C2-symmetric bicyclo[2.2.2]octadienes as chiral ligands: Their high performance in rhodium-catalyzed asymmetric arylation of N-tosylarylimines

Article abstract of DOI:10.1021/ja044790e

Asymmetric synthesis of diarylmethylamines with high enantioselectivity (95-99% ee) was realized by use of a new C₂-symmetric diene ligand, (1R,4R)-2,5-diphenylbicyclo[2.2.2]octa-2,5-diene (Ph-bod*), for the rhodium-catalyzed asymmetric arylati

Full text of DOI:10.1021/ja044790e

Published on Web 10/05/2004
C2-Symmetric Bicyclo[2.2.2]octadienes as Chiral Ligands: Their High
Performance in Rhodium-Catalyzed Asymmetric Arylation of
N-Tosylarylimines
Norihito Tokunaga, Yusuke Otomaru, Kazuhiro Okamoto, Kazuhito Ueyama, Ryo Shintani, and
Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan
Received August 29, 2004; E-mail: thayashi@kuchem.kyoto-u.ac.jp
Table 1. Rhodium-Catalyzed Asymmetric Arylation of Imines 3
with Arylboroxines 4a
Recent developments on the chiral diene ligands have opened
the door to an exciting new research area of catalytic asymmetric
reactions.1 The C2-symmetric bicyclo[2.2.1]heptadienes (nbd*),
which we have reported in 2003,2 are highly enantioselective chiral
ligands for rhodium-catalyzed asymmetric addition of organoboronic
acids to R,â-unsaturated ketones2 and fumaric and maleic com-
pounds.3 More recently, Carreira reported C1-symmetric bicyclo-
[2.2.2]octadienes and their successful use for iridium-catalyzed
kinetic resolution of allyl carbonates.4 We have continued our
studies on the preparation of new chiral diene ligands and their
application to catalytic asymmetric reactions. Here we wish to report
our new C2-symmetric bicyclo[2.2.2]octadienes (bod*), which have
a clear superiority over chiral phosphorus ligands in both enantio-
selectivity and catalytic activity in the rhodium-catalyzed arylation
of N-tosylarylimines giving diarylmethylamines.
The synthetic pathway to the C2-symmetric bicyclo[2.2.2]-
octadienes is straightforward as shown in Scheme 1. Enantiomeri-
cally pure (1R,4R)-bicyclo[2.2.2]octa-2,5-dione ((R,R)-1)5 was
obtained by optical resolution of racemic diketone 1 through
fractional recrystallization of its dihydrazone of (R)-5-(1-phenyl-
ethyl)semioxamazide.6 Ditriflate formation with excess LDA and
N-(2-pyridyl)triflimide followed by cross-coupling of (R,R)-ditriflate
2 with PhCH2MgBr and PhMgBr in the presence of PdCl2(dppf)
as a catalyst7 gave the 2,5-disubstituted (1R,4R)-bicyclo[2.2.2]-
octadienes, (R,R)-Bn-bod* and (R,R)-Ph-bod*, respectively.8 It
should be noted that the 2,5-diphenyl diene, Ph-bod*, is a stable
compound, while its [2.2.1] (nbd) analogue readily undergoes
decomposition in the air under light.
yield (%)b
of amine
% eec
of amined
entry
imine 3
boroxine 4
ligand
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
3g
3h
3h
3h
3c
4m
4m
4m
4m
4m
4m
4m
4m
4m
4m
4m
4m
4n
(R,R)-Ph-bod*
(R,R)-Bn-bod*
(R,R)-Bn-nbd*
(R)-binap
(R)-segphos
(S)-phosphoramidite 44 (5am)
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
(R,R)-Ph-bod*
96 (5am)
98 (5am)
98 (5am)
28 (5am)
30 (5am)
98 (S)
94 (S)
92 (S)
31 (S)
70 (S)
6 (S)
95 (S)
99 (S)
98 (S)
99 (S)
98 (S)
99 (S)
99 (R)
96 (R)
99 (R)
99 (R)
97 (5bm)
96 (5cm)
94 (5dm)
98 (5em)
95 (5fm)
99 (5gm)
99 (5hn)
97 (5ho)
96 (5hp)
98 (5cn)
4o
4p
4n
a Reaction was carried out in dioxane at 60 °C for 6 h with 1.2 equiv of
boroxine 4 in the presence of 20 mol % KOH, 1 equiv (with respect to
boron) of H2O, and 3 mol % the catalyst generated from [RhCl(C2H4)2]2
and a chiral ligand. b Isolated yields by column chromatography on silica
gel (hexane/ethyl acetate ) 2/1). c Determined by HPLC analysis with a
chiral stationary phase column (Chiralcel OD-H: hexane/2-propanol ) 80/
20 for 5am, 5bm, 5cm, 5dm, 5em, 5fm, 5hn, 5ho, 5hp, and 5cn; hexane/
2-propanol ) 90/10 for 5gm). d Absolute configuration of 5am was
determined by conversion into known free amine (S)-phenyl(4-chlorophen-
yl)methylamine. The configurations of other amines were assigned by
consideration of the stereochemical reaction pathway.
ylboroxine (4m).12 Thus, a rhodium catalyst13 generated from
[RhCl(C2H4)]2]2 (3 mol % Rh), Ph-bod* (1.1 equiv with respect to
Rh), and aqueous KOH (20 mol %) in dioxane was added to a
solution of imine 3a and boroxine 4m (1.2 equiv with respect to
3a) in dioxane, and the mixture was heated at 60 °C for 6 h.
Chromatography on silica gel gave phenyl(4-chlorophenyl)meth-
ylamine tosylamide 5am in 96% yield, whose enantiomeric purity
was determined to be 98% ee by HPLC analysis with a chiral
stationary phase column (entry 1 in Table 1). Deprotection of
tosylamide 5am with SmI2 in HMPA gave free amine (S)-phenyl-
(4-chlorophenyl)methylamine in 64% yield as a mixture with 14%
of diphenylmethylamine (not optimized). The enantioselectivity was
still high but a little lower with benzyl-disubstituted diene ligands,
Bn-bod* and Bn-nbd*, which gave a high yield of tosylamide 5am
in 94 and 92% ee, respectively (entries 2 and 3). On the contrary,
(R)-binap, which is a chiral bisphosphine ligand successfully used
for the rhodium-catalyzed asymmetric 1,4-addition to electron-
deficient olefins,14 was a poor ligand in terms of both enantiose-
lectivity and catalytic activity in the present arylation reaction15
(entry 4). Low efficiency was also observed with segphos16 and a
phosphoramidite17 as a ligand (entries 5 and 6).
Scheme 1a
a Reagents and conditions: (a) (i) LDA/THF, -78 °C; (ii) Tf2Npy-2,
-78 °Cfrt. Yield ) 70%. (b) RMgBr, PdCl2(dppf) (1 mol %), Et2O reflux.
Yield ) 59% for R ) PhCH2. Yield ) 78% for R ) Ph.
The asymmetric synthesis of diarylmethylamines by the catalytic
asymmetric arylation9,10 has attracted growing attention due to their
importance in biological activity.11 Unfortunately, however, enan-
tioselectivity as high as 95% has not been reported yet for the
asymmetric synthesis of phenyl(4-chlorophenyl)methylamine, which
is a potential key intermediate to Cetirizine hydrochloride,11
although considerable efforts have been made, for example, by steric
tuning of aryl groups in arenesulfonamides of arylimines.9b,c
It was found that the enantioselectivity as high as 98% was
readily attained by use of chiral diene ligand (R,R)-Ph-bod* for
the reaction of 4-chlorobenzaldehyde N-tosylimine 3a with phen-
9
13584
J. AM. CHEM. SOC. 2004, 126, 13584-13585
10.1021/ja044790e CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
using arylboroxines where the aryl groups are 4-chloro (4n),
4-methoxy (4o), and 2-methyl (4p) phenyls (entries 13-15). The
diarylmethylamines where both of the aryl groups are substituted
phenyls can be prepared as well by combination of substituted
arylimines and substituted arylboroxines. One example giving (R)-
4-chlorophenyl(4-methoxyphenyl)methylamine (5cn) with 99%
enantioselectivity is shown in entry 16.
In summary, asymmetric synthesis of diarylmethylamines with
high enantioselectivity (95-99% ee) was realized for the first time
by use of a C2-symmetric diene ligand, Ph-bod*, for the rhodium-
catalyzed asymmetric arylation of N-tosylarylimines.
Acknowledgment. This work has been supported in part by a
Grant-in-Aid for Scientific Research, the Ministry of Education,
Culture, Sports, Science and Technology, Japan (21 COE on Kyoto
University Alliance for Chemistry). N.T. thanks the Japan Society
for the Promotion of Science for the award of a fellowship for
graduate students.
Supporting Information Available: Experimental procedures and
spectroscopic and analytical data for the products (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
References
(1) For a short review: Glorius, F. Angew. Chem., Int. Ed. 2004, 43, 3364.
(2) Hayashi, T.; Ueyama, K.; Tokunaga, N.; Yoshida, K. J. Am. Chem. Soc.
2003, 125, 11508.
(3) Shintani, R.; Ueyama, K.; Yamada, I.; Hayashi, T. Org. Lett. 2004, 6,
3425.
(4) Fischer, C.; Defieber, C.; Suzuki, T.; Carreira, E. M. J. Am. Chem. Soc.
2004, 126, 1628.
(5) Some other methods for the optical resolution have been reported: (a)
Hill, R. K.; Morton, G. H.; Peterson, J. R.; Walsh, J. A.; Paquette, L. A.
J. Org. Chem. 1985, 50, 5528. (b) Naemura, K.; Takahashi, N.; Tanaka,
S.; Ida, H. J. Chem. Soc., Perkin Trans. 1 1992, 2337. (c) Kinoshita, T.;
Haga, K.; Ikai, K.; Takeuchi, K.; Okamoto, K. Tetrahedron Lett. 1990,
31, 4057. (d) Almqvist, F.; Ekman, N.; Frejd, T. J. Org. Chem. 1996, 61,
3794 and references therein.
Scheme 3
(6) Optical resolution of ketones using (R)-5-(1-phenylethyl)semioxamazide
has been reported: Leonard, N. L.; Boyer, J. H. J. Org. Chem. 1950, 15,
42.
(7) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.; Hirotsu,
K. J. Am. Chem. Soc. 1984, 106, 158.
(8) (R,R)-Bn-bod*: [R]20D +86 (c 0.71, CHCl3). (R,R)-Ph-bod*: [R]20D -30
(c 0.72, CHCl3).
(9) Rhodium-catalyzed arylation. (a) With arylstannane: Hayashi, T.; Ishige-
dani, M. J. Am. Chem. Soc. 2000, 122, 976. (b) With arylboroxine:
Kuriyama, M.; Soeta, T.; Hao, X.; Chen, Q.; Tomioka, K. J. Am. Chem.
Soc. 2004, 126, 8128. (c) With aryltitanium: Hayashi, T.; Kawai, M.;
Tokunaga, N. Angew. Chem., Int. Ed. 2004, 43, in press.
(10) Addition of phenylzinc catalyzed by a chiral ketimine: Hermanns, N.;
Dahmen, S.; Bolm, C.; Bra¨se, S. Angew. Chem., Int. Ed. 2002, 41, 3692.
(11) For examples: (a) Bishop, M. J.; McNutt, R. W. Bioorg. Med. Chem.
Lett. 1995, 5, 1311. (b) Spencer, C. M.; Foulds, D.; Peters, D. H. Drugs
1993, 46, 1055. (c) Sakurai, S.; Ogawa, N.; Suzuki, T.; Kato, K.; Ohashi,
T.; Yasuda, S.; Kato, H.; Ito, Y. Chem. Pharm. Bull. 1996, 44, 765. (d)
Cetirizine is currently prepared as a racemate via an alkylation of a
piperazine derivative with a benzhydryl chloride derivative: Baltes, E.;
De Lannoy, J.; Rodriguez, L. Eur. Patent Appl. 58146, 1982.
(12) Rhodium-catalyzed arylation of N-tosylimines with arylboron reagents has
been reported by Tomioka (ref 9b) and by Miyaura: (a) Ueda, M.;
Miyaura, N. J. Organomet. Chem. 2000, 595, 31. (b) Ueda, M.; Saito,
A.; Miyaura, N. Synlett 2000, 1637.
The high enantioselectivity observed with chiral diene ligands,
especially Ph-bod*, demonstrates that the chirality recognition
ability brought about by two phenyl groups at the 2- and 5-positions
of the diene is substantially high and is higher for the N-
tosylarylimine than the ability brought about by the face-and-edge
orientation of four phenyl groups18 on the chelating bisphosphine
ligands represented by binap. The S configuration of the arylation
product 5am obtained with (R,R)-dienes is rationalized by the
coordination of imine 3a to a rhodium with its si-face.19 The
coordination with the other face is much less favorable due to the
steric repulsions caused by both of two phenyl groups on the diene
ligand (Scheme 3).
The scope of the present rhodium-catalyzed asymmetric arylation
using Ph-bod* as a chiral ligand is limited to aryl-derived imines
but is tolerant of a range of functional groups. Phenylation of the
aromatic imines substituted with trifluoromethyl (3b), methoxy (3c),
and dimethylamino (3d) at the 4-position of phenyl gave the
corresponding sulfonamides of aryl(phenyl)methylamines (S)-5 in
high yields with over 95% enantioselectivity (entries 7-9 in Table
1). High enantioselectivity (98-99% ee) was also observed in the
phenylation of imines 3e, 3f, and 3g, which were derived from
2-methoxybenzaldehyde, 1-naphthaldehyde, and 2-furaldehyde,
respectively (entries 10-12). The asymmetric arylation of benzal-
dehyde imine 3h with substituted phenyl groups was also successful
(13) We presume the formation of [Rh(OH)(Ph-bod*)]2 as an active catalyst.
(14) For reviews: (a) Hayashi, T.; Yamasaki, K. Chem. ReV. 2003, 103, 2829.
(b) Fagnou, K.; Lautens, M. Chem. ReV. 2003, 103, 169. (c) Bolm, C.;
Hildebrand, J. P.; Mun˜iz, K.; Hermanns, N. Angew. Chem., Int. Ed. 2001,
40, 3284.
(15) Low enantioselectivity of binap in the rhodium-catalyzed asymmetric
arylation of imines has been reported in refs 9b and 9c.
(16) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.; Sayo, N.; Miura, T.;
Kumobayashi, H. AdV. Synth. Catal. 2001, 343, 264.
(17) Boiteau, J.-G.; Minnaar, A. J.; Feringa, B. L. J. Org. Chem. 2003, 68,
9481.
(18) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T. Organometallics 1993,
12, 4188, and references therein.
(19) Insertion of imine into an aryl-rhodium bond forming an amido complex
has been reported: Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2004,
126, 2694.
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