A novel chiral super-Lewis acidic catalyst for enantioselective synthesis

Full text of DOI:10.1021/ja960766s

5502
J. Am. Chem. Soc. 1996, 118, 5502-5503
A Novel Chiral Super-Lewis Acidic Catalyst for
Enantioselective Synthesis
Yujiro Hayashi, Jeffrey J. Rohde, and E. J. Corey*
Department of Chemistry, HarVard UniVersity
Cambridge, Massachusetts, 02138
ReceiVed March 8, 1996
The exceptional power of the Diels-Alder reaction in the
synthesis of complex organic molecules has been greatly
enhanced by newly developed enantioselective versions, espe-
cially chiral Lewis acid catalyzed Diels-Alder reactions using
recoverable chiral ligands.1 Catalytic enantioselective Diels-
Alder reactions have amply demonstrated their effectiveness
recently in the direct and simplified construction of target
molecules such as prostaglandins, gibberellic acid, cassiol, and
gracillin B.2,3 However, there are major limitations of currently
available chiral Diels-Alder catalysts with regard to the range
of dienes to which they can be applied successfully. In fact,
most of the reported catalytic enantioselective Diels-Alder
reactions have involved reactive dienes such as cyclopentadi-
ene,4 and, as far as we are aware, 1,3-butadiene and 1,3-
cyclohexadiene have not been successfully used. In order to
expand the scope and utility of the catalytic enantioselective
Diels-Alder reaction, we set out to develop a new class of
super-reactiVe chiral Lewis acid catalysts. This paper reports
a very promising discovery which has emerged from these
studies.
The most useful chiral ligand developed in this investigation
was the chiral amino alcohol 4, best employed as the corre-
sponding trimethylsilyl ether, 5. The synthetic route to 4 and
5 followed the path shown herein. The starting material 1,
prepared in 93% yield by heating (R)-1-phenylethylamine,
2-carboethoxycyclohexanone, and 1 mol % Yb(OTf)3 in benzene
at reflux for 3 h with removal of water, was reduced as
previously described5 to the cis-â-amino ester which was then
isomerized by tert-butoxide to the more stable trans-â-amino
ester 2 (oil, obtained in diastereomerically pure form by
chromatography on silica). Reduction of the ester function in
2, hydrogenolysis of the N-R-phenylethyl group, and N-
dialkylation with 3,5-dimethylbenzyl bromide produced the
tertiary amino alcohol 4.
of this research was the idea that the cationic oxazaborinane 7
would be a much stronger Lewis acid than any of the previously
studied1,4 neutral chiral Lewis acids. After innumerable experi-
ments on the conversion of 5 to the oxazaborinane system,
several key findings emerged: (1) boron tribromide reacts with
5 at -78 °C in dry CH2Cl2 within 1 h with cleavage of the
trimethylsilyl ether and formation of Me3SiBr (as monitored
1
by H NMR at -80 °C); (2) the resulting oxazaborinane is a
mixture of 6 (mainly) and the cation 7; (3) a molar ratio of
BBr3 to silyl ether of between 0.9:1 and 1.6:1 can be used to
generate the oxazaborinane system 6 a 7; (4) 11B NMR
-
spectroscopy revealed increased formation of BBr4 as the
amount of BBr3 reagent was raised from 1 to 1.6 equiv,
indicating that BBr3 probably enhances the conversion of 6 to
the cationic form 7 (with BBr4- counterion); (5) the oxazabori-
nane system 6 a 7 is unstable and undergoes gradual decom-
position at temperatures above -60 °C with formation of the
primary bromide corresponding to amino alcohol 4; (6) reaction
of the oxazaborinane from ligand 5 and 0.9-1.0 equiv of BBr3
-
with 0.9 equiv of dry Ag+B[C6H3-3,5-(CF3)2]4 (8)7 in CH2-
Cl2 solution affords the most active catalyst system as expected
for the conversion of 6 to the tetraarylborate of cation 7 (and
AgBr). Two catalyst preparations were used in the experiments
described herein; the first, designated as catalyst A, is the
mixture of 6 and 7 produced from 5 with 0.9-1 equiv of BBr3,
and the second, designated as catalyst B, is the tetraarylborate
-
salt, 7+B[C6H3-3,5-(CF3)2]4
.
The next objective of our research was the conversion of the
trimethylsilyl ether 5 of chiral ligand 4 to the cationic ox-
azaborinane 7 via the neutral oxazaborinane 6.6 A key element
The application of the oxazaborinane catalytic system 6 a 7
(10 mol %) to Diels-Alder reactions of cyclopentadiene with
a variety of R,â-unsaturated aldehydes in CH2Cl2 has been very
successful as shown by the results summarized in Table 1. The
effectiveness of the cation 7 as a chiral catalyst is evident from
these data; the reactions in CH2Cl2 are fast even at -94 °C,
and the enantioselectivities are very good. In general, with
cyclopentadiene, an unusually reactive diene, there is no
advantage in using silver tetraarylborate enhancement (catalyst
B), and the procedure with catalyst A is simpler.8
(1) For recent reviews, see: (a) Kagan, H. B.; Riant, O. Chem. ReV.
1992, 92, 1007. (b) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl.
1994, 33, 497. (c) Pindur, U.; Lutz, G.; Otto, C. Chem. ReV. 1993, 93, 741.
(d) Deloux, L.; Srebnik, M. Chem. ReV. 1993, 93, 763. (e) Noyori, R.
Asymmetric Catalysis in Organic Synthesis; John Wiley: New York, 1993.
(2) Corey, E. J.; Guzman-Perez, A.; Loh, T.-P. J. Am. Chem. Soc. 1994,
116, 3611.
(3) Corey, E. J.; Letavic, M. A. J. Am. Chem. Soc. 1995, 117, 9616.
(4) For previous studies of catalytic enantioselective Diels-Alder
reactions, see: (a) Iwasawa, N.; Sugimori, J.; Kawase, Y.; Narasaka, K.
Chem. Lett. 1989, 1947 and references cited therein. (b) Corey, E. J.;
Imwinkelried, R.; Pikul, S.; Xiang, Y. B. J. Am. Chem. Soc. 1989, 111,
5493. (c) Furuta, K.; Shimizu, S.; Miwa, Y.; Yamamoto, H. J. Org. Chem.
1989, 54, 1481. (d) Takemura, H.; Komeshima, N.; Takahashi, I.;
Hashimoto, S.-I.; Ikota, N.; Tomioka, K.; Koga, K. Tetrahedron Lett. 1987,
28, 5687. (e) Hashimoto, S.-I.; Komeshima, N.; Koga, K. J. Chem. Soc.,
Chem. Commun. 1979, 437. (f) Takasu, M.; Yamamoto, H. Synlett 1990,
194. (g) Sartor, D.; Saffrich, J.; Helmchen, G. Synlett 1990, 197. (h) Corey,
E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (i) Corey,
E. J.; Loh, T.-P. J. Am. Chem. Soc. 1991, 113, 8966. (j) Corey, E. J.; Loh,
T.-P.; Roper, T. D.; Azimioara, M. D.; Noe, M. C. J. Am. Chem. Soc. 1992,
114, 8290. (k) Ishihara, K.; Gao, Q.; Yamamoto, H. J. Am. Chem. Soc.
1993, 115, 10412. (l) Maruoka, K.; Murase, N.; Yamamoto, H. J. Org.
Chem. 1993, 58, 2938. (m) Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc.
1994, 116, 1561. (n) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem.
Soc. 1993, 115, 6460. (o) Hawkins, J. M.; Loren, S. J. Am. Chem. Soc.
1991, 113, 7794. (p) Bav, J.; Wulff, W. D.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 3814. (q) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am.
Chem. Soc. 1988, 110, 1238.
Less reactive dienes, for example 1,3-cyclohexadiene or
isoprene, are unreactive with 2-bromoacrolein using catalyst A
at -94 or -78 °C. At higher temperatures the reactions do
not proceed well, probably due to decomposition of the catalyst.
(6) Parallel experimentation on the synthesis of the oxazaborinanes 6
and 7 directly from the amino alcohol 4 afforded much poorer results than
with the trimethylsilyl ether 5.
(7) The silver salt 8 was prepared by a modification of a previous method
(Gorden, J. H.; Mutolo, P. F.; Lobkovsky, E. B.; DiSalvo, F. J. Inorg. Chem.
1994, 33, 5374) as follows. An ethereal solution of Na B[C6H3-3,5-(CF3)2]4
(Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920)
was shaken with 2 equiv of aqueous AgNO3 in a separatory funnel for 5
min, and the layers were separated. Evaporation of the ether layer afforded
a quantitative yield of the colorless silver salt which was dissolved in ether
to give a clear 0.1 M solution which was stored at -78 °C in a flask wrapped
with aluminum foil to exclude light. For the preparation of the catalyst, a
measured amount of this ethereal solution was concentrated in Vacuo to
remove ether, dissolved in dry CH2Cl2, and dried over activated molecular
sieves, 4 Å, for 1 h at room temperature (with constant protection from
light). The appropriate amount of the dry CH2Cl2 solution of Ag+ B[C6H3-
(5) Cimarelli, C.; Palmieri, G.; Bartoli, G. Tetrahedron: Asymmetry 1994,
5, 1455.
-
3,5-(CF3)2]4 (8) was then used for reaction with 6 to form catalyst B.
S0002-7863(96)00766-4 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5503
Table 1. Enantioselective Diels-Alder Reaction of
1,3-Cyclopentadiene with R,â-Unsaturated Aldehydes in CH2Cl2 at
-94 °C for 2 h Catalyzed by Cationic Lewis Acid 7
The two 3,5-dimethylbenzyl substituents on the nitrogen of
the ligand are important determinants of enantioselectivity. This
fact was first discovered using a series of oxazaborinane
catalysts based on the related ligand 9, which was synthesized
starting with commercially available perillyl alcohol. The
following enantioselectivities were observed in the test reaction
of cyclopentadiene and 2-bromoacrolein at -94 °C using the
type A catalyst (10) (without addition of Ag+), under which
conditions the major product was the (2S)-adduct 11: R1 ) R2
) 3,5-dimethylbenzyl, 94% ee; R1 ) R2 ) CH2C6H5, 90% ee;
R1 ) CH3, R2 ) CH2C6H5, 45% ee; R1 ) CH3, R2 ) C2H5,
53% ee; R1 ) CH3, R2 ) â-naphthylmethyl, 65% ee.
1
a Exo-endo ratios were determined by H-NMR analysis. b Yields
refer to the isolated amount of exo-endo mixture (from silica gel
chromatography). c Enantioselectivities were determined by reduction
to the primary alcohol (NaBH4), conversion to the Mosher ester, and
1H-NMR (500 MHz) analysis. d Reference 4i. e Reference 4c.
These results are relevant to the question of the nature of the
transition state assembly leading to the predominating Diels-
Alder enantiomer. Although it is not possible to decide this
issue at present, one possible arrangement which is consistent
with presently available information is shown in 12. In this
hypothetical transition state model, one of the N-CH2Ar
substituents serves to block attack on the lower face of the
s-trans-coordinated dienophile while the other screens off
another region in space and limits the rotational position of both
dienophile and N-CH2Ar moieties.4i,4j,11 It is interesting in
connection with structure 12 that a very similar arrangement of
the two N-CH2Ar groups and the cyclohexane ring is observed
in the X-ray structure of 4‚HCl.12
Table 2. Diels-Alder Reaction of 1,3-Dienes with
-
2-Bromoacrolein Catalyzed by 7+B[C6H3-3,5-(CF3)2]4 (10 mol %)
in CH2Cl2 (Catalyst B)
In conclusion, a new and effective type of very strong chiral
Lewis acid has been developed, and its utility has been
demonstrated in Diels-Alder reactions with both reactive and
unreactive 1,3-dienes. Exploratory studies indicate that this area
of research will produce other catalyst structures and other useful
enantioselective reactions.
a Isolated yield by column chromatography for the endo/exo mixture.
b Diastereoselectivity was determined by 1H-NMR analysis of Diels-
Alder adducts. c Enantioselectivity was determined by reduction with
NaBH4, conversion to the benzoyl ester, and HPLC analysis (Chiracel
AD). d Enantioselectivity was determined by reduction with NaBH4,
conversion to the benzoyl ester, and HPLC analysis (Chiracel OD).
e Enantioselectivity was determined by reduction with NaBH4, conver-
Acknowledgment. This research was supported generously by
grants from the National Science Foundation, the National Institutes
of Health, the Uehara Foundation, and the Eli Lilly Co. We thank
Mr. Mihai Azimioara for the X-ray crystallographic data.
Supporting Information Available: Experimental procedures for
the synthesis of 4 along with spectral data on Diels-Alder adducts
(21 pages). Ordering information is given on any current masthead
page.
1
sion to Mosher ester, and H-NMR (500 MHz) analysis. f Reference
4i. g Configuration was determined by conversion to (-)-(S)-
bicyclo[2.2.2]oct-5-en-2-one.
However, unreactive dienes do react readily with 2-bromoac-
rolein using catalyst B, the tetraarylborate of cation 7, at -94
°C with excellent yields and enantioselectivity as indicated in
Table 2. Even 1,3-butadiene, which is considerably less reactive
than isoprene and very much less reactive than cyclopentadiene,
reacts completely with 2-bromoacrolein within 1 h at -94 °C.9
This result provides a clear indication of the super-Lewis acidity
of the uncoordinated cation 7.10 Because of this high reactivity,
catalyst 7 and structural variants represent a promising new
generation of chiral Lewis acids for enantioselective catalysis.
These catalysts could substantially broaden the range of possible
enantioselective Diels-Alder additions.
(8) The following procedure for the reaction of cyclopentadiene with
2-bromoacrolein is illustrative. A solution of 56 mg (0.128 mmol) of silyl
ether 5 in 1 mL of CH2Cl2 at -94 °C (hexane-liquid N2 bath) under dry
argon was treated with a solution of BBr3 in CH2Cl2 (0.115 mmol in 250
µL of CH2Cl2), and after 5 min the temperature was brought to -78 °C by
replacing the cooling bath by dry ice-acetone. After 1 h at -78 °C the
bath temperature was again changed to -94 °C, and 93 µL (1.15 mmol) of
2-bromoacrolein was added. A cold solution of 355 µL of cyclopentadiene
in 0.5 mL of CH2Cl2 was then added slowly down the wall of the flask,
and the reaction mixture was maintained at -94 °C for 1 h and then quench-
ed with 150 µL (1 mmol) of triethylamine. Removal of solvent in Vacuo
and column chromatography on silica gel afforded 229 mg (99%) of the
adduct4i shown in Table 1 of 95% enantiomeric purity with respect to the
exo-formyl diastereomer and 40 mg (85%) of recovered amino alcohol 4.
JA960766S
(9) The following experiment illustrates the procedure using the tet-
raarylborate of 7 as catalyst (catalyst B). A solution of the oxazaborinane
7 (0.086 mmol) in CH2Cl2 at -78 °C was prepared as described in ref 8.
A freshly prepared dry solution of Ag+B[C6H3-3,5-(CF3)2]4 (8) (0.086
-
mmol) in 1 mL of CH2Cl2 was added, and the reaction mixture was stirred
for 20 min at -78 °C and then cooled to -94 °C. 2-Bromoacrolein (76
µL, 0.94 mmol) and isoprene (500 µL) were added successively (each
dropwise), and the reaction mixture was stirred for 1 h at -94 °C and then
quenched by addition of 150 µL of triethylamine. After the mixture warmed
to room temperature and the inorganic salts were removed by filtration,
the solvent was removed in Vacuo, and the residue was chromatographed
on a column of silica gel to give 191 mg (99%) of the isoprene Diels-
Alder adduct shown in Table 2, [R]23 + 82.2° (c 0.8, CH2Cl2), and also
D
recovered ligand 4 (28.6 mg, 83%).
(10) Other silver salts have also been investigated in place of Ag+B-
[C6H3-3,5-(CF3)2]4- (8), including AgOSO2CF3 and AgSbF6, but were less
effective.
(11) (a) Corey, E. J.; Sarshar, S.; Bordner, J. J. Am. Chem. Soc. 1992,
114, 7938. (b) Corey, E. J.; Sarshar, S.; Lee, D.-H. J. Am. Chem. Soc. 1994,
116, 12089. (c) Corey, E. J. Proceedings 33rd National Organic Chemistry
Symposium of the American Chemical Society, 1993; p 30. (d) Corey, E.
J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (e) Corey, E.
J.; Ishihara, K. Tetrahedron Lett. 1992, 33, 6807.
(12) The ammonium and hydroxyl groups in the hydrochloride of 4 are
hydrogen bonded. Detailed X-ray crystallographic data are available from
the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge,
CB2 1EZ, U.K.