J. Am. Chem. Soc. 1997, 119, 4535-4536
4535
Communications to the Editor
dextrins to bind olefins hydrophobically and perform selective
epoxidations.7 We now find that this latter system can perform
the directed hydroxylation of an unactivated saturated carbon
of a reversibly bound steroid.
Selective Catalytic Hydroxylation of a Steroid by an
Artificial Cytochrome P-450 Enzyme
The preparation and characterization of catalyst 1 has been
described previously.7 Molecular models suggested that substrate
3 would bind its two tert-butylphenyl groups into two trans
cyclodextrin rings of 1 and place steroid ring B directly above
the metalloporphyrin ring. Thus we carried out the oxidation
of 1.0 mM 3 with 10 mol % of 1 and 5 mM iodosobenzene (5
equiv; with 1 equiv the conversion was lower, ca. 10-15%,
and less than 5% of product was formed with 0.3 equiv) in
water solution in the presence of 10 equiv of pyridine.8 After
2 h at room temperature the esters were hydrolyzed with 25%
KOH and the steroid products examined by thin layer chroma-
tography. Only two significant compounds were presentsstarting
diol 2 and a new product that was identified as triol 4.
Triol 4 had the expected mass spectrum, and showed three
Ronald Breslow,* Xiaojun Zhang, and Ying Huang
Department of Chemistry, Columbia UniVersity
New York, New York 10027
ReceiVed February 17, 1997
The ability of oxidizing enzymes, particularly those of the
cytochrome P-450 class, to perform selective hydroxylations
of unactivated carbons in substrates such as steroids is of great
practical importance.1 It also represents a great challenge for
biomimetic chemistry. Indeed the phrase biomimetic chemistry
was first coined in 1972 with respect to efforts to achieve
selective functionalization of steroids and other hydrocarbon
derivatives with use of geometric control to mimic that in
enzymes.2 However, the earliest work involved the function-
alization of steroids by reagents or catalysts that were covalently
attached to a steroid hydroxyl group.3 Thus catalytic turnover
was not possible. Furthermore, the reactions first examined
were directed photolytic insertions, or directed free radical
halogenations.
1
CH-OH groups in the H NMR at δ 3.57 (H-17), 3.47 (H-3),
and 3.33 (H-6, an axial H with 10.7 Hz coupling to two axial
protons and 4.4 Hz coupling to one equatorial proton). The
13C NMR spectrum of 4 showed only three signals between 69
and 83 ppm for monooxygenated carbon, and a DEPT experi-
ment confirmed that all were from CH-OH groups. The H at
δ 3.33 showed NOEs with Me-19, H-4, and H-8. Final proof
came from spectroscopic comparison with an authentic sample
of 4.9
In Nature the relevant enzymatic reactions involve oxidation
by metalloporphyrins, with reversible enzyme binding of the
substrate in such a geometry that specific substrate positions
are within reach of the oxygen atom on the metal. After
oxidation the product is released, so catalytic turnover is seen.
To date there is no true mimic of this entire process. Groves
has shown that he can use organization in a bilayer to achieve
hydroxylation of a steroid,4 but catalytic turnover was blocked
by strong binding of the product. Grieco has shown that a
metalloporphyrin can hydroxylate a steroid in an intramolecular
reaction if it is covalently attached,5 but again this is not a
catalytic process with turnover. We now wish to describe a
system that indeed binds a substrate, performs a hydroxylation
catalyzed by a metalloporphyrin, and then releases the product
to perform true turnover catalysis. In our best example, the
hydroxylation is highly selective for an otherwise unreactive
and unremarkable steroid position. Geometry within the
complex of substrate with artificial enzyme determines the
product formed.
Compound 4 was isolated in ca. 40% conversion, along with
recovered 2, so each catalyst performed at least 4 turnovers.10
The catalyst was destroyed at that point, but other work makes
it clear how to stabilize such metalloporphyrins to oxidation.11
Controls showed that 3 was not hydroxylated in the absence of
oxidant or of catalyst, and that substrate 5, which lacks the tert-
butylphenyl binding groups, was not hydroxylated under the
same conditions.
We also examined the hydroxylation of substrate 6, related
to a stilbene we had epoxidized with this catalyst previously.7
We found that 6 was converted quantitatively to 7 with at least
14 turnovers with use of 7% of catalyst 1 and 3 equiv (relative
to substrate) of iodosobenzene. An analog of 6 lacking the tert-
butylphenyl groups was not oxidized under the same conditions.
Other controls also exclude a free-radical chain process: the
yield of 7 was unaffected in the presence of 12 mol % added
potassium nitrosodisulfonate, and with only 0.3 equiv of
iodosobenzene the yield of 7 was below 7%. Thus the normal
catalytic mechanism12 is operating.
The catalyst builds on our earlier work in which substrates
were reversibly bound to metalloporphyrins, and then selectively
oxidized. Our first case used auxilliary metal ions to bind
coordinating substrates,6 and our most recent case used a
manganese porphyrin that used two (or four) attached cyclo-
Our highly selective hydroxylation of 3 depends critically
(7) Breslow, R.; Zhang, X.; Xu, R.; Maletic, M.; Merger, R. J. Am. Chem.
Soc, 1996, 118, 11678-11679. The preparation of catalyst 1 is described
in the Supporting Information of this paper.
(8) (a) Meunier, B.; Guilmet, E.; De Carvalho, M.-E.; Poilblanc, R. J.
Am. Chem. Soc. 1984, 106, 6668-6676. (b) Montanari, F.; Penso, M.; Quici,
S.; Vigano`, P. J. Org. Chem. 1985, 50, 4888-4893.
(1) (a) Woggon, W.-D. Cytochrome P450: Significance, Reaction
Mechanisms and ActiVe Site Analogues; Schmidtchen, F. P., Ed.; Springer:
Berlin Heidelberg, 1996; pp 39-96. (b) Groves, J. T.; Han, Y.-Z. Models
and Mechanism of Cytochrome P-450 Action; Ortiz de Montellano, P. R.,
Ed.; Plenum Press: New York, 1995; pp 3-48. (c) Meunier, B. Chem.
ReV. 1992, 92, 1411-1456.
(9) Obtained from Aldrich Chemical Company.
(10) We have also prepared the Fe(III) analog of catalyst 1, which
catalyzes the selective overall conversion of 3 to 4 but in only 5-10%
conversion under our standard conditions. Groves and Neumann (ref 4c)
report that iron porphyrin catalysts are less effective than are manganese
porphyrin analogs.
(2) Breslow, R. Chem. Soc. ReV. 1972, 1, 553.
(3) Breslow, R. Acc. Chem. Res. 1980, 13, 170.
(4) (a) Groves, J. T.; Neumann, R. J. Am. Chem. Soc. 1989, 111, 2900-
2909. (b) Groves, J. T.; Neumann, R. J. Am. Chem. Soc. 1987, 109, 5045-
5047. (c) Groves, J. T.; Neumann, R. J. Org. Chem. 1988, 53, 3891-3893.
(5) (a) Grieco, P. A.; Stuk, T. L. J. Am. Chem. Soc. 1990, 112, 7799-
7801. (b) Kaufman, M. D.; Grieco, P. A.; Bougie, D. W. J. Am. Chem.
Soc. 1993, 115, 11648-11649. (c) Stuk, T. L.; Grieco, P. A.; Marsh, M.
M. J. Org. Chem. 1991, 56, 2957-2959.
(11) (a) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26, 1338-1339.
(b) Chang, C. K.; Ebina, F. J. Chem. Soc., Chem. Commun. 1981, 778-
779. (c) Nappa, M. J.; Tolman, C. A. Inorg. Chem. 1985, 24, 4711-4719.
(d) Ellis, P. E., Jr.; Lyons, J. E. J. Chem. Soc., Chem. Commun. 1989, 1189-
1190. (e) Ellis, P. E., Jr.; Lyons, J. E. J. Chem. Soc., Chem. Commun. 1989,
1315-1316.
(12) (a) Groves, J. T.; Kruper, W. J., Jr.; Haushalter, R. C. J. Am. Chem.
Soc. 1980, 102, 6375-6377. (b) Hill, C. L.; Schardt, B. C. J. Am. Chem.
Soc. 1980, 102, 6374-6375.
(6) Breslow, R.; Brown, A. B.; McCullough, R. D.; White, P. W. J. Am.
Chem. Soc. 1989, 111, 4517-4518.
S0002-7863(97)00495-2 CCC: $14.00 © 1997 American Chemical Society
Breslow
