Formal palladium-catalyzed asymmetric hydrogenolysis of racemic N -sulfonyloxaziridines

Article abstract of DOI:10.1021/ol503118v

Highly enantioselective palladium-catalyzed formal hydrogenolysis of racemic N-sulfonyloxaziridines has been realized, providing a novel access to sultams with up to 99% ee. Preliminary mechanistic insights revealed that the reaction proceeded through a N-O bond cleavage, dehydration, and the sequential asymmetric hydrogenation.

Full text of DOI:10.1021/ol503118v

Letter
pubs.acs.org/OrgLett
Formal Palladium-Catalyzed Asymmetric Hydrogenolysis of Racemic
N‑Sulfonyloxaziridines
Bo Song, Chang-Bin Yu, Wen-Xue Huang, Mu-Wang Chen, and Yong-Gui Zhou*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, China
S
* Supporting Information
ABSTRACT: Highly enantioselective palladium-catalyzed formal
hydrogenolysis of racemic N-sulfonyloxaziridines has been realized,
providing a novel access to sultams with up to 99% ee. Preliminary
mechanistic insights revealed that the reaction proceeded through a
N−O bond cleavage, dehydration, and the sequential asymmetric
hydrogenation.
ydrogenolysis is a powerful method with extensive use in
industrial applications, pharmaceutical synthesis, and
into consideration, we cogitated if N-sulfonyloxaziridines could
be controlled to disconnect the N−O bond and then
transformed to imines with the aid of Brønsted acids, which
have been successfully applied in homogeneous Pd-catalyzed
asymmetric hydrogenation (Scheme 1).11 Herein, we document
the enantioselective synthesis of sultams by a Pd-catalyzed formal
hydrogenolysis of racemic N-sulfonyloxaziridines with up to 99%
ee.
H
organic synthesis.1 While the hydrogenolysis reaction is widely
employed to synthesize racemic or achiral compounds, very few
asymmetric versions have been developed to date. Initial efforts
for asymmetric hydrogenolysis focused on the development of
desymmetrization of epoxides2 and meso dihalide complexes.3
Recently, asymmetric hydrogenolysis of racemic tertiary alcohols
has also been achieved with high optical enrichment.4 Owing to
the variability and fragility of compounds in hydrogenolysis
conditions, the synthetic utility of hydrogenolysis is difficult to
conduct in asymmetric form. Therefore, the enlargement of the
range of substrates poses a key challenge for the development of
asymmetric hydrogenolysis reactions.
Scheme 1. Asymmetric Hydrogenolysis of N-
Sulfonyloxaziridines
Soon after their first preparation by Emmons in 1957,5
oxaziridines6 became regarded as fascinating objectives for novel
and unusual chemistry and were utilized in oxygen or nitrogen
atom transfer reactions, rearrangements, and cycloadditions. N-
sulfonyloxaziridines, commonly referred to as “Davis’ oxazir-
idines”,7 were the most extensively utilized class of oxaziridines in
organic synthesis attributing to their stability, ready availability,
and superior reactivity.6j Facilitated by ring strain releasing, a ring
opening reaction easily occurs to the strained three-membered
heterocyclic compounds containing two adjacent electronegative
heteroatoms. The experimental and theoretical mechanistic
investigations corroborate that the oxygen transfer reaction of
the oxaziridines is usually deemed to be a concerted process with
simultaneous disconnection of N−O and C−O bonds.8
Furthermore, an alternative transition-state model, in which
the N−O bond cleaves ahead of a C−O bond, is also proposed by
Evans, Davis, and Dmitrienko.9
Very recently, we reported a Pd-catalyzed asymmetric
hydrogenolysis of N-sulfonyl aminoalcohols.4d Mechanism
studies indicated that N-sulfonyl aminoalcohols were dehydrated
in the presence of trifluoroacetic acid to afford achiral
enesulfonamide intermediates, followed by asymmetric hydro-
genation to provide the desired products. Taking the convenient
synthesis of N-sulfonyl oxaziridines from commercially available
starting marterials10 and ready cleavage of a C−O or N−O bond
We began our investigation with 3-phenyl-1,2-benzisothiazole
1,1-dioxide oxide 1a as the model substrate using the
Pd(OCOCF3)2/(S,S′,R,R′)-TangPhos complex as the catalyst.
To our delight, the desired sultam 2a was obtained in moderate
yield and excellent enantioselectivity employing catalytic
amounts of benzoic acid as an additive (Table 1, entry 1). The
activity of this protocol can be improved by changing Brønsted
acids. With di-p-toluoyl-D-tartaric acid (D-DTTA), a higher yield
but with a somewhat lower ee value was obtained (entry 2).
Several other acids were then examined. Of the acid additives, L-
camphorsulfonic acid (L-CSA) afforded both an excellent yield
and ee value, resembling D-camphorsulfonic acid (D-CSA) which
gave a slightly lower enantioseletivity (entries 3−5). The choice
Received: October 24, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol503118v
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Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Substrate Scopes
b
c
b
c
entry
solvent
ligand
acid
yield (%)
ee (%)
entry
R
yield (%)
ee (%)
d
1
DCM
DCM
DCM
DCM
DCM
PhMe
THF
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L1
L1
L1
PhCOOH
D-DTTA
TsOH·H2O
D-CSA
L-CSA
53
63
92
95
96
97
91
28
97
75
<5
97
98
97
96
93
94
96
98
96
91
93
84
90
1
C6H5
97 (2a)
96 (2b)
97 (2c)
94 (2d)
96 (2e)
98 (2f)
96 (2g)
98 (2h)
95 (2i)
96 (2j)
94 (2k)
97 (2l)
97 (S)
99 (S)
96 (S)
98 (S)
97 (S)
98 (S)
97 (S)
93 (S)
94 (S)
97 (S)
97 (S)
95 (S)
e
2
2
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-FC6H4
d
3
3
4
4
5
5
6
L-CSA
6
2-MeOC6H4
7
L-CSA
7
3,5-(MeO)2C6H3
8
TFE
L-CSA
8
Me
nBu
iBu
9
DCM
DCM
DCM
DCM
DCM
DCM
L-CSA
9
10
11
12
13
14
L-CSA
10
g
e
L-CSA
N/A
11
Cy
d
L-CSA
96
97
97
12
C6H5CH2CH2
de
,
L-CSA
a
Conditions: Oxaziridine 1 (0.20 mmol), Pd(OCOCF3)2 (3.0 mol %),
(S,S′,R,R′)-TangPhos (3.3 mol %), L-CSA (10.0 mol %), H2 (600 psi),
DCM (3.0 mL), 50 °C, 20 h. Isolated yields. Determined by HPLC.
def
, ,
L-CSA
a
b
c
Conditions: Oxaziridine 1a (0.20 mmol), Pd(OCOCF3)2 (5.0 mol
%), (S,S′,R,R′)-TangPhos (5.5 mol %), acid (10.0 mol %), H2 (1000
psi), solvent (3.0 mL), 50 °C, 20 h. Determined by H NMR with
1,3,5-trimethoxybenzene as the internal standard. Determined by
HPLC. The amount of catalyst is 3.0 mol %. 600 psi. 30 °C. N/A:
Not analyzed.
d
e
30 °C. 1000 psi.
b
1
c
a
Table 3. Substrate Scopes
d
e
f
g
b
c
entry
R
yield (%)
ee (%)
1
2
3
4
5
C6H5
87 (4a)
93 (4b)
90 (4c)
80 (4d)
91 (4e)
96 (4f)
94 (4g)
98 (4h)
93 (4i)
91 (R)
97 (R)
91 (R)
90 (R)
98 (S)
94 (R)
94 (R)
94 (R)
94 (R)
2-MeC6H4
3-MeC6H4
4-MeC6H4
nBu
of solvents was also found to have a dramatic influence on the
reaction activities while maintaining the prominent enantiose-
lectivities, with DCM proving to be optimal (entries 6−8).
Toluene was not endowed with good solubility for the catalyst,
notwithstanding it provided a similar remarkable result. Next, we
tested the performance of some other commercially available
bisphosphine ligands. It turns out that no ligand gave better
results other than (S,S′,R,R′)-TangPhos (entries 9−11).
Fortunately, decreasing the amount of catalyst, reducing the
pressure of hydrogen gas, and lowing the temperature failed to
bring about deterioration in yields and enantioselectivities
(entries 12−14).
d
6
C6H5OCH2
d
7
2-MeC6H4OCH2
4-MeC6H4OCH2
2-Naphthyl-OCH2
d
8
d
9
a
Conditions: Oxaziridine 3 (0.20 mmol), Pd(OCOCF3)2 (5.0 mol %),
(S,S′,R,R′)-TangPhos (5.5 mol %), L-CSA (10.0 mol %), H2 (1000
psi), DCM (3.0 mL), 70 °C, 20 h. Isolated yields. Determined by
b
c
d
HPLC. The amount of catalyst is 3.0 mol %, 50 °C.
With the optimized reaction conditions established, the scope
of this transformation was subsequently investigated. As shown
in Table 2, a range of 3-substituted-1,2-benzisothiazole 1,1-
dioxide oxides 1 were found to be competent substrates. Not
only alkyl but also aryl substituted substrates could be converted
to products with excellent reactivities and enantioselectivities. It
is of note that slightly higher ee values were obtained for aryl
substituents than alkyl substituents of N-sulfonyloxaziridines,
which may be ascribed to the distinct electronic properties.
Especially, the sterically hindered 2-methyl and 2-methoxy group
on the phenyl ring for the substrates 1b and 1f, respectively,
culminated in the best enantioselectivities (entries 2 and 6).
To further determine the substrate generality of this protocol,
multifarious nonbenzofused N-sulfonyloxaziridines were also
subjected to the modified optimal conditions, and the results are
summarized in Table 3. To our satisfaction, various oxaziridines 3
can serve as efficacious substrates. With respect to aryl-
substituted substrates, the impact that steric effect had on
activities and enantioselectivities was moderate (entries 1−4). It
is worthwhile to note that the hydrogenolysis of simple alkyl-
substituted substrate 3e proceeded smoothly and the expected
good yield and high ee value were obtained (entry 5). Moreover,
mild reaction conditions with a reduced catalyst loading and low
temperature could be applied to an array of aryloxymethyl-
substituted substrates, providing the chiral sultams 4 with
excellent yields and enantioselectivities (entries 6−9).12
To obtain further insight into the reaction pathway, several
control experiments were carried out, as summarized in Table 4.
When the reaction was conducted under a reduced pressure of
hydrogen gas for 1 h, dynamic kinetic resolution of N-
sulfonyloxaziridines was not observed and racemic starting
material 1a was recovered.13 In addition, 2-benzoylbenzene-
sulfonamide 5 and imine 6a were detected, illustrating that imine
6a and 2-benzoylbenzenesulfonamide 5 might participate in this
process (entry 1). If the reaction carried out only for 1 h was
quenched, imine 6a and 2-benzoylbenzene-sulfonamide 5 were
B
DOI: 10.1021/ol503118v
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Letter
smoothly hydrogenated to desired product 2a. L-CSA not only
plays the part of acid for transformations but also acts as an
activator for asymmetric hydrogenation.14 It is worthwhile to
mention that the transformation of 1a to B and B to 6a are faster
than that of 6a to 2a.
Table 4. Investigation of the Pathway for Asymmetric
Hydrogenolysis of N-Sulfonyloxaziridines
a
In conclusion, we have developed the first homogeneous
formal Pd-catalyzed asymmetric hydrogenolysis for a broad
range of racemic aryl- and alkyl-substituted N-sulfonyl-
oxaziridines, providing novel access to the chiral sultams with
up to 99% ee. Explicit mechanism studies reveal that the reaction
proceeds by a stepwise sequence initiated by the ring opening of
N-sulfonyloxaziridine through hydrogenolysis, followed by a
dehydration of the tertiary alcohol intermediate and then an
asymmetric hydrogenation of the imine intermediate. Further
work will be devoted to the synthetic application of the
developed strategy.
b
selectivity (%)
c
entry
t (h)
conv (%)
2a
5
6a
ee of 2a (%)
d
e
f
1
1
54
−
30
32
70
41
44
15
11
N/A
2
3
4
5
1
>95
>95
>95
>95
27
56
85
89
96
97
96
96
e
5
−
e
10
15
−
e
−
a
Conditions: Oxaziridine 1a (0.20 mmol), Pd(OCOCF3)2 (3.0 mol
%), (S,S′,R,R′)-TangPhos (3.3 mol %), L-CSA (10.0 mol %), H2 (600
b
c
psi), DCM (3.0 mL), 30 °C. Determined by 1H NMR. Determined
d
e
f
by HPLC. 100 psi. Not observed. N/A: Not analyzed.
ASSOCIATED CONTENT
■
S
* Supporting Information
monitored besides sultam 2a with almost the same ee value
(entry 2). If reactions were conducted for more than 5 h, only the
desired product and imine intermediate at various ratios were
obtained (entries 3−5).
Experimental details and full spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
To obtain mechanistic information regarding the course of the
hydrogenolysis, further control experiments were conducted.
Intriguingly, imine 6 was submitted to the standard conditions
furnishing the desired product 2a with full conversion and an
identical ee value, while the reaction conducted without L-CSA
provided the desired product with 75% conversion and a lower ee
value (Scheme 2). The mechanistic details of our proposed
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ygzhou@dicp.ac.cn.
Notes
The authors declare no competing financial interest.
Scheme 2. Mechanistic Investigation of the Asymmetric
Hydrogenolysis of N-Sulfonyloxaziridines
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (21125208 and 21032003).
REFERENCES
■
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Scheme 3. Plausible Reaction Pathway for the Asymmetric
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D
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