Rhodium/chiral diene-catalyzed asymmetric methylation of N-sulfonylarylimines with trimethylboroxine

Article abstract of DOI:10.1016/j.tetasy.2012.04.021

A hydroxorhodium complex coordinated with a chiral diene ligand catalyzed the asymmetric addition of trimethylboroxine to N-sulfonylarylimines to give high yields of chiral 1-aryl-1-ethylamines with high enantioselectivity.

Full text of DOI:10.1016/j.tetasy.2012.04.021

Tetrahedron: Asymmetry 23 (2012) 655–658
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Rhodium/chiral diene-catalyzed asymmetric methylation
of N-sulfonylarylimines with trimethylboroxine
⇑
⇑
Takahiro Nishimura , Akram Ashouri, Yusuke Ebe, Yuko Maeda, Tamio Hayashi
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A hydroxorhodium complex coordinated with a chiral diene ligand catalyzed the asymmetric addition of
trimethylboroxine to N-sulfonylarylimines to give high yields of chiral 1-aryl-1-ethylamines with high
enantioselectivity.
Received 21 March 2012
Accepted 27 April 2012
Available online 12 June 2012
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
a
-Chiral amines are important structural motifs in several chiral
Ts
Ts
H
[RhCl((R,R)-Ph-bod*)]2
HN
N
H2O
drugs and natural products, and their enantioselective synthesis
(3 mol % Rh)
+
Me2Zn
has attracted much attention from organic chemists.1 Of the many
dioxane
50 °C, 3 h
Ar
Me
Ar
(1.5 equiv)
approaches for the synthesis of a-chiral amines, the catalytic enan-
94–98% ee
tioselective addition of dialkylzinc reagents to imines has been
developed by using chiral copper catalysts;1c most of the studies
reported so far have focused on the catalytic ethylation of imines
because of its high reactivity and selectivity. Conversely, efficient
methods for the asymmetric methylation of imines have been
limited due to the lower reactivity of dimethylzinc, and the large
amount of dimethylzinc often required for high yielding methyla-
tions.2–5 Leading studies of the asymmetric methylation of imines
with dimethylzinc have been developed in copper-catalyzed asym-
metric methylations of N-tosylimines with an amidophosphine
ligand (Tomioka)2b and of N-phosphinoylimines with a chiral
bis(phosphine) monoxide ligand (Charette),3a,b,f and in methyla-
tions catalyzed by a Lewis acid complexed with chiral amino
acid–based ligands (Hoveyda/Snapper).4 We have also reported
the rhodium-catalyzed asymmetric methylation of N-sulfonylaryli-
mines with dimethylzinc, which was realized by the use of a
rhodium/chiral diene catalyst (Scheme 1).6,7
Organoboronic acids and their derivatives are attractive carbon
nucleophiles because of their wide availability and stability,8 while
aryl- and alkenylboron compounds have frequently been used in
the rhodium-catalyzed asymmetric addition to electron-deficient
alkenes and imines.9 On the other hand, although methylboronic
acid and its derivatives are stable in air and moisture and are easy
to handle, there have been only a few examples of their use as
methylating reagents in rhodium catalysis.10 Herein we report the
(R,R)-Ph-bod*
Scheme 1. Asymmetric addition of dimethylzinc to N-tosylarylimines catalyzed by
Rh/(R,R)-Ph-bod*.
first example of a rhodium-catalyzed asymmetric methylation of
N-sulfonylarylimines by using trimethylboroxine.
2. Results and discussion
We found that a hydroxorhodium complex coordinated with a
diene ligand has high catalytic activity in the addition of trim-
ethylboroxine to N-tosylarylimines (Table 1). Thus, treatment of
N-tosylimine 1a with trimethylboroxine (3 equiv B) in the pres-
ence of [Rh(OH)(cod)]2 (5 mol % of Rh) and tert-amyl alcohol
(3 equiv) in 1,4-dioxane at 80 °C for 15 h gave the methylation
product 2a in 74% yield (entry 1). The reaction of 1a with meth-
ylboronic acid instead of trimethylboroxine gave a lower yield
(22%) of 2a (entry 2). When carrying out the reaction by using a
chlororhodium complex [RhCl(cod)]2 combined with aqueous
KOH, which are typical reaction conditions for the addition of
arylboronic acids,11 it did not give 2a at all, either in the addition
of trimethylboroxine (entry 3) or methylboronic acid (entry 4).
The hydroxorhodium/bisphosphine complex [Rh(OH)((R)-bin-
ap)]2,12 which is one of the most efficient catalysts in the addition
⇑
Corresponding authors.
E-mail addresses:
tnishi@kuchem.kyoto-u.ac.jp (T. Nishimura),
thayashi@
of arylboronic acids to
a,b-unsaturated ketones, had no catalytic
kuchem.kyoto-u.ac.jp (T. Hayashi).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.04.021

656
T. Nishimura et al. / Tetrahedron: Asymmetry 23 (2012) 655–658
Table 1
N-nosylamides 2j and 2k in 92% and 89% yields, respectively (98%
ee, entries 10 and 11).
Rhodium-catalyzed asymmetric methylation of 1aa
Ts
N
Ts
HN
Rh catalyst
(5 mol %Rh)
Table 2
H
Me
Rhodium-catalyzed asymmetric methylation of N-sulfonylarylimines 1a
(MeBO)3
t-amyl alcohol
(3 equiv)
1,4-dioxane
80 °C, 15 h
Cl
Cl
SO2Ar'
SO2Ar'
Me
[RhCl((S,S)-Fc-tfb*)]2
(3 equiv B)
N
HN
(5 mol% Rh)
1a
2a
(MeBO)3
t-amyl alcohol
(3 equiv)
Ar
H
Ar
(3 equiv B)
F
F
1,4-dioxane
80 °C, 15 h
1
2
F
PPh2
PPh2
R = Bn (benzyl)
Entry
Ar
SO2Ar0
Isolated yield (%)
eeb (%)
F
Ph
1
2
3
4
5
6
7
8
4-ClC6H4
3-ClC6H4
2-ClC6H4
4-BrC6H4
4-CF3C6H4
4-MeOC6H4
2-MeOC6H4
1-Naphthyl
2-Naphthyl
4-ClC6H4
Ts, 1a
Ts, 1b
Ts, 1c
Ts, 1d
Ts, 1e
Ts, 1f
Ts, 1g
Ts, 1h
Ts, 1i
Ns, 1j
Ns, 1k
96, 2a
95, 2b
96, 2c
91, 2d
96, 2e
91, 2f
97, 2g
91, 2h
95, 2i
92, 2j
89, 2k
99
98
99
98
99
99
98
97
98
98
98
R
Fc (ferrocenyl)
R
(R)-binap
Catalyst
R-tfb*
Entry
Isolated yield (%)
eeb (%)
1
[Rh(OH)(cod)]2
[Rh(OH)(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[Rh(OH)((R)-binap)]2
[Rh(OH)((R,R)-Bn-tfb*)]2
[Rh(OH)((R,R)-Ph-tfb*)]2
[Rh(OH)((S,S)-Fc-tfb*)]2
74
22d
0
0
0
79
90
96
—
—
—
—
2c
3e
4c,e
5
9
10
11
4-MeOC6H4
—
a
6
7
8
65
96
98
Reaction conditions:
(MeBO)3 (0.20 mmol), t-amyl alcohol (0.60 mmol) in 1,4-dioxane (0.8 mL) at 80 °C
for 15 h.
1 (0.20 mmol), [RhCl((S,S)-Fc-tfb*)]2 (5 mol % of Rh),
b
Determined by chiral HPLC analysis.
a
Reaction conditions: 1a (0.10 mmol), [RhClL*]2 (5 mol % of Rh), (MeBO)3
(0.10 mmol), t-amyl alcohol (0.30 mmol) in 1,4-dioxane (0.4 mL) at 80 °C for 15 h.
b
Determined by HPLC analysis with a chiral stationary phase column: Chiralcel
OD-H.
3. Conclusion
c
Performed with MeB(OH)2 (0.30 mmol) instead of (MeBO)3 in the absence of t-
amyl alcohol.
d
Determined by 1H NMR.
In conclusion, we have developed a rhodium-catalyzed asym-
metric methylation of N-sulfonylarylimines by using trimethylbo-
roxine as a methylating reagent to give chiral 1-aryl-1-ethylamines
in high yields and with high enantioselectivity; this was achieved
by using hydroxorhodium/chiral diene catalyst.
e
Performed in the presence of 1.5 M KOH (aq) (20 mol %) in the absence of t-
amyl alcohol.
activity (entry 5). These results prompted us to use a hydroxorho-
dium complex coordinated with a chiral diene ligand13 for the
development of the asymmetric reaction. We have recently devel-
oped stable hydroxorhodium complexes coordinated with a chiral
diene based on a tetrafluorobenzobarrelene (tfb*) skeleton;14 in
the present reaction we found them to display high catalytic activ-
ity and enantioselectivity. Thus, the reactions using hydroxorhodi-
um complexes coordinated with (R,R)-Bn-tfb* (Bn = benzyl),14b
(R,R)-Ph-tfb*,14c and (S,S)-Fc-tfb*14e (Fc = ferrocenyl) gave 2a in
79%, 90%, and 96% yield, respectively (entries 6–8), where (S,S)-
Fc-tfb* displayed the highest enantioselectivity (98% ee, entry 8).
The absolute configuration of 2a obtained when using (S,S)-Fc-
tfb* was determined to be (S) by comparison of its specific rotation
4. Experimental
4.1. General and materials
All anaerobic and moisture-sensitive manipulations were carried
out with standard Schlenk techniques under predried nitrogen.
NMR spectra were recorded on a JEOL JNM ECA-600 spectrometer
(600 MHz for 1H, 150 MHz for 13C). Chemical shifts are reported in
d (ppm) referenced to the residual peaks of CDCl3 (d 7.26) for 1H
NMR and CDCl3 (d 77.00) for 13C NMR. Optical rotations were mea-
sured on a JASCO P-2200 polarimeter. Flash column chromatogra-
phy was performed with Silica Gel 60 N (spherical, neutral)
(Cica-Reagent). 1,4-Dioxane was purified by passing through a
neutral alumina column under N2. Rhodium complexes,
[RhCl(cod)]2,15 [Rh(OH)(cod)]2,16 [Rh(OH)((R)-binap)]2,12 [Rh(OH)
{½a 2D0
ꢀ
¼ ꢁ79 (c 0.55, CHCl3) with 99% ee; lit. 6 (½a D20
¼ ꢁ70:7 (c 1.02,
ꢀ
CHCl3) with 96% ee (S)}.
Table 2 summarizes the results obtained for the methylation of
several N-sulfonylarylimines 1, where the yields and ee of 2 were
higher than those observed in our previous studies on the addition
of dimethylzinc.6 The methylation of arylimines substituted with
not only para-1a but also meta-1b and ortho-chloro 1c proceeded
to give the corresponding addition products 2a–2c in high yields
with high enantioselectivity (entries 1–3). The arylimines having
bromo 1d, trifluoromethyl 1e, and methoxy at para 1f and ortho
1g gave the corresponding addition products 2d–2g in high yields
(91–97%) with over 98% ee (entries 4–7). Naphthylimines 1h
and 1i were also good substrates and gave 2h and 2i in high yields
and with 97% and 98% ee, respectively (entries 8 and 9). The
present catalytic system can also be applied to the methylation of
N-nosylarylimines (Ns = p-nitrobenzenesulfonyl) with high enanti-
oselectivity. Thus, the reaction of N-nosylimines 1j and 1k gave
((R,R)-Bn-tfb*)]2,14b [Rh(OH)((R,R)-Ph-tfb*)]2,14c and [Rh(OH)((S,S)-
14e
Fc-tfb*)]2
were prepared according to reported procedures.
Trimethylboroxine and methylboronic acids (Aldrich) were pur-
chased and used as received. N-Sulfonylarylimines were prepared
from aldehydes and arenesulfonamides according to reported
procedures.17
4.2. A general procedure for the asymmetric addition of
trimethylboroxine to imines 1 (Table 2)
To a mixture of [Rh(OH)((S,S)-Fc-tfb*)]2 (0.0050 mmol, 7.2 mg,
5 mol % of Rh), imine 1 (0.20 mmol), and t-amyl alcohol (66
0.60 mmol) in 1,4-dioxane (0.8 mL) was added trimethylboroxine
(28 L, 0.20 mmol) under N2, and the mixture was stirred at
lL,
l

T. Nishimura et al. / Tetrahedron: Asymmetry 23 (2012) 655–658
657
80 °C for 15 h. The mixture was passed through a short column of
silica gel with ethyl acetate and concentrated on a rotary evapora-
tor. The residue was subjected to flash column chromatography on
silica gel with hexane/ethyl acetate to give 2.
4.3.10. (S)-N-(1-(4-Chlorophenyl)ethyl)-4-nitrobenzenesulfonamide
2j 20
The ee was measured by HPLC (Chiralcel OJ-H column, hexane/
2-propanol = 1/1, flow 0.3 mL/min, 230 nm, t1 = 36.9 min (S),
t2 = 42.0 min (R); ½a D20
¼ ꢁ37 (c 0.56, CHCl3) with 98% ee (S).
ꢀ
4.3. Characterization of the products
4.3.11. (S)-N-(1-(4-Methoxyphenyl)ethyl)-4-nitrobenzenesulfonamide
2k 20
All products are known compounds and were characterized by
comparison of the spectroscopic data with those reported previ-
ously. Conditions for HPLC analysis and specific rotation values
are shown below.
The ee was measured by HPLC (Chiralcel OJ-H column, hexane/
2-propanol = 1/1, flow 0.3 mL/min, 230 nm, t1 = 46.5 min (S),
t2 = 60.6 min (R); ½a D20
¼ ꢁ32 (c 0.47, CHCl3) with 98% ee (S).
ꢀ
Acknowledgement
4.3.1. (S)-N-(1-(4-Chlorophenyl)ethyl)-4-methylbenzenesulfonamide
2a 6
This work was supported by a Grant-in-Aid for Scientific
Research from the MEXT, Japan. AA thanks the Iran Ministry of
Science Research and Technology for an academic scholarship.
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 25.7 min (R),
t2 = 28.6 min (S); ½a D20
¼ ꢁ79 (c 0.55, CHCl3) with 99% ee (S).
ꢀ
References
4.3.2. (S)-N-(1-(3-Chlorophenyl)ethyl)-4-methylbenzenesulfonamide
2b 6
1. For reviews, see: (a) Kobayashi, S.; Mori, Y.; Fossey, J. S.; Salter, M. M. Chem. Rev.
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2001, 123, 984; (b) Porter, J. R.; Traverse, J. F.; Hoveyda, A. H.; Snapper, M. L. J.
Am. Chem. Soc. 2001, 123, 10409; (c) Akullian, L. C.; Snapper, M. L.; Hoveyda, A.
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J. F.; Snapper, M. L.; Hoveyda, A. H. Adv. Synth. Catal. 2005, 347, 417; (e) Fu, P.;
Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2008, 130, 5530.
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 23.1 min (R),
t2 = 26.4 min (S); ½a D20
¼ ꢁ73 (c 0.50, CHCl3) with 98% ee (S).
ꢀ
4.3.3. (S)-N-(1-(2-Chlorophenyl)ethyl)-4-methylbenzenesulfonamide
2c 18
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.5 mL/min, 254 nm, t1 = 14.5 min (R),
t2 = 16.6 min (S); ½a D20
¼ ꢁ53 (c 0.40, CHCl3) with 99% ee (S).
ꢀ
4.3.4. (S)-N-(1-(4-Bromophenyl)ethyl)-4-methylbenzenesulfonamide
2d 18
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 25.8 min (R),
t2 = 29.8 min (S); ½a D20
¼ ꢁ62 (c 0.33, CHCl3) with 98% ee (S).
ꢀ
5. For selected examples of catalytic enantioselective methylations of imines with
dimethylzinc or trimethylaluminium, see: (a) Soai, K.; Hatanaka, T.; Miyazawa,
T. J. Chem. Soc., Chem. Commun. 1992, 1097; (b) Dahmen, S.; Bräse, S. J. Am.
Chem. Soc. 2002, 124, 5940; (c) Almansa, R.; Guijarro, D.; Yus, M. Tetrahedron:
Asymmetry 2007, 18, 2828; (d) Pizzuti, M. G.; Minnaard, A. J.; Feringa, B. L. J. Org.
Chem. 2008, 73, 940.
4.3.5. (S)-4-Methyl-N-(1-(4-(trifluoromethyl)phenyl)ethyl)benzene-
sulfonamide 2e 6
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 22.4 min (R),
t2 = 24.9 min (S); ½a D20
¼ ꢁ49 (c 0.51, CHCl3) with 99% ee (S).
ꢀ
6. Nishimura, T.; Yasuhara, Y.; Hayashi, T. Org. Lett. 2006, 8, 979.
7. For examples of rhodium-catalyzed asymmetric 1,2-additions of dimethylzinc
or trimethylaluminium, see: (a) Crampton, R. H.; Hajjaji, S. E.; Fox, M. E.;
Woodward, S. Tetrahedron: Asymmetry 2009, 20, 2497; (b) Yoshida, K.; Akashi,
N.; Yanagisawa, A. Tetrahedron: Asymmetry 2011, 22, 1225; (c) Siewert, J.;
Sandmann, R.; von Zezschwitz, P. Angew. Chem., Int. Ed. 2007, 46, 7122.
8. Boronic Acids; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2005.
4.3.6. (S)-N-(1-(4-Methoxyphenyl)ethyl)-4-methylbenzenesulfona-
19
mide 2f
The ee was measured by HPLC (Chiralcel OD-H column ꢂ 2,
hexane/2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 64.5 min
9. For reviews, see: (a) Sibi, M. P. Ed.; Manyem, S. Tetrahedron 2000, 56, 8033; (b)
Krause, N.; Hoffmann-Röder, A. Synthesis 2001, 171; (c) Fagnou, K.; Lautens, M.
Chem. Rev. 2003, 103, 169; (d) Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103,
2829; (e) Darses, S.; Genet, J.-P. Eur. J. Org. Chem. 2003, 4313; (f) Christoffers, J.;
Koripelly, G.; Rosiak, A.; Rössle, M. Synthesis 2007, 1279; (g) Miyaura, N. Bull.
Chem. Soc. Jpn. 2008, 81, 1535; (h) Berthon, G.; Hayashi, T. In Catalytic
Asymmetric Conjugate Reactions; Córdova, A., Ed.; Wiley-VCH: Weinheim, 2010;
p 1; (i) Hargrave, J. D.; Allen, J. C.; Frost, C. G. Chem. Asian J. 2010, 5, 386; (j)
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39, 2093; (k) Tian, P.; Dong, H.-Q.; Lin, G.-Q. ACS Catal. 2012, 2, 95.
10. For an example of rhodium-catalyzed additions of methylboronic acid to
alkynoates, see: (a) Yasuhara, Y.; Nishimura, T.; Hayashi, T. Chem. Commun.
2010, 46, 2130; For an example of rhodium-catalyzed 1,2-additions of chiral
secondary and tertiary benzylic potassium trifluoroborate salts to aldehydes,
see: (b) Ros, A.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2009, 48, 6289.
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13. For reviews, see: (a) Defieber, C.; Grützmacher, H.; Carreira, E. M. Angew. Chem.,
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E. M. J. Am. Chem. Soc. 2003, 126, 1628; (h) Paquin, J.-F.; Defieber, C.;
(R), t2 = 68.7 min (S); ½a D20
¼ ꢁ82 (c 0.51, CHCl3) with 99% ee (S).
ꢀ
4.3.7. (S)-N-(1-(2-Methoxyphenyl)ethyl)-4-methylbenzenesulfonamide
2g 19
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 25.4 min (R),
t2 = 28.4 min (S); ½a D20
¼ ꢁ66 (c 0.42, CHCl3) with 98% ee (S).
ꢀ
4.3.8. (S)-4-Methyl-N-(1-(naphthalen-1-yl)ethyl)benzenesulfonamide
2h 18
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.3 mL/min, 254 nm, t1 = 29.8 min (R),
t2 = 39.3 min (S); ½a D20
¼ þ14 (c 0.52, CHCl3) with 97% ee (S).
ꢀ
4.3.9. (S)-4-Methyl-N-(1-(naphthalen-2-yl)ethyl)benzenesulfonamide
2i 6
The ee was measured by HPLC (Chiralcel OD-H column, hexane/
2-propanol = 4/1, flow 0.5 mL/min, 254 nm, t1 = 19.0 min (R),
t2 = 23.1 min (S); ½a D20
¼ ꢁ79 (c 0.46, CHCl3) with 98% ee (S).
ꢀ

658
T. Nishimura et al. / Tetrahedron: Asymmetry 23 (2012) 655–658
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