Hydroformylation and Hydrogenation with Platinum Phosphinito

25 Hydroformylation and Hydrogenation with Platinum Phosphinito Complexes

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch025

Piet W. Ν. M . van Leeuwen and Cornelis F. Roobeek Koninklijke/Shell-Laboratorium, Amsterdam, Shell Research B.V., Postbus 3003, 1003 AA Amsterdam, Netherlands

Platinum complexes containing phosphinito ligands are moderately active hydroformylation catalysts (30-100 bar, 80-100 °C). The products consist of a mixture of alcohols and aldehydes with linearities in excess of 90%. Internal alkenes can also be hydroformylated, with linearities as high as 70%. Alkyl and acyl complex intermediates have been identified. Aldehyde reduction proceeds via metal alkoxide spe cies rather than hydroxymethyl fragments. Aldehyde reduction can be greatly accelerated by the addition of carboxylic acids. In sum mary, diphenylphosphinous acid turns out to be an interesting ligand with peculiar electronic properties capable of inducing catalytic hy droformylation and hydrogenation. It may have a function in promoting the activation of dihydrogen, which seems to be a prerequisite for platinum complexes to be catalytically active.

HYDROFORMYLATION OF INTERNAL ALKENES

to linear products is a key process for the industrial production of higher alcohols. Two metals are commercially applied, rhodium and cobalt. O f these, only cobalt is used for converting internal alkenes to terminal hydroformylation products. Platinum is the third metal active in hydroformylation (1-11). Out of the plethora of known platinum (hydride) complexes, only those containing trichlorostannate as the ligand-anion show activity as hydroformylation catalysts. The classical, but still rare, example of a platinum complex active as catalyst for the hydrogénation of alkenes (12) also requires the presence of trichlorostannate as the ligand. Cationic complexes have been reported as active hydroformylation catalysts, yielding only branched products (13). A combination of the cationic character and trichlorostannate anion led to the 0065-2393/92/0230-0367$06.00/0 © 1992 American Chemical Society

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

368

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

discovery of the conversion of internal alkenes with platinum catalysts (5), although the selectivity to alkanes was rather high (>40%). In a preliminary communication (14, 15) we reported on the formation of platinum hydroformylation catalysts with P P h O H (phosphinous acids or diphenylphosphine oxide) as the ligands. Quite significantly, they are also active for internal alkenes. In this report we review some of the results of the catalytic hydroformylation studies as well as the isolation of several intermediates and hydride precursors. The synthesis, not the catalysis, of one of the hydride catalyst precursors was described earlier (16). In addition, we report on the use of a new catalyst (viz., platinum phosphinito complexes in the presence of carboxylic acids) for the hydrogénation of aldehydes. Hydrogénation of aldehydes during hydroformylation is a secondary reaction, which may well be promoted by the carboxylic acids formed by partial decomposition of the catalyst.


2

Catalytic Hydroformylation Reactions In situ mixing of Pt(l,5-cod) (cod is 1,5-cyclooctadiene), P P h O H , and various other donor ligands led to active catalysts. Hydroformylation of 1-heptene gave rise to a high linearity and appreciable rates (see Table I, entries 1-4). When no P P h O H was added (entry 5) the catalytic activity was negligible. Two hydroformylation products were formed, aldehydes and alcohols. The formation of alcohols requires 2 mol of H , and therefore most experiments were carried out with C O and H in a 1:2 ratio. 2

2

2

2

2

The composition of the actual catalyst cannot be derived from the ratios of P P h and P P h O H applied and the resulting activities, although a presumably unsaturated system such as that of entry 1 seems to give the highest activity. The lowest activity for isomerization was found when more saturated platinum complexes could be formed (as in, for instance, entry 2). A n excess of diphenylphosphinous acid only temporarily slows down the reaction. The excess reacts with the aldehyde product to form an ot-hydroxyphosphine oxide that does not interfere with the platinum complexes. Several other ligand combinations were tested; entries 8 (with P h P C H C 0 H ) and 9 (with P C y ) serve as examples. 3

2

2

2

2

3

Bidentate phosphines were reported (10, 11) to have a pronounced and accelerating effect on hydroformylation with platinum trichlorostannate catalysts. In combination with platinum phosphinito complexes, the bidentate ligands dppe, dppp, and dppb (see Table I, entries 6 and 7) led to catalysts with higher rates. However, the effect is not as spectacular as that seen with the trichlorostannate complexes. The systems based on P P h and P P h O H led to isolable species derived from complex 1 (Scheme I), previously reported by Roundhill et al. (16). Complexes l a - l e can be used as the catalyst precursor; the results in this case are in line with those obtained from in situ mixing (Table I, entry 10). 3

2


25.

VAN L E E U W E N & ROOBEEK

1

CO

00

-fH o

o

00

r-H (M

1 1

ci

CO

H f

CD

I

o ^

d

-H

co

ίο ο*

O

(06

00

-fH

S

«53 « a

N

H

M

4

H

4

d io io

Q

h

H

' N

·Ο

ïν

^ .JS

t>

^

Q

CM T f

r*-

1Λ n

lO i C CM*

^

-fH·*->c

^

a: ·O

H

»—5

.

O

S- — S-— n ^ « E ffi « S3

' ^ O ^ O T J O ' D ^ ' U - w w w ϋ J3 HS 0 - f i U f C Ofi3 O - C ÇU CD Û f f i ^ S-fi ^ 0 Η o-i^rpu, C U ^ P M ^ P X I ^ O H ^ O H ^ 4^Ê« JS y Q M p k P H C l p p Q a Q C l 4 C X Q H ( ^ & ' ^ CU(^PHQHPHP-I 4

-H

ΙΟ

d, · —

d d

ΙΟ

PH

•Ο Ο —' ©, —

CO

Φ 05 ΙΟ W o i ^ oc* oc* ο ό

g

CO *

CD CD

Φ>

S

5- ^

g:

o

i-H »H

S

f-î

CM

P-H

CM

in ic oq o ^ i/S io

ph

is.

Scheme II.

ethyl complexes 2a-2e can also be converted to the propionyl derivatives by admission of 1 bar of C O at 25 ° C . Acyl complexes 3a-3e were prepared by this method. From consideration of the H N M R spectra of the butyric complex 7 (Scheme III), we conclude that the linear isomer is formed exclusively, not only in the catalytic experiment but also in the stoichiometric reaction. We assume that the formation of the linear isomer of the acyl is kinetically controlled because we do not expect a large difference in stability between the linear and branched acyl complexes. In the alkyl complexes the linear isomers are most likely thermodynamically favored. Comparison of the p N M R spectra obtained from the solutions after a catalytic hydroformylation run revealed that they are the sum of the spectra of the hydride and acyl species together with some decomposition products. !

3 1

0 -p

4

Η %

0 -p

L

2

\

y

/

\

Ph

Η

C H

6

20

bar

3

CO H

/ Ph

P

t

\ C - n - C 3"7 ,H

0 —P

90° C

2

'

2

o'

la Scheme HI.


372

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

Catalysts based on dppe display somewhat higher activity (entries 6 and 7, Table I) than those based on monophosphines such as P P h . Accordingly, we synthesized platinum hydride complexes based on dppe (or dppp) and P P h O H that turned out to have structure 8 (Scheme IV). The hydride 8 did not lead to ethyl complexes after reaction with ethene at 30 bar. Reaction at 85 °C under 20 bar of ethene:CO (1:1) gave the propionyl complex 9 within 0.5 h. 3


2

9 L = PPh 0 2

Scheme IV.

Decomposition Studies; Formation of Phosphido Species Prolonged hydroformylation (24 h) caused complete decomposition, and a very complex P N M R spectrum was recorded. This spectrum showed the presence of at least four species, all containing platinum dimers with a phosphido bridge. During work-up through column chromatography all were converted into 12 (Scheme V). This compounds structure was established by an X-ray determination (18). 3 1

O-P, i i

H O - P

10

11

O-P, H

H O - P

12

Scheme V i: 90 ° C , 20-50 bar ofH , C O , and C H ; ii: le added, C H COOH and PPh OH removed. 2

2

4

2


2

5

25.

VAN L E E U W E N & ROOBEEK

Platinum Phosphinito Complexes

373

Decomposition of triarylphosphine to phosphido species is a common reaction often encountered in catalytic reactions using phosphine complexes of noble metals (17). When the present hydroformylation reaction for ethene was run for 24 h, traces of propionic acid were found instead of the decom position products to be expected from aryl groups of the added phosphines. H N M R analysis showed that indeed roughly one propionic acid mol ecule had been formed per mole of platinum dimer after work-up. From this and other evidence, it was concluded that the diphenylphosphido anion originates from diphenylphosphinous acid ("diphenylphosphine oxide"; see Scheme V, complexes 10-12). This reaction is quite surprising because the phosphine-oxygen double bond is extremely stable toward cleavage. The scheme involves the formation of a mixed anhydride of a carboxylic acid and phosphinous acid. Such complexes have been observed in the reaction mix ture. An alternative way to form complexes of phosphinous carboxylic acid anhydrides 11 is the treatment of phosphinous acid complexes with acetic anhydride. Indeed, addition of acetic anhydride to refluxing l a or l e in toluene gave the characteristic red color and p N M R spectra of the dimeric complex 12 and its analogues within 15 min.


l

3 1

These results show that p ί ( μ - Η ) ( μ - p p η ) p ί is a stable bonding unit that is preferentially formed under severe conditions. In addition, we have seen that diphenylphosphinous acid can be reduced by platinum acyl complexes. 2

Aldehyde Reduction The catalytic conversion of aldehydes into alcohols during the hydrofor mylation of alkenes suggests that the present platinum phosphinito com plexes are catalysts for the reduction of aldehydes in the presence of C O . Very few known catalysts affect this reaction in the presence of C O ; phos phine-modified cobalt catalysts are the most familiar example. Ruthenium and rhodium catalysts (notably in the absence of CO) have also been reported (19, 20). When complex l e was tested as a hydrogénation catalyst for aldehydes, we accidentally found that very fast catalysts can be obtained when carboxylic acids are present in these catalyst systems (see Table II, entries 2 and 3). The highest rates were obtained in the absence of C O , but at 5 bar of C O the rates were acceptable (entry 8). In the hydroformylation experiments the rate of hydrogénation was only in the order of ten moles of aldehyde per mole of catalyst per hour, but this time turnover frequencies of several thousands were obtained. Even for acetone a turnover frequency of 500 mol mol h was found, but in the absence of acetic acid the rate of 2-propanol formation was negligible. The addition of carboxylic acids has no rate-enhancing effect on the hydroformylation reaction. This fact was shown in a hydroformylation experiment with ethene as the substrate; the rate remained the same and the 1

1


374

HOMOGENEOUS TRANSITION M E T A L CATALYZED REACTIONS Table II. Hydrogénation Results

Exp. No. 1 2 3 4 5 6 7 8

le (mmol)

Acid (mmol)

0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01

\-C HTCOH (mmol)

Solvent (mL)

110 110 220 22 275 275 220

ethanol (20) toluene (20) none toluene (20) acetone acetone none

3

t - C a H y C O O H (4) i - C H C O O H (9) Î - C 3 H 7 C O O H (9) CH COOH(0.1) none CH COOH(10) t-C H7COOH (5) as 7, with 5 bar C O 3

b

7

b

3

3

3

c c

Rate" 1000 4500 9000 220

Hydroformylation and Hydrogenation with Platinum Phosphinito

Recommend Documents