Enantioselective intramolecular amidation of sulfamate esters catalyzed by chiral manganese(III) Schiff-base complexes

Article abstract of DOI:10.1016/j.tetlet.2005.05.146

Enantioselective intramolecular amidation of sulfamate esters catalyzed by chiral manganese(III) Schiff-base complexes under mild conditions (PhI(OAc) ₂, Al₂O₃, C₆H₆, 5°C) was achieved in moderate to

Full text of DOI:10.1016/j.tetlet.2005.05.146

Tetrahedron
Letters
Tetrahedron Letters 46(2005) 5403–5408
Enantioselective intramolecular amidation of sulfamate
esters catalyzed by chiral manganese(III) Schiff-base complexes
Ji Zhang, Philip Wai Hong Chan and Chi-Ming Che*
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug
Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
Received 15 April 2005; revised 26May 2005; accepted 30 May 2005
Abstract—Enantioselective intramolecular amidation of sulfamate esters catalyzed by chiral manganese(III) Schiff-base complexes
under mild conditions (PhI(OAc)2, Al2O3, C6H6, 5 °C) was achieved in moderate to good yields (up to 92%), substrate conversions
(up to 99%), with virtually complete cis-selectivity and with ee values up to 79% ee.
Ó 2005 Elsevier Ltd. All rights reserved.
Nitrogen atom insertion to saturated C–H bonds cata-
lyzed by transition metal complexes provide a conve-
nient synthetic route to amines and amine derivatives.1
Seminal works by Jacobsen2 and Katsuki3 showed that
chiral Schiff-base complexes of Cu(II), Ru(II), and
Mn(III) are effective catalysts for enantioselective inter-
molecular amidation of saturated C–H bonds and azir-
idination of alkenes with PhI@NTs as nitrogen source.
The asymmetric amidation and aziridination of alkenes
by nitridomanganese(V) Schiff-base complexes were first
reported by Carreira4 and subsequently by Komatsu,5
and Jørgensen.6 Studies in our laboratory demonstrated
that highly enantioselective intermolecular amidation of
silyl enol ethers and intramolecular aziridination of
C@C bonds can be accomplished by chiral ruthe-
nium(II) Schiff-base and dirhodium(II,II) catalysts,
respectively.7 Despite these advances, examples on
asymmetric intramolecular amidation of saturated C–
H bonds using chiral metal catalysts remain sparse. A
recent notable achievement by Du Bois and co-workers
showed that dirhodium(II,II) catalysts can effect stereo-
specific intramolecular amidation of sulfamate esters
and carbamates, allowing the synthesis of enantiomeri-
cally pure synthetically useful amidation products from
enantiomerically pure sulfamate esters or carbamates.8
We and others subsequently demonstrated that intramo-
lecular amidation of sulfamate esters and carbamates
can be achieved in the presence of chiral Ru(II),9
Rh2(II,II)10 or achiral Ag(I)11 catalyst. Using the chiral
ruthenium(II) D4-symmetric porphyrin catalyst, high
enantioselectivity with ee value up to 88% ee was
found.9 However, to further improve ee values through
structural modification of the metalloporphyrin catalyst,
a drawback to the protocol remains the need to prepare
the expensive chiral porphyrin ligands, which can be
arduous and time consuming. The chiral Rh2(II,II)-cat-
alyzed intramolecular amidation of sulfamate esters
were shown by Muller and Fruit to give low to moderate
¨
product yields and ee values.10 In this context and con-
sidering the lower cost of Mn versus Ru or Rh, we envi-
sioned that Mn(III) catalysts containing inexpensive
chiral Schiff-base ligands could hold promising pros-
pects for asymmetric intramolecular C–N bond forma-
tion reactions. Herein, we describe enantioselective
amidation of sulfamate esters using chiral manga-
nese(III) Schiff-base complexes as catalysts and with
PhI(OAc)2 as oxidant.
At the outset, we examined the effect of several chiral
manganese(III) Schiff-base catalysts on the intramole-
cular amidation reactions studied in this work (Fig. 1).
Chiral manganese(III) catalysts 1–7 and 9–12 were
prepared following literature methods;12 treatment of
[Mn(OAc)2]Æ4H2O (0.5 mmol) with the chiral H2-
(Schiff-base) ligand (1 mmol) in refluxing EtOH
(10 mL) gave Mn(III) complexes 1–7 and 9–12 in 70–
85% yield. The chiral Mn(III) salt 8 used in this work
was prepared from reaction of 3 with AgOTf in 95%
yield and structurally characterized by X-ray crystal
analysis.13 With 13a as probe substrate, a survey of
*
Corresponding author. Tel.: +852 2859 2154; fax: +852 2857 1586;
e-mail: cmche@hku.hk
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.146

5404
J. Zhang et al. / Tetrahedron Letters 46 (2005) 5403–5408
Ph
1R
Ph
2R
1R
2R
N
N
N
O
N
O
Mn
L
Mn
Cl
R1
O
O
R1
R1
R1
R2
R2
R2
R2
1: R1 = R2 = Br, L = Cl;
2: R1 = R2 = H, L = Cl;
3: R1 = R2 = L = Cl;
9: R1 = R2 = Br;
10: R1 = R2 = I;
11: R1 = R2 = tBu;
12: R1 = Br, R2 = H.
4: R1 = R2 = I, L = Cl;
5: R1 = R2 =tBu, L = Cl;
6: R1 = H, R2 = Br, L = Cl;
7: R1 = Br, R2 = H, L = Cl;
8: R1 = R2 = Br, L = OTf.
Figure 1. Chiral manganese(III) Schiff-base catalysts used in this work.
different reaction conditions revealed amidation of 13a
would be best performed in the presence of 1.5 equiv
of PhI(OAc)2 and Al2O3 (2.5 equiv) in C6H6 at 5 °C
for 10 h with 1 (10 mol %) as catalyst (Table 1, entry
1). Under these conditions, cyclic sulfamidate 14a was
furnished with an ee value of 55% in 68% yield based
on 95% substrate conversion. Examination of other
manganese(III) Schiff-base catalysts revealed the perfor-
mances of 3, 4, 6, and 8 similar to that of 1 in terms of
product yields and substrate conversions but gave lower
ee values of 45–53% (entries 3–5, 7, and 9). In addition,
reaction with (1S,2S)-3 as catalyst gave the epimer of
14a in a comparable product yield and ee value to that
obtained for the same reaction catalyzed by 3 (in
(1R,2R) form) (cf. entries 3 and 4). In contrast, the ana-
logous reactions catalyzed by either 2 or 7 were found to
give markedly lower ee values (23–36% ee); the latter
catalyst was found to be less reactive and afforded 14a
in 49% yield with 79% substrate conversion (entries 2
and 8).
Inspection of entries 10–13 in Table 1 reveals intra-
molecular amidation of 13a with catalysts 9–12 contain-
ing a chiral 1,2-diphenylethylenediamine backbone
structure exhibited similar reactivities but are much less
enantioselective. Under the conditions: PhI(OAc)2
(1.5 equiv) and Al2O3 (2.5 equiv) in C6H6 at 5 °C for
10 h, intramolecular amidation of 13a using either 9,
10, or 12 as catalyst (10 mol %) gave 14a in 39–67% yield
with 82–96% substrate conversions and ee values of 6–
22% ee (entries 10–11, and 13). Reactions of 13a with
either 5 or 11 as catalyst are the only instances where
no product formation could be detected (entries 6and
12).
To explore the scope of these chiral manganese(III)
Schiff-base catalysts in intramolecular amidation reac-
tions, we examined the similar reactions of other sulfa-
mate esters 13b–p. The results are summarized in
Table 2.
Table 1. Enantioselective intramolecular amidation of 13a catalyzed
by manganese(III) Schiff-bases 1–12a
O
O
catalyst, PhI(OAc)2
S
HN
O
Ph
OSO2NH2
13a
Al2O3, C6H6, 5oC
Ph
It is evident that sulfamate esters 13b–n are good sub-
strates for 1-catalyzed intramolecular amidation, as the
corresponding cyclic sulfamidates 14b–n were obtained
in reasonable to good yields (48–92%) and with ee values
up to 79% ee (entries 1–18). The highest enantioselectiv-
ity of 79% ee attained in this work is for the intramole-
cular amidation of 13i, which, to our knowledge, is also
the highest enantiocontrol so far achieved for intra-
molecular amidation of saturated C–H bonds using a
nonporphyrin based metal catalyst (entry 13). More
notably, intramolecular amidation of 13b with 1
(10 mol %), PhI(OAc)2 (1.5 equiv) and Al2O3 (2.5 equiv)
in CH2Cl2 at 5 °C for 10 h, was found to proceed with
virtually complete cis-selectivity (entry 1). Likewise,
intramolecular amidation of 13c and 13d gave the cis-
diastereomers of 14c and 14d as the sole products based
on 1H and 13C NMR measurements, and chiral GC
analysis14 (entries 4 and 5). In this work, the analogous
reaction of 13c with [Rh2(CH3CO2)4] (5 mol %) as
catalyst gave 14c as a 1:1.9 cis:trans mixture of
1
14a
Entry
Catalyst
Conversion (%)
Yield (%)
eeb (%)
1
1
95
93
95
93
94
—
81
79
70
68
66
71
68
68
—
51
49
66
55
23
51
53c
45
—
52
36
49
2
2
3
3
ent-3
4
5
4
5
6
7
8
6
7
8
9
10
11
12
13
7 9
20966
10
11
12
95
—
82
63
—
39
22
—
6
a All reactions were performed for 10 h with catalyst–13a–PhI(OAc)2–
Al2O3 molar ratio = 0.1:1:1.5:2.5 in C6H6 at 5 °C.
b Determined by HPLC analysis (Chiralcel OD column,
hexane–iPrOH = 4:1).
c Gave epi-14a.

J. Zhang et al. / Tetrahedron Letters 46 (2005) 5403–5408
5405
Table 2. Enantioselective intramolecular amidation of sulfamate esters 13b–p catalyzed by manganese(III) Schiff-base catalyst 1a
Entry
Substrate
Product
% Yield (conversion)
% eeb
H
N
1
48 (82)
60 (94)
68 (98)
52
35
42
OSO2NH2
SO2
2c
3d
O
13b
14b
H
OSO2NH2
O
SO2
e
4
5
55 (92)
92 (99)
—
N
H
H
13c
14c
Me
Me
O
SO2
OSO2NH2
e
—
N
H
H
13d
14d
HN
SO2
O
OSO2NH2
Ph
13e
6
7
57 (84)
67 (85)
54
61
Ph
14e
HN
SO2
O
Br
OSO2NH2
Br
14f
13f
H
N
8
64 (91)
60 (89)
59 (87)
76(42)
71
72
45
9c
SO2
O
OSO2NH2
OSO2NH2
Cl
10d
11f
Cl
13g
14g
69
HN
SO2
F
12
13
14
15
16
17
58 (81)
63 (90)
62 (93)
64 (97)
68 (74)
69 (94)
52
79
O
F
13h
14h
HN
SO2
O
O2N
OSO2NH2
OSO2NH2
O2N
14i
13i
HN
SO2
O
Me
27
Me
13j
14j
HN
SO2
O
MeO
23g
47
OSO2NH2
OSO2NH2
MeO
13k
14k
HN SO2
F
O
F
13l
14l
HN SO2
O
OSO2NH2
MeO
36
MeO
13m
14m
O
S
Me
Me
Me
O
e
N
H
18
19
58 (87)
40 (72)
—
Me
O
OSO2NH2
13n
14n
14o
O
35g
SO2
NH
OSO2NH2
13o
(continued on next page)

5406
J. Zhang et al. / Tetrahedron Letters 46 (2005) 5403–5408
Table 2 (continued)
Entry
Substrate
Product
% Yield (conversion)
% eeb
45
O
SO2
NH
20
19 (83)
OSO2NH2
13p
14p
a All reactions were performed in C6H6 at 5 °C for 10 h with 1–substrate–PhI(OAc)2–Al2O3 molar ratio = 0.1:1.0:1.5:2.5.
b Determined by HPLC analysis using a Chiralcel OD column unless specially noted.
c Reaction conducted with 3 as catalyst.
d Reaction conducted with 4 as catalyst.
e Not determined.
f Reaction conducted in toluene at ꢀ20 °C.
g Determined by HPLC analysis using a Chiralcel OJ column.
diastereomers in 87% yield based on 99% conversion.
The 1-catalyzed intramolecular amidations of 13b and
13d also compare favorably to the same reactions of
13b and 13d using either ruthenium(II) porphyrin9 or
dirhodium(II,II) catalysts,8b which were reported to
proceed with either only cis-selectivity or 8:1 cis:trans
selectivity in the case of the Rh2(II,II)-catalyzed intra-
molecular cyclization of 13d.
of 13a–n under these conditions gave product yields of
48–92%.
We attempt to rationalize the virtually complete cis-
selectivity observed for the intramolecular amidation
of 13b–d with a putative manganese(V)-imido species
having a conformation depicted in Figure 2. This shows
the cis-hydrogen atom of 13b is able to interact with the
imido-nitrogen atom to form a three-atom centered
transition state. In contrast, the trans-hydrogen atom
of 13b always points away during any combinations of
the internal rotations. For sulfamate esters 13o and
13p, the unfavorable steric interactions between the
naphthyl ring of the substrate and cyclohexane ring of
1 rendering it difficult for the benzylic C–H bonds to ap-
proach the imido-nitrogen atom. The observed trend in
ee values found for the amidation products 14e–k sug-
gests that through-space electrostatic interactions, that
is, the field effects,15 in addition to steric effects could
be operative.
Inspection of entries 6–8 and 12–17 in Table 2 reveals
that the electron-withdrawing or -donating capability
of the substituent on the acyclic sulfamate ester can af-
fect the enantioselectivity of the intramolecular amida-
tion process. With the exception of 13h, on going from
13f ! 13g ! 13i, the ee values of the resultant cyclic sul-
famidates increased from 61% to 79% ee as the substitu-
ent on the substrate becomes more electron-withdrawing
(cf. entries 7–8, and 13). For sulfamate esters with
electron-donating substituents as in 13j and 13k, a
significant drop in enantioselectivity was observed (cf.
entries 14 and 15). A similar trend can be found on com-
paring the ee values obtained for amidation of 13l and
13m catalyzed by 1, which afforded cyclic sulfamidates
14l and 14m in 47% and 36% ee, respectively (entries
16and 17).
To shed more information on the rate-limiting step, we
performed competition experiments on the intramole-
cular amidations of p-X–C6H4(CH2)2OSO2NH2 [X =
H (13e), Br (13f), Cl (13g), F (13h) and NO2 (13i),
A comparison of isolated product yields shows the posi-
tion of the substituent on the substrate can also affect
the efficiency of the intramolecular amidation process.
Under the conditions: 10 mol % of 1, PhI(OAc)2
(1.5 equiv), Al2O3 (2.5 equiv), 5 °C, 10 h, reaction of
13o and 13p afforded 14o and 14p in 40% and 19% yield,
respectively (entries 19 and 20). The analogous reactions
0.2
MeO
R = 0.99
0.1
Me
H
0.0
Cl
Br
F
-0.1
-0.2
-0.3
O
O
H
S
H
H
O
NO2
-0.2
N
N
Br
Br
N
H
-0.1
0.0
0.1
0.2
Mn
.
O
O
0.250σmb+0.0035σJJ
Br
Br
Figure 3. Linear free-energy correlation of logkX/kH versus (rmb,rꢁJJ)
plot for intramolecular amidation of para-substituted sulfamate esters
p-X–C6H4(CH2)2OSO2NH2 13e–k catalyzed by 1 with PhI(OAc)2.
Figure 2. Proposed transition state for intramolecular amidation of
13b catalyzed by 1.

J. Zhang et al. / Tetrahedron Letters 46 (2005) 5403–5408
5407
Scheme 1. Intramolecular amidation of 13q with PhI(OAc)2 catalyzed by 1. Gas chromatograms of cyclic sulfamidate 14q furnished from racemic
and optically pure 13q.
Me (13j), OMe (13k)]. This gave logkX/kH values of 0.16
(13k), 0.047 (13j), ꢀ0.01 (13h), ꢀ0.055 (13g), ꢀ0.074
(13f), and ꢀ0.253 (13i), suggesting that electron-
donating groups accelerate whereas electron-withdrawing
groups retard the nitrogen atom insertion reaction. A
dual-parameter (rmb,rꢁJJ) fitting of logkX/kH, as estab-
lished by Jiang,16 through multiple regression gave rise
to good linearity (R = 0.99) with qmb and qꢁJJ values of
ꢀ0.25 and 0.0035, respectively. The logkX/kH versus
(rmb,rꢁJJ) plot is shown in Figure 3.
Grants Council (HKU7011/04P), HKSAR, and The
University of Hong Kong (University Development
Fund). P.W.H.C. wishes to thank The University of
Hong Kong (Small Project Funding Programme) for
funding.
References and notes
1. (a) Muller, P. In Advances in Catalytic Processes; Doyle,
¨
M. P., Ed.; JAI Press: Greenwich, CT, 1997; Vol. 2, p 113;
(b) Jacobsen, E. N. In Comprehensive Asymmetric Cataly-
sis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999; Vol. 2, p 607; (c) Katsuki, T.
To gain further insight into the mechanism, we exam-
ined the reactions of racemic and enantiopure 13q with
1 as catalyst under the same conditions as that for
13a–p. Intramolecular cyclization of 13q afforded 14q
in 60% yield with 95% substrate conversion (Scheme
1). For the amidation of (S)-13q, only a single isomer
of 14q was detected, whose absolute configuration is
identical to that observed for the same reactions using
either Rh2(II,II),8 Ru(II),9 or Ag(I)11 as catalyst, as
determined by chiral GC14 (see Scheme 1) and 1H
NMR (in the presence of (+)-Eu(hfc)3) analyses of the
products. This result indicates the Mn(III)-catalyzed
intramolecular amidation reaction is stereospecific.
Synlett 2003, 281; (d) Muller, P.; Fruit, C. Chem. Rev.
2003, 103, 2905.
¨
2. Li, Z.; Conser, K. R.; Jacobsen, E. N. J. Am. Chem. Soc.
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In summary, the first chiral metallosalen catalyzed enan-
tioselective intramolecular amidation of sulfamate esters
that proceeded in moderate to good yields, substrate
conversions, with exclusive cis-selectivity and with mod-
erate to good enantioselectivity is reported. The manga-
nese(III) Schiff-base catalyzed reaction also represents
the first step toward the development of a general cata-
lytic system for asymmetric intramolecular C–N bond
formation. Efforts are currently underway to examine
the scope of the present Mn(III)-catalyzed intramole-
cular amidation protocol with respect to fine-tuning
the ee values obtained through structural modification
of the chiral Schiff-base ligand.
9. (a) Liang, J.-L.; Yuan, S.-X.; Huang, J.-S.; Yu, W.-Y.;
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Acknowledgements
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This work is supported by the Area of Excellence
Scheme (AoE/P-10-01) established under the University
Grants Committee, HKSAR, the Hong Kong Research
1607.
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J. Zhang et al. / Tetrahedron Letters 46 (2005) 5403–5408
12. Robert, A.; Tsapara, A.; Meunier, B. J. Mol. Catal. 1993,
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13. CCDC 245227 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033; e-mail: deposit@
ccdc.cam.ac.uk).