Substituent effect of binaphthyl-modified spiro-type chiral phase-transfer catalysts

Article abstract of DOI:10.1016/S0040-4039(03)00608-7

Binaphthyl-modified spiro-type phase-transfer catalysts possessing 4,4′-diaryl substituents are shown to exhibit high asymmetric induction in asymmetric benzylation of benzophenone imine glycine tert-butyl ester under phase-transfer conditions.

Full text of DOI:10.1016/S0040-4039(03)00608-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3313–3316
Substituent effect of binaphthyl-modified spiro-type chiral
phase-transfer catalysts
Takuya Hashimoto and Keiji Maruoka*
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Received 27 January 2003; revised 27 February 2003; accepted 28 February 2003
Abstract—Binaphthyl-modified spiro-type phase-transfer catalysts possessing 4,4%-diaryl substituents are shown to exhibit high
asymmetric induction in asymmetric benzylation of benzophenone imine glycine tert-butyl ester under phase-transfer conditions.
©
2003 Elsevier Science Ltd. All rights reserved.
We recently communicated the design of new binaph-
thyl-modified spiro-type phase-transfer catalysts of type
We first prepared (S,S)-4,4%,6,6%-tetraphenylbinaphthyl
derivative 2a from a synthetic point of view rather than
(S,S)-4,4%-diphenylbinaphthyl derivative 2b. The reac-
tion sequences for the synthesis of 2a starting from the
1
, and their application to the highly practical asym-
metric synthesis of various a-alkyl- and a,a-dialkyl-a-
1
,2
3
amino acids. In the series of this work, introduction
of 3,3%-diaryl substituents to the parent symmetrical
ammonium bromide 1a is found to be crucially impor-
tant for obtaining higher enantioselectivity. Indeed,
asymmetric benzylation of benzophenone imine glycine
tert-butyl ester in toluene/50% aqueous KOH with 1
mol% of catalysts 1a–c at 0°C gave rise to phenylala-
known (S)-4,4%,6,6%-tetraphenylbinaphthol (3) are
shown in Scheme 1. Thus, (S)-4,4%,6,6%-tetraphenyl-
binaphthol (3) is transformed with Tf O and NEt to
2
3
the corresponding (S)-bis-triflate 4 which is then sus-
ceptible to the Ni-catalyzed cross-coupling with MeMgI
and catalytic NiCl (dppp) to furnish (S)-bis-methyl
2
derivative 5. Radical bromination of 5 is effected with
NBS and catalytic AIBN as radical initiator to afford
(S)-dibromide 6. Treatment of 6 with cyclic secondary
2a,j
nine derivative in 79, 89 and 98% ee’s, respectively.
During the course of this study, we have been interested
in the possibility of examining the effect of adjacent
4
amine 7 as a right-hand side molecule gives the desired
4,4%-substituents of the catalyst rather than 3,3%-sub-
spiro-type (S,S)-ammonium bromide 2a.
stituents in the asymmetric phase-transfer alkylations.
Here we wish to report that even 4,4%-diaryl substituents
of the catalysts of type 2 exhibited a meaningful effect
on such asymmetric phase-transfer alkylations.
The chiral efficiency of the spiro-type phase-transfer
catalyst 2a was examined by carrying out asymmetric
alkylation of benzophenone imine glycine tert-butyl
Keywords: asymmetric alkylation; phase-transfer catalyst; binaphthol; glycine ester.
*
Corresponding author. Tel./fax: +81 75 753 4041; e-mail: maruoka@kuchem.kyoto-u.ac.jp
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00608-7

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T. Hashimoto, K. Maruoka / Tetrahedron Letters 44 (2003) 3313–3316
Scheme 1. Reagents and conditions: (a) Tf O (3 equiv.), Et N (3 equiv.), CH Cl , −78°C to rt; (b) MeMgI (4 equiv.), NiCl (dppp)
2
2
3
2
2
2
(
5 mol%), ether reflux; (c) NBS (2.2 equiv.), AIBN (10 mol%), benzene reflux; (d) K CO (3 equiv.), MeCN, 50°C.
2 3
ester (8). For example, reaction of 8 with benzyl bro-
mide in toluene/50% aqueous KOH under the influence
of 1 mol% of catalyst 2a at 0°C for 7 h gave rise to
The (S,S)-4,4%-diphenylbinaphthyl derivative 2b was
found to exhibit similar reactivity and a little bit lower
selectivity compared to 2a in the asymmetric alkylation
of glycine derivative 8 (entry 2 versus 1). We also
prepared sterically more hindered (S,S)-4,4%,6,6%-tetra-
kis(3,5-diphenylphenyl)-binaphthyl analogue 2c and
applied it to the asymmetric alkylation of glycine
derivative 8 to furnish the alkylation product 9 with
slightly higher enantioselectivity and shorter reaction
time (entries 3, 5, 8 and 10). However, the observed
enantioselectivity is not as appealing as that with 3,3%-
bis(3,5-diphenylphenyl) analogue 1c (e.g. 98% ee in the
asymmetric benzylation of 8 with 1c under similar
phase transfer conditions).
benzylation product 9 (R=CH Ph) in 90% yield with
2
9
1% ee. The absolute configuration of 9 (R=CH Ph)
2
was determined to be R by comparison of the HPLC
retention time with the authentic sample independently
2
synthesized by the reported procedure. The observed
enantioselectivity is rather surprising compared to that
(
89% ee) using 3,3%-diphenyl-substituted 1b under simi-
lar reaction conditions. Other selected examples are
listed in Table 1. With several other alkyl halides, good
to high enantioselectivity is observable in the catalytic
enantioselective phase transfer alkylation of glycine
derivative 8.
We also prepared (S,S)-4,4%-diphenylbinaphthyl deriva-
tive 2b in order to examine the substituent effect of
Although elucidation of the effect of 4,4%-diaryl sub-
stituents in chiral phase transfer catalyst 2 must await
further research, the new finding described herein fur-
ther expands the potential in the design of new chiral
phase transfer catalysts.
6
,6%-diphenyl moieties (Scheme 2). The known (S)-4,4%-
5
dibromo-6,6%-dichlorobinaphthyl ether 10 is selectively
converted to (S)-4,4%-diphenyl-6-6%-dichlorobinaphthyl
ether 11 by Suzuki–Miyaura coupling with PhB(OH)2,
aqueous K CO and catalytic Pd(PPh ) , and 6,6%-
A typical experimental procedure is as follows (entry 1
in Table 1): To a mixture of benzophenone imine
glycine tert-butyl ester (8) (148 mg, 0.5 mmol) and
chiral catalyst 2a (4.8 mg, 0.005 mmol) in toluene (3.0
mL)–50% KOH aqueous solution (1.0 mL) was added
benzyl bromide (72.1 mL, 0.6 mmol) dropwise at 0°C.
The reaction mixture was stirred vigorously at the same
temperature for 7 h. The mixture was then poured into
2
3
3 4
dichloro groups are then removed by catalytic hydro-
genation with Pd/C and ammonium formate to furnish
(
S)-4,4%-diphenylbinaphthyl ether 12a which is cleaved
with BBr to furnish (S)-4,4%-diphenylbinaphthol (12b)
3
in 79% overall yield. Transformation of 12b to 2b via
1
2c, 13, and 14 was accomplished in a similar manner
as described in Scheme 1.

T. Hashimoto, K. Maruoka / Tetrahedron Letters 44 (2003) 3313–3316
3315
a
Table 1. Catalytic enantioselective phase transfer alkylation
water and extracted with ether. The organic extracts
were washed with brine and dried over Na SO . Evapo-
1:10 as eluant) gave the alkylation product 9 (R=
CH Ph) (173.5 mg, 0.45 mmol, 90% yield) as a colorless
2
4
2
ration of solvents and purification of the residual oil by
column chromatography on silica gel (ether/hexane=
oil. The enantiomeric excess was determined by chiral
HPLC analysis (DAICEL CHIRALCEL OD, hex-

3
316
T. Hashimoto, K. Maruoka / Tetrahedron Letters 44 (2003) 3313–3316
Scheme 2. Reagents and conditions: (a) PhB(OH) (2.4 equiv.), Pd(PPh ) (3 mol%), aq. K CO (2 M), THF reflux; (b) HCO NH
4
2
3 4
2
3
2
(
16 equiv.), Pd/C (5 mol%), MeOCH CH OH, 60°C; (c) BBr (2 equiv.), CH Cl , −78°C to rt; (d) Tf O (3 equiv.), Et N (3 equiv.),
2 2 3 2 2 2 3
CH Cl , −78°C to rt; (e) MeMgI (9 equiv.), NiCl (dppp) (5 mol%), ether reflux; (f) NBS (2.2 equiv.), AIBN (10 mol%), benzene
2
2
2
reflux; (g) 7, K CO (3 equiv.), MeCN, rt.
2
3
ane:isopropanol=100:1, flow rate=0.5 mL/min, reten-
tion time; 14.8 min (R) and 28.2 min (S)).
p. 5; (e) Shioiri T.; Arai, S. In Stimulating Concepts in
Chemistry; Vogtle, F.; Stoddart, J. F.; Shibasaki, M., Eds.;
Wiley-VCH: Weinheim, 2000; p. 123; (f) O’Donnell, M. J.
Aldrichimica Acta 2001, 34, 3.
Acknowledgements
2. (a) Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem. Soc.
1999, 121, 6519; (b) Ooi, T.; Takeuchi, M.; Kameda, M.;
This work was supported by the Asahi Glass Founda-
tion and a Grant-in-Aid for Scientific Research (No.
Maruoka, K. J. Am. Chem. Soc. 2000, 122, 5228; (c) Ooi,
T.; Kameda, M.; Tannai, H.; Maruoka, K. Tetrahedron
Lett. 2000, 41, 8339; (d) Ooi, T.; Doda, K.; Maruoka, K.
Org. Lett. 2001, 3, 1273; (e) Maruoka, K. J. Fluorine
Chem. 2001, 112, 95; (f) Ooi, T.; Takeuchi, M.; Maruoka,
K. Synthesis 2001, 1716; (g) Ooi, T.; Uematsu, Y.;
Maruoka, K. Advanced Synth. Cat. 2002, 344, 288; (h)
Ooi, T.; Uematsu, Y.; Kameda, M.; Maruoka, K. Angew.
Chem., Int. Ed. 2002, 41, 1621; (i) Ooi, T.; Takahashi, M.;
Doda, K.; Maruoka, K. J. Am. Chem. Soc. 2002, 124,
7640; (j) Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem.
Soc., in press.
1
3853003) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
References
1
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O’Donnell, M. J. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; Verlag Chemie: New York, 1993; Chapter
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