-Fink), G. J. Chem. Soc., Dalton Trans. 1979, 1356. 6. Süss-Fink, G.; Herrmann, G.; Thewalt, U. Angew. Chem. 1983, 95, 899; Angew. Chem., Int. Ed. Engl. 1983, 22, 880; Angew. Chem. Suppl. 1983, 1203. 7. Süss-Fink, G.; Schmidt, G. F. J. Mol. Catal. 1987, 42, 361. 8. Kampe, C. E.; Boag, N. M.; Kaesz, H. D. J. Am. Chem. Soc. 1983, 105, 2896. 9. Süss-Fink, C.; Herrmann, G. J. Chem. Soc., Chem. Commun. 1985, 735. 10. Cornils, B. In New Syntheses with Carbon Monoxide; Falbe, J., Ed.; Reactivity and Structure, Concepts in Organic Chemistry 11; Springer-Vcrlag: Berlin, 1980. 11. Taube, D. J.; Ford, P. C. Organometallics 1986, 5, 99. 12. Taube, D. J.; van Eldik, R.; Ford, P. C. Organometallics 1987, 6, 125. RECEIVED

for review October 19, 1990.

ACCEPTED

revised manuscript June 6, 1991.


Homogeneous Transition Metal Catalyzed Reactions - American

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