Initiation of Polymerization - American Chemical Society

12 Rational Design of Catalysts with Interacting Supports 1

2

Downloaded by STANFORD UNIV GREEN LIBR on March 15, 2013 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch012

G. L. BAKER, S. J. FRITSCHEL, and J. K. STILLE Colorado State University, Department of Chemistry, Fort Collins, CO 80523

Polymer bound catalyst systems have become highly sophisti cated. For supports, commercial resins have in cases given way to custom tailored polymers designed to optimize a sup ported catalyst's performance. This progression from simple to complex systems is illustrated by advances in supported catalysts used for the asymmetric hydrogenation of enamides. Early supports were designed so that the support had no influence on the catalysts' activity. An optically active sup port, designed to interact with the catalyst and improve its per formance, has now been synthesized and used in asymmetric hydrogenations. The influence of the support on the catalyst was surprisingly low. This result can be interpreted in terms of the mechanism of asymmetric hydrogenation. Soluble homogeneous catalysts can perform useful chemical transformations under mild conditions. One of their chief disadvantages is that their solubility in the reaction medium makes the separation, recovery, and recycling of the catalysts difficult. This would be a minor concern, but for the fact that the most useful catalysts often use costly metals such as platimum, palladium and rhodium. The solution to this problem has been to attach these catalysts to polymer supports. The ideal polymer-bound catalyst must satisfy a formidable list of requirements. It should be easily prepared from low cost materials. The support must be compatible with the solvent system employed, and be chemically and thermally stable under the reaction conditions. The catalyst should show minimal losses in reaction rate or selec tivity when bound to the support, and should be able to be recycled many times without loss of activity. Finally, the interactions between the catalytic site and the support must be either negligible or beneficial. The development of polymer sup ported rhodium-phosphine catalysts for the asymmetric hydrogenation of amino acid precursors illustrates the incremental process which has led to supports which approach the ideal support. Early catalysts were bound to crosslinked polystyrenes (1,2,3). and shared the same swelling characteristics as polystyrene. These catalysts proved inferior to their 1

Current address: Bell Laboratories, Murray Hill, Ν J 07974.

Current address: Ε. I. duPont de Nemours and Company, Polymer Products De partment, Wilmington, D E 19898. 2

0097-6156/83/0212-0137$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

138

INITIATION OF POLYMERIZATION

homogeneous analogs, chiefly due to the incompatibility of the support with the sol vent of choice, ethanol. Reactions demanding a non-polar medium, such as asym metric hydrosilylation, however, could be carried out (3). To surmount this problem, monomers were prepared which contain the desired ligand, and upon polymerization with suitable comonomers, the ligand is incorporated into the polymer. This has been accomplished with monomers such as 1 and 2 (Fig-


(4,5,6)

(7,8)

ure 1) to give polymers containing either DIOP or BPPM-type ligands respectively. The comonomers can be chosen to give the optimum ligand density, crosslink density, and swelling characteristics for the supported catalyst. In addition, this route assures a high degree of ligand purity, since few reactions are carried out on a crosslinked support. Using catalysts derived from these polymers, products can be obtained in optical purities comparable to those achieved with their homogeneous ana logs. Interacting Supports Although most supports have been designed to minimize catalyst-support interac tions, an appropriate designed support might be expected to interact with the catalytic site in a favorable way, enhancing enantioselectivity. This can be tested by providing an additional optical center at the catalyst and then observing any change in the enatiomeric excess of the product. A previous approach to this problem demonstrated that such an effect was indeed possible (7). A polymer containing an optically active ligand and methyl ketones was reduced by asymmetric hydrosilylation to give the opti cally active support. Unfortunately the enantioselectivity of this reduction could not be evaluated, and catalysts derived from this polymer gave low optical yields. A superior method for preparing polymer bound phosphines of high optical purity is to polymerize an optically pure phosphine monomer with the desired comonomers. This approach was extended to the comonomer. An optically active comonomer suit able for use in preparing a polymer containing optically active pendant alcohols should be available in both the R and S enantiomers so that the chirality of the alcohol and that of the catalyst may be matched to provide a synergistic effect. A suitable starting material for the synthesis of optically active comonomers is 2,3-butanediol, since the R,R isomer is commercially available, and the S,S isomer can be synthesized in a straightforward manner from tartaric acid (9). Optically active acrylates 3a-c were prepared from the diols to give optically pure monomers (Fig. 2). An inactive mono mer was synthesized from racemic 2,3-butanediol for use as a standard. Two types of supported phosphine polymers were prepared. Phosphinopyrrolidinecontaining polymers were prepared by copolymerizing 2 with acrylates 3a-c and ethylene dimethacrylate to give white free-flowing powders. In a similar fashion 1 was copolymerized with 3a-c and ethylene dimethacrylate. Treatment of these poly mers with a large excess of sodium diphenylphosphide gave polymers containing DIOP-type ligands. All of the polymers swell in both tetrahydrofuran and ethanol. Since ethanol would be expected to compete with the polymer bound alcohols for sites at the catalyst, tetrahydrofuran was chosen as the reaction solvent. The polymer bound catalysts were prepared by stirring the polymer and μ^ίοη1θΓθθΐ8(1,5cyclooctadiene)dirhodium(I) in tetrahydrofuran for several hours, and after filtration the yellow catalyst was then transferred under argon to the reaction vessel containing the substrate. Solvent was added, and the reaction vessel was pressurized with hydro gen. At the end of the reaction, the pressure was released and the product was iso-

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

BAKER ET AL.

Catalysis

with Interacting

Supports


12.

Figure 1.

Synthesis of polymers with optically active supports.


139



Me

Me

OH

OH

Me

11 BuLi 2) Me SiCI 3

f HO

* O: OSiMe

3

Ο

CCLi3A sieves 4

Me

Me

Me f

OH

Τ—O

*

OSiMe

RR

3a b

SS

c

racemic

Figure 2. Synthesis of monoacrylate comonomers from butanediols.


3


141

Catalysts with Interacting Supports

BAKER ET A L .

(?) - B P P M / H E M A

®-BPPM/acrylate

////(////////////// PPh

2

\

ι

/

PPh

HC-Me

I

PPh

^NHAc \(

^COOH

\

?

/

CH PPh

, ™ HO Me

2

2

2

I

0

CH OH 2

H

2

THF

NHAc

*l

MeCH COOH ee

(config.)

®-

BPPM/RR

acrylate

10

S

®©©-

B P P M / S S acrylate

2

R

B P P M / r a c acrylate

6

S

B P P M / HEMA

12

S

B P P M / THF

15

R

- B P P M / EtOH

28

S

Figure 3. Rhodium-catalyzed hydrogénation of 2-acetamidoacrylic acid with a BPPM-type phosphine on an optically active support.


142



© - D I O P / a e r y late

///////////////////

©-DIOP/HEMA

///////////////////

©-DIOP/RR

acrylate

10

S

©-DIOP/SS

acrylate

1

R

® - D I O P / rac.

acrylate

6

S

26

R

-DIOP/THF

59

R

-DIOP/EtOH

68

R

©-DIOP/HEMA

Figure 4. Rhodium-catalyzed hydrogénation of 2-acetamidoacrylic acid with a DIOP-type phosphine on an optically active support.



12.

BAKER ET AL.

Catalysts with Interacting

Supports

143

lated. The workup consisted of filtration to remove the supported catalyst and evaporation of the solvent to dryness. The substrate chosen to probe the effect of the optically active alcohols was 2-acetamidoacrylic acid. The hydrogénation product, N acetylalanine, was converted to the methyl ester by diazomethane and was then analyzed by G L C using a column containing a chiral stationary phase. The effect of the additional optical center was smaller than anticipated (Figure 3). Different enantiomeric excesses were observed for BPPM type catalysts containing R,R, S,S or racemic alcohols, with the racemic support falling between the optically active supports. Far greater was the solvent effect due to the alcohols, shifting the enantiomeric excesses to a position midway between those obtained homogeneously in ethanol and those obtained with tetrahydrofuran as the solvent. The hydrogénation reaction is also sensitive to the structure of the alcohol comonomer, with the primary alcohols of the hydroxyethyl methacrylate polymer interacting with the catalyst to give results more closely resembling those found when ethanol is used as the solvent. Similar results were found with DIOP-type ligands (Figure 4). The weak effect of the secondary optical center can be explained by a consideration of the mechanism of asymmetric hydrogénation. It was demonstrated that the rate determining step is the oxidative addition of hydrogen to the rhodium-olefin complex (10). During this step, solvent would not be expected to be coordinated to the complex. Thus any influence of the secondary optical center would not be through direct coordination to the metal, but rather by a lesser effect in the surrounding medium. This suggests that it will be difficult to prepare a system in which a secondary optical center will have much influence in rhodium catalyzed hydrogénations of enamides. One important observation is the ability of the polymer to greatly affect the solvent environment at the catalytic site. This can occur even though the polymer is highly swollen in solvent. Although this is a complicating factor at times, it is likely that a carefully designed system would be able to exploit this characteristic of supported catalysts. Acknowledgment This work was supported by Grant DMR-77-14447 from the National Science Foundation, and by the Phillips Petroleum Company.

Literature Cited 1. Strukal, G.; Bonivento, M.; Graziani, M.; Cernia, E.; Palladino, N. Inorg. Chim. Acta 1975, 12, 15. 2. Krause, H. W. React. Kinet. Catal. Lett. 1979, 10, 243. 3. Dumont, W.; Poulin, J.-C.; Dang, T.-P.; Kagan, H. B. J. Am. Chem. Soc. 1973, 95, 8295. 4. Fritschel, S. J.; Ackerman, J. J. H.; Keyser, T.; Stille, J. K. J. Org. Chem. 1979, 44, 3152. 5. Takaishi, N.; Imai, H.; Bertelo, C. A.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 264. 6. Masuda, T.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 268. 7. Achiwa, K. Chem. Lett. 1978, 905.



144

8. Baker, G. L.; Fritschel, S. J.; Stille, J. K. J. Org. Chem. 1981, 46, 2954. 9. Schurig, V.; Koppenhoefer, B.; Buerkle, W. J. Org. Chem. 1980, 45, 538. 10. Chan, A. S. C.; Pluth, J. J.; Halpern, J. J. Am. Chem. Soc. 1980, 102, 5952.


RECEIVED October 15, 1982


Initiation of Polymerization - American Chemical Society

Recommend Documents