Enantioselective Catalysis with Transition Metal Compounds

9 Enantioselective Catalysis with Transition Metal Compounds Downloaded by UNIV OF ARIZONA on November 8, 2012 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch009

Right or Left—This Is the Question Henri Brunner Institut für Anorganische Chemie, Universität Regensburg, 31, D-8400 Regensburg, Germany

Universitätsstrasse

Extension of the scope of enantioselective catalysis with transition metal complexes from well-established reaction types, such as the hydrogenation of dehydroamino acids and the hydrosilylation of ketones, to new reaction types is a challenging goal. The extension to (i) the transfer hydrogenation of itaconic acid with formic acid, (ii) the hydrophenylation of norbornene, and (iii) the homo-Diels-Alder reaction of norbornadiene with acetylenes is described. In reactions i and iii, there is virtually complete optical induction.

V ^ P T I C A L L Y P U R E S U B S T A N C E S are increasingly in demand for human food additives, animal food supplements, pharmaceuticals, and agrochemicals. Enantioselective catalysis with transition metal compounds is a promising approach to meet this demand. An optically active catalyst is required in only small quantities, an economically important point. It reenters each catalytic cycle with its chiral information. Therefore, large amounts of optically active compounds can be prepared by using only small amounts of an optically active catalyst. Preferably, the enantioselective cocatalyst is applied as an in situ catalyst. Such an in situ catalyst consists of a metal compound (the procatalyst) and an optically active ligand (the cocatalyst), both of which (in favorable cases) are stable and commercially available. Use of in situ catalysts does not require the synthesis of the actual catalyst prior to the catalytic reaction. 0065-2393/92/0230-0143$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

144

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

Hydrogénation

and Hydrosilyhtion

Well-established reaction types for enantioselective catalysis with transition metal complexes are the hydrogénation of dehydroamino acids (Scheme I) and the hydrosilylation of ketones (Scheme II). Hydrogénation of (Z)-a-N-acetamidocinnamic acid to give N-acetylphenylalanine (Scheme I) is a frequently used standard reaction. A variety of Rh complexes catalyze this reaction under mild conditions (room temperature, no hydrogen pressure), with optical inductions close to 100%. A n example of an in situ catalyst is [Rh(cod)Cl] -Norphos (see Abbreviations list) (I, 2). [Rh(cod)Cl] is the procatalyst, and the optically active chelate phosphine, Norphos, is the cocatalyst. Both are shown in Scheme I.

Downloaded by UNIV OF ARIZONA on November 8, 2012 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch009

2

2

In the hydrosilylation of acetophenone with diphenylsilane, first a catalytic addition of a S i - Η bond to the C = 0 bond occurs. This addition gives rise to a silyl ether, which is subsequently hydrolyzed at the O - S i bond.

procatalyst

Norphos Scheme I.


9.

BRUNNER

Enantioselective Catalysis with Transition Metal Compounds 145

0 II H C^ 3

^C H 6

6

5

H

5

2

I.CQt. 2.H 0 2

OH

HO

\ /

HC

C H


3

H V

6

5

HC

C H

3

5

6

5

95

COOC H 2

5

Scheme II.

1-Phenylethanol is the ultimate product (Scheme II). In this reaction, the celebrated optically active chelate phosphines are inefficient cocatalysts as far as enantioselectivity is concerned. Therefore new types of nitrogen l i gands have been introduced as optically active cocatalysts, such as the pyridinethiazolidines. In situ catalysts consisting of [Rh(cod)Cl] and the pyri2

dinethiazolidine shown in Scheme II give optical inductions close to 100% in the hydrosilylation of acetophenone with diphenylsilane (3, 4). To extend the scope of enantioselective catalysis with transition metal complexes from the established reaction types (hydrogénation and hydrosilylation, exemplified in Schemes I and II) to new reaction types is a challenging goal. This extension is the topic of the following paragraphs.

Enantioselective Hydrogénation

with Formic Acid

Itaconic acid is a frequently used substrate in enantioselective hydrogénation. The best results have been obtained with rhodium catalysts of the ligand B P P M (see Abbreviations list) and B P P M derivatives. Molecular hydrogen can be replaced by formic acid as the transfer hydrogénation agent (5, 6). The most convenient choice is the azeotrope H C O O H - N E t

3

(5:2),

which is commercially available. This transfer hydrogénation takes place in D M SO (dimethyl sulfoxide) at room temperature. It avoids the inconvenience and risks of molecular hydrogen and pressure.


146


The in situ catalyst [Rh(cod)Cl] -BPPM (Scheme III) gives methylsuccinic acid of 85% ee (enantiomeric excess), similar to the enantioselectivity of the hydrogénation with molecular hydrogen (7). In the transfer hydrogénation, chelate phosphines giving seven-membered rings are superior to chelate phosphines giving six- or five-membered rings. In addition to Rh(I) compounds such as [Rh(cod)Cl] , Rh(II) compounds such as R h ( O A c ) or Rh(III) compounds such as R h C l are suitable procatalysts (6). Triethylamine can be replaced by other amines. The use of (fi)-l-phenylethylamine decreases the enantioselectivity of all the in situ catalysts, whereas (S)-l-phenylethylamine increases the enantioselectivity of all the systems, and thus gives virtually complete optical induction (Scheme III). Another type of reaction with H C O O H - N E t as the reducing agent is the hydroarylation of norbornene with iodobenzene. This reaction is catalyzed by phosphine palladium acetate complexes (8, 9). The product is exo2

2

2

4


3

3

H CH COOH \ / C=C / \ H COOH 2

•

HCOOH/NR3

procat. • cocat.

5 : 2

H

CH COOH

Η

2

*3 W

»%

* V

COOH

cocat. :

Ν

Λ

procat.:

NR3 NEt

[Rh(cod)Cl] 3

2

Rh (0Ac)^ 2

RhCI

3

84.9

92.2

82.2 */. ee

(R)-PhMeCHNH

2

7A.6

87.0

80.1

·/· ee

(S)-PhMeCHNH

2

90.5

98.7

99.5

V. ee

Scheme III.


9.

BRUNNER

Enantioselective Catalysis with Transition Metal Compounds

147

phenylnorbornane, a chiral molecule. With an in situ catalyst consisting of Pd(OAc) and Norphos (Scheme IV) we obtain phenylnorbornane in 60% chemical yield and 45% ee (10). The optical purity of the hydrocarbon phen ylnorbornane can be determined by gas chromatography using a permethylated β-cyclodextrin column (11).


2

cat. : LSV* ee ! 6 0 * / · yield)

Pd(OAc) /Norphos 2

1 Mol % DMF, 6 0 ° C ,

16h Scheme IV.

Other new reaction types have been opened up to enantioselective catalysis with transition metal compounds, such as the Michael addition of methyl l-indanone-2-carboxylate to methyl vinyl ketone (12, 13), the monophenylation of meso-diols with P h B i ( O A c ) (14,15), and the hydrosilylation 3

2

of oximes with diphenylsilane (16, 17). Because the results have been pub lished previously, they will not be repeated here.

Homo-Dieh-Alder

Reactions of Norbornadiene

The reaction of norbornadiene with olefins and acetylenes will be described in the final paragraphs. Reaction with acetylenes gives rise to extremely high optical inductions. The reaction of norbornadiene with acrylonitrile is catalyzed by phos phine nickel cyano complexes (18). It produces the deltacyclanes shown in Scheme V, which form diastereomers that differ in the orientation of the C N group with respect to the deltacyclane skeleton. Each diastereomer consists of a pair of enantiomers. To render the reaction enantioselective, P P h is replaced by the optically active phosphines Diop (see Abbreviations list) and Norphos (19). With Diop the chemical yield is 100% (diastereomer 3

Amerïcao Chemteil Society

library

115S i i tCatalyzed h St, MM* In Homogeneous Transition Metal Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Washington* DCSociety: 20036Washington, DC, 1992.

148


ratio 60:40), but the optical induction is only 4 and 3% (Scheme V). With a Ni(CN) -Norphos catalyst a poor (10%) chemical yield is obtained (diastereomer ratio 55:45), and the optical inductions of 12 and 15% are only slightly higher (19).


2

cat.: Ni(CN) /Diop 2

Ni(CN) /Norphos 2

yield

ratio diastereomers

100%

60 : L0

10%

55:45

optical induction 4 and

3%ee

12and15%ee

Scheme V. A much more favorable situation is the reaction of norbornadiene with phenylacetylene, in which phenyldeltacyclenes are formed (Scheme VI). In this reaction there are no diastereomers, only a pair of enantiomers being the possible product. In situ catalysts for this reaction are cobalt(III) compounds such as Co(acac) (see Abbreviations list) in combination with phosphine ligands and diethylaluminum chloride (20, 21). With Norphos as the optically active phosphine, a catalyst quantity of 0.2 mol % is sufficient for a quantitative formation of the phenyldeltacyclene in tetrahydrofuran (THF) at 35 ° C during 4 h (Scheme VI). According to a gas chromatographic (GC) analysis with a chiral cyclodextrin column (22), the optical purity is 99.2:0.8 (19). Scheme VII shows the reaction of norbornadiene with 1-hexyne. This reaction is quantitative, giving 99% ee with a catalyst Co(acae) -NorphosE t A l C l . The G C traces are shown at the bottom of Scheme VII for the racemic mixture, for the Diop-containing catalyst (ca. 20% ee), and for the Norphos-containing catalyst (99% ee). Five-membered chelate rings containing compounds such as Prophos, Chiraphos, and Norphos (see Abbreviations list) are puckered (23-27). A ring 3

3

2


9.

BRUNNER

Enantioselective Catalysis with Transition Metal Compounds 149

+

£ j L / 7

HCECPh

cat.

Ph cat.: Cotacac^ /Norphos/Et AlCl 0.2 MolV. THF, 3 5 ° ε , 4h

99.2


2

(yield

0.8 (98.^ /eee) e

quantitative )

Scheme VI.

f/\

*

77

HC=C Bu

cat.: Co(acac) / Norphos/Et 2Al CI

99

3

:

1

(98% ee)

[yield quantitative)

Racemate

CFiop

Norphos

Scheme VII. conformation that places the large substituents in an equatorial position is favored, as shown in structure A , which is also a suitable cocatalyst for the homo-Diels-Alder reaction of norbornadiene and acetylenes. The puckering of the chelate ring results in an equatorial-axial differentiation of the phenyl substituents at the phosphorus atoms. This puckering allows the phenylacetylene to bind in the preferred orientation shown in structure B, in which the phenyl substituent of the acetylene is in the neighborhood of the equaIn Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.



150

torial phenyl substituent at phosphorus. The formation of the two dashed carbon-carbon bonds completes the deltacyclene skeleton in an almost enantiospecific way.

Abbreviations and Chemical Names acac BPPM

acetylacetonate 1,1-dimethylethyl

ester of

(2R-4R)-4-(diphenylphosphino)-2-

[(diphenylphosphino)methyl]-l-pyrrolidinecarboxylic acid; see cocat. in Scheme III



9.

BRUNNER

Enantioselective Catalysis with Transition Metal Compounds

Chiraphos

(S, S)-(l,2-dimethyl-1,2-ethanediyl)bis[diphenylphosphine] ; see

cod Diop

structures A and Β 1,5-cyclooctadiene (S,S)-[(2,2-dimethyl-l,3-dioxolane-4,5-diyl)bis(methylene)]bi

Norphos Prophos

[diphenylphosphine] (S,S)-bicyclo[2.2.1]hept-5-ene-2,3-diylbis[diphenylphosphine] (fl)-(l-methyl-l,2-ethanediyl)bis[diphenylphosphine]

151

Acknowledgment The following students participated in the work described: W. Pieronczyk, G . Riepl, H . Weitzer, W. Leitner, K. Wutz, K. Kramler, M . Muschiol, and F. Prester.

References 1. Brunner, H.; Pieronczyk, W. Angew. Chem., Int. Ed. Engl. 1979, 18, 620. 2. Brunner, H.; Pieronczyk, W.; Schönhammer, B.; Streng, K.; Bernal, I.; Korp, J. Chem. Ber. 1981, 114, 1137. 3. Brunner, H.; Riepl, G.; Weitzer, H. Angew. Chem., Int. Ed. Engl. 1983, 22, 331; Angew. Chem. Suppl. 1983, 445. 4. Brunner, H.; Becker, R.; Riepl, G. Organometallics 1984, 3, 1354. 5. Brunner, H.; Leitner, W. Angew. Chem., Int. Ed. Engl. 1988, 27, 1180. 6. Brunner, H.; Graf, E.; Leitner, W.; Wutz, K. Synthesis 1989, 743. 7. Ojima, I.; Kogure, T.; Yoda, N. J. Org. Chem. 1980, 45, 4728. 8. Arcadi, Α.; Marinelli, F.; Bernocchi, E.; Cacchi, S.; Ortar,G.J.Organomet. Chem. 1989, 368, 249. 9. Larock, R. C.; Johnson, P. L. J. Chem. Soc., Chem. Commun. 1989, 1368. 10. Brunner, H.; Kramler, K. Synthesis in press. 11. Schurig, V.; Nowotny, H.-P.; Schmalzing, D. Angew. Chem., Int. Ed. Engl. 1989, 28, 736. 12. Brunner, H.; Hammer, B. Angew. Chem., Int. Ed. Engl. 1984, 23, 312. 13. Brunner, H.; Kraus, J. J. Mol. Catal. 1989, 49, 133. 14. Brunner, H.; Obermann, U.; Wimmer, P. J. Organomet. Chem. 1986, 316, C1. 15. Brunner, H.; Obermann, U.; Wimmer, P. Organometallics 1989, 8, 812. 16. Brunner, H.; Becker, R. Angew. Chem., Int. Ed. Engl. 1984, 23, 222. 17. Brunner, H.; Becker, R.; Gauder, S. Organometallics 1986, 5, 739. 18. Schrauzer, G. N.; Glockner, P. Chem. Ber. 1964, 97, 2451. 19. Brunner, H.; Muschiol, M.; Prester, F. Angew. Chem., Int. Ed. Engl. 1990, 29, 653. 20. Lyons, J. E.; Myers, J. E.; Schneider, A. Ann. Ν. Y. Acad. Sci. 1980, 333, 273. 21. Lautens, M.; Crudden, C. M. Organometallics 1989, 8, 2733. 22. Ehlers, J.; König, W. Α.; Lutz, S.; Wenz, G.;tomDieck, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1556. 23. Vineyard, B. D.; Knowles, W. S.; Sabacky, M. J.; Bachman, G. L.; Weinkauff, D. J. J. Am. Chem. Soc. 1977, 99, 5946. 24. Fryzuk, M. D.; Bosnich, B. J. Am. Chem. Soc. 1977, 99, 6262.


152


25. Knowles, W. S.; Vineyard, B. D.; Sabacky, M. J.; Stults, B. R. In Fundamental Research in Homogeneous Catalysis; Ishii, Y.; Tsutsui, M., Eds.; Plenum: New York, 1979; Vol. 3, p 537. 26. Slack, D. Α.; Greveling, I.; Baird, M. C. Inorg. Chem. 1979, 18, 3125. 27. Koenig, Κ. E.; Sabacky, M. J.; Bachman, G. L.; Christopfel, W. C.; Barnstorff, H. D.; Friedman, R. B.; Knowles, W. S.; Stults, B. R.; Vineyard, B. D.; Weinkauff, D. J. Ann. Ν. Y. Acad. Sci. 1980, 333, 16.


RECEIVED for review October 19, 1990. ACCEPTED revised manuscript July 15, 1991.


Enantioselective Catalysis with Transition Metal Compounds

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