Symmetrical 4,4′,6,6′-tetraarylbinaphthyl-substituted ammonium bromide as a new, chiral phase-transfer catalyst

Article abstract of DOI:10.1016/S0957-4166(03)00271-4

Binaphthyl-modified spiro-type symmetrical phase-transfer catalysts possessing 4,4′,6,6′-tetraaryl substituents are shown to exhibit high asymmetric induction in asymmetric alkylation of benzophenone imine glycine tert-butyl ester under ordinary phase-tra

Full text of DOI:10.1016/S0957-4166(03)00271-4

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 1599–1602
Symmetrical 4,4%,6,6%-tetraarylbinaphthyl-substituted ammonium
bromide as a new, chiral phase-transfer catalyst
Takuya Hashimoto, Youhei Tanaka and Keiji Maruoka*
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Received 21 February 2003; accepted 13 March 2003
Abstract—Binaphthyl-modified spiro-type symmetrical phase-transfer catalysts possessing 4,4%,6,6%-tetraaryl substituents are shown
to exhibit high asymmetric induction in asymmetric alkylation of benzophenone imine glycine tert-butyl ester under ordinary
phase-transfer conditions. © 2003 Elsevier Science Ltd. All rights reserved.
unsuccessful due to the steric repulsion of such tetra-
phenyl substituents. Accordingly, we prepared symmetri-
cal 4,4%-diaryl-substituted ammonium salts 3 by mixing
2 equiv. of dibromide 4 and ammonia, and evaluated
their chiral efficiency in the asymmetric alkylation of
glycine derivative.3
1. Introduction
We recently reported the design of binaphthyl-modified,
spiro-type chiral ammonium salts of type 1 and their
application to the catalytic asymmetric synthesis of
various a-alkyl- and a,a-dialkyl-a-amino acids in addi-
tion to b-hydroxy-a-amino acids.1,2 In this part of the
study, introduction of 3,3%-diaryl substituents to the
parent symmetrical ammonium bromide 1a was found to
be highly effective for obtaining higher enantioselectivity.
For example, in a typical asymmetric benzylation of
benzophenone imine glycine tert-butyl ester, 1 mol% of
unsymmetrical catalyst 1b and 1c afforded 89% ee (81%
yield) and 96% ee (95% yield), respectively, compared to
the lower enantioselectivity (79% ee) by using the parent
symmetrical 1a.2a From the viewpoint of catalyst design,
such an unsymmetrical catalyst 1 requires the indepen-
dent synthesis of both right-hand and left-hand sides of
the molecules as illustrated in Scheme 1. Obviously,
preparation of symmetrical ammonium salts of type 2 has
a distinct advantage over unsymmetrical 1 (Ar"H),
because synthesis of only one side of the symmetrical
catalyst 2 is required. Unfortunately, however, attempted
synthesis of symmetrical bis(3,3%-diphenylbinaphthyl-
modified) ammnonium salt was found to be totally
2. Results
The requisite catalysts 3a and 3b can be prepared as
outlined in Schemes 2 and 3. Thus, the known (S)-
4,4%,6,6%-tetraphenylbinaphthol 54 is transformed with
Tf2O and Et3N to the corresponding (S)-bis-triflate 6
which is susceptible to the Ni-catalyzed cross coupling
with MeMgI and catalytic NiCl2(dppp) to furnish (S)-
bis-methyl derivative 7. Radical bromination of 7 is
effected with NBS and catalytic AIBN as a radical
initiator to afford (S)-dibromide 4a. Treatment of 4a (2
equiv.) with aqueous ammonia in CH3CN directly gives
the desired spiro-type (S,S)-ammonium bromide 3a.
We also prepared (S)-4,4%-diphenylbinaphthyl derivative
3b in order to examine the substituent effects of 6,6%-
diphenyl moieties (Scheme 3). The known (S)-4,4%-
* Corresponding author. E-mail: maruoka@kuchem.kyoto-u.ac.jp
0957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00271-4

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T. Hashimoto et al. / Tetrahedron: Asymmetry 14 (2003) 1599–1602
Scheme 1.
Scheme 2. Reagents and conditions: (a) Tf2O (3 equiv.), Et3N (3 equiv.), CH2Cl2, −78°C to rt; (b) MeMgI (4 equiv.), NiCl2(dppp)
(5 mol%), ether reflux; (c) NBS (2.2 equiv.), AIBN (10 mol%), benzene reflux; (d) 28% aq. NH3 (4 equiv.), CH3CN.
Scheme 3. Reagents and conditions: (a) PhB(OH)2 (2.4 equiv.), Pd(PPh3)4 (3 mol%), aq. K2CO3 (2 M), THF reflux; (b) HCO2NH4
(16 equiv.), Pd/C (5 mol%), MeOCH2CH2OH, 60°C; (c) BBr3 (2 equiv.), CH2Cl2, 0°C; (d) Tf2O (3 equiv.), Et3N (3 equiv.),
CH2Cl2, −78°C to rt; (e) MeMgI (4 equiv.), NiCl2(dppp) (5 mol%), ether reflux; (f) NBS (2.2 equiv.), AIBN (10 mol%), benzene
reflux; (g) 28% aq. NH3 (4 equiv.), CH3CN.

T. Hashimoto et al. / Tetrahedron: Asymmetry 14 (2003) 1599–1602
1601
Scheme 4. Reagents and conditions: (a) Pd(OAc)2 (15 mol%), dppp (16.5 mol%), i-Pr2NEt (5 equiv.), CO, MeOH/DMSO, 80°C;
(b) LiAlH4 (2 equiv.), THF, 0°C; (c) BBr3 (2 equiv.), CH2Cl2, 0°C; (d) 28% aq. NH3 (4 equiv.), CH3CN.
Table 1. Catalytic enantioselective phase-transfer alkylationa
Entry
Catalyst
RX
Conditions (°C, h)
Yield (%)b
% eec (config.)d
1
2
3
4
3a
3b
3c
3d
PhCH2Br
0, 3.5
0, 3.5
0, 24
0, 6
85
82
87
86
92 (R)
90 (R)
97 (R)
96 (R)
5
6
7
8
3a
3b
3c
3d
0, 1
0, 2
0, 20
0, 5
81
83
76
91
91 (R)
87 (R)
93 (R)
92 (R)
9
3a
3b
3c
3d
0, 2
0, 4
0, 20
0, 7
90
92
83
56
94 (R)
94 (R)
94 (R)
95 (R)
10
11
12
13
14
3a
3c
0, 3
0, 24
88
91
90 (R)
95 (R)
15
3a
CH3CH2Ie
0, 24
12
88 (R)
a Unless otherwise specified, the reaction was carried out with 1.2 equiv. of RX in the presence of 1 mol% of 3 in 50% aq. KOH/toluene (volume
ratio=1:3) under the given reaction conditions.
b Isolated yield.
c Enantiopurity of 15 was determined by HPLC analysis of the alkylated imine using a chiral column (DAICEL Chiralcel OD) with
hexane–isopropanol as solvent.
d Absolute configuration was determined by comparison of the HPLC retention time with the authentic sample independently synthesized by the
reported procedure.2a
e Use of 5 equiv. of alkyl halide.
dibromo-6,6%-dichlorobinaphthyl ether 85 is selectively
converted to (S)-4,4%-diphenyl-6,6%-dichlorobinaphthyl
ether 9 by Suzuki–Miyaura coupling with PhB(OH)2,
aqueous K2CO3 and catalytic Pd(PPh3)4, and 6,6%-
dichloro groups are then removed by catalytic hydro-
genation with Pd/C and ammonium formate to furnish
(S)-4,4%-diphenylbinaphthyl ether 10 which is cleaved
with BBr3 to furnish (S)-4,4%-diphenylbinaphthol 11 in
79% overall yield. Transformation of 11 to 3b via 12,
13, and 4b was accomplished in a similar manner as
described above.
The synthetic potential of these catalysts 3a and 3b was
evaluated by the asymmetric phase-transfer alkyl-
ation of benzophenone imine glycine tert-butyl ester 14
under ordinary phase-transfer conditions. Thus,

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T. Hashimoto et al. / Tetrahedron: Asymmetry 14 (2003) 1599–1602
treatment of protected glycine derivative 14 with benz-
yl bromide (1.2 equiv.) and 50% aqueous KOH/toluene
(volume ratio=1:3) under the influence of 1 mol% 3a at
0°C for 3.5 h resulted in formation of a-phenylalanine
derivative 15 (R=CH2Ph) in 85% yield with 92% ee.6
When we use 4,4%-diphenylbinaphthyl derivative 3b,
similar reactivity and selectivity (82% yield; 90% ee) was
observed in the asymmetric benzylation of glycine deriva-
tive 14.
References
1. For recent reviews on chiral phase-transfer catalysis, see: (a)
O’Donnell, M. J. In Catalytic Asymmetric Synthesis; Ojima,
I., Ed.; Verlag Chemie: New York, 1993; Chapter 8; (b)
Shioiri, T. In Handbook of Phase-Transfer Catalysis; Sasson,
Y.; Neumann, R., Eds.; Blackie Academic & Professional:
London, 1997; Chapter 14; (c) O’Donnell, M. J. Phases—The
Sachem Phase Transfer Catalysis Review 1998, Issue 4, p. 5;
(d) O’Donnell, M. J. Phases—The Sachem Phase Transfer
Catalysis Review 1999, Issue 5, 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. Aldrichim. Acta 2001, 34, 3.
2. (a) Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem. Soc.
1999, 121, 6519; (b) Ooi, T.; Takeuchi, M.; Kameda, M.;
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. Adv.
Synth. Catal. 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.
3. Recently, C2-symmetric guanidine based and tartrate-
derived chiral phase-transfer catalysts have been developed.
See: (a) Kita, T.; Georgieva, A.; Hashimoto, U.; Nakata, T.;
Nagasawa, K. Angew. Chem., Int. Ed. 2002, 41, 2832; (b)
Arai, S.; Tsuji, R.; Nishida, A. Tetrahedron Lett. 2002, 43,
9535; (c) Shibuguchi, T.; Fukuta, Y.; Akachi, Y.; Sekine, A.;
Ohshima, T.; Shibasaki, M. Tetrahedron Lett. 2002, 43, 9539.
4. Gong, L.-Z.; Hu, Q.-S.; Pu, L. J. Org. Chem. 2001, 66, 2358.
5. Cui, Y.; Evans, O. R.; Ngo, H. L.; White, P. S.; Lin, W.
Angew. Chem., Int. Ed. 2002, 41, 1159.
Since the observed enantioselectivity is not excellent with
3a or 3b, we then prepared 4,4%,6,6%-tetrakis(3,5-
diphenylphenyl)binaphthyl analogue 3c as shown in
Scheme 4.7 Thus, (S)-bis-triflate 16 (Ar=3,5-
diphenylphenyl) can be prepared in a similar manner as
described in Scheme 2 and Ref. 4, and then converted by
catalytic Pd(OAc)2, dppp, i-Pr2NEt, CO (gas), and
MeOH to the corresponding (S)-dicarboxylic acid methyl
ester 17, which is further reduced with LiAlH4 to give
(S)-diol 18. Bromination of 16 with BBr3 afforded
(S)-dibromide 4c which is reacted with aqueous ammonia
in CH3CN to furnish the desired spiro-type (S,S)-ammo-
nium bromide 3c. This carbonylation/reduction route is
also applicable to the synthesis of 3d.
The asymmetric benzylation of glycine derivative 14 was
effected with new catalysts 3c and 3d to furnish the
alkylation product 15 (R=CH2Ph) with higher enantiose-
lectivity (96–97% ee) under similar phase-transfer condi-
tions. Other selected examples are also included in Table
1.
Inconclusion, we have developedseveral new and efficient
catalysts 3a–d, via a simplified catalyst preparation, for
effecting asymmetric phase-transfer alkylation of glycine
derivative 14.
6. The catalyst 3a was recovered in ꢀ50% yield, suggesting the
partial decomposition of 3a under the phase-transfer condi-
tions. In contrast, catalyst 1 having 3,3-diaryl substituents
is stable and gives higher recovery yield than 3. See Ref. 2c.
7. Because of the difficulty for 4,4%,6,6%-tetrakis(3,5-
diphenylphenyl)binaphthyl analogue in radical bromination
as shown in Scheme 2, we developed a new synthetic route
to 4c as indicated in Scheme 4.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (No. 13853003) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.