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
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan
Received August 29, 2004; E-mail: email@example.com
Recent developments on the chiral diene ligands have opened
Table 1. Rhodium-Catalyzed Asymmetric Arylation of Imines 3
the door to an exciting new research area of catalytic asymmetric
with Arylboroxines 4a
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, -unsaturated ketones2 and fumaric and maleic com-
pounds.3 More recently, Carreira reported C
[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.
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 a Reaction was carried out in dioxane at 60 °C for 6 h with 1.2 equiv of
fractional recrystallization of its dihydrazone of (R)-5-(1-phenyl-
boroxine 4 in the presence of 20 mol % KOH, 1 equiv (with respect to boron) of H
ethyl)semioxamazide.6 Ditriflate formation with excess LDA and
2O, and 3 mol % the catalyst generated from [RhCl(C2H4)2]2
and a chiral ligand. b Isolated yields by column chromatography on silica
N-(2-pyridyl)triflimide followed by cross-coupling of (R,R)-ditriflate
gel (hexane/ethyl acetate ) 2/1). c Determined by HPLC analysis with a
2 with PhCH
chiral stationary phase column (Chiralcel OD-H: hexane/2-propanol ) 80/
2MgBr and PhMgBr in the presence of PdCl2(dppf)
20 for 5am, 5bm, 5cm, 5dm, 5em, 5fm, 5hn, 5ho, 5hp, and 5cn; hexane/
as a catalyst7 gave the 2,5-disubstituted (1R,4R)-bicyclo[2.2.2]-
2-propanol ) 90/10 for 5gm). d Absolute configuration of 5am was
octadienes, (R,R)-Bn-bod* and (R,R)-Ph-bod*, respectively.8 It
determined by conversion into known free amine (S)-phenyl(4-chlorophen-
should be noted that the 2,5-diphenyl diene, Ph-bod*, is a stable
yl)methylamine. The configurations of other amines were assigned by
compound, while its [2.2.1] (nbd) analogue readily undergoes
consideration of the stereochemical reaction pathway.
decomposition in the air under light.
ylboroxine (4m).12 Thus, a rhodium catalyst13 generated from Scheme 1 a
[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 a Reagents and conditions: (a) (i) LDA/THF, -78 °C; (ii) Tf2Npy-2,
was determined to be 98% ee by HPLC analysis with a chiral
-78 °Cfrt. Yield ) 70%. (b) RMgBr, PdCl2(dppf) (1 mol %), Et2O reflux.
stationary phase column (entry 1 in Table 1). Deprotection of
Yield ) 59% for R ) PhCH2. Yield ) 78% for R ) Ph.
tosylamide 5am with SmI2 in HMPA gave free amine (S)-phenyl-
The asymmetric synthesis of diarylmethylamines by the catalytic
(4-chlorophenyl)methylamine in 64% yield as a mixture with 14%
asymmetric arylation9,10 has attracted growing attention due to their
of diphenylmethylamine (not optimized). The enantioselectivity was
importance in biological activity.11 Unfortunately, however, enan-
still high but a little lower with benzyl-disubstituted diene ligands,
tioselectivity as high as 95% has not been reported yet for the
Bn-bod* and Bn-nbd*, which gave a high yield of tosylamide 5am
asymmetric synthesis of phenyl(4-chlorophenyl)methylamine, which
in 94 and 92% ee, respectively (entries 2 and 3). On the contrary,
is a potential key intermediate to Cetirizine hydrochloride,11
(R)-binap, which is a chiral bisphosphine ligand successfully used
although considerable efforts have been made, for example, by steric
for the rhodium-catalyzed asymmetric 1,4-addition to electron-
tuning of aryl groups in arenesulfonamides of arylimines.9b,c
deficient olefins,14 was a poor ligand in terms of both enantiose-
It was found that the enantioselectivity as high as 98% was
lectivity and catalytic activity in the present arylation reaction15
readily attained by use of chiral diene ligand (R,R)-Ph-bod* for
(entry 4). Low efficiency was also observed with segphos16 and a
the reaction of 4-chlorobenzaldehyde N-tosylimine 3a with phen-
phosphoramidite17 as a ligand (entries 5 and 6). 13584 9 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
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 timeby 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 KyotoUniversity Alliance for Chemistry). N.T. thanks the Japan Societyfor the Promotion of Science for the award of a fellowship forgraduate students. Supporting Information Available: Experimental procedures and
spectroscopic and analytical data for the products (PDF). This materialis 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.
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(5) Some other methods for the optical resolution have been reported: (a)
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(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,
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86 (c 0.71, CHCl3). (R,R)-Ph-bod*: [R]20D
(9) Rhodium-catalyzed arylation. (a) With arylstannane: Hayashi, T.; Ishige-
The high enantioselectivity observed with chiral diene ligands,
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.
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ability brought about by two phenyl groups at the 2- and 5-positions
Tokunaga, N. Angew. Chem., Int. Ed. 2004, 43, in press.
of the diene is substantially high and is higher for the N-
(10) Addition of phenylzinc catalyzed by a chiral ketimine: Hermanns, N.;
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(11) For examples: (a) Bishop, M. J.; McNutt, R. W. Bioorg. Med. Chem.
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Cetirizine is currently prepared as a racemate via an alkylation of apiperazine derivative with a benzhydryl chloride derivative: Baltes, E.;
coordination of imine 3a to a rhodium with its si-face.19 The
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(16) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.; Sayo, N.; Miura, T.;
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The collection of proceedings of Institute Topical problems of NLS diagnostics (theoretical and clinical) CONTENTS: Computer non linear diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Clinic Tech Inc. — ascending into the XXI century . . . . . . . . . . . . . . . . . 7Method of computer non linear analysis and its role in diagnostics . . . . . 9Pote
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