A new and extremely active Corey's chiral oxazaborolidine catalyst

Article abstract of DOI:10.1055/s-1999-2804

A new chiral oxazaborolidine catalyst has been prepared in situ from 3,5-bis(trifluoromethyl)phenylboron dichloride and N-(p-toluenesulfonyl)- (S)-tryptophan. This catalyst is much more active than Corey's original catalyst for Mukaiyama aldol reaction of

Full text of DOI:10.1055/s-1999-2804

LETTER
1283
A New and Extremely Active Corey’s Chiral Oxazaborolidine Catalyst
Kazuaki Ishihara, Shoichi Kondo, Hisashi Yamamoto*
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan, CREST, Japan Science and Technology
Corporation (JST), and Research Center for Advanced Waste and Emission Management (ResCWE), Furo-cho, Chikusa, Nagoya
464-8603, Japan
Fax +81-52-789-3222; E-mail: j45988a@nucc.cc.nagoya-u.ac.jp
Received 7 June 1999
A new chiral oxazaborolidine catalyst 2d was prepared
Abstract: A new chiral oxazaborolidine catalyst has been prepared
simply by treating of N-(p-toluenesulfonyl)-(S)-tryp-
in situ from 3,5-bis(trifluoromethyl)phenylboron dichloride and N-
tophan (1) with an equimolar amount of 3,5-bis(trifluo-
(p-toluenesulfonyl)-(S)-tryptophan. This catalyst is much more ac-
romethyl)phenylboron dichloride (5) in dichloromethane
and subsequent removal of produced HCl and the solvent
in vacuo (Scheme 1).12 Moisture sensitive boron dichlo-
ride 5 was synthesized by dehydration of 3 to trimeric an-
hydride 4 and subsequent chlorination of 4 with 2
equivalents of boron trichloride.13 The preparation of ox-
azaborolidines from arylboron dichlorides was previously
reported by Reilly and Oh9a and Harada et al.9b–i Although
B-butyloxazaborolidine 2b has been prepared from 1 and
butylboronic acid by dehydration,5 B-aryloxazaborolidine
could not be prepared from arylboronic acid as observed
by Nevalainen et al.6p and Harada et al.9c In contrast, CAB
derived from 2,6-di(isopropoxy)benzoyltartaric acid has
been easily prepared by adding an equimolar amount of
the corresponding arylboronic acid at room temperature.10
tive than Corey’s original catalyst for Mukaiyama aldol reaction of
aldehydes with silyl enol ethers.
Key words: Mukaiyama aldol, enantioselective, arylboron dichlo-
ride, chiral oxazaborolidine, Lewis acid
Since our group1 and Helmchen’s2a independently an-
nounced a new class of chiral acyloxyboranes (CAB)3 de-
rived from N-sulfonylamino acids and borane•THF, chiral
1,3,2-oxazaborolidines, their utility as chiral Lewis acid
catalysts in enantioselective synthesis has been convinc-
ingly demonstrated.4-9 In particular, Coreyís tryptophan-
derived chiral oxazaborolidines 2a and 2b are highly ef-
fective for not only Mukaiyama aldol reaction of alde-
hydes with silyl enol ethers5d but also Diels–Alder
reaction of a-substituted a,b-enals with dienes (eq 1).5a–c,e
More than 20 mol% of 2b is required for the former reac-
tion, however. Other chiral oxazaborolidines developed
for the enantioselective aldol reaction of aldehydes with
relatively more reactive ketene silyl acetals also require
more than 20 mol% amounts to give aldol adducts in good
yield.6,7 We recently succeeded in enhancing the catalytic
activities of CAB derived from 2,6-di(isopropoxy)ben-
zoyltartaric acid and borane•THF and Brønsted acid-as-
sisted chiral Lewis acid (BLA) derived from chiral tetrol
and borane∑THF by a modified method using 3,5-bis(trif-
luoromethyl)phenylboronic acid (3) instead of bo-
rane∑THF.10,11 The utility of 3 for the design of more
active boron catalysts encouraged us to seek a new and ex-
tremely active Corey’s catalyst 2d. This paper describes a
successful example of designer chiral Lewis acid catalyst
modified using arylboron dichlorides bearing electron-
withdrawing substituents as Lewis acid components.
Scheme 1 Synthesis of 3,5-Bis(trifluoromethyl)phenylboron Dichlo-
ride (5) and Preparation of Catalyst 2d
According to Corey’s paper,5d terminal trimethylsilyloxy
(vinylidene) olefins appear to be the most favorable sub-
strates for the enantioselective Mukaiyama aldol coupling
catalyzed by 2b as compared to more highly substituted
olefins like RCH=(OSiMe3)R’ or R2C=C(OSiMe3)R’.
Actually, the reaction of trimethylsilyl enol ether derived
from cyclopentanone with benzaldehyde afforded the al-
dol products in only 71% yield even in the presence of 40
mol% of 2b.5d
Synlett 1999, No. 8, 1283–1285 ISSN 0936-5214 © Thieme Stuttgart · New York

1284
K. Ishihara et al.
LETTER
Our initial studies, summarized in Table 1, were conduct- 99% ee.7a The syn selection observed in both reactions of
ed with benzaldehyde and trimethylsilyl enol ether de- aldehydes with (E)- and (Z)-trimethylsilyl enol ethers sug-
rived from acetophenone at –78 °C in propionitrile as gests that the reaction occurs via extended-transition state
solvent in the presence of 2 as catalyst (eq 2).14 Following assemblies. It is noteworthy that the anti selection has
Corey’s procedure using 10 mol% of 2b, we obtained the been observed in the reaction of aldehydes with (E)-
trimethylsilyl ether of aldol 6 and the free aldol 7 in only ketene trimethylsilyl acetals catalyzed by other chiral ox-
38% and 15% yields, respectively. However, when the B- azaborolidines.6,7 Thus, we were able to expand the scope
phenyl analog 2c was used as catalyst, the chemical yield of the substrates which were usable for the enantioselec-
was improved strikingly. Furthermore, when the B-3,5- tive Mukaiyama-aldol reaction by developing B-3,5-
bis(trifluoromethyl)phenyl analog 2d was used, catalytic bis(trifluoromethyl)phenyloxazaborolidine 2d.
activity and enantioselectivity were increased to a turn-
over of 25 and 91~93% ee respectively. The absolute con-
figuration of aldol adducts indicated in the Table was
uniformly R.
Next, the reaction of several aldehydes with trisubstituted
trimethylsilyl enol ethers was conducted at –78 °C in pro-
pionitrile in the presence of 10 mol% of 2d as catalyst (eq
3).14 The results of these experiments are summarized in
Table 2. In the reaction of benzaldehyde with the trimeth-
ylsilyl enol ether of cyclohexanone, both substrates were
sequentially added in a solution of 2d in propionitrile at
In summary, it has been demonstrated that the introduc-
tion of an electron-withdrawing substituent such as 3,5-
bis(trifluoromethyl)phenyl group on the B atom of chiral
boron catalysts is one of the most effective methods for
–78 °C according to Coreyís procedure.5d The reaction
proceeded quantitatively to give only the aldol products in
78:22 diastereomeric ratio 8:9, and the optical yield of 8
was 89% ee. The reaction of butyraldehyde with the (Z)-
trimethylsilyl enol ether derived from propiophenone,
however, did not proceed well, probably due to the de-
composition of butyraldehyde in the presence of the
strong Lewis acid 2d before addition of the trimethylsilyl
enol ether. Fortunately, the reaction proceeded cleanly by
adding trimethylsilyl enol ether followed by butyralde-
hyde to afford only the syn aldol adduct with more than
enhancement of their catalytic activity.
References and Notes
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(b) Sartor, D.; Saffrich, J.; Helmchen, G.; Richards, C. J.;
Lambert, H. Tetrahedron: Asymmetry 1991, 2, 639.
Synlett 1999, No. 8, 1283–1285 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A New and Extremely Active Coreyís Chiral Oxazaborolidine Catalyst
1285
(3) CAB is a general name of chiral Lewis acid prepared from
boron reagents and chiral carboxylic acids such as a-
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(12) Experimental procedure for the preparation of 2d: To a
solution of 1 (32.3 mg, 0.09 mmol) in dichloromethane (0.75
mL) was added 5 (22.1 mg, 0.075 mmol) at room temperature
under argon. After being stirred for 1 h the mixture was
concentrated in vacuo to give 2d as a white solid, which was
dissolved in propionitrile and used for Mukaiyama aldol
reactions.
(13) Experimental procedure for the synthesis of 5: After a solution
of 3 (3.86 g, 15 mmol) in benzene (30 mL) was heated at
reflux with removal of water (CaH2 in a Soxhlet thimble) for
2~4 h (oil bath: 100~105 °C), the solution was cooled to room
temperature and concentrated in vacuo to give trimeric
anhydride 4 as white solid. 3: 1H NMR (C6D6, 300 MHz) d
7.81 (s, 1H), 8.01 (s, 2H); 4: 1H NMR (C6D6, 300 MHz) d 8.01
(s, 1H), 8.46 (s, 2H). A 1 M solution of boron trichloride (30
mL, 30 mmol) in hexane was added to 4 at room temperature
under argon. After the reaction mixture was heated at reflux
for 4 h (oil bath: 100–105 °C), hexane was removed by
heating. Dichloroboron compound 5 was isolated as a
colorless oil by distillation from the residue at 38~40 °C under
0.05~0.06 torr. 5: 1H NMR (C6D6, 300 MHz) d 7.80 (s, 1H),
8.12 (s, 2H); 11B NMR (C6D6, 96 MHz) d 53.2 (d 0.0 for
BF3•Et2O); 13C NMR (C6D6, 75.5 MHz) d 123.1 (q, J=272.8
Hz, 2C), 127.1 (s, 1C), 131.0 (q, J=33.5 Hz, 2C), 134.8-135.2
(m, 1C), 135.5 (s, 2C); 19F NMR (C6D6, 282 MHz) d 129.1 (d
129.5 for CF3C6H5).
(14) Experimental procedure for the reaction of benzaldehyde with
1-phenyl-1-(trimethylsiloxy)ethylene catalyzed by 2d is
representative. To 2d (0.075 mmol, 6 mol%) prepared
according to ref. 12 was added propionitrile (1 mL) at room
temperature. After being cooled to –78 °C, benzaldehyde (127
mL, 1.25 mmol) was added, and a solution of the trimethylsilyl
enol ether (308 mL, 1.5 mmol) in propionitrile (0.5 mL) was
subsequently added dropwise over 2 min. The reaction
mixture was stirred at –78 °C for 7 h and then quenched by the
addition of saturated aqueous NaHCO3. The mixture was
extracted with ether and then the combined organic phases
were dried over MgSO4 and evaporated. The residue was
dissolved in THF (2 mL) and 1 M aq HCl (2 mL), and the
resulting solution was allowed to stand for 30 min. Saturated
aqueous NaHCO3 was added and the mixture was extracted
with ether. The combined organic phases were dried over
MgSO4 and evaporated to an oily residue. Silica gel
chromatography (hexane:EtOAc = 4:1) afforded 282 mg
(>99% yield) of the known aldol product. HPLC analysis
(Daicel OD-H column with hexane:i-PrOH = 20:1, flow
rate=1.0 mL/min) indicated an enantiomeric excess of 90.4%
(tR major 19.6 min (S); minor 23.1 min (R)).
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Article Identifier:
1437-2096,E;1999,0,08,1283,1285,ftx,en;G15399ST.pdf
Synlett 1999, No. 8, 1283–1285 ISSN 0936-5214 © Thieme Stuttgart · New York