Rhodium-catalyzed stereoselective amination of thioethers with n-mesyloxycarbamates: DMAP and Bis(DMAP)CH2Cl2 as key additives

Article abstract of DOI:10.1002/anie.201402961

A stereoselective Rh-catalyzed intermolecular amination of thioethers using a readily available chiral N-mesyloxycarbamate to produce sulfilimines in excellent yields and diastereomeric ratio is described. A catalytic mixture of 4-dimethylaminopyridine (DMAP) and bis(DMAP)CH₂Cl₂ proved pivotal in achieving high selectivity. The X-ray crystal structure of the (DMAP)₂·[Rh₂{(S)-nttl}₄] complex was obtained and mechanistic studies suggested a Rh^II-Rh^III complex as the catalytically active species.

Full text of DOI:10.1002/anie.201402961

.
Angewandte
Communications
DOI: 10.1002/anie.201402961
Chiral Sulfilimines
Rhodium-Catalyzed Stereoselective Amination of Thioethers with
N-Mesyloxycarbamates: DMAP and Bis(DMAP)CH2Cl2 as Key
Additives**
Hꢀlꢁne Lebel,* Henri Piras, and Johan Bartholomꢀꢂs
Abstract: A stereoselective Rh-catalyzed intermolecular ami-
nation of thioethers using readily available chiral
N-mesyloxycarbamate to produce sulfilimines in excellent
yields and diastereomeric ratio is described. A catalytic mixture
nation of thioethers with iminoiodinanes has been developed
using chiral Cu,[9] Mn,[10] Ru,[11] and Fe complexes.[12]
Although high yields and selectivities are typically observed
in the synthesis of aromatic sulfilimines, only modest yields
and selectivities are reported with hindered and aliphatic
substrates. Consequently, there remains a need for a general
and highly stereoselective method to prepare chiral sulfil-
imines.
a
of
4-dimethylaminopyridine
(DMAP)
and
bis-
(DMAP)CH2Cl2 proved pivotal in achieving high selectivity.
The X-ray crystal structure of the (DMAP)2·[Rh2{(S)-nttl}4]
complex was obtained and mechanistic studies suggested
a RhII-RhIII complex as the catalytically active species.
Our research group has been interested in the use of
N-sulfonyloxycarbamates as metal–nitrene precursors.[13]
Stable and available chiral N-tosyl- and N-mesyloxycarba-
C
hiral sulfilimines[1] are characterized as bearing a dative
À
À
N S bond with strong polarization towards the nitrogen
mates 1 and 2 readily undergo asymmetric C H amination
atom.[2] Relative to chiral sulfoxides, the chemistry of chiral
sulfilimines[3] remains largely underdeveloped because of the
limited synthetic methods currently available. The recent
discovery of sulfilimine cross-links in collagen IV networks
has illustrated the importance of this class of compounds, thus
highlighting the need for the development of methods to
generate this moiety.[4] The amination of thioethers with
electrophilic aza reagents is the oldest known and most
and aziridination reactions in the presence of chiral RhII
dimers or CuI catalysts (Scheme 1).[14]
applied method for preparing chiral sulfilimines.[1a,5]
A
number of transition-metal complexes have been reported
to catalyze the amination of thioethers, presumably via the
formation of metal–nitrene species.[5e,6] Although metal-
catalyzed diastereoselective reactions with chiral substrates[7]
or chiral reagents[8] have been delineated, they often have
limited substrate scope. The catalytic enantioselective ami-
Scheme 1. Stereoselective aminations with chiral N-tosyl- and N-mesyl-
oxycarbamate. LG=leaving group, Ts=toluene-4-sulfonyl, Ms=mesyl.
Herein, we report the use of chiral N-mesyloxycarbamate
2 to perform the stereoselective intermolecular amination of
thioethers in the presence of a chiral dirhodium(II) carbox-
ylate catalyst. A catalytic mixture of achiral 4-dimethylami-
nopyridine (DMAP) and bis(DMAP)CH2Cl2 was found to be
crucial for the stereoinduction of the process.
The amination of thioanisole was first investigated using
the tert-leucine-derived Rh catalyst [Rh2{(S)-nttl}4][15,16] in the
presence of both enantiomers of 2 [Eqs. (1) and (2)].
Although the rate of reaction for (R)-2 and (S)-2 were
similar, improved selectivity was noted with the latter.
[*] Prof. Dr. H. Lebel, H. Piras, J. Bartholomꢀꢁs
Dꢀpartement de Chimie, Center for Green Chemistry and Catalysis
Universitꢀ de Montrꢀal
C.P. 6128, Succ. Centre-ville
Montrꢀal, Quꢀbec, H3C 3J7 Canada
E-mail: helene.lebel@umontreal.ca
[**] This research was supported by the NSERC (Canada), the Canada
Foundation for Innovation, the Canada Research Chair Program, the
Universitꢀ de Montrꢀal, and the Centre in Green Chemistry and
Catalysis (CGCC). We thank Dr. O. Leogane for preliminary
investigations of Rh-catalyzed amination reactions of thioethers
with N-tosyloxycarbamates. We also thank Prof. D. Zargarian and
his research group for the use of their potentiostat, and Prof. D.
Rochefort for his help interpreting cyclic voltammetric results. We
thank also F. Bꢀlanger-Gariꢀpy for the resolution of X-ray crystal
structures, as well as A. Hamel, C. Malveau, and M. Tan Phan Viet
from the Universitꢀ de Montrꢀal NMR Center. The X-ray crystal
structure is displayed using CYLview (Legault, C.Y.; CYLview, 1.0b,
Universitꢀ de Sherbrooke, 2009 (http://www.cylview.org)). We
would like to thank Dr. V. N. G. Lindsay and Dr. J. J. Mousseau for
fruitful discussions. DMAP=4-dimethylaminopyridine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402961.
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As it has previously been described that the addition of
Lewis bases can improve the stereoselectivity of processes
catalyzed by chiral RhII complexes (presumably through
binding at the apical position),[17] several of these additives
were evaluated in the reaction. Interestingly, they proved to
have a significant effect on the stereoselectivity (Table 1).
these two species are present (entries 1 and 5 versus entries 3
and 7), it suggests that both a Lewis base additive and
a pyridinium salt are necessary. Indeed, the best result was
obtained when a mixture of DMAP and 4 was used, which
produced 3 in 87% yield and a diastereomeric ratio of 95:5
(entry 9). Similar results were obtained using bis-
(DMAP)CH2Br2 or methylviologen(PF6)2 in the presence of
DMAP (entries 10 and 11).
Table 1: Effect of additives in rhodium-catalyzed amination of thioani-
A variety of thioethers were explored under the optimal
reaction conditions (Table 2). Gram-scale amination with
a catalyst loading of 1 mol% afforded 1.38 g of 3 with d.r. 94:6
(entry 2). Various diversely substituted aryl methyl thioethers
produced sulfilimines in excellent yields and d.r. values
(entries 3–9). Sulfilimine 12 was isolated in high yield and
diastereomeric ratio, with no undesired reactions at the
sole.
Entry[a]
Additives (mol%)
Solvent
Yield [%][b]
d.r.[c]
1
2
3
4
5
6
7
8
9
DMAP (5)
–
DMAP (7)
PPh3 (5)
bis(DMAP)CH2Cl2 (4) (5)
nBu4NCl (5)
Me-viologen(PF6)2 (5)
octyl-viologen(PF6)2 (5)
bis(DMAP)CH2Cl2 (4) (2.5)
DMAP (7)
CH2Cl2
iPrOAc
iPrOAc
iPrOAc
iPrOAc
iPrOAc
iPrOAc
iPrOAc
iPrOAc
92
85
52
41
77
62
82
70
87
88:12
75:25
80:20
76:24
94:6
73:27
90:10
88:12
95:5
Table 2: Rh-catalyzed amination of thioethers with (S)-2.
Entry[a] Thioether
Crude d.r.[b] Yield [%][c]
d.r.[d]
10
11
bis(DMAP)CH2Br2 (2.5)
DMAP (7)
Me-viologen(PF6)2 (5)
DMAP (7)
iPrOAc
iPrOAc
85
85
95:5
94:6
[a] Thioether (0.15 mmol). [b] Yield of isolated product. [c] Crude S,S/R,S
1
2
R=H
R=H
95:5
94:6
(3) 87
(3) 88
96:4
94:6
ratio determined by 1H NMR spectroscopy.
(gram-scale)[e]
(5) 90
3
4
5
R=Me
R=OMe
R=NO2
R=Br
92:8
93:7
91:9
93:7
93:7
94:6
94:6
94:6
94:6
93:7
The use of 5 mol% DMAP in CH2Cl2 led to 3 being
isolated in a yield of 92% with an improved d.r. value of 88:12
(entry 1). Notably, the effect was not as significant when the
reaction was conducted in iPrOAc instead of CH2Cl2
[entries 2 and 3 versus Eq. (2) and entry 1]. Notably, lower
yields are observed with a stronger coordinating ligand,
namely PPh3 (entry 4). DMAP and CH2Cl2 are known to react
to produce the corresponding bis(DMAP)CH2Cl2 [4;
Eq. (3)].[18]
(6) 86
(7) 84
(8) 81
(9) 89
6
7[f]
R=F
8
9
R1 =H, R2 =Br
R1 =Br, R2 =H
92:8
95:5
(10) 83
(11) 89
92:8
95:5
10
95:5
(12) 90
99:1
11
94:6
(13) 77
94:6
We hypothesized that 4 could be formed in situ (entry 1),
and be responsible for the increase in the diastereomeric
ratio. Indeed, when the amination of thioanisole was per-
formed with 4 in iPrOAc, the d.r. value improved to 94:6
(Table 1, entry 5). Other ammonium salts were tested to
understand the role of 4. The use of nBu4NCl had a detri-
mental effect, which suggests a negligible role of the ClÀ
counterion. Other pyridinium salts were tested, but none
proved as effective as 4 (entries 7 and 8). In contrast to
viologen salts,[16] bis(DMAP)CH2Cl2 in solution partially
reverts into DMAP and CH2Cl2, thus leading to a mixture
of DMAP and 4.[19] As improved results are obtained when
12
82:18
71:29
89:11
(14) 76
83:18
93:7[g]
96:4[g]
13[f]
14[f]
(15) 58[g]
(16) 78[g]
1
[a] Thioether (1 mmol). [b] Crude S,S/R,S ratio determined by H NMR
spectroscopy. [c] Yield of isolated product. [d] Purified S,S/R,S ratio
determined by 1H NMR spectroscopy. [e] Thioether (4.2 mmol), [Rh2{(S)-
nttl}4] (1 mol%), DMAP (1.4 mol%), and 4 (0.5 mol%). [f] [Rh2{(S)-4-
NO2-nttl}4] (3 mol%). [g] Recrystallization from CHCl3/hexanes.
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thiophene center (entry 10). Nitrogen heterocycles, such as
pyridines, were also compatible with the reaction conditions
(entry 11). Previously, sterically hindered and aliphatic thio-
ethers have been problematic. Herein we demonstrate that
14, which contains bulky isopropyl groups, was produced in
76% yield and a good d.r. value (entry 12). Other aliphatic
sulfilimines could also be synthesized in excellent yields.
Sulfilimine 15 was isolated in 88% yield, albeit with a modest
diastereomeric ratio,[19] although recrystallization afforded 15
in 58% yield with d.r. 93:7 (entry 13). Sulfilimine 16 was
obtained in 92% yield and d.r. 89:11,[19] and could also be
isolated in 78% yield after recrystallization as a mixture with
d.r. 96:4 (entry 14).
Figure 1. UV/Vis spectra of [Rh2{(S)-nttl}4] in iPrOAc (b); with 2
(20 equiv; a), with thioanisole (20 equiv; c), with DMAP
(1.4 equiv; g); with thioanisole (20 equiv) and DMAP (1.4 equiv;
d); with DMAP (2.8 equiv; c).[21]
Ru-catalyzed oxidation of sulfilimine 3 afforded sulfox-
imine 17 in 98% yield with complete retention of the
d.r. value (Scheme 2).[12,20] Cleavage of the carbamate was
achieved using Zn in AcOH to produce free sulfoximine 18 in
94% yield as a single enantiomer.
Scheme 2. Synthesis of free sulfoximine 18.
The effect of DMAP and bis(DMAP)CH2Cl2 on the
diastereomeric ratio of the amination of thioethers is very
intriguing. Our optimization suggests that both species are
required for optimal results. As a color change was observed
when thioanisole and DMAP were added to [Rh2{(S)-
nttl}4],[19] UV/Vis spectra were recorded to study the coordi-
nation of the additives. A solution of [Rh2{(S)-nttl}4] displayed
a lmax value at 630 nm (green solution; Figure 1). The addition
of 2 (20 equiv) did not change the lmax value, which suggests
no coordination with the metal. Such a binding appears to be
required to form the metal–nitrene species,[13d] but this event
is still kinetically possible. The addition of thioanisole
(20 equiv) to [Rh2{(S)-nttl}4] shifted the lmax value to
560 nm. The same lmax value was measured when 1.4 equiv
DMAP was added, whereas a solution of 2.8 equiv DMAP
displayed a lmax value at 530 nm. These data suggest that only
one molecule of thioether can coordinate to the Rh center. In
contrast, DMAP can form a 1:1 complex with the Rh catalyst
or form a 2:1 DMAP/Rh complex (see below). Furthermore,
when DMAP (1.4 equiv) was added to a solution of thio-
anisole (20 equiv) and [Rh2{(S)-nttl}4], the lmax value was also
shifted to 530 nm, thus suggesting the formation of a thio-
ether-Rh-DMAP complex.
Figure 2. Crystal structure of the (DMAP)2·[Rh2{(S)-nttl}4] complex.
Thermal ellipsoids are shown at 50% probability. Hydrogen atoms and
cocrystallized solvent are removed for clarity. One naphthalimide
moiety was found disordered over two positions of which only the one
with higher occupancy is shown.[21]
molecules of DMAP were bound to the complex (one to each
Rh atom), and importantly, the all-up (or chiral crown)
conformation typically observed for these types of complexes
was preserved.[22] Although a similar coordination of DMAP
to an analogous all-up chiral Rh dimer has been propo-
sed,[17d,e] this represents the first crystallographic proof of its
existence.
Pyridinium salts are known for their oxidative properties
(notably viologen salts).[23] As such, we hypothesized that they
could play a role as single-electron oxidants. The redox
potentials of [Rh2{(S)-nttl}4] were measured by cyclic voltam-
metry (CV; Figure 3).[19] The dimer displayed a quasireversi-
ble oxidation at 1070 mV for the redox couple RhII-RhII/RhII-
RhIII. The addition of either thioanisole or 2 had no effect on
the redox potential. Conversely, the addition of 1.4 equiv
Suitable crystals for X-ray analysis were obtained from
a solution of [Rh2{(S)-nttl}4] and DMAP (Figure 2). Two
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vacuum. The crude product was purified by flash chromatography on
silica gel using EtOAc in hexanes as the eluent.
Received: March 3, 2014
Published online: May 30, 2014
Keywords: asymmetric catalysis · chiral sulfilimines ·
.
metal nitrenes · rhodium · sulfoximines
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In conclusion, high yields and d.r. values were observed
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Experimental Section
General procedure: [Rh2{(S)-nttl}4] (3 mol%), K2CO3aqsat.
(1.5 mmol), DMAP (5.1 mg), and 4 (4.9 mg) were added successively
to a solution of thioether (1.05 mmol) in iPrOAc (10 mL) at RT. (S)-2
(1.00 mmol) was added with vigorous stirring and the mixture stirred
vigorously for 4 h at RT. The reaction mixture was filtered over a pad
of celite, washed with EtOAc, and the filtrate was concentrated under
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À
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b) K. P. Kornecki, J. F. Berry, Chem. Commun. 2012, 48, 12097 –
12099.
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