Phosphoric acid-catalyzed enantioselective transfer hydrogenation of N-aryl-ortho-hydroxybenzophenone ketimines

Article abstract of DOI:10.1002/adsc.201000754

The first enantioselective chiral phosphoric acid-catalyzed transfer hydrogenation of N-aryl-ortho-hydroxybenzophenone ketimines using a Hantzsch ester as hydrogen source was developed to afford, after removal of the N-aryl group, the corresponding chiral diarylmethylamines in high yields and enantioselectivities.

Full text of DOI:10.1002/adsc.201000754

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DOI: 10.1002/adsc.201000754
Phosphoric Acid-Catalyzed Enantioselective Transfer
Hydrogenation of N-Aryl-ortho-Hydroxybenzophenone
Ketimines
Thanh Binh Nguyen,a Hadjira Bousserouel,a Qian Wang,a,b,*
and FranÅoise Guꢀrittea
a
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
E-mail: qian.wang@icsn.cnrs-gif.fr
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL), 1015 Lausanne,
b
Switzerland
Fax : (+41)-21-693-9740; e-mail: q.wang@epfl.ch
Received: October 8, 2010; Published online: February 9, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000754.
Abstract: The first enantioselective chiral phospho-
ric acid-catalyzed transfer hydrogenation of N-aryl-
catalytic enantioselective reduction of N-protected
aryl alkyl ketimines using a Hantzsch ester as reduc-
ing agent have been reported.[6,7,8,9] However, the
metal-free variants of the asymmetric hydrogenation
of substituted diarylketimines remain unknown.
ortho-hydroxybenzophenone ketimines using
a
Hantzsch ester as hydrogen source was developed
to afford, after removal of the N-aryl group, the
corresponding chiral diarylmethylamines in high
yields and enantioselectivities.
We recently reported a highly enantioselective
chiral phosphoric acid-catalyzed reduction of ortho-
À
hydroxyaryl alkyl N H ketimines using a Hantzsch
Keywords: diarylmethylamines; Hantzsch esters;
ortho-hydroxybenzophenone ketimine; organocatal-
ysis; phosphoric acids; transfer hydrogenation
ester as hydrogen source.[10] Herein we describe the
first chiral phosphoric acid-catalyzed transfer hydro-
genation of N-aryl-ortho-hydroxybenzophenone ket-
AHCTUNGTRENNUNG
imines using a Hantzsch ester as hydrogen source.[11]
The N-PMP protected ortho-hydroxybenzophenone
ketimine 2 was initially chosen as a substrate for the
screening of (S)-binol-derived phosphoric acid cata-
Chiral diarylmethylamines are important synthetic in- lysts 1[12] using Hantzsch dimethyl ester as reducing
termediates and are substructures of many medicinal- agent. As can be seen, performing the reduction at
ly relevant compounds.[1] Methodologies for their room temperature in benzene afforded the corre-
asymmetric syntheses have relied upon the chiral aux- sponding amine 3 with complete conversion within
iliary-based addition of arylmetallic reagents to C=N 2 h (entries 1–4, Table 1). Among the four phosphoric
bonds,[2] or metal-catalyzed enantioselective addition acids tested, only 1d afforded 3 with reasonable enan-
of arylstannanes, aryltitaniums, and arylboron re- tioselectivity (ee 63%) (entry 4). A low conversion
agents to N-sulfonyl-, N-sulfamoyl-, and N-tert-butyl- rate was observed in acetonitrile under otherwise
sulfinylimines.[3]
identical conditions. Performing the same reaction in
The enantioselective reduction of diaryl ketimines[4] other solvents (1,2-dichloroethane, diethyl ether) af-
would provide another convenient way for the prepa- forded amine 3 with complete conversion but lower
ration of chiral non-racemic diarylmethylamines. In enantioselectivity (data not shown).
2010, Zhang and Gosselin et al. reported the asym-
Reasoning that the steric and electronic environ-
metric hydrogenation of diarylimines in the presence ment at the ketimine nitrogen might modify the inter-
of an iridium-monodentate phosphine complex under action of the catalyst, the ketimine substrate and the
high pressure (up to 1500 psi). To the best of our Hantzsch ester, the reduction of diarylimines with dif-
knowledge, this is the only example known to date.[5] ferent N-aryl groups was next examined. The imines
The chiral Brønsted acid-catalyzed reduction of derived from ortho-substituted anilines (entries 5 and
ket
ACHTUNGTRENNUNiG mines has become an important alternative for 6) were reduced to amines with negligible enantiose-
the metal-catalyzed process. Examples of the organo- lectivity. The nature of para substituent of anilines did
Adv. Synth. Catal. 2011, 353, 257 – 262
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257

Thanh Binh Nguyen et al.
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Table 1. Survey of catalysts and N-protecting groups.[a]
Entry
Catalyst
R1
R2
R3
ee[b] [%]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1d
1d
1d
1d
1d
H
H
H
H
MeO
MeO
H
H
H
H
H
H
H
H
H
H
H
MeO
MeO
MeO
MeO
H
MeO
H
i-Pr
8
10
0
63
10
4
62
64
76
MeO
MeO
[a]
Reaction conditions: imine (2) (0.125 mmol), Hantzsch dimethyl ester (0.165 mmol), catalyst (0.0125 mmol) in benzene
(5 mL) for 2 h.
Determined by chiral HPLC analysis.
[b]
Table 2. Survey of Hantzsch esters.[a]
Entry
R
Temperature [8C]
ee [%][b]
1
2
3
4
Me
Et
t-Bu
Me
r.t.[c]
r.t.[c]
r.t.[c]
0[d]
76
76
64
89
[a]
Reaction conditions: imine (2a) (0.125 mmol), Hantzsch ester (0.165 mmol), catalyst 1d (0.0125 mmol) in toluene (5 mL).
Determined by chiral HPLC analysis.
2 h.
16 h.
[b]
[c]
[d]
not influence the enantioselectivity of the process as
The Hantzsch ester was next screened using the
N-Ph and N-4-i-Pr-C6H4 imines were reduced to 3,4,5-trimethoxyphenyl (TMP)-derived imine as sub-
amines with a similar enantioselectivity as that ob- strate (Table 2). Using the hindered Hantzsch di-tert-
served for the N-PMP imine (entries 4, 7 and 8). butyl ester as hydrogen source led to reduced enan-
However, the presence of meta-substituents in the N- tioselectivity as compared to its dimethyl and diethyl
aryl group influenced considerably the enantioselec- counterparts. (entry 3 vs. 1 and 2). Finally, when the
tivities. Indeed, the N-(3,4,5-trimethoxyphenyl)imine reaction was carried out at 08C using the Hantzsch di-
was reduced to the corresponding amine with 76% ee methyl ester as hydrogen source, a remarkable ee en-
(entry 9). A similar result was obtained when toluene hancement was observed leading to the amine in ex-
was used as solvent.
258
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Phosphoric Acid-Catalyzed Enantioselective Transfer Hydrogenation
Table 3. Chiral phosphoric acid-catalyzed transfer hydrogenation of N-aryl-ortho-hydroxybenzophenone ketimines 2.[a]
Entry
Imine
R
R1
R2
R3
Yield [%][b]
ee[c] [%]
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
TMP
TMP
TMP
TMP
TMP
TMP
TMP
TMP
TMP
TMP
TMP
TMP
PMP
PMP
PMP
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
H
H
H
H
H
H
H
H
5-Me
5-OMe
4-Me
4-Me
H
H
89
74
93
94
74
90
74
82
76
81
80
93
0
89
85
88
88
91
84
81
88
84
80
80
89
–
4’-Me
4’-Ph
4’-Br
4’-NO2
3’-Me
3’-MeO
3’-NO2
H
H
H
4’-Cl
H
H
9
10
11
12
13
14
15
Me
Cl
H
H
0
0
–
–
H
[a]
Reaction conditions: imine (2) (0.125 mmol), Hantzsch dimethyl ester (0.165 mmol), catalyst (0.0125 mmol) in toluene
(5 mL) for 16 h at 08C for imines 2a–l, for 72 h at room temperature for imines 2m–o.
After column chromatography.
[b]
[c]
Determined by chiral HPLC analysis.
cellent isolated yield (89%) and enantioselectivity
(89%, entry 4).
Under optimized conditions [Hantzsch dimethyl
ester (1.3 equiv.), 1d (0.1 equiv.), toluene, 08C, 16 h],
the scope of this reduction was next explored with a
variety of differently substituted N-TMP-ortho-hy-
droxybenzophenone ketimines. As shown in Table 3,
ketimines with different electron-withdrawing and
electron-donating groups in different positions of
both aromatic rings were reduced to the correspond-
ing N-TMP-diarylmethylamines with good to excel-
Scheme 1. Removal of the trimethoxyphenyl group.
lent yields (74–94%) and enantioselectivity 80–91% failed to produce the desired product. The absolute
ee). Functional groups (OMe, Cl, Br, NO2) are well configuration of amine was determined to be “S” by
tolerated. The pivotal role of the ortho-hydroxy group comparison of the sign of the optical rotation of 4
is readily seen since substrates lacking this function with that reported in the literature.[14]
were inert under the established conditions (en-
The N-aryl-ortho-hydroxybenzophenone ketimines
tries 13–15). It is thus interesting to note that the used in this study were synthesized in good to excel-
presence of the ortho-hydroxy group not only acceler- lent yields by simply heating ortho-hydroxyacetophe-
ated the reduction, but also allowed efficient differen- nones and anilines under solventless conditions. These
tiation of the two sterically similar aryl groups.
ketimines existed as unique E-isomers as evidenced
Finally, the trimethoxyphenyl group can be re- by the downfield shift of the phenol group due to the
moved using trichloroisocyanuric acid[13a] as oxidizing intramolecular H-bond (dOH =13.6–14.7 ppm in
agent in wet acidic acetonitrile at room temperature CDCl3). As the presence of the ortho-hydroxy moiety
(Scheme 1). The unprotected aminophenol 4 was ob- is essential for both the reactivity and enantioselectiv-
tained in moderate yield with unchanged enantiomer- ity of the present transformation, we postulated that
ic purity. Attempts with cerium ammonium nitrate[13b] the reduction may proceed via a transition state A
Adv. Synth. Catal. 2011, 353, 257 – 262
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asc.wiley-vch.de
259

Thanh Binh Nguyen et al.
COMMUNICATIONS
Procedure for Removal of the 3,4,5-Trimethoxy-
phenyl Group from Amine 3a
To a solution of 3a (183 mg, 0.5 mmol) in MeCN (8 mL) and
H2O (0.5 mL) were added trichloroisocyanuric acid (58 mg,
0.25 mmol) and camphorsulfonic acid (116 mg, 0.5 mmol).
The mixture was stirred for 16 h at room temperature and
then partitioned in CH2Cl2 (100 mL) and a saturated aque-
ous solution of NaHCO3 (20 mL). The organic layer was
separated, dried (MgSO4), and concentrated. The residue
was purified by silica gel column chromatography to afford
4 as a white solid; yield: 67.0 mg (67%).
Figure 1. Proposed transition state.
Acknowledgements
whereby phosphoric acid acted as a bifunctional cata-
lyst to form hydrogen bonds with both the imine and
the hydroxy groups of the substrate.[10,15] Approach of
the nucleophile from the less hindered re-face of the
imine would then give the observed (S)-N-aryldiaryl-
methylamine (Figure 1). Alternatively, the phosphate
oxygen could form an additional H-bond with the NH
group of the Hantzsch ester leading to a ternary com-
plex. Delivering the hydride in a pseudo-intramolecu-
lar fashion would then afford the same observed
enantiomer.[16]
Financial support from CNRS and ICSN is gratefully ac-
knowledged. T.B.N. thanks ICSN for a postdoctoral fellow-
ship.
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