10 Shape-Selective Olefin Epoxidation Catalyzed by Metallo "Picnic-Basket" Porphyrins James P. Collman , Xumu Zhang , Virgil Lee , Robert T. Hembre , and John I. Brauman 1
1
1
2
1
Department of Chemistry, Stanford University, Stanford, C A 94305 Department of Chemistry, University of Nebraska, Lincoln, N E 68588
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2
Dramatic selectivities are observed in the epoxidation of several olefin pairs with a series of manganese "picnic basket" porphyrin catalysts, which have a rigid cavity of variable dimensions on one side of the porphyrin ring and a bulky anionic ligand (3,5-di-t-butyl phenoxide) on the other side. The pronounced selectivities are in contrast to those obtained with flat porphyrins such as tetraphenylporphyrin (TPP) and sterically hindered porphyrins such as tetramesitylporphyrin (TMP). When substrates of varying shapes and sizes are used, the selectivities obtained from the picnic-basket porphyrins with a synthetic cavity mimic the substrate specificity of enzymes with a protein cavity.
S H A P E S E L E C T I V I T Y is an important characteristic of enzymatic reactions. The substrate specificity of an enzyme generally results from the interaction of the three-dimensional protein structure and the unique shape of the substrate. In the past, much attention has been devoted to understanding binding between a substrate and an enzyme. Host-guest chemistry and molecular recognition are terms used to describe these and related studies. Many host-guest systems such as crown ethers, cyclodextrins, cyclophanes, and molecular clefts have been synthesized to elucidate the nature of substrate-enzyme interactions (1-6). Although these systems successfully mimic substrate binding in enzymes, few of the systems incorporate catalytic cen0065-2393/92/0230-0153$06.00/0 © 1992 American Chemical Society
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REACTIONS
ters in the host cavities. Therefore, these systems have failed to address the problem of substrate selectivities in catalytic processes. In contrast, zeolites, a type of heterogeneous shape-selective catalyst, have been applied successfully as important catalysts in the petroleum industry. Zeolites can have acid-base or metallic catalytic centers in an extended pore structure. The framework of zeolites controls the shape selectivity of substrates and influences the product distribution in the catalysis. Xylene isomerization and the methanol-to-gasoline process (MTG) are two such examples (7). The development of homogeneous shape-selective catalysts is important and remains a challenging problem in homogeneous catalysis.
"Picnic-Basket" Porphyrins An effective strategy in the development of shape-selective homogeneous catalysts is to mimic enzymes. In the past, we have been involved in modeling the chemistry of cytochrome P-450. Cytochrome P-450, a family of enzymes, is crucial in oxidative metabolism. The active site contains an iron protoporphyrin moiety (structure 1) with an axial cysteine thiolate ligand. These enzymes catalyze the reaction of hydrocarbons with molecular oxygen, incorporating one oxygen atom into the substrate and reducing the other oxygen atom to water. The hydroxylation of alkanes and the epoxidation of olefins are well-studied reactions catalyzed by these enzymes. As do many other enzymatic systems, cytochrome P-450 controls its substrate selectivity exclusively by its surrounding protein cavity (8-9). The catalytic cycle of cytochrome P-450 is shown in Scheme I (10).
1
10.
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Scheme I. Catalytic cycle of cytochrome P-450.
Although nature uses oxygen as the oxygen-atom source, it is possible to use other oxygen-transfer agents in what is called the shunt pathway. Synthetic metalloporphyrins have similar activities in alkane hydroxylation and in olefin epoxidation when such oxygen-transfer reagents are employed (Scheme II) (IJ). O n the basis of these findings, shape-selective oxygenation catalysts were developed. The shape-selective catalysts employ diverse strategies, including sterically hindered porphyrins (12-17), membrane-spanning porphyrins (18), and zeolite-encapsulated macrocyclic complexes (19) and metal ions (20). To mimic the substrate selectivity found with cytochrome P-450, we developed a new porphyrin system, the "picnic-basket" porphyrins (PBP) (Chart I) (21-23). The PBP complexes have a rigid cavity of variable dimensions on one face of the porphyrin ring. A bulky axial ligand is used to block the open face of the porphyrin. This conformation allows for the formation of the active oxygenating species inside the porphyrin superstructure. The interaction between this superstructure and the olefin leads to shape-selective epoxidation. For example, some cyclic olefins like cyclooctene have a larger size than comparable acyclic olefins like cw-2-octene. The difference results in the latter being epoxidized at a faster rate in certain PBP cavities (Scheme III).
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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
Ar =
-o
tetraphenylporphyrin (TPP)
CH,
Ar =
CH
3
tetramesitylpoφhyrin (TMP)
PhIO, H 0 , NaOCl, etc. 2
V
/
/
^
2
M(TPP)X or M(TMP)X
Ν
M = Fe, Mn, Cr X = C I Br
/ ^
Scheme II. Metalloporphyrin models for cytochrome P-450.
Results We failed to achieve shape selectivity in olefin epoxidation by using Mn(PBP)Br and bulky neutral axial ligands such as 1,5-diphenylimidazole to block the open face of the porphyrin (8). The reason for this failure seems to be as follows. Suppose the bulky imidazole ligand coordinates to the open face of the porphyrin and bromide coordinates inside the basket cavity. For epoxidation to occur inside the cavity, the bromide must be displaced from manganese by coordination of an oxygen-transfer agent such as PhIO. This process might be thermodynamically unfavorable, as suggested in a related system (24). In contrast, the displacement of a neutral imidazole by PhIO could be a more favorable pathway, and this reaction is likely to take place on the more accessible open face of the porphyrin. To solve this problem a bulky anionic axial ligand, 3,5-di-f-butyl phenoxide, was used to block the open face of PBP (Scheme IV).
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R
Chart I. "Picnic-basket" porphyrins.
Four olefin pairs were studied in competitive epoxidations. A 1:1 ratio of two olefins was used as a substrate mixture; manganese complexes of tetraphenylporphyrin (TPP), tetramesitylporphyrin (TMP), or P B P were used as the catalyst. The oxidations were carried out in dry acetonitrile with PhIO as the oxygen-atom source. The results are summarized in Table I. A catalytic asymmetric epoxidation of styrene was also studied with (R)binapPBP at 0 °C; S( + )- styrene oxide is formed in 13% enantiomeric excess (ee), as determined with an N M R chiral shift reagent.
Discussion Very slow epoxidation is observed with the Mn(C PBP)(OAr) or Mn(C PBP)(OAr) as catalyst. The competitive epoxidation ratios of four olefin pairs with either Mn(C PBP)(OAr) or Mn(C PBP)(OAr) are close to those obtained with Mn(TPP)(OAr) (Table I). The only exception is the ratio of the third olefin pair with Mn(C PBP)(OAr). The slow rate and low selectivity may be a manifestation of reaction "leaking', in which the oxidation occurs on the open unhindered face of the porphyrin. Dramatic selectivity is found in the epoxidation of ds-2-octene and transβ-methylstyrene (the first olefin pair in Table I). When Mn(C PBP)(OAr) or Mn(PXYLPBP)(OAr) is used as the catalyst, the competitive epoxidation 2
4
2
4
4
6
Scheme III. Competitive epoxidation with the PBP.
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What is the problem ?
Solution: Bulky anionic ligand ?
Scheme IV. Problems with the PBP strategy.
ratios are 70 and 29, respectively. These ratios are larger than those obtained with the sterically hindered porphyrin catalyst, Mn(TMP)(OAr), and much larger than that obtained with the flat porphyrin catalyst, Mn(TPP)(OAr). The latter ratios are 14.4 and 1.2, respectively. Selectivity decreases as the size of P B P increases to C P B P and C P B P with competitive ratios of 12.7 and 8.8, respectively. This selectivity can be 8
1 0
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REACTIONS
Table I. Competitive Olefin Epoxidation Results
Catalyst"
(Ratio)
Mn(TPP)(OAr) Mn(TMP)(OAr) Mn(C PBP)(OAr) Mn(C,PBP)(OAr) Mn(C PBP)(OAr) Mn((PXYLPBP)(OAr) Mn(C PBP)(OAr) Mn(C PBP)(OAr) 2
6
H
1()
(Ratio)
0.9 2.5 1.3* 1.8* >1000 >1000 21.1 17.9
0.03 0.04 0.05* 0.4* 1.7 7.0 0.06 0.04
1.1 0.7 1.1* 2.1* 67 >1000 1.6 0.2
1.2 14.4 0.4* 1.0* 70 29 12.7 8.8
(*·*>)
"OAr is 3,5-di-f-butyl phenoxide. ''These reactions are very slow.
attributed to a combination of steric effects, the ability of a particular olefin to attain a favorable geometry for epoxidation, and size effects, which determine whether the olefin can approach the active site of the cavity. C o m pared to cw-2-octene, tnms-p-methylstyrene
is less reactive toward sterically
hindered porphyrins. Presumably, its larger size makes it difficult for it to react inside the porphyrin cavity. The
second olefin pair, cis-2-octene
and cyclooctene, are both cis-
olefins. They have a similar reactivity toward the flat porphyrin catalyst Mn(TPP)(OAr) and the sterically hindered porphyrin catalyst Mn(TMP)(OAr). T h e competitive epoxidation ratios are 1.1 with Mn(TPP)(OAr) and
0.7 with Mn(TMP)(OAr). However, with Mn(C PBP)(OAr) or M n 6
(PXYLPBP)(OAr) these ratios are 67 or >1000, respectively. The ratios drop
dramatically to 1.6 with
(C PBP)(OAr). 10
These results
Mn(C PBP)(OAr) 8
and 0.2
with M n -
indicate that the cavities of both M n -
(C PBP)(OAr) and Mn(PXYLPBP)(OAr) tend to exclude cyclooctene from 6
the catalytic centers, presumably because of the greater steric bulk of this cyclic olefin relative to the linear ds-2-octene. The third olefin pair is 1-octene and cyclooctene. As terminal olefins are generally more electron-deficient than internal olefins, 1-octene is less reactive
than cyclooctene
toward electrophilic attack by M = 0 . With
Mn(TPP)(OAr) or Mn(TMP)(OAr), the competitive ratios for this olefin pair are
0.03
or 0.04,
respectively.
However,
with
Mn(C PBP)(OAr) or 6
Mn(PXYLPBP)(OAr), the respective ratios are 1.7 or 7.0. This contrast indicates that cyclooctene is excluded from the cavities of Mn(C PBP)(OAr) 6
and
Mn(PXYLPBP)(OAr).
With
the
larger
Mn(C PBP)(OAr) 8
or
Mn(C PBP)(OAr) the selectivity in the competitive epoxidation of cis-210
octene and cyclooctene decreases to 0.06 or 0.04, respectively. Both olefins are now able to get inside the cavity. This result is consistent with our earlier argument. The final olefin pair is cis-2-octene and 2-methyl-2-pentene. This pair allows us to compare the shape selectivity of a disubstituted versus a tri-
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substituted olefin. With Mn(TPP)(OAr) or Mn(TMP)(OAr) the competitive epoxidation ratios are 0.9 or 2.5, respectively. However, with Mn(C PBP)(OAr) or Mn(PXYLPBP)(OAr), the ratios are >1000 in each case. These values demonstrate that the disubstituted cis-2-oetene is much more reactive than the trisubstituted 2-methyl-2-pentene. The ratios using Mn(C PBP)(OAr) or Mn(C PBP)(OAr) are also larger than those with Mn(TPP)(OAr) or Mn(TMP)(OAr). This reactivity pattern reflects the required orientation of the M n = 0 group with the olefin axis. Perhaps the alkene approaches the M n = 0 bond from the side and is parallel to the porphyrin plane, as has been suggested by Groves and Nemo (12) in related F e = 0 systems and further discussed by Ostovic and Bruice (25, 26). The oxidation of trisubstituted olefins is allowed with the hindered Mn(TMP)(OAr), probably because the substituents on the olefin face can orient themselves between the methyl groups of T M P . However, the steric interactions between the walls of the P B P and the trisubstituted olefins disfavor the oxidation of these trisubstituted systems. 6
8
10
Styrene was oxidized to (S)-styrene oxide in 13% ee with the Mn(R)binapPBP(OAr) catalyst. Because the chiral site is far above the reaction locus, the low ee value is not surprising (27-30). However, this geometry demonstrates that some, perhaps all, of the reaction occurs within the cavity. This result suggests that we may be able to develop synthetically useful and efficient chiral catalysts on the basis of these systems.
Conclusions These studies have led to dramatic shape-selective catalytic epoxidation of several olefin pairs and chiral epoxidation of a prochiral olefin, albeit in modest yield. These selectivities depend upon the dimensions of the cavity and are different from those achieved with simple porphyrin catalysts derived from T P P or T M P . We seem to be on the verge of predictable, shapeselective, catalytic oxygenation with a readily available, variable series of synthetic catalysts. For example, highly regioselective epoxidation of poly enes such as steroids and terpenoids may be possible. By applying variable chiral groups on the porphyrins, we hope to develop efficient asymmetric epoxidation catalysts for unsubstituted olefins.
Acknowledgments The support for this work was provided by the National Science Foundation (CHE8&-14949) and the National Institutes of Health (5R37-GM17880). Helpful discussions with Scott Bohle and Jeff Fitzgerald are gratefully ac knowledged.
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References 1. 2. 3. 4. 5. 6. 7. 8.
Lehn, J. M. Science (Washington, D.C.) 1985, 227, 849. Rebek, J., Jr. Science (Washington, D.C.) 1987, 235, 1478. Cram, D. J. Science (Washington, D.C.) 1988, 240, 760. Gutsche, C. D. Acc. Chem. Res. 1983, 16, 161. D'Souza, V. T.; Bender, M. L. Acc. Chem. Res. 1987, 20, 146. Sutherland, I. O. Chem. Soc. Rev. 1986, 15, 63. Newsman, J. M. Science (Washington, D.C.) 1986, 231, 1093. Jefcoate, C. R. In Cytochrome P-450: Reactions, Mechanism, and Biochemis Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1986;p387. 9. Sligar, S. G.; Murry, R. I. In Cytochrome P-450: Reactions, Mechanism, and Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1986;p429. 10. Cytochrome P-450: Reactions, Mechanism, and Biochemistry; Ortiz de Mo lano, P. R., Ed.; Plenum: New York, 1986. 11. McMurry, T. J.; Groves, J. T. In Cytochrome P-450: Reactions, Mechanism, and Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1986;p1. 12. Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 5786. 13. Suslick, K. S.; Cook, B. R.; Fox, M. M. J. Chem. Soc., Chem. Commun. 1985, 580. 14. Suslick, K. S.; Cook, B. R. J. Chem. Soc., Chem. Commun. 1987, 200. 15. Cook, B. R.; Reinert, T. J.; Suslick, K. S. J. Am. Chem. Soc. 1986, 108, 7281. 16. Collman, J. P.; Brauman, J. T.; Meunier, B.; Hayashi, T.; Kodadek, T.; Raybuck, S. Α. J. Am. Chem. Soc. 1985, 107, 2000. 17. Nappa, M. J.; Tolman, C. A. Inorg. Chem. 1985, 24, 4711. 18. Groves, J. T.; Newmann, R. J. Am. Chem. Soc. 1987, 109, 5045. 19. Herron, N.; Stucky, G. D.; Tolman, C. A. J. Chem. Soc., Chem. Commun. 1986, 1521. 20. Herron, N.; Tolman, C. A. J. Am. Chem. Soc. 1987, 109, 2837. 21. Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Hampton, P. D.; Naruta, Y.; Michina, T. Bull. Chem. Soc. Jpn. 1988, 61, 47. 22. Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Hampton, P. D.; Naruta, Y.; Sparapany, J. W.; Ibers, J. A. J. Am. Chem. Soc. 1988, 110, 3411. 23. Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Sparapany, J. W.; Ibers, J. A. J. Am. Chem.Soc.1988, 110, 3486. 24. Powell, M. F.; Pai, E. F.; Bruice, T. C. J. Am. Chem. Soc. 1984, 106, 3277. 25. Ostovic, D.; Bruice, T. C. J. Am. Chem. Soc. 1988, 110, 6906. 26. Ostovic, D.; Bruice, T. C. J. Am. Chem. Soc. 1989, 111, 6511. 27. Groves, J. T.; Myers, R. S. J. Am. Chem. Soc. 1983, 105, 5791. 28. Mansuy, D.; Battioni, P.; Renaud, J. P.; Guerin, P. J. Chem. Soc., Chem. Commun. 1985, 155. 29. Naruta, Y.; Tani, F.; Maruyama, K. Chem. Lett. 1989, 1269. 30. O'Malley, S.; Kodadek, T. J. Am. Chem. Soc. 1989, 111, 9116. RECEIVED for review October 19, 1990. ACCEPTED revised manuscript May 29, 1991.
