Synthesis of Chiral Fluorinated Propargylamines via Chemoselective Biomimetic Hydrogenation

Article abstract of DOI:10.1021/acs.orglett.6b02283

A highly enantioselective synthesis of chiral fluorinated propargylamines was developed through phosphoric acid and ruthenium-catalyzed chemoselective biomimetic hydrogenation of the carbon-nitrogen double bond of fluorinated alkynyl ketimines in the presence of a carbon-carbon triple bond. This reaction features high chemoselectivity and slow background reaction. In addition, selective transformations of the chiral fluorinated propargylamines were also reported.

Full text of DOI:10.1021/acs.orglett.6b02283

Letter
pubs.acs.org/OrgLett
Synthesis of Chiral Fluorinated Propargylamines via Chemoselective
Biomimetic Hydrogenation
Mu-Wang Chen,† Bo Wu,† Zhang-Pei Chen,† Lei Shi,† 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
ABSTRACT: A highly enantioselective synthesis of chiral
fluorinated propargylamines was developed through phosphoric
acid and ruthenium-catalyzed chemoselective biomimetic hydro-
genation of the carbon−nitrogen double bond of fluorinated
alkynyl ketimines in the presence of a carbon−carbon triple bond.
This reaction features high chemoselectivity and slow background
reaction. In addition, selective transformations of the chiral fluorinated propargylamines were also reported.
hiral propargylamines are valuable and important synthetic
and complete reduction of the triple bond (Scheme 1, eq 1).
Cintermediates for the synthesis of biologically active You’s group reported an asymmetric transfer hydrogenation of
compounds and natural products.1 As a result, continuous
efforts have been devoted to explore the facile and efficient
methodologies for their construction.2 Among these syntheti-
cally and biologically important compounds, fluorinated
propargylamines occupy a particularly significant position and
are expected to be a class of substantial building blocks for further
derivatizations and pratical utilities, due to the fact that
introduction of fluorine atoms into organic molecules can greatly
modify their physical, chemical, and biological properties.3
Despite their impressive significance, synthesis of fluorinated
propargylamines is still under-explored to date, and existing
strategies are typically concentrated in the asymmetric
alkynylation of fluorinated imines. For instance, in 2004, the
Zanda and Qing groups reported the synthesis of fluorinated
propargylamines through an acetylide addition to trifluorometh-
yl-substituted ketimines using an N-sulfinyl moiety as the chiral
auxiliary.4 Later, the Zn-mediated5a−e and Rh-catalyzed5f
enantioselective alkynylation of fluorinated imines were also
developed. Therefore, a convenient and straightforward
procedure for their preparation in optically pure form is highly
demanded.
Scheme 1. Synthesis of Chiral Fluorinated Propargylamines
through Chemoselective Reduction
Asymmetric (transfer) hydrogenation of the alkynyl-sub-
stituted fluorinated ketimines is a potentially efficient method for
the synthesis of chiral fluorinated propargylamines because of
simple operation and high atom-economy. Although asymmetric
hydrogenation,6a−d transfer hydrogenation,6e−g and hydro-
silylation6h of simple fluorinated ketimines have been success-
fully achieved, hydrogenation of alkynyl-substituted fluorinated
ketimines for the synthesis of chiral fluorinated propargylamines
is still a great challenge. These difficulties can be attributed to the
following factors. First, chemoselectivity for asymmetric hydro-
genation of the alkynyl ketimines is difficult to control for the
multiple possible reactive sites (CN and CC bonds),
including reduction of imine, partial reduction of the triple bond,
β,γ-alkynyl-α-imino esters to afford trans-alkenyl-α-imino esters,
which involved both CN reduction and partial reduction of the
CC bond (eq 2).7a,b Second, the C−F bond is easily cleaved in
transition-metal-catalyzed systems.7c Herein, we report an
enantioselective synthesis of fluorinated propargylamines by
highly chemoselective biomimetic hydrogenation of the CN
Received: August 1, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02283
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Letter
bond of fluorinated alkynyl ketimines in the presence of a CC
bond with up to 98% ee (eq 3).
To probe the feasibility of selective asymmetric reduction
synthesis of fluorinated propargylamines, 4-methoxy-N-(1,1,1-
trifluoro-4-phenylbut-3-yn-2-ylidene)aniline (1a) was chosen as
the model substrate (Table 1). First, we focused our attention on
may cause a background reaction and the reduction of the CC
bond in the presence of a transition metal catalyst.
With this consideration in mind, we further optimized the
reaction conditions. Gratifyingly, 2a was obtained with 59%
conversion and 66% ee in the presence of (R)-CPA 3a, [Ru(p-
cymene)I2]2, and PD (phenanthridine) under hydrogen gas at
room temperature. Notably, 1a was inactive without the addition
of PD. Subsequently, different solvents were examined, and it was
found that solvents played a crucial role in reactivity and
enantioselectivity; 1,2-dichloroethane was the best choice (Table
2, entries 1−4). Then, some commercially available chiral
Table 1. Selective Asymmetric Reduction
a
Table 2. Evaluation of Reaction Parameters
catalyst
hydrogen source
conv (%)
<5
ee (%)
Pd(OCOCF3)/(R)-BINAP
[Ir(COD)Cl]2/(R)-BINAP
CPA 3a
H2
H2
HEH
<5
6
CPA 3a
benzothiazoline
DHPD
31
71
CPA 3a
52
the most predominant approach for the hydrogenation of imine
catalyzed by transition metal. Unfortunately, no desired product
was observed with the combination of Pd(OCOCF3)2/(R)-
BINAP. Meanwhile, in the presence of [Ir(COD)Cl]2/(R)-
BINAP, most of the starting material remained and a small amout
of mixed reducted products was observed. Considering the
excellent substrate tolerance and selectivity of chiral phosphoric
acid catalyzed asymmetric transfer hydrogenation, we turned our
attention to organocatalytic asymmetric transfer hydrogenation.
Subsequently, a series of asymmetric transfer hydrogenation
systems were tested. For instance, under a chiral phosphoric acid
(CPA)/Hantzsch ester system originally developed by Rueping,
List, and MacMillan,8 no conversion was observed. Because the
hydride transfer mode (1,4 and 1,2) and hydrogen transfer ability
exerts a significant influence on the reactivity and enantiose-
lectivity,9 we turn to the 1,2-hydrogen sources, such as
benzothiazolines, which were developed by Akiyama and serve
as a versatile hydrogen source for organocatalytic transfer
hydrogenation and have been widely used for asymmetric
reduction of imines.10 Moderate 31% ee of enantioselectivity was
obtained using CPA/benzothiazoline for alkynyl ketimine
reduction, albeit with low conversion (6%). Additionally, another
simple and commercially available 1,2-hydride transfer hydrogen
source was examined. Moderate reactivity (52% conversion) and
enantioselectivity (71% ee) could be obtained using the CPA/
dihydrophenanthridine (DHPD) system in which DHPD acts as
a good biomimetic 1,2-hydrogen source.
Biomimetic hydrogenation was successfully used for the
preparation of chiral compounds due to the mild conditions, a
wide range of substrates, and high reactivity and enantioselectiv-
ities. Very recently, our group9d,11 developed several models of in
situ regeneration of nicotinamide adenine dinucleotide phos-
phate (NAD(P)H),12 which was applied to the asymmetric
biomimetic hydrogenation of a series of cyclic imines and
heteroaromatics. Later, Beller and co-workers13 developed an
elegant iron-catalyzed hydrogenation of benzoxazinones with
our NAD(P)H model/DHPD. The 1,2-hydride transfer hydro-
gen source of DHPD could be regenerated in situ under mild
conditions with a unique performance.11c Therefore, we assumed
that in situ regeneration of DHPD could be employed for the
asymmetric synthesis of 2a due to cost and availability, but this
a
b
c
entry
solvent
CH2Cl2
CPA
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
3a
3a
3a
3a
3b
3c
3d
3e
3f
59
4
66
77
74
79
91
93
84
93
94
94
95
THF
toluene
28
80
79
79
88
83
84
98
>95
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
d
10
3f
de
,
11
3f
a
Conditions: 1a (0.1 mmol), CPA 3 (2 mol %), solvent (2.0 mL),
phenanthridine (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), H2 (500
psi), rt, 48 h. Conversion was determined by HNMR. Determined
b
c
1
d
e
by chiral HPLC analysis. H2 (1000 psi). 8-Methylphenanthridine.
phosphoric acids were evaluated, and the best enantioselectivity
was obtained with 3f (entries 4−9). The conversion was further
improved, and excellent enantioselectivity was retained with
increased hydrogen pressure (entry 10). Fortunately, excellent
conversion and enantioselectivity could be obtained when
phenanthridine was replaced by 8-methylphenanthridine (entry
11). Thus, the optimized condition was established: [Ru(p-
cymene)I2]2/(R)-3f/ClCH2CH2Cl/H2 (1000 psi)/8-methyl-
phenanthridine/RT.
With the optimized conditions in hand, exploration of
substrate scope was carried out (Scheme 2). Various substrates
performed very well under the standard reaction conditions. It
was noteworthy that the electronic properties and position of
substituents on the aromatic ring (R) had a marginal effect on
reactivity and enantioselectivity (2a−e). Subsequently, sub-
strates with different alkyl and alkenyl at R were tested, and high
yields (98%) and excellent enantioselectivities (94−95% ee)
could be obtained (2f,g). Impressively, the diyne-substituted
fluorinated ketimine gave 95% yield and 94% enantioselectivity
(2h), which is a useful building block for the synthesis of
agrochemicals and compounds with potential biological activity.
Furthermore, various kinds of N-aryl-substituted substrates were
also examined; both electron-donating and electron-withdrawing
B
DOI: 10.1021/acs.orglett.6b02283
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Letter
Scheme 2. Biomimetic Hydrogenation of Fluorinated Imines
1
Scheme 3. Gram Scale and Derivatization of (R)-2a
a
promoted by Ru(II)/CPA relay catalysis (Scheme 4). First, the
DHPD is generated from PD by Ru-catalyzed hydrogenation.
Scheme 4. Proposed Mechanism of Biomimetric Asymmetric
Hydrogenation Process
a
Conditions: Alkynyl-substituted fluorinated ketimines 1a−o (0.2
mmol), CPA 3f (2 mol %), ClCH2CH2Cl (2.0 mL), 8-methylphenan-
thridine (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), H2 (1000 psi),
rt, 48 h. Isolated yields. The ee values were determined by chiral
b
HPLC analysis. Toluene was used instead of ClCH2CH2Cl.
groups as well as meta-substituents did not inhibit the reactions,
and the corresponding products were obtained in 88−96% yields
and 92−95% ee. In addition, different perfluoroalkyl-substituted
imines 1n and 1o were also tested, with 98 and 97% ee obtained,
respectively. Meanwhile, phenylacetylenyl-substituted α-imino
ester 1p was tested for the reaction with moderate reactivity and
enantioselectivity, but the carbon−carbon triple bond also
remains, which is different from result reported by You’s group.7a
After establishing the facile approach for the synthesis of
fluorinated propargylamines by biomimetic asymmetric hydro-
genation, we further evaluated the practical utility (Scheme 3).
The biomimetic asymmetric hydrogenation of 1a was carried out
on gram scale, and the desired product was furnished with 94% ee
and 99% yield. Meanwhile, the remaining CC bond is an
attractive functional group for further modifications.14 For
example, reduction of 2a in Pd/CaCO3/H2 and LiAlH4 systems
provided (Z)-alkene 4 and (E)-alkene 5 in 92 and 88% yield,
respectively. Chiral amine 6 was obtained in 92% yield by
complete reduction of 2a with Pd/C. Significantly, optical purity
of these reduction products still remained. The absolute
configuration was determined as R by comparison with the
sign of optical rotation of 6 reported in the literature.6c In
addition, the p-methoxyphenyl group of hydrogenated products
can be readily removed through oxidative cleavage of the
hydrogenated product 2a, giving the chiral primary amine (R)-7.
Concerning the mechanism, the chemoselective biomimetric
asymmetric hydrogenation comprises two cascade redox cycles
Second, the selective asymmetric transfer hydrogenation of 1 by
DHPD gave the product in the presence of chiral Brønsted acid.
The excellent enantioselectivity achieved in this biomimetic
chemoselective asymmetric hydrogenation was attributed to the
very slow background reaction and high chiral induction of
organocatalyst 3.
In summary, we have successfully developed a novel and
efficient method for enantioselective synthesis of fluorinated
propargylamines by chemoselective biomimetic hydrogenation
with up to 98% ee. The DHPD can be generated from PD under
mild conditions. Keys to success include the CPA/DHPD
selective hydrogenation of the CN bond in the presence of a
CC bond and very slow background reactions. In addition, a
series of transformations of the chiral fluorinated propargyl-
amines have been developed. Further investigations on the
applications of this method are currently ongoing in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02283.
C
DOI: 10.1021/acs.orglett.6b02283
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Organic Letters
Experimental procedures, characterization data, and NMR
Letter
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spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ygzhou@dicp.ac.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science Foundation
of China (21502188, 21532006) and Dalian Institute of
Chemical Physics (DMTO201501) is acknowledged.
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