Palladium-catalyzed asymmetric hydrogenolysis of N-sulfonyl aminoalcohols via achiral enesulfonamide intermediates

Article abstract of DOI:10.1002/anie.201307036

Giving the cat complete control: A stereoconvergent formal hydrogenolysis of N-sulfonyl aminoalcohols in the presence of a chiral Pd catalyst and trifluoroacetic acid (TFA) gave chiral amines with two contiguous stereocenters (see scheme; TFE=trifluoroethanol, Ts=p-tosyl). The products were formed with up to 94 %ee by acid-catalyzed dehydration to afford an enesulfonamide, enamine/imine isomerization, and Pd-catalyzed asymmetric hydrogenation.

Full text of DOI:10.1002/anie.201307036

Angewandte
Chemie
DOI: 10.1002/anie.201307036
Asymmetric Synthesis
Palladium-Catalyzed Asymmetric Hydrogenolysis of N-Sulfonyl
Aminoalcohols via Achiral Enesulfonamide Intermediates**
Chang-Bin Yu and Yong-Gui Zhou*
Hydrogenolysis is an important synthetic method in organic
chemistry,[1] and has been widely applied in industrial
processes,[2] natural product synthesis,[3] and the removal of
waste materials.[4] In practice, most hydrogenolysis reactions
have been used for the synthesis of racemic or achiral
compounds. Very few studies on asymmetric homogeneous
Scheme 1. Stereoconvergent palladium-catalyzed formal asymmetric
hydrogenolysis of N-sulfonyl aminoalcohols.
hydrogenolysis have been reported. The first example,
reported by Chan and Coleman, was a homogeneous asym-
metric hydrogenolysis of sodium epoxysuccinate through
desymmetrization with chiral rhodium catalysts with moder-
ate enantioselectivity.[5] Subsequently, a water-soluble sulfo-
nated-phosphine–rhodium catalyst system for the asymmetric
hydrogenolysis of epoxides was described by Bakos et al., but
the products were obtained with only 39% ee.[6] Kꢀndig and
co-workers documented a desymmetrization of meso dihalide
complexes by asymmetric hydrogenolysis, which led to planar
chiral organometallic complexes.[7] Very recently, asymmetric
hydrogenolysis of racemic tertiary alcohols was also success-
fully developed.[8] Despite considerable efforts, these reac-
tions are far from ideal because of low optical enrichment of
the hydrogenolysis product and a relatively limited range of
substrates. Therefore, the search for an efficient strategy for
homogeneous asymmetric hydrogenolysis and extension of
the scope of such processes to a wider range of substrates is
still of great significance.
Palladium has been extensively investigated as a hetero-
geneous hydrogenolysis catalyst.[9] In contrast, palladium-
catalyzed homogeneous asymmetric hydrogenolysis is still
rare.[7] Recently, homogeneous palladium catalysts were
successfully utilized for the asymmetric hydrogenation of
ketones,[10] imines,[11] heteroaromatic compounds,[12] and ene-
sulfonamides[13] by us and other research groups. Considering
the straightforward synthesis of N-sulfonyl aminoalcohols
from commercially available starting materials and the
usefulness of the hydrogenolysis products, chiral amines
with two contiguous stereogenic centers,[14] we speculated
that a chiral palladium catalyst system could be applied for
the homogeneous asymmetric hydrogenolysis of N-sulfonyl
aminoalcohols (Scheme 1). Herein, we report the palladium-
catalyzed formal asymmetric hydrogenolysis of N-sulfonyl
aminoalcohols to give chiral amines with two contiguous
stereocenters with up to 94% ee. Furthermore, both
“dynamic kinetic asymmetric transformation” and “dynamic
kinetic resolution” phenomena were observed.
Prior to our study, we wanted to synthesize the chiral N-
((1R,2S)-1-butyl-2,3-dihydro-1H-inden-2-yl)-4-methylbenze-
nesulfonamide 2a by hydrogenolysis of the benzylic hydroxy
group of the N-sulfonyl aminoalcohol (1S,2S)-1a[15] with Pd/C
as the catalyst in the presence of trifluoroacetic acid (TFA).
Surprisingly, although the reaction proceeded smoothly
À
[Eq. (1)], the stereogenic center of the C N bond was also
racemized: only racemic cis-2a was obtained in 76% yield.
This observation suggested that an achiral intermediate might
be involved in this process, and that the asymmetric hydro-
genolysis of racemic N-sulfonyl aminoalcohols might be
possible. In this context, we began to study the homogeneous
palladium-catalyzed asymmetric hydrogenolysis of racemic
N-sulfonyl aminoalcohols.
First, racemic 1a was chosen as a model substrate.
Asymmetric hydrogenolysis proceeded in the presence of
trifluoroacetic acid (TFA) in trifluoroethanol (TFE) with
chiral Pd(OCOCF3)2/(R)-synphos (L1) as the catalyst to
afford the desired product in 84% yield with 69% ee and
higher than 20:1 diastereoselectivity (Table 1, entry 1). As
expected, use of the antipode of the (R)-synphos ligand
resulted in the opposite configuration (Table 1, entry 2), thus
suggesting that the stereoselectivity is completely controlled
by the chiral catalyst. These observations excited our interest
and encouraged us to further explore this formal hydro-
genolysis reaction. Subsequently, the effect of the Brønsted
acid on the enantioselectivity and reactivity was investigated.
[*] Dr. C.-B. Yu, Prof. Y.-G. Zhou
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
Dalian 116023 (China)
E-mail: ygzhou@dicp.ac.cn
[**] Financial support from the National Natural Science Foundation of
China (21125208 & 21032003) and the National Basic Research
Program of China (2010CB833300) is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307036.
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Table 1: Optimization of the reaction conditions for the formal asym-
metric hydrogenolysis of N-sulfonyl aminoalcohol 1a.[a]
Entry
Solvent
Ligand Acid
Yield [%][b] ee [%][c]
1
2
3
4
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
(R)-L1 TFA
84
87
N/A
69
68 (+)
–
(S)-L1
(R)-L1 TsOH
(R)-L1
TFA
PhCO2H N/A
–
5[d]
6[d]
7[d]
8[d]
9[d,e]
(R)-L1 TFA
(R)-L2 TFA
(R)-L3 TFA
(R)-L4 TFA
90
92
95
75
73
95
70
59
62
86
92
92
TFE/CH2Cl2 (3:1) (R)-L4 TFA
10[d,e,f] TFE/CH2Cl2 (3:1) (R)-L4 TFA
[a] Reaction conditions: 1a (45 mg, 0.125 mmol), Pd(OCOCF3)2 (0.9 mg,
2.0 mol%), ligand (2.4 mol%), acid (1 equiv), H2 (400 psi), solvent
(3 mL), 708C, 20 h. [b] The yield and diastereomeric ratio (d.r.>20:1 in
1
all cases) were determined by H NMR spectroscopy. N/A=not
applicable. [c] The ee value was determined by HPLC. [d] The reaction
was carried out with 2 equivalents of TFA. [e] The reaction was carried
out at 608C. [f] The reaction was carried out for 60 h.
Scheme 2. Palladium-catalyzed formal asymmetric hydrogenolysis of
N-sulfonyl aminoalcohols 1. Reaction conditions: 1 (0.125 mmol), Pd-
(OCOCF3)2 (0.9 mg, 2.0 mol%), (R)-difluorphos (2.1 mg, 2.4 mol%),
TFA (2.0 equiv), H2 (400 psi), TFE/CH2Cl2 (3:1; 3 mL), 608C, 60 h. All
products were obtained with d.r.>20:1, as determined by 1H NMR
spectroscopy. [a] The opposite absolute configuration was observed
with the chiral ligand (S)-difluorphos.
Strong Brønsted acids, such as p-toluenesulfonic acid
(Table 1, entry 3), afforded complex mixtures. None of the
desired product but instead the dehydration product enesul-
fonamide 3 was observed in the presence of weak benzoic acid
(Table 1, entry 4). Fortunately, trifluoroacetic acid provided
the desired product, albeit with moderate enantioselectivity.
When the amount of acid was increased to 2.0 equivalents, the
yield and ee value were slightly improved (Table 1, entry 1
versus 5).
We further evaluated the influence of the chiral ligand. Of
various commercially available chiral diphosphine ligands,
electron-withdrawing (R)-difluorphos (L4), which was
reported by GenÞt and co-workers in 2004,[16] proved to be
the most favorable in view of enantioselectivity (Table 1,
entry 8). A survey of solvents indicated that the combination
of TFE and dichloromethane in a ratio of 3:1 was the best
choice with respect to enantioselectivity and reactivity
(92% ee; Table 1, entries 8 and 9). Full conversion was
observed without loss of enantioselectivity when the reaction
was prolonged to 60 h (Table 1, entry 10).
isopropyl group was introduced at the 6-position of substrates,
the transformation still displayed high reactivity and enantio-
selectivity (products 2g and 2h). As expected, the replace-
ment of (R)-difluorphos with (S)-difluorphos resulted in the
formation of product 2a with the opposite configuration and
essentially the same ee value (93%).
The asymmetric formal hydrogenolysis of cyclic N-sulfo-
nyl aminoalcohol 1k with a six-membered ring was also
successful [Eq. (2)]. The desired hydrogenolysis product 2k
was obtained with moderate enantioselectivity and diaste-
reoselectivity with Pd(OCOCF3)2/Duanphos as the catalyst.
Under the optimized conditions, the palladium-catalyzed
asymmetric hydrogenolysis of N-sulfonyl aminoalcohols 1a–
i was explored (Scheme 2). Generally, alkyl-substituted sub-
strates performed very well, and the length of the alkyl chain
had little influence on enantioselectivity. Interestingly, the
benzyl-substituted product 2 f was formed smoothly with
excellent enantioselectivity (89% ee) by hydrogenolysis of
the corresponding aminoalcohol. Moreover, when a methyl or
We were able to determine the absolute configuration of
the products of asymmetric hydrogenolysis on the basis of
product 2c. Slow diffusion of hexane into a solution of
optically pure cis-(À)-2c with 99% ee in dichloromethane
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gave a single crystal suitable for X-ray diffraction, which
established the absolute configuration of this isomer as
1S,2R.[17]
To gain insight into the reaction process, we conducted
several further experiments. During our evaluation of the
influence of acids, the dehydration product was obtained with
benzoic acid [Scheme 3, Eq. (3)]. This result suggested that
To demonstrate the practical utility of our methodology,
we focused on the development of a facile and expeditious
route to a chiral calcium-channel antagonist. Removal of the
p-toluenesulfonyl group in 2 f with sodium/naphthalene
furnished the chiral amine 4 [Eq. (5)], which has been
shown to be a calcium-channel antagonist for the treatment
of ischemic stroke.[20]
In conclusion, a stereoconvergent palladium-catalyzed
formal asymmetric hydrogenolysis of N-sulfonyl aminoalco-
hols was successfully developed with a chiral palladium
complex as the catalyst in the presence of trifluoroacetic acid
to provide direct and facile access to chiral 2,3-dihydro-1H-
inden-2-amine derivatives with two contiguous stereogenic
centers with up to 94% ee. The reaction pathway involves
Brønsted acid catalyzed dehydration to afford the enesulfon-
amide, acid-catalyzed enamine/imine isomerization, and
palladium-catalyzed asymmetric hydrogenation of the ene-
Scheme 3. Mechanistic investigation.
the achiral enesulfonamide 3 might be involved in the
transformation. To our delight, when the enesulfonamide
intermediate 3 was subjected to asymmetric hydrogenation
under the standard reaction conditions, the desired product
cis-(1S,2R)-2a was obtained with identical enantioselectivity
[Scheme 3, Eq. (4)]. On the basis of these experimental
results, deuterium-labeling experiments,[18] and previous stud-
ies on the asymmetric hydrogenation of enesulfonamides,[13]
we propose a stepwise hydrogenation process (Scheme 4):
sulfonamide and N-sulfonylimine:
a process based on
a dynamic kinetic asymmetric transformation. Notably, the
present enantioselective reactions represent the first success
in the homogeneous palladium-catalyzed formal asymmetric
hydrogenolysis of N-sulfonyl aminoalcohols.
Received: August 10, 2013
Revised: October 8, 2013
Published online: November 12, 2013
Keywords: a-aminoalcohols · asymmetric catalysis ·
.
homogeneous catalysis · hydrogenolysis · palladium
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