Rational design of a new chiral Lewis acid catalyst for enantioselective Diels-Alder reaction: Optically active 2-dichloroboryl-1,1′-binaphthyl

Article abstract of DOI:10.1055/s-1998-1860

A novel chiral aryldichloroborane catalyst bearing binaphthyl skeletons with axial chirality was developed as an asymmetric catalyst for the Diels-Alder reaction of dienes and α,β-unsaturated esters, and could be reused as the corresponding boronic acid. In addition, a new convenient method for preparing arylboronic acids from aryl alcohols is described.

Full text of DOI:10.1055/s-1998-1860

October 1998
SYNLETT
1053
Rational Design of a New Chiral Lewis Acid Catalyst for Enantioselective Diels-Alder
Reaction: Optically Active 2-Dichloroboryl-1,1´-binaphthyl
a
b
b
b
b
Kazuaki Ishihara, Kazato Inanaga, Shoichi Kondo, Miyuki Funahashi, and Hisashi Yamamoto*
aResearch Center for Advanced Waste and Emission Management (ResCWE), Nagoya University, CREST, Japan Science and Technology
Corporation (JST), Furo-cho, Chikusa, Nagoya 464-8603, JAPAN
bGraduate School of Engineering, Nagoya University, CREST, JST, Furo-cho, Chikusa, Nagoya 464-8603, JAPAN
Fax: 81-52-789-3222
Received 1 July 1998
Abstract: A novel chiral aryldichloroborane catalyst bearing binaphthyl
skeletons with axial chirality was developed as an asymmetric catalyst
for the Diels-Alder reaction of dienes and α,β-unsaturated esters, and
could be reused as the corresponding boronic acid. In addition, a new
convenient method for preparing arylboronic acids from aryl alcohols is
described.
Spectacular advances have been achieved in recent years in the
enantioselective Diels-Alder reaction of dienes and α,β-unsaturated
1
,2
aldehydes catalyzed by chiral Lewis acids. However, there has been
relatively less progress in the development of asymmetric catalysts for
the reaction with α,β-unsaturated esters as dienophiles because of their
3
,4
low reactivities. Alkyldihaloborane is one of the most powerful Lewis
acids which can catalyze the latter reaction.3c,d For example, Hawkins
and co-workers previously reported an effective chiral
alkyldicholoroborane catalyst prepared by hydroboration of 1-(1-
naphthyl)cyclohexene for the latter reaction, which gave high
3d
enantioselectivity. However, in general, since alkyldichloroboranes
readily decompose to alkanes or alkenes by protonolysis or β-hydride
elimination, it is difficult to recover them as alkylboronic acids
5
quantitatively. In addition, in the preparation of optically active
alkyldihaloboranes, optical resolution or asymmetric hydroboration is
3
c,d,5
required since they involve chiral carbons adjacent to boron.
Thus,
the utility of chiral alkyldichloroboranes as strong Lewis acid catalysts
in enantioselective synthesis encouraged us to seek new members of this
class which would not have the disadvantages mentioned above. This
paper describes novel and practical chiral aryldichloroborane catalysts
7
a-c bearing binaphthyl skeletons with axial chirality which can be
6
reused as the corresponding boronic acids 5a-c.
For the synthesis of optically pure 2-dihydroxyboryl-1,1’-binaphthyl
(
5a), (R)-1,1’-bi-2-naphthol (1a) was first treated with N-
phenyltrifluoromethanesulfonimide and the resulting monotriflate 2a
7
was subjected to hydrogenolysis to give mono-ol 3a (Scheme 1). After
transformation of the phenolic hydroxyl group to a diethylphosphate, 4a
12
exchange of the anhydrides of boronic acids with trichloroborane. The
latter procedure is simpler and more convenient.1
0b
was treated with lithium-naphthalene and trimethylborate successively
8
to give binaphthylboronic acid 5a. As far as we know, this is a new
method for preparing arylboronic acid from aryl alcohol.9
Unfortunately, slight epimerization of 5a was revealed by conversion to
1
the (2R,4R)-2,4-pentanediol ester and H NMR analysis or by
conversion to the pinacol ester and HPLC analysis. Therefore, boronic
acid 5a was esterified with diethanolamine in 2-propanol, from which
the boronate ester 6a was crystallized in good yield. Recrystallization of
6a and subsequent hydrolysis with dilute acid readily provided
enantiomerically pure (R)-5a. (R)-6,6’-Diarylbinaphthylboronic acids
8
8
5
b and 5c were synthesized from 1b and 1c, respectively, in a similar
manner.
To activate chiral arylboronic acids 5a-c as Lewis acid catalysts,
conversion to the corresponding aryldichloroboranes 7a-c was
3
d,10
investigated.
As a result, two different procedures, Method A and
Method B, were developed as shown in Scheme 2: one is via exchange
1
1
of the methanol boronate with trichloroborane, and the other is via

1
054
LETTERS
SYNLETT
With these complexes 7a-c in hand, we first explored their asymmetry-
inducing ability in the Diels-Alder reaction of cyclopentadiene and
methyl acrylate as test materials. The results are summarized in Table 1.
The reaction proceeded smoothly at -78 °C in the presence of 10 mol%
of catalysts 7a-c to give the endo-adduct in high yield with >99%
diastereoselectivity. Catalyst 7b showed the highest asymmetric
induction, but even this was insufficient (entry 3). The substituents at the
6,6’-positions of the catalysts had remarkable effects on
enantioselectivity and absolute stereochemistry. Sterically bulky
substituents such as a mesityl group increased enantioselectivity (entry 1
vs. entry 3), while
a phenyl substituent inverted the absolute
enantioselectivity (entries 1-3 vs. entry 4).
e
The correlation between the enantioselectivity and the alkoxy moiety of
alkyl acrylates in the Diels-Alder reaction with cyclopentadiene
catalyzed by 7a was also investigated. Sterically bulky alkoxy groups
lowered the enantioselectivity: 48% ee (R) for ethyl acrylate, 11% ee (S)
for t-butyl acrylate. However, the reaction of ethylene diacrylate (8),
which is commercially available, and the following reduction with
lithium aluminum hydride gave endo-5-norbornene-2-methanol with
7
8% ee (eq. 1). According to a control experiment (eq. 2), the
enantioselectivity in the initial addition and the diastereoselectivity
double asymmetric induction) in the second addition were higher than
(
that of methyl acrylate (Table 1, entry 2). Although it is not clear why
the selectivities are increased by the link between dienophiles, similar
effects are expected for other asymmetric reactions.
The proposed mechanism shown in Figure 1 can explain the absolute
stereopreference in the above Diels-Alder reaction. The following
features of the intermediates were predicted by a conformational
analysis: boron complexes the carbonyl oxygen anti to the C-O bond of
exposed as shown in B is disfavored due to steric interaction of the
alkene with the naphthyl. Increased enantioselectivity with the use of 7b
can be easily understood in terms of steric repulsion between the alkene
and the mesityl group (see C). Although the reason for the inverse
absolute stereopreference with the use of 7c is not yet clear,
intermediate D may be slightly favored due to some π-π attractive
interaction between the alkene and the 6-phenylnaphthyl group.
10
13
the ester, the enone unit is s-trans, and the carbonyl group is
positioned over and approximately parallel to the naphthalene ring
3b
within the van der Waals radii (ca. 3 Å). Thus, the absolute
configuration of the major endo product using 7a as a catalyst is
consistent with the naphthyl shielding the re face of the coordinated
methyl acrylate leading to attack by cyclopentadiene at the si face, as
shown in A. Coordination of the methyl acrylate with the re face

October 1998
SYNLETT
1055
References and Notes
1
.
For a recent review, see: Ishihara, K.; Yamamoto, H. In Advances
in Catalytic Processes; Doyle, M. P., Ed.; JAI Press: London,
1
995; Vol. 1 p. 29.
2
.
For recent articles on enantioselective Lewis acid-catalyzed Diels-
Alder reactions of dienes and α,β-unsaturated aldehydes, see: (a)
Ishihara, K.; Kurihara, H.; Yamamoto, H. J. Am. Chem. Soc. 1996,
1
18, 3049. (b) Hayashi, Y.; Rohde, J. J.; Corey, E. J. J. Am. Chem.
Soc. 1996, 118, 5502. (c) Ishihara, K.; Kondo, S.; Kurihara, H.;
Yamamoto, H.; Ohashi, S.; Inagaki, S. J. Org. Chem. 1997, 62,
3
026. (c) Corey, E. J.; Lee, T. W. Tetrahedron Lett. 1997, 38,
5
755.
3
.
For previous articles on enantioselective Lewis acid-catalyzed
Diels-Alder reactions of dienes and α,β-unsaturated esters, see:
(
a) Hashimoto, S.-H.; Komeshima, N.; Koga, K. J. Chem. Soc.
Chem. Commun. 1979, 437. (b) Rebiere, F.; Riant, O.; Kagan, H.
B. Tetrahedron: Asymmetry 1990, 1, 199. (c) Bir, G.; Kaufmann,
D. J. Organomet. Chem. 1990, 390, 1. (d) Hawkins, J. M.; Loren,
S. J. Am. Chem. Soc. 1991, 113, 7794; Hawkins, J. M.; Loren, S.;
Nambu, M. J. Am. Chem. Soc. 1994, 116, 1657. (e) Devine, P. N.;
Taeboem, O. J. Org. Chem. 1992, 57, 396. (f) Maruoka, K.;
Concepcion, A. B.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1992, 65,
3
501.
4
.
Very recently, Wulff and co-workers reported the highly
enantioselective Diels-Alder reaction of cyclopentadiene and
methyl acrylate catalyzed by a VAPOL-aluminum catalyst in the
presence of an additional ligand. Heller, D. P.; Goldberg, D. R.;
Wulff, W. D. J. Am. Chem. Soc. 1997, 119, 10551.
5
6
.
.
Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic
Press: New York, 1988.
Recently, the synthesis of racemic 5a has been announced, see: (a)
Schilling, B.; Kaiser, V.; Kaufmann, D. E. Chem. Ber. 1997, 130,
9
7
23. (b) Schilling, B.; Kaufmann, D. E. Eur. J. Org. Chem. 1998,
01.
7
.
.
Sasaki, H.; Irie, R.; Katsuki, T. Synlett 1993, 300.
8
(R)-1,1’-Binaphthyl-2-boronic Acid (5a): IR (Kbr) 3670-3150
-1 1
(br), 3057, 1470, 1458, 1375, 1350, 1317, 1248 cm ; H NMR
(
300 MHz, CDCl +D O) δ 7.15 (d, J=8.7 Hz, 1H), 7.23-7.36 (m,
3
2
3
1
H), 7.48-7.56 (m, 3H), 7.67 (t, J=7.2 Hz, 1H), 7.95 (d, J=8.3 Hz,
H), 7.99 (d, J=8.5 Hz, 1H), 8.00 (d, J=8.3 Hz, 1H), 8.06 (d, J=8.5
Next, the enantioselectivity and absolute stereochemistry in the Diels-
Alder reaction of cyclopentadiene and other dienophiles were explored
using non-substituted catalyst 7a. The results are summarized in Table
1
3
Hz, 1H), 8.14 (d, J=8.3 Hz, 1H);
C NMR (75 MHz,
CDCl +D O) δ 126.0, 126.5, 127.1, 127.3, 127.4, 127.8, 128.1,
3
2
1
1
28.3, 128.8, 129.4, 131.3, 133.1, 133.2, 134.1, 135.1, 138.6,
2. Dimethyl fumarate reacted with cyclopentadiene with better
44.7. 11B NMR (96 MHz, C D ) δ 30.5; HREIMS calcd for
6
6
enantioselectivity than methyl acrylate, and the absolute
stereopreference was consistent with that of methyl acrylate. The
+
C20H15O B [M ] 298.1165, found 298.1159.
2
(
3
1
R)-6,6’-Dimesityl-1,1’-binaphthyl-2-boronic Acid (5b): IR (KBr)
675-3150 (br), 2920, 1622, 1470, 1377, 1360, 1350, 1312 cm ;
reaction with 2,2-dimethyl-5-methylene-1,3-dioxolan-4-one14 as
a
-1
cyclic dienophile also gave good enantioselectivity but low yield, and
the absolute stereopreference could be explained in terms of the
sterically favored intermediate E. The enhanced enantioselectivity in
both cases may be ascribed to conformational restriction of the enone
unit. Low enantioselectivity was seen with methacrolein as a dienophile.
Nevertheless, the absolute stereopreference could be explained in terms
of the most favorable intermediate F.
H NMR (300 MHz, CDCl +D O) δ 2.02 (s, 6H), 2.06 (s, 6H),
3
2
2.35 (s, 6H), 6.99 (s, 4H), 7.12 (dd, J=1.6, 8.6 Hz, 1H), 7.18 (dd,
J=1.6, 8.6 Hz, 1H), 7.32 (d, J=8.8 Hz, 1H), 7.43 (d, J=8.6 Hz,
1
1
H), 7.58 (dd, J=1.1, 6.9 Hz, 1H), 7.70 (t, J=8.2 Hz, 1H), 7.72 (s,
H), 7.77 (d, J=0.8 Hz, 1H), 7.97 (d, J=8.5 Hz, 1H), 8.02 (d, J=8.4
1
3
Hz, 1H), 8.16 (d, J=8.4 Hz, 1H);
C NMR (75 MHz,
CDCl +D O) δ 21.8 (6C), 126.2, 126.7, 127.6, 127.9, 128.1,
3
2
In conclusion, a new and effective chiral arylboronic acid and its
dichloroborane derivative with a binaphthyl skeleton was developed,
and its utility has been demonstrated in Diels-Alder reactions with both
α,β-unsaturated esters and aldehydes, although the enantioselectivity is
insufficient. Preliminary studies indicate that this area of research may
yield other catalyst structures and other useful enantioselective
reactions.
1
1
1
28.5, 128.7, 128.9, 129.5, 129.7, 131.7, 132.1, 132.2, 134.4,
35.4, 136.49, 136.53, 136.6, 136.7, 137.34, 137.39, 138.6, 138.9,
39.0, 140.1, 140.3, 144.9.
(R)-6,6’-Diphenyl-1,1’-binaphthyl-2-boronic Acid (5c): IR (KBr)
-1
3
675-3250 (br), 3050, 1489, 1395, 1375, 1360, 1352, 1320 cm ;
1
H NMR (300 MHz, CDCl +D O) δ 7.28 (d, J=8.8 Hz, 1H), 7.35-
3
2
7
.43 (m, 3H), 7.44-7.62 (m, 7H), 7.67-7.75 (m, 5H), 8.07 (d,

1
056
LETTERS
SYNLETT
J=8.0 Hz, 1H), 8.12 (d, J=9.1 Hz, 1H), 8.17 (d, J=9.7 Hz, 1H),
12. Method B: A dry, 25-mL round flask fitted with a stirring bar and
13
8.18 (d, J=9.8 Hz, 1H), 8.19 (d, J=8.3 Hz, 1H); C NMR (75
a 10-mL pressure-equalized addition funnel (containing a cotton
MHz, CDCl +D O) δ 125.8, 125.9, 126.2, 126.3, 126.80, 126.84,
plug and CaH and functioning as a Soxhlet extractor) surmounted
3
2
2
1
1
27.5, 127.7, 127.8, 129.0, 129.4, 131.5, 131.9, 132.1, 134.1,
35.1, 138.1, 139.5, 139.6, 140.65, 140.73, 144.2.
by a reflux condenser was charged with chiral boronic acid 5a
(14.9 mg, 0.05 mmol) and benzene (3 mL). An argon atmosphere
was secured, and the solution was brought to reflux. After 7 h, the
reaction mixture was cooled to room temperature and boron
trichloride (1 M in hexane, 300 µL, 0.3 mmol) was added. After
being refluxed for 3 h, the resulting mixture was cooled to room
temperature and the addition funnel and condenser were quickly
removed and replaced with a septum. The volatile boron
compounds and solvents were then removed under vacuum at
room temperature. The residual oil obtained as 7a was
9
1
1
.
For the dehydroxylation of phenols by cleavage of their diethyl
phosphate esters, see: Rossi, R. A.; Bunnett, J. F. J. Org. Chem.
1
973, 38, 2314.
0. (a) Katz, H. E. J. Org. Chem. 1985, 50, 2575. (b) Cole, T. E.;
Quintanilla, R.; Smith, B. M.; Hurst, D. Tetrahedron Lett. 1992,
33, 2761.
1. Method A: A dry, 25-mL round flask fitted with a stirring bar and
a reflux condenser was charged with chiral boronic acid 5a (14.9
mg, 0.05 mmol) and dry methanol (1 mL). An argon atmosphere
was secured, and the solution was brought to reflux. After being
refluxed for 2 h, the methanol and water generated were removed
under vacuum at room temperature. After the above treatment of
immediately used as a catalyst for the Diels-Alder reaction
11
without isolation. (R)-2-Dichloroboryl-1,1’-binaphthyl (7a):
NMR (96MHz, C D ) δ 57.4.
B
6
6
1
3. S-trans enone conformation with the metal located anti to the
alkoxy group has been observed for complexes of α,β-unsaturated
esters with Lewis acids: (a) Lewis, F. D.; Oxman, J. D.; Gibson, L.
L.; Hampsch, H. L.; Quillen, S. L. J. Am. Chem. Soc. 1986, 108,
5a with methanol was repeated three times, the resulting solid was
dissolved in dichloromethane (150 µL) and boron trichloride
solution in hexane (1 M , 150 µL, 0.15 mmol) at 0 °C. After being
stirred at 0 °C for 12 h, the volatile boron compounds and solvents
were removed under vacuum at room temperature. The treatment
with boron trichloride was repeated twice. The residual oil
obtained as 7a was immediately used as a catalyst for the Diels-
Alder reaction without isolation.
3
005. (b) Lewis, F. D.; Quillen, S. L.; Hale, P. D.; Oxman, J. D. J.
Am. Chem. Soc. 1988, 110, 1261. (c) see reference 3b.
14. For preparation of 2,2-dimethyl-5-methylene-1,3-dioxolan-4-one,
see: (a) Bailey, W. J.; Feng, P. Polym. Prepr. 1987, 28, 154. (b)
Mattay, J.; Kneer, G.; Mertes, J. Synlett 1990, 145.