Asymmetric hydrogenation via capture of active intermediates generated from aza-pinacol rearrangement

Article abstract of DOI:10.1021/ja5075745

An efficient palladium-catalyzed asymmetric hydrogenation via capture of an active intermediate generated in situ from acid-catalyzed aza-Pinacol rearrangement has been successfully developed, providing efficient access to chiral exocyclic amines with up to 98% ee. Three-, four-, and five-membered cyclic N-sulfonyl amino alcohols are viable substrates. This study opens a new window to the application of asymmetric hydrogenation.

Full text of DOI:10.1021/ja5075745

Communication
pubs.acs.org/JACS
Asymmetric Hydrogenation via Capture of Active Intermediates
Generated from Aza-Pinacol Rearrangement
Chang-Bin Yu,† Wen-Xue Huang,† Lei Shi,† Mu-Wang Chen,† Bo Wu,† and Yong-Gui Zhou*,†,‡
†State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
S
* Supporting Information
reactions.6 Over the past decade, chiral Pd complexes have been
ABSTRACT: An efficient palladium-catalyzed asymmetric
hydrogenation via capture of an active intermediate
generated in situ from acid-catalyzed aza-Pinacol rear-
rangement has been successfully developed, providing
efficient access to chiral exocyclic amines with up to 98%
ee. Three-, four-, and five-membered cyclic N-sulfonyl
amino alcohols are viable substrates. This study opens a
new window to the application of asymmetric hydro-
genation.
successfully applied by us and other groups in asymmetric
hydrogenation of aromatic compounds,7 ketones,8 ketimines,9
and enesulfonamides10 and asymmetric hydrogenolysis of N-
sulfonyl amino alcohols11 with Brønsted acids as additives. In a
mechanistic study of Pd-catalyzed asymmetric hydrogenation,7f
we found that 1 equiv of trifluoroacetic acid is generated in the
formation of active palladium hydride species (Scheme 2) and
Scheme 2. Asymmetric Hydrogenation via Capture of Active
Intermediates Generated in Situ from Brønsted Acid-
Catalyzed Aza-Pinacol Rearrangement
symmetric hydrogenation is undoubtedly one of the most
A
important synthetic methods in organic chemistry, as it
provides general access to versatile building blocks for chemical
synthesis, medicines, and functionalized materials.1 Generally,
the substrate scope includes stable and isolable olefins, ketones,
imines, and aromatics. Extension of the substrate scope is still
highly desirable in asymmetric hydrogenation.2 A novel type is
asymmetric hydrogenation via capture of inseparable and
instable active intermediates (Scheme 1), especially for active
Scheme 1. Combination of Hydrogenation and C−C Bond
Formations
that the Pd catalytic system is relatively tolerant to acid, oxygen,
and water. On the basis of these findings, we envisioned that the
combination of a Pd catalyst and a Brønsted acid could be
suitable for asymmetric hydrogenation via the capture of the
carbonium ion active intermediate generated in situ from the
Brønsted acid-catalyzed aza-pinacol rearrangement of cyclic N-
sulfonyl amino alcohols (Scheme 2). Notably, convergent
asymmetric hydrogenation of several active intermediates gives
a single product in the presence of acid; the side reactions can be
inhibited. Herein we report an enantioselective Pd-catalyzed
hydrogenation of cyclic N-sulfonyl amino alcohols. The present
strategy opens a new window to the application of asymmetric
hydrogenation.
intermediates involving C−C bond formation. One obvious
advantage is the systematic utilization of inseparable, unstable,
and reversible active intermediates due to the irreversibility of
hydrogenation; the other is rapid construction of chiral
compounds that are difficult to synthesize in traditional ways.
However, few reports on asymmetric hydrogenation of active
intermediates involving C−C bond formation have appeared.3
As powerful strategies for forming α-quaternary carbonyl
structures, the pinacol4 and semipinacol5 rearrangements play an
important role in organic chemistry. However, their analogue,
the aza-pinacol rearrangement, is relatively limited, mainly
because of the instability of the rearrangement product imine,
iminium, or enamine (the pinacol rearrangement product ketone
is relatively stable) and the weak driving force, which leads to
poor regio- and diastereoselectivity and unpredictable side
Received: July 31, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja5075745 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
First, four-membered N-sulfonyl amino alcohol 1a12 was used
as a model substrate to assay the best reaction conditions.
Pd[(S)-SynPhos](OCOCF3)2 and p-toluenesulfonic acid
(TsOH·H2O) (200 mol %) were selected as the hydrogenation
catalyst and acid to begin the investigation, and the reaction was
run in trifluoroethanol (TFE) at 50 °C. 1a was fully consumed,
but in addition to the decomposition product p-toluenesulfona-
mide generated by TsOH, no desired product was obtained
(Table 1, entry 1). Since p-toulenesulfonamide could be a leaving
Table 2. Palladium-Catalyzed Asymmetric Hydrogenation of
Cyclic N-Sulfonyl Amino Alcohols 1
a
b
c
entry
R
Ar
yield (%)
ee (%)
d
1
2
3
4
Me
Et
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-FC6H4
81 (2a)
86 (2b)
89 (2c)
81 (2d)
95 (2e)
97 (2f)
85 (2g)
72 (2h)
87 (2i)
79 (2j)
79 (2k)
69 (2l)
94 (R)
96 (+)
n-Pr
96 (−)
97 (−)
96 (+)
96 (+)
94 (+)
94 (+)
93 (+)
96 (+)
94 (+)
96 (−)
93 (−)
a
Table 1. Optimization of the Reaction Parameters
n-hexyl
Ph
e
5
6
7
8
4-MeC6H4
3-MeC6H4
4-MeOC6H4
Ph
e
b
c
9
e
entry ligand
solvent
equiv of TsOH·H2O yield (%)
ee (%)
10
Ph
4-MeOC6H4
Ph
1
2
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
TFE
TFE
TFE
TFE
DCM
toluene
THF
TFE
TFE
TFE
TFE
2.00
0.20
0.10
0.05
0.10
0.10
0.10
0.10
0.10
0.10
0.10
−
51
−
11
12
13
Ph
91 (−)
92 (−)
92 (−)
−
−
−
(CH2)4
(CH2)5
4-MeC6H4
4-MeC6H4
3
80
51 (2m)
4
33
a
d
d
d
d
Reaction conditions: 1 (0.125 mmol), Pd[(S)-SegPhos](OCOCF3)2
(2.0 mol %), TsOH·H2O (0.1 equiv), H2 (400 psi), TFE (3 mL), 50
5
NR
NR
NR
NR
80
6
b
c
d
°C, 24 h. Isolated yields. Determined by chiral HPLC. The absolute
configuration was determined by X-ray analysis. A 3/1 TFE/CH2Cl2
7
e
e
8
−
mixed solvent (4 mL) was added.
9
94 (−)
94 (−)
93 (+)
10
11
81
alcohol motif performed smoothly, giving the corresponding
amines bearing spirocyclic skeletons with excellent enantiose-
lectivity (entries 12 and 13).
f
74
In addition, we also studied three-membered cyclic N-sulfonyl
amino alcohols 3 (Table 3). It was found that excellent
Table 3. Palladium-Catalyzed Asymmetric Hydrogenation of
Cyclic N-Sulfonyl Amino Alcohols 3
a
a
Reaction conditions: 1a (0.125 mmol), Pd(L*)(OCOCF3)2 (2.0
mol %), TsOH·H2O (0.1 equiv), H2 (400 psi), solvent (3 mL), 50 °C,
b
c
d
14−24 h. Isolated yields. Determined by chiral HPLC. NR = no
reactivity. Benzoic acid was used instead of TsOH. The opposite
enantiomer was obtained.
e
f
b
c
entry
R
Ar
yield (%)
ee (%)
d
group under acidic conditions, we thought that a catalytic
amount of Brønsted acid might inhibit the side reaction. To our
delight, when 20 mol % acid was added, the desired product 2a
was obtained in 51% isolated yield with 91% ee (entry 2). Next,
the effect of the amount of acid on the enantioselectivity and
reactivity was investigated (entries 3 and 4). When the amount of
Brønsted acid was reduced to 10 mol %, the highest yield was
obtained (entry 3). The study of the solvent effect suggested that
only TFE gave the desired product (entries 3 and 5−7). When
benzoic acid was introduced as the additive, the first step of the
reaction could not occur (entry 8). Evaluation of various
commercially available axially chiral diphosphine ligands (entries
9−11) showed that (S)-SegPhos (L3) was the most favorable in
view of enantioselectivity and reactivity (entry 10).
1
Et
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
52 (4a)
68 (4b)
63 (4c)
76 (4d)
90 (4e)
91 (4f)
71 (4g)
66 (4h)
69 (4i)
98 (−)
98 (−)
98 (−)
98 (−)
89 (+)
89 (+)
98 (−)
98 (−)
97 (−)
d
2
n-Pr
3
4
5
6
7
8
9
n-Bu
n-hexyl
Ph
4-MeC6H4
n-hexyl
n-hexyl
n-hexyl
4-MeOC6H4
4-FC6H4
a
Reaction conditions: 3 (0.125 mmol), Pd[(S)-DifluorPhos]-
(OCOCF3)2 (2.0 mol %), TsOH·H2O (0.1 equiv), H2 (400 psi),
TFE (3 mL), 50 °C, 24 h. Isolated yields. Determined by chiral
b
c
d
HPLC. 40 °C.
With a set of optimized conditions in hand, the substrate scope
was studied (Table 2). Generally, alkyl-substituted substrates
performed very well, and the alkyl chain length had little
influence on the enantioselectivity (entries 1−4). The aryl-
substituted substrates were all good partners for the reaction, and
the corresponding products were obtained with 94−96% ee
(entries 5−8). Moreover, electron-donating and electron-
withdrawing groups in the arylsulfonyl moiety exhibited little
influence on the enantioselectivity (entries 9−11). Interestingly,
substrates 1m and 1l bearing a five- or six-membered cyclic
enantioselectivities were obtained regardless of the length of
the alkyl substituent (entries 1−4), and the n-hexyl group proved
to be optimal with respect to the yield. Tolerance with respect to
variation of the aryl group was also investigated. All aryl-
substituted substrates gave high ee values and yields (entries 5
and 6). The electronic properties of the sulfonyl group exhibited
little influence on the enantioselectivity (entries 7−9). It is
noteworthy that the direct synthesis of these sterically hindered
N-tosyl imine or N-tosyl enamine intermediates is very difficult
B
dx.doi.org/10.1021/ja5075745 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
using cyclic sterically hindered α,α-dialkyl ketones as starting
materials. For N-tosyl imines from cyclobutanones, no report has
appeared in the literature. This transformation provided a clever
detour to the synthesis of enantioenriched α,α-dialkyl cyclic
amine derivatives.13
Scheme 4. Plausible Reaction Pathway
This methodology was also suitable for five-membered cyclic
N-toluenesulfonyl amino alcohol 5 (eq 1), providing the desired
subsequent deprotonation affords enesulfonamide C and N-
sulfonylimine D. Meanwhile, a fast and reversible process of
enesulfonamide/N-sulfonylimine isomerization by Brønsted
acid-catalyzed protonation and deprotonation exists. The final
step is the Pd-catalyzed asymmetric hydrogenation of N-
sulfonylimine D to give the chiral amine.
It should be noted that the p-toluenesulfonyl group could be
easily removed without racemization by treatment with sodium/
ammonia (Scheme 5).
product 6 with excellent enantioselectivity in a low 23% yield
using Pd[(S)-SegPhos](OCOCF3)2 as the catalyst. The low
yield might be due to the relatively weak driving force from ring
strain release.14
The absolute configuration of the product 2a15 was
determined to be S on the basis of single-crystal X-ray analysis
after recrystallization from dichloromethane and hexanes (Figure
1). The absolute configurations of the other products were
assigned by analogy.
Scheme 5. Removal of the p-Toluenesulfonyl Group in 2a
Figure 1. X-ray crystallographic analysis of product 2a.
Generally, because of poor selectivity, unpredictable side
reactions, and instability of the products, the aza-pinacol
rearrangement is rarely applied in organic synthesis. By the
combination of asymmetric hydrogenation and the aza-pinacol
rearrangement, convergent asymmetric hydrogenation of several
active intermediates leads to a single product, and the side
reactions can also be inhibited. This strategy extends the scope of
application of the aza-pinacol rearrangement and asymmetric
hydrogenation in organic synthesis.
To investigate the detailed reaction pathway, two isotopic
labeling experiments were carried out. When 1a was subjected to
the reaction conditions with D2, the deuterium atoms were fully
incorporated at the α-position (Scheme 3). When the reaction
Scheme 3. Isotopic Labeling Experiments Using D2 and TFE-
d1
In summary, we have successfully developed the first highly
enantioselective Pd-catalyzed hydrogenation via capture of active
intermediates generated in situ from Brønsted acid-catalyzed aza-
pinacol rearrangement of N-sulfonyl amino alcohols, providing
efficient access to chiral exocyclic amine derivatives with up to
98% ee. This study opens a completely new window to the
application of asymmetric hydrogenation and provides some
useful hints for the design of new reactions. Future work will
focus on mechanistic studies and applications in the synthesis of
optically active drugs and natural products.
1
was carried out in TFE-d1 with H2, H NMR analysis of the
ASSOCIATED CONTENT
■
product showed that two deuterium atoms were taken up at the
β-position with >95% incorporation, suggesting that a reversible
process of Brønsted acid-catalyzed enesulfonamide/N-sulfony-
limine isomerization via protonation and deprotonation exists
and that the equilibrium is faster than the following asymmetric
hydrogenation.
On the basis of the above experimental results and the putative
pinacol rearrangement mechanism,4,5 a plausible reaction
pathway is proposed (Scheme 4). First, Brønsted acid-catalyzed
dehydration and aza-pinacol rearrangement of the cyclic N-
sulfonyl amino alcohol forms iminium ion intermediate B, and
S
* Supporting Information
Procedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
ygzhou@dicp.ac.cn
Notes
The authors declare no competing financial interest.
C
dx.doi.org/10.1021/ja5075745 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21032003 and 21125208) and the
National Basic Research Program of China (2010CB833300).
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