Enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes catalyzed by chiral dirhodium(II) carboxylates: asymmetric synthesis of phenylglycine derivatives

Article abstract of DOI:10.1016/j.tetlet.2007.10.087

The first catalytic enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes is described. The reaction of silylketene acetals derived from methyl phenylacetates with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsN =

Full text of DOI:10.1016/j.tetlet.2007.10.087

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 8799–8802
Enantioselective amination of silylketene acetals with
(N-arylsulfonylimino)phenyliodinanes catalyzed by chiral
dirhodium(II) carboxylates: asymmetric synthesis of
phenylglycine derivatives
Masahiko Tanaka, Yasunobu Kurosaki, Takuya Washio, Masahiro Anada
and Shunichi Hashimoto*
Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Received 21 September 2007; revised 15 October 2007; accepted 18 October 2007
Available online 22 October 2007
Abstract—The first catalytic enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes is
described. The reaction of silylketene acetals derived from methyl phenylacetates with [N-(2-nitrophenylsulfonyl)imino]phenyliod-
inane (NsN = IPh) under the catalysis of dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh2(S-TCPTTL)4, pro-
ceeds in benzene at room temperature to give N-(2-nitrophenylsulfonyl)phenylglycine derivatives in high yields and with
enantioselectivities of up to 99% ee.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The catalytic asymmetric amination of silylketene ace-
tals is one of the most straightforward methods for the
preparation of enantioenriched a-amino esters.1,2 In
1999, Evans and Johnson described the first catalytic
enantioselective amination of thioester silylketene
acetals or silylketene aminals of acylpyrroles with azodi-
carboxylate derivatives, in which enantioselectivities up
to 99% ee were achieved with the use of a Cu(OTf)2-
bis(oxazoline) catalyst.3,4 Kobayashi and co-workers
later reported that a AgClO4-BINAP system catalyzed
the amination of the silylketene acetal of phenyl propio-
nate with dibenzyl azodicarboxylate with good enanti-
oselectivity (51% ee).5 In this context, the aziridination
of silylketene acetals followed by ring opening of the azi-
ridine intermediate provides a prototypical approach to
catalytic asymmetric synthesis of a-amino esters.6 In
1994, Evans et al. reported the first copper(I)-catalyzed
amination of silylketene acetals using [(p-tolylsulfo-
nyl)imino]phenyliodinane (TsN = IPh, 2a) as a nitrene
precursor,7 and they concluded that this protocol does
not represent a practical approach to the synthesis of
a-amino esters. While high levels of enantiocontrol in
aziridinations of alkenes have already been achieved
using a variety of different chiral transition metal-cata-
lysts,8 the catalytic enantioselective amination reaction
of silylketene acetals with (N-arylsulfonylimino)phenyl-
iodinanes has, to the best of our knowledge, not been
reported.9
Very recently, we demonstrated that the enantioselective
amination of silyl enol ethers derived from acyclic
ketones or enones with [(2-nitrophenylsulfonyl)-
imino]phenyliodinane (NsN = IPh, 2b) catalyzed by chi-
ral dirhodium(II) carboxylates provides N-(2-nitrophen-
ylsulfonyl)-a-amino ketones with enantioselectivities of
up to 95% ee.10 In this process, Rh2(S-TFPTTL)4
(1a),11 characterized by the substitution of fluorine
atoms for four hydrogen atoms on the phthalimido
group in the parent dirhodium(II) complex, Rh2 (S-
PTTL)4 (1c),12 proved to be the catalyst of choice in
terms of product yield and enantioselectivity as well as
catalytic activity. As a logical extension of our studies,
we herein report that dirhodium(II) tetrakis[N-tetra-
chlorophthaloyl-(S)-tert-leucinate], Rh2(S-TCPTTL)4
(1b),13 catalyzes enantioselective aminations of silylke-
tene acetals derived from methyl phenylacetates with
NsN = IPh (2b), to provide N-(2-nitrophenylsulfo-
nyl)phenylglycine derivatives in high yields and with
enantioselectivities of up to 99% ee.14 While a number
of methods,15 such as the asymmetric Strecker
*
Corresponding author. Tel.: +81 117 063 236; fax: +81 117 064
981; e-mail: hsmt@pharm.hokudai.ac.jp
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.087

8800
M. Tanaka et al. / Tetrahedron Letters 48 (2007) 8799–8802
reaction,16,17 the Sharpless asymmetric aminohydroxy-
lation,18 and the catalytic enantioselective hydrogena-
tion of a-aryl imino esters,19 have been used to
synthesize optically active arylglycine derivatives,20 this
protocol provides a new, catalytic asymmetric approach
to phenylglycine derivatives.
prepared from (R)-phenylglycine methyl ester hydro-
chloride (NsCl, pyridine, CH2Cl2, 0 ꢁC, 2 h, 87%).21,22
Gratifyingly, switching the catalyst from Rh2(S-
TFPTTL)4 to Rh2(S-TCPTTL)4 greatly improved the
enantioselectivity, providing 4b in 98% yield with 97%
ee (entry 2), although lowering the reaction temperature
to –20 ꢁC had little impact on the enantioselectivity
(98% ee, entry 3). In this system, Rh2(S-PTTL)4 was also
less effective in terms of both reactivity and enantioselec-
tivity (entry 4). Using Rh2(S-TCPTTL)4 as the catalyst,
we next evaluated the performance of other nitrene pre-
cursors, [(4-nitrophenylsulfonyl)imino]phenyliodinane
(pNsN = IPh, 2c), [(2,4-dinitrophenylsulfonyl)imino]-
phenyliodinane (DNsN = IPh, 2d), and TsN = IPh
(2a) (entries 5–7). This screening revealed that NsN =
IPh (2b) was the optimal nitrene precursor for this trans-
formation (entries 2 vs 5–7), although the reason for the
advantage of 2b is not clear at this time. A survey of sol-
vents revealed that the use of benzene was the superior
choice as nearly perfect enantiocontrol (99% ee) was
achieved at room temperature (entry 8). The product
4b was also formed with a similar high enantioselectivity
in toluene (99% ee, entry 9), but the reaction time (6 h)
was longer than that in benzene (3 h). It is noteworthy
that the amination of silylketene acetal 3a (Z:E =
23:77) with 2b using Rh2(S-TCPTTL)4 produced 4b with
the same sense of asymmetric induction as above in only
18% yield with 99% ee, along with 60% recovery of
methyl phenylacetate (entry 10). This observation indi-
cates that only the Z-isomer of 3a is responsible for
the formation of 4b in the present catalytic process.
Therefore, it is not necessary to use geometrically pure
X
X
X
O
N
X
H
O
O
O
Rh Rh
X = F: Rh2(S-TFPTTL)4 (1a)
X = Cl: Rh2(S-TCPTTL)4 (1b)
X = H: Rh2(S-PTTL)4 (1c)
On the basis of our previous work,10 we initially ex-
plored the amination of triethylsilylketene acetal 3a
(Z:E = 97:3, 1.2 equiv) derived from methyl phenylace-
tate with NsN = IPh (2b) using 1 mol % of Rh2(S-
TFPTTL)4. The reaction in CH2Cl2 proceeded smoothly
at 0 ꢁC to completion in less than 15 min, giving N-(2-
nitrophenylsulfonyl)phenylglycine methyl ester (4b)21
in 95% yield after column chromatography on silica
gel (Table 1, entry 1). The enantioselectivity of this reac-
tion was determined to be 88% ee by HPLC analysis
with a connected series of Daicel Chiralcel OD-H and
Chiralpak IB columns. The preferred absolute stereo-
24
chemistry of 4b ½½aꢀD ꢁ156.4 (c 1.36, CHCl3)] was
assigned as R by comparing the sign of optical rotation
24
with (R)-4b ½½aꢀD ꢁ172.3 (c 1.01, CHCl3)], which was
Table 1. Enantioselective amination of silylketene acetals with [N-(arylsufonyl)imino]phenyliodinanes catalyzed by chiral dirhodium(II)
carboxylatesa
OSiR3
OMe
Rh(II) catalyst (1 mol %)
Ph
CO2Me
+
ArSO2N=IPh
Ph
NHSO2Ar
3
2
4
Entry
Silylketene acetal
Iminoiodinane
Ar
Rh(II) catalyst
Solvent
Temperature Time
a-Amino ester
(ꢁC)
(h)
R3Si
Z:E
Yieldb eec
(%)
(%)
1
2
3
4
5
6
7
8
9
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3a Et3Si
3b Me3Si
97:3
97:3
97:3
97:3
97:3
97:3
97:3
97:3
97:3
2b 2-NO2C6H4
2b 2-NO2C6H4
2b 2-NO2C6H4
2b 2-NO2C6H4
2c 4-NO2C6H4
Rh2(S-TFPTTL)4 (1a) CH2Cl2
Rh2(S-TCPTTL)4 (1b) CH2Cl2
Rh2(S-TCPTTL)4 (1b) CH2Cl2
0
0
0.2
0.2
2.5
1
0.2
2
16
3
6
4b 95
88
97
98
64
59d
92d
88e
99
99
99
95
97
4b 98
4b 91
4b 80
4c 92
4d 70
4a 40
4b 98
4b 91
4b 18f
4b 88
4b 63
ꢁ20
Rh2(S-PTTL)4 (1c)
CH2Cl2
0
0
0
0
Rh2(S-TCPTTL)4 (1b) CH2Cl2
2d 2,4-(NO2)2C6H3 Rh2(S-TCPTTL)4 (1b) CH2Cl2
2a 4-MeC6H4
2b 2-NO2C6H4
2b 2-NO2C6H4
Rh2(S-TCPTTL)4 (1b) CH2Cl2
Rh2(S-TCPTTL)4 (1b) Benzene 23
Rh2(S-TCPTTL)4 (1b) Toluene 23
Rh2(S-TCPTTL)4 (1b) Benzene 23
Rh2(S-TCPTTL)4 (1b) Benzene 23
Rh2(S-TCPTTL)4 (1b) Benzene 23
10
11
12
23:77 2b 2-NO2C6H4
97:3
3
3
3
2b 2-NO2C6H4
2b 2-NO2C6H4
3c t-BuMe2Si 96:4
a The following is a representative procedure (entry 8): 2b (80.8 mg, 0.2 mmol) was added in one portion to a solution of 3a (63.4 mg, 0.24 mmol) and
Rh2(S-TCPTTL)4Æ2EtOAc (1b) (3.8 mg, 0.002 mmol, 1 mol %) in benzene (0.5 mL) at 23 ꢁC. The mixture was concentrated in vacuo and
chromatographed on silica gel to afford 4b (68.7 mg, 98%) as a white solid.
b Isolated yield.
c Determined by HPLC (column: Daicel Chiralcel OD-H followed by Daicel Chiralpak IB, eluent: 5:1 hexane/i-PrOH, flow rate: 1.0 mL/min) unless
otherwise stated.
d Determined by HPLC (column: Daicel Chiralpak AD-H, eluent: 1:1 hexane/i-PrOH, flow rate: 1.0 mL/min).
e Determined by HPLC (column: Daicel Chiralpak AD-H (2 · 250 mm), eluent: 9:1 hexane/i-PrOH, flow rate: 1.0 mL/min).
f 60% of methyl phenylacetate was recovered.

M. Tanaka et al. / Tetrahedron Letters 48 (2007) 8799–8802
8801
Table 2. Enantioselective amination of silylketene acetals with 2b
catalyzed by Rh2(S-TCPTTL)4 (1b)
Ph
R
b,c
Ph
CO2Me
O
N
NHNs
Rh2(S-TCPTTL)4 (1b)
O
4b
(1 mol %)
OSiEt3
OMe
3 (Z:E = >95:5)
7 (R = Ns)
R
CO2Me
NHNs
99% ee
d
+
NsN=IPh
a
R
8 (R = H)
CO2H
benzene, 23 °C
>99% ee
2b
Ph
e
4
Ns = 2-NO2C6H4SO2
NHNs
9
Entry Silylketene acetal Time (h) Phenylglycine derivatives
Scheme 1. Reagents and conditions: (a) recrystallization from EtOH,
91%; (b) DIBAL-H, CH2Cl2, ꢁ78–0 ꢁC, 0.5 h; (c) triphosgene, Et3N,
CH2Cl2, 0 ꢁC, 1 h, 88% (two steps); (d) PhSH, Cs2CO3, DMF, 23 ꢁC,
1 h, 81%; (e) LiI, EtOAc, reflux, 6 h, 94%.
R
Yielda (%) eeb (%)
1
2
3
4
5
6
7
8
9
3a C6H5
3d 4-CF3C6H4
3e 4-FC6H4
4-ClC6H4
3g 4-BrC6H4
3h 4-MeC6H4
4-MeOC6H4
3-CF3C6H4
3k 3-ClC6H4
3l 3-MeC6H4
3m 3-MeOC6H4
3
3
3
2
3
6
4
2
3
4
3
4b 98
4e 95
4f 95
4g 98
4h 93
95
98
4k 92
4l 94
99
97
97
96
96
98
80
97
98
98
98
3f
at the ortho position on the benzene ring gave aromatic
C–H amination products 5a and 5b as the sole products
instead of the expected phenylglycine derivatives (Eq.
1).23
4i
4j
3i
3j
The N-(2-nitrophenylsulfonyl)phenylglycine derivatives
are potentially useful for the synthesis of chiral ligands24
and novel oligopeptide structures21,25 (Scheme 1). A sin-
10
11
4m 93
4n 48
a Isolated yield.
b Determined by HPLC. See the Supplementary data for details.
gle recrystallization of 4b (99% ee) from EtOH produced
23
an optically pure sample [mp = 103.0–104.0 ꢁC, ½aꢀD
ꢁ172.0 (c 1.00, CHCl3)] in 91% yield. Reduction of 4b
silylketene acetals to achieve high enantioselection. We
also examined the effect of the silicon substituents of
silylketene acetals 3a–c on enantioselectivity. Variation
in the silyl group revealed that the triethylsilyl function-
ality was optimal for this process (entries 8 vs 11 and
12).
with DIBAL-H followed by treatment with triphosgene
23
produced the N-Ns-protected oxazolidinone 7 ½½aꢀD
ꢁ629.7 (c 0.50, CHCl3)] in 93% yield with no racemiza-
tion.22 Removal of the Ns-group under standard Fuku-
yama conditions26 furnished an Evans auxiliary (R)-8,24
23
22
½½aꢀD ꢁ57.0 (c 1.01, CHCl3), lit.,24b ½aꢀD +58.0 (c 1.00,
CHCl3) for (S)-8] in 81% yield. In addition, methyl ester
cleavage of 4b with LiI in refluxing ethyl acetate25 pro-
Having demonstrated the effectiveness of the combina-
tion of Rh2(S-TCPTTL)4 as the catalyst, NsN = IPh
as the nitrene precursor, and benzene as the solvent,
the applicability of this catalytic system to a range of
silylketene acetals was then investigated (Table 2). High
yields and excellent enantioselectivities were consistently
observed at room temperature with electron-withdraw-
ing substituents such as trifluoromethyl and chlorine at
the para or meta position on the benzene ring (96–98%
ee, entries 2–5, 8, and 9). The reaction with silylketene
acetals 3h and 3l bearing a methyl group at the para
or meta position also afforded the corresponding phen-
ylglycine derivatives 4i and 4m in high yields and very
high enantioselectivities (98% ee, entries 6 and 10). How-
ever, the introduction of a methoxy group at the para or
meta position had a detrimental effect on product yield
or enantioselectivity. The use of para-methoxy-substi-
tuted silylketene acetal 3i resulted in only modest enan-
tioselection (80% ee, entry 7). Although a high
enantioselectivity (98% ee) was achieved with meta-
methoxy-substituted silylketene acetal 3m, a marked de-
crease in product yield was observed (entry 11). Surpris-
ingly, the introduction of methyl or chlorine substituents
vided (R)-N-(2-nitrophenylsulfonyl)phenylglycine (9)
25
½½aꢀD ꢁ181.5 (c 1.01, CHCl3)] in 94% yield and in enan-
tiomerically pure form.22
In summary, we have developed the first catalytic enan-
tioselective amination of silylketene acetals with (N-
arylsulfonylimino)phenyliodinanes. The reaction of
Z -triethylsilylketene acetals derived from methyl pheny-
lacetates with NsN = IPh is catalyzed by Rh2(S-
TCPTTL)4 at room temperature to give N-Ns-protected
phenylglycine derivatives in high yields and with excel-
lent enantioselectivities, although the efficiency of the
present protocol depends on the substitution pattern
on the benzene ring. Further studies to expand the range
of substrates as well as mechanistic and stereochemical
studies are currently underway.
Acknowledgements
This research was supported in part by a Grant-in-Aid
for Scientific Research on Priority Areas ‘Advanced
Molecular Transformations of Carbon Resources’ from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan and by Grant-in-Aid from Innova-
tion Plaza Hokkaido in Japan Science and Technology
Agency. We thank S. Oka, M. Kiuchi, and T. Hirose
of the Center for Instrumental Analysis at Hokkaido
University, for technical assistance in MS and elemental
analysis.
NsNH
OSiEt3
OMe
1b (1 mol %)
+
NsN=IPh
2b
CO2Me
benzene, 23 °C
16-18 h
R
R
3n, R = Cl
Ns = 2-NO2C6H4SO2
5a (R = Cl, 53%)
5b (R = Me, 62%)
3o, R = Me
ð1Þ

8802
M. Tanaka et al. / Tetrahedron Letters 48 (2007) 8799–8802
15. For a review on asymmetric synthesis of arylglycines, see:
Supplementary data
Williams, R. M.; Hendric, J. A. Chem. Rev. 1992, 92, 889–
917.
16. For recent reviews on asymmetric Strecker reactions, see:
(a) Yet, L. Angew. Chem., Int. Ed. 2001, 40, 875–877; (b)
Gro¨ger, H. Chem. Rev. 2003, 103, 2795–2827; (c) Spino, C.
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Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.10.087.
17. For selected examples of catalytic enantioselective synthe-
sis of arylglycine derivatives via the Strecker reaction, see:
(a) Iyer, M. S.; Gigstad, K. M.; Namdev, N. D.; Lipton,
M. J. Am. Chem. Soc. 1996, 118, 4910–4911; (b) Sigman,
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21. Albanese, D.; Landini, D.; Lupi, V.; Penso, M. Eur. J.
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22. See the Supplementary data for experimental details.
23. It was found that the presence of an aryl group in
silylketene acetals is responsible for the high enantioselec-
tivity. For example, the amination of silylketene acetal
derived from methyl cyclohexylacetate (Z:E = 64:36,
2 equiv) with NsN = IPh (2b) in the presence of Rh2(S-
TCPTTL)4 afforded the corresponding a-amino ester6 in
92% yield with 48% ee.
24. (a) Evans, D. A.; Sjogren, E. B. Tetrahedron Lett. 1985,
´
26, 3783–3786; (b) Nicolas, E.; Russell, K. C.; Hruby, V. J.
J. Org. Chem. 1993, 58, 766–770.
25. Biron, E.; Kessler, H. J. Org. Chem. 2005, 70, 5183–5189.
26. (a) Fukuyama, T.; Jow, C.-K.; Cheung, M. Tetrahedron
Lett. 1995, 36, 6373–6374; For a review on the nitrophe-
nylsulfonamide chemistry, see: (b) Kan, T.; Fukuyama, T.
Chem. Commun. 2004, 353–359.
14. Recently, Davies and Reddy reported highly enantio-
selective benzylic C–H aminations using dirhodium(II) tet-
rakis[N-tetrachlorophthaloyl-(S)-(1-adamantyl)glycinate],
[Rh2(S-TCPTAD)4], as a catalyst, see: Reddy, R. P.;
Davies, H. M. L. Org. Lett. 2006, 8, 5013–5016.