Pd-catalyzed enantioselective C-H iodination: Asymmetric synthesis of chiral diarylmethylamines

Article abstract of DOI:10.1021/ja408864c

An enantioselective C-H iodination reaction using a mono-N-benzoyl- protected amino acid has been developed for the synthesis of chiral diarylmethylamines. The reaction uses iodine as the sole oxidant and proceeds at ambient temperature and under air.

Full text of DOI:10.1021/ja408864c

Communication
pubs.acs.org/JACS
Pd-Catalyzed Enantioselective C−H Iodination: Asymmetric Synthesis
of Chiral Diarylmethylamines
Ling Chu,† Xiao-Chen Wang,† Curtis E. Moore,‡ Arnold L. Rheingold,‡ and Jin-Quan Yu*,†
†Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States
‡Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California
92093-0358, United States
S
* Supporting Information
Scheme 1. Strategies Toward Chiral Diarylmethylamines
ABSTRACT: An enantioselective C−H iodination reac-
tion using a mono-N-benzoyl-protected amino acid has
been developed for the synthesis of chiral diarylmethyl-
amines. The reaction uses iodine as the sole oxidant and
proceeds at ambient temperature and under air.
nantiopure diarylmethylamine is an important motif in
bioactive compounds, as exemplified by the antihistamine
drug, Certirizine hydrochloride, and the promising drug
candidate, SNC-80, which contain a stereogenic diarylmethyl-
amine core (Figure 1).1 Extensive efforts have led to the
E
reported, enantioselective iodination remains to be demon-
strated.11 Herein we report the first example of an
enantioselective C−H iodination to provide a new route for
the synthesis of diarylmethylamines. The use of inexpensive
molecular iodine as the sole oxidant and a readily available
chiral amino acid-derived ligand renders this reaction
potentially practical for a large-scale production of enantiopure
diarylmethylamines.
Figure 1. Biologically active diarylmethylamine compounds.
Recently, we developed a Pd-catalyzed C−H iodination of
benzamides using inexpensive I2 as the sole oxidant.12 We
found that product separation with this catalytic system is much
simpler as there are no byproducts from the oxidants and the
inert reactivity of I2, relative to other commonly used highly
electrophilic halogenating reagents, largely prevents back-
ground reactions. Therefore, we surmised that this catalytic
system may be suitable for the development of an
enantioselective C−H iodination reaction. In light of the
importance of the chiral diarylmethylamine scaffolds, we
embarked on the development of enantioselective C−H
iodination of trifluoromethanesulfonyl-protected diarylmethyl-
amines. The superior reactivity of triflamide compared to other
protected amines (NHAc, NHTFA, NHBoc) was previously
demonstrated.13
development of two key strategies for the enantioselective
synthesis of diarylmethylamines (Scheme 1).2,3 The first
approach involves the asymmetric addition of an arylmetal or
arylboron species to aldimines using either a chiral ligand4 or an
auxiliary.5 The Ellman auxiliary has found extensive applications
due to its ready availability and reliability.5a−h Alternatively,
asymmetric hydrogenation has been employed for the
enantioselective synthesis of this important scaffold.6 Unfortu-
nately, an ortho-substitution is often required on at least one of
the arenes to achieve high levels of enantioselection, except for
a few examples relying on the use of a high pressure of
hydrogen.6c
We have previously reported several examples of Pd(II)-
catalyzed enantioselective C−H activation reactions in which a
mono-N-protected amino acid (MPAA) ligand is used to
control the stereochemistry during the asymmetric cleavage of a
prochiral C−H bond.7−10 While the majority of these reactions
proceed via a Pd(II)/Pd(0) catalytic cycle, MPAA has also been
shown to be an effective ligand in an intramolecular
enantioselective C−H activation/C−O bond-forming reaction
based on Pd(II)/Pd(IV) catalysis.8f Although an early example
of asymmetric C−H iodination using a chiral auxiliary was
Screening of our chiral MPAA ligands and various reaction
parameters using 1a as the model substrate led to an
encouraging finding. The use of the Boc-Leu-OH ligand with
CsOAc as a base in DMF provided the chiral diarylmethyl-
Received: September 3, 2013
Published: October 23, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
a
a
Table 1. Ligand Screening
Table 2. Base Screening
b
c
entry
ligand
yield (%)
ee (%)
b
c
entry
base
yield (%)
ee (%)
1
Boc-Leu-OH
Boc-Val-OH
Boc-Ala-OH
Boc-Ile-OH
25
20
11
30
26
18
20
20
n.r.
34
10
n.r.
10
13
n.r.
n.r.
43
30
20
25
33
23
26
25
−33
15
35
n.r.
2
1
none
n.r.
47
n.r.
12
40
n.r.
n.r.
n.r.
6
n.r.
78
2
2
CsOAc
LiOAc
NaOAc
KOAc
3
3
n.r.
53
4
4
5
Boc-Phe-OH
Boc-D-Val-OH
Boc-Nle-OH
Boc-Try(tBu)-OH
Boc-Asn-OH
Boc-MeAla-OH
Form-Leu-OH
Ac-Leu-OH
5
60
6
6
NaHCO3
KHCO3
Li2CO3
n.r.
n.r.
n.r.
77
7
7
8
8
9
9
Na2CO3
K2CO3
10
11
12
13
14
15
16
17
18
19
10
11
12
13
14
15
16
38
40
n.r.
20
33
58
82
99
30
n.r.
3
Cs2CO3
Na2HPO4
K2HPO4
K3PO4
98
n.r.
83
TFA-Leu-OH
TcBoc-Leu-OH
Me-Leu-OH
Bn-Leu-OH
8
97
n.r.
n.r.
15
0
LiH2PO4
75
d
CsOAc + Na2CO3
89
MeO2C-Leu-OH
Piv-Leu-OH
e
d
f
f
17
CsOAc + Na2CO3
81 (80)
98 (98 )
e
18
CsOAc
45
90
Bz-Leu-OH
67
a
b
d
e
e
1
20
Bz-Leu-OH
18 (47)
89 (78)
Conducted on 0.1 mmol scale. Determined by H NMR analysis
c d
a
b
1
using CH2Br2 as the internal standard. By chiral HPLC analysis.
equiv of CsOAc and 3 equiv of Na2CO3. 15 equiv of DMSO as an
additive instead of DMF. Conducted at 30 °C, 48 h.
3
Conducted on 0.1 mmol scale. Determined by H NMR analysis
using CH2Br2 as the internal standard. Determined by chiral HPLC
analysis. t-Amyl-OH used as solvent with 15 equiv of DMF as an
e
c
f
d
e
additive. Conducted at 50 °C, 12 h.
amine 2a in 25% yield with 25% ee (Table 1, entry 1).
Encouraged by this result, we screened a variety of Boc
protected amino acid ligands with different backbones (entries
2−10). Unfortunately, no significant improvement was
observed with these ligands, and the best enantioselectivity
obtained was 33% ee when Boc-Val-OH was used (entry 2).
The low yield observed in general is also a major concern at this
stage.
proved critical in this transformation with Cs+ and K+ being
generally superior to Na+ and Li+ (entries 2−5). Both the
conversions and ees were also significantly influenced by the
anions of the base present (entries 6−15). The use of either
sodium or potassium bicarbonate resulted in no reaction
(entries 6 and 7) whereas sodium, potassium or cesium acetates
and carbonates promoted the reaction to various extents
(entries 2, 4, 5, 9, 10, and 11). Tripotassium phosphate was
more effective than dipotassium phosphate in terms of both
yield and enantioselectivity (entries 13 and 14). Surprisingly,
monolithium phosphate gave moderate yields and enantiose-
lectivity (entry 15). None of these conditions, however,
provided synthetically useful yields. Ultimately we discovered
that combination of Na2CO3 and CsOAc effectively promotes
the iodination reaction, affording the desired product 2a in 82%
yield and 89% ee (entry 16). Replacing the 15 equiv of DMF
with the same amount of DMSO improved the enantiose-
lectivity to 98% ee while yield remained as high as 81% (entry
17).14 DMF or DMSO could sequester the small amount of
free Pd(II) species not coordinated to chiral ligand thereby
avoiding racemic iodination. Removal of Na2CO3 from the
reaction under these new conditions decreased the yield to
45%, while maintaining the high enantioselectivity (entry 18).
These combined experimental results suggest that the use of
mixed bases CsOAc and Na2CO3, and DMSO as an additive
has significant beneficial effect on both the yield and
enantioselectivity.
Since we have shown previously that the N-protecting group
on the MPAA ligand exerts significant influence on the
enantioselectivity, we tested leucine ligands protected with
various protecting groups that are available from our library
(entries 11−20) and found that the use of benzoyl-protected
leucine improved the enantioselectivity to 67% ee, albeit in low
yield (entry 19). Through further optimization, however, we
discovered that the use of a binary solvent system, t-amyl-OH,
as the main solvent with 15 equiv of DMF as the additive
provides a significant improvement in enantioselectivity in this
transformation, affording the desired product in 89% ee, albeit
in low yield (entry 20). Raising the reaction temperature to 50
°C increased the yield to 47%, with the enantioselectivity
dropping to 78% ee (entry 20). Based on this finding, we also
reexamined other amino acids protected with a benzoyl group
and found that leucine backbone appeared to be the optimal.
We also modified the benzoyl protecting group on the leucine
ligand, attempting to improve the yields and enantioselectivity
but without further success (see SI for a detailed study). In
summary, Bz-Leu-OH was identified as the optimal ligand at
this stage (Table 1).
The iodination reaction also proceeded at 30 °C to give high
yield and enantioselectivity (Table 2, entry 17). The excellent
reactivity of this reaction at low temperature is a significant
advantage when enantioselection of certain substrates become
We subsequently investigated the effect of the base on this
transformation (Table 2). The identity of the metal counterion
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Journal of the American Chemical Society
Communication
challenging. Tolerance of air also simplifies the experimental
operation of this reaction.
Scheme 2. Gram-Scale Synthesis
With these optimized conditions in hand, we began to
examine the substrate scope of this reaction. The achiral
starting materials are readily prepared on gram-scale following
literature procedures.15 While the conditions for enantioselec-
tive iodination are effective with majority of the substrates, we
found that the use of 40 mol% ligand gave slightly improved
enantioselectivity overall (Table 3). Iodination of the ortho-
of 2a (75% yield) in 95% ee (Scheme 2). The minor decrease
of the enantioselectivity in gram-scale could be due to the
heterogeneity of this reaction and lack of highly efficient
stirring. We were subsequently able to deprotect the
trifluoromethylsulfonyl amine using a modification of con-
ditions previously reported separately by Hendrickson,
Danheiser, and Blakey.17 Thus, activation of the triflamide
with p-nitrobenzylbromide facilitated the deprotection of 2h, as
shown in Scheme 3.
a
Table 3. Substrate Scope
Scheme 3. Deprotection
Finally, the coordination mode of the triflamide directing
group is especially intriguing. Based on our previous stereo-
chemical model with Pd(II)/MPAA catalysts, the directing
group coordinates with Pd(II) as a neutral σ-donor similar to
pyridine,8a,d carbonyl8b and imidate.8c These results have
prompted us to invoke an unusual, weakly coordinating
sulfonaimine structure 4 in the C−H activation step (Figure
2). The absolute configuration of the iodinated product 2a (R,
a
Conducted on 0.2 mmol scale. Reaction conditions: 10 mol %
Pd(OAc)2, 40 mol % Bz-Leu-OH, 3 equiv of CsOAc, 3 equiv of
Na2CO3, 3 equiv of I2, 15 equiv of DMSO, 2 mL of t-amyl-OH, 30 °C,
air, 48 h. Isolated yield are given; ee’s were determined by chiral
HPLC analysis.
substituted substrates 1a−1f consistently affords the desired
products 2a−2f in moderate to good yields with excellent
enantioselectivity (Table 3, 54−85% yield, 97−99% ee).
Substitution of the aryl rings with electron-donating (OMe)
or electron-withdrawing (F, Cl) substituents does not
significantly affect the yield or enantioselectivity. In the absence
of ortho-substitution, the iodination reaction proceeds to give a
mixture of mono- and di-iodinated products 2g−2k in 54−71%
yields. Further ligand development will be required to suppress
the di-iodination. The enantiomeric purity of the di-iodinated
product 2gdi is higher than that of the mono-iodinated product
2gmono (99% ee vs 87% ee), presumably due to a chiral
Horeau’s amplification during the second iodination.16 The
origin of low enantiomeric purity of 2k (77% ee) remains to be
understood at this stage. We were pleased to find that, under
these conditions, di-(2-thiophenyl)methylamine 1l can be
selectively mono-iodinated to provide 2l in 51% yield and
99% ee.
Figure 2. Proposed intermediate and X-ray structure of 2a (R).
determined by X-ray) is also consistent with the structure of the
proposed reactive intermediate 4. We hope that further
investigation into this coordination mode and the remarkable
effects of the metal counterions will shed light onto
enantioselective C−H activation reactions.
In summary, we have developed the first example of an
enantioselective C−H iodination reaction, which provides a
useful method for the preparation of chiral diarylmethylamines.
The newly introduced iodides should be amenable to a variety
of transformations leading to diarylmethylamine analogues. The
use of a readily available mono-N-protected amino acid ligand
in conjunction with the practical halogenating reagent I2 should
render this reaction practical for large-scale asymmetric
iodination. Mechanistically, the ambient temperature and
absence of strongly electron-donating ligands in this C−H
This new enantioselective iodination reaction was also
performed on gram-scale using substrate 1a to provide 1.0 g
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Journal of the American Chemical Society
Communication
Cogan, D. A.; Ellman, J. A. J. Am. Chem. Soc. 1997, 119, 9913. For its
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iodination reaction begs further investigation whether Pd(IV)
intermediate is involved in the functionalization step.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectral data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yu200@scripps.edu
■
(6) (a) Hou, G.; Tao, R.; Sun, Y.; Zhang, X.; Gosselin, F. J. Am.
Chem. Soc. 2010, 132, 2124. (b) Nguyen, T. B.; Wang, Q.; Guer
Chem.Eur. J. 2011, 17, 9576. (c) Amezquita-Valencia, M.; Cabrera,
A. J. Mol. Catal. A: Chem. 2013, 366, 17.
́
itte, F.
́
Notes
(7) A recent review on enantioselective C−H activation: Giri, R.; Shi,
B. F.; Engle, K. M.; Maugel, N.; Yu, J.-Q. Chem. Soc. Rev. 2009, 38,
3242.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge The Scripps Research Institute and
the U.S. NSF (CHE-1011898) for financial support.
■
(8) Enantioselective C−H activation using Pd(II)/MPAA catalysts:
(a) Shi, B. F.; Maugel, N.; Zhang, Y. H.; Yu, J.-Q. Angew. Chem., Int.
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H.; Yu, J.-Q. J. Am. Chem. Soc. 2010, 132, 460. (c) Wasa, M.; Engle, K.
M.; Lin, D. W.; Yoo, E. J.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133,
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Chem. Soc. 2012, 134, 1690. (e) Gao, D.-W.; Shi, Y.-C.; Gu, Q.; Zhao,
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Y.; Su, Y.-M.; Yin, F.; Wang, J.-Y.; Sheng, J.; Vora, H. U.; Wang, X.-S.;
Yu, J.-Q. J. Am. Chem. Soc. 2013, 135, 1236.
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