Enantioselective Synthesis of 1,2-Diamines Containing Tertiary and Quaternary Centers through Rhodium-Catalyzed DYKAT of Racemic Allylic Trichloroacetimidates

Article abstract of DOI:10.1021/acs.orglett.7b02256

The amination of racemic secondary and tertiary allylic trichloroacetimidates possessing β-nitrogen substituents and proximal nitrogen-containing heterocycles, via chiral diene-ligated rhodium-catalyzed dynamic kinetic asymmetric transformations (DYKAT),

Full text of DOI:10.1021/acs.orglett.7b02256

Letter
pubs.acs.org/OrgLett
Enantioselective Synthesis of 1,2-Diamines Containing Tertiary and
Quaternary Centers through Rhodium-Catalyzed DYKAT of Racemic
Allylic Trichloroacetimidates
Edward T. Mwenda and Hien M. Nguyen*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
S
* Supporting Information
ABSTRACT: The amination of racemic secondary and tertiary allylic
trichloroacetimidates possessing β-nitrogen substituents and proximal
nitrogen-containing heterocycles, via chiral diene-ligated rhodium-
catalyzed dynamic kinetic asymmetric transformations (DYKAT),
provides branched allylic 1,2-diamines with high enantioselectivity. The
catalytic system can be applied to the synthesis of 1,2-diamines
possessing two contiguous stereocenters with excellent diastereoselec-
tivity. Furthermore, the nitrogen-containing heterocycles suppress
competing vinyl azirdine formation, allowing for the high enantiose-
lective syntheses of 1,2-diamines possessing tertiary and quaternary
centers.
he 1,2-diamines are found in a wide range of bioactive
natural products and pharmaceuticals.1 In addition, their
the current limitations associated with the synthesis of 1,2-
diamines possessing tertiary and quaternary centers. Herein, we
report an enantioselective synthesis of 1,2-diamines 2 (Figure 1)
via rhodium-catalyzed dynamic kinetic asymmetric transforma-
tions (DYKAT) of racemic allylic trichloroacetimidates 1 with
anilines.
T
enantiopure motifs are utilized as chiral ligands, auxiliaries, and
catalysts for enantioselective transformations.1 Because of the
privileged nature of the 1,2-diamines, significant efforts have been
focused on the development of asymmetric methods for their
synthesis.2 To date, enantioselective syntheses of 1,2-diamines
(2, Figure 1) bearing a tertiary or quaternary center and an
Previous developments in our group have provided methods
for the facile catalytic asymmetric amination of racemic allylic
trichloroacetimidates using chiral diene-ligated rhodium cata-
lysts.8 Following our work and related studies in the area of
transition-metal-catalyzed enantioselective amination of allylic
electrophiles,9−12 we recognize that allylic substrates, bearing β-
nitrogen substituents and β-nitrogen-containing heterocycles,
could undergo amination to generate 1,2-diamines. However,
due to an amino group proximal to the electrophilic site of π-
allylrhodium complexes 3 and 4 (Figure 1), competing vinyl
aziridine intermediates 5 and 6 could be formed, resulting in an
erosion of the enantioselectivity of the product 2.13 We
hypothesize that it may be possible to address this shortcoming
through the judicious choice of protecting and functional groups,
that could decrease nitrogen nucleophilicity and, consequently,
suppress formation of vinyl aziridine. Overcoming this foreseen
challenge would allow access to allylic 1,2-diamine 2 via rhodium-
catalyzed DYKAT reactions.14−16
Figure 1. A proposed approach to the enantioselective synthesis of 1,2-
diamines 2 via Rh-catalyzed DYKAT.
unsubstituted β-aminomethyl group remain underveloped.3,4 For
instance, the asymmetric synthesis of quaternary-containing 1,2-
diamines3 could be achieved through the addition of azomethine
ylides to chiral N-sulfinylketimines.5 However, several factors
complicate this method. First, ketimines possessing an α-
hydrogen are prone to enolize.5b,6 Second, ketimines can exist
as a mixture of E- and Z-isomers, and thus differentiation of their
enantiotopic faces can be challenging.7 Third, ketimines often
require activation by Lewis or Bronsted acids because they are
much less reactive than corresponding aldimines.3,5b Thus, there
is a need to develop catalytic asymmetric methods that overcome
To validate vinyl aziridine formation, we selected benzoyl
protected β-nitrogen substituted allylic trichloroacetimidate 7
(Scheme 1) in our studies. Treatment of 7 with 5 mol %
norbornadiene rhodium chloride dimer, [RhCl(NBD)]2, in the
absence of aniline nucleophile yielded exclusively vinyl aziridine 8
(Scheme 1a) in 69% yield. Reaction of 7 with p-methoxyaniline
Received: July 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02256
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Organic Letters
Letter
MTBE as a solvent and a slightly higher temperature (40 °C)
further improved the yield and enantioselectivity (95%, 90% ee)
of 14 (entry 5). Furthermore, lowering the catalyst loading to 2
mol % (entry 12) caused no erosion in yield or selectivity. The
application to a larger scale (1.4 mmol of 13) was also explored
(entry 13), and 1,2-diamine 14 was obtained in similar yield and
selectivity (99%, 91% ee).
Scheme 1. Studies with β-Nitrogen Substituted Imidates
We next sought to examine the substrate scope of the reaction
(Table 2). Allylic 1,2-diamines 23−29 were obtained in moderate
Table 2. Scope of β-Nitrogen and β-Azido Substituted 1,2-
Diamines Bearing Tertiary and Quaternary Centers
a
a
Table 1. Optimization of the Reaction Conditions
c
d
[Rh]
temp
yield
(%)
ee
entry
1
mol %
ligand
solvent
dioxane
(°C)
(%)
5
5
5
5
5
5
5
5
5
5
5
10 mol %
L1
25
40
40
40
40
40
40
40
40
40
40
70
99
99
91
95
91
99
99
43
74
99
97
71
2
10 mol %
L1
dioxane
THF
88
82
89
90
89
79
84
77
67
89
90
3
10 mol %
L1
4
10 mol %
L1
CPME
MTBE
toluene
PhCF3
5
10 mol %
L1
6
10 mol %
L1
7
10 mol %
L1
8
10 mol %
L1
CH2Cl2
cyclohexane
MTBE
MTBE
9
10 mol %
L1
10
11
10 mol %
L2
10 mol %
L3
12
13
2
2
4 mol % L1 MTBE
4 mol % L1 MTBE
40
40
b
b
99
91
a
All reactions were conducted at 0.1 M using 0.15−0.3 mmol of allylic
b
imidate 13 (1 equiv) and p-methoxylaniline 10 (1.5 equiv). Reaction
c
was carried out on a 1.4 mmol scale of 13. Isolated yield.
d
Determined by chiral HPLC.
a
All reactions were conducted at 0.1 M using 0.15−0.3 mmol of allylic
b
imidates (1 equiv) and anilines (1.5 equiv). Isolated yield.
(10, Scheme 1b) was then tested under rhodium-catalyzed
DYKAT conditions (vide infra, Table 1). The 1,2-diamine 11 was
isolated with 21% ee. We next explored tosyl-protected (Ts)
substrate 9 (Scheme 1c) since the sulfonamide group attenuates
nitrogen nucleophilicity.17 As expected, allylic amine 12 (Scheme
1c) was isolated with improved ee (49%). Collectively, these
results support that competing vinyl aziridine plays a role in
eroding the enantioselectivity of the product.
To overcome the above shortcomings, we predicted that the
use of β-phthalimide-substituted allylic trichloroacetimidate 13
(Table 1) could hinder vinyl aziridine formation. We commenced
our studies with reaction of 13 and aniline 10 in 5 mol %
[RhCl(ethylene)2]2 and 10 mol % L1 at 25 °C for 1 h (entry 1).
The desired 1,2-diamine 14 was obtained in 70% yield and 71%
ee. After examining each reaction parameter (temperature,
solvent, and ligand, entries 2−11), it was determined that use of
c
Determined by chiral HPLC.
to good yields (57−93%) with high enantioselectivities (81−93%
ee) when using substrates with fully protected β-nitrogen (entries
1−7). Although the catalytic system tolerates both electron-
donating and electron-withdrawing anilines, the use of the more
electron-rich aniline nucleophile 20 further improved the
enantioselectivity of the product (entry 5 vs entry 6). We
hypothesize that the electron-rich aniline accelerates the rate of
nucleophilic attack onto the π-allylrhodium complex (Figure
1),15 consequently suppressing vinyl aziridine formation. The β-
azido group is also well-tolerated, providing allylic 1,2-diamines
30 and 31 (entries 8 and 9) in high yields (88% and 99%) and
enantioselectivies (88% ee and 92% ee).
B
DOI: 10.1021/acs.orglett.7b02256
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Letter
We next explored the synthesis of allylic 1,2-diamines bearing
sterically challenging quaternary centers using 18 as the model
compound (entry 10). Our previous results showed that only
tertiary allylic imidate substrates bearing β-hydroxy substituents
provided high selectivity.8a We questioned whether the
sulfonamide could serve as a directing group17 to provide
additional chelation control, decreasing conformational flexibility
of the π-allylrhodium intermediates and facilitating the trans-
mission of asymmetric induction from the catalyst to the
substrate.18 Unfortunately, quaternary-containing 1,2-diamine
32 (entry 10) was formed in only 57% ee, but with excellent yield
(99%). We hypothesize that the modest enantioselectivity is
likely due to the high energy barrier of π-allylrhodium
intermediate isomerization (3 and 4, Figure 1).19
nitrogen-containing heterocycles.26 As illustrated in Table 3,
amination of allylic imidates 33−38 provided pyrrole, carbazole,
and indole-containing diamines 43−48 (entries 1−6) with high
enantioselectivities (86−94% ee). Use of triazole-containing
substrate 39 (entry 7) provided 49 with 66% ee. To our delight,
the method is also amendable to the asymmetric synthesis of
congested quaternary-containing 1,2-diamines 50−52 (entries
8−10) in high enantiomeric excess (85−91% ee). Overall, most
allylic 1,2-diamines were obtained in good yields. Because of the
decomposition of their starting materials, some products (entries
2−4 and 10) were obtained near 50% yield.
To determine whether the chiral diene-ligated rhodium
complex could influence the diasteresoelectivity of the amination,
we investigated L- and D-proline-derived substrates 53 and 54
(Scheme 2)27 which possess the ability to generate two
We further investigated the scope with nitrogen-heterocycle
containing substrates 33−42 (Table 3). We hypothesize that the
Scheme 2. Diastereoselective Amination of Both L- and D-
Proline Derived Trichloroacetimidate Substrates
Table 3. Scope of Nitrogen-Heterocycle Substituted Allylic
1,2-Diamines Bearing Tertiary and Quaternary Centers
a
contiguous stereogenic centers. In the case of L-proline imidate
53, use of achiral [RhCl(NBD)]2 provided 1,2-diamine 55
(Scheme 2a) with excellent diastereoselectivity (dr > 99:1), while
use of chiral diene ligated rhodium catalyst resulted in a 1:1
mixture of 55. In stark contrast, both achiral and chiral rhodium
catalysts were effective at promoting the amination of D-proline
imidate 54 to provide 56 (Scheme 2b) with excellent
diastereomeric ratio (dr > 1:99). The major diastereomer of 55
and 56 could be recrystallized allowing for X-ray crystallographic
determination of the allylic carbon to be in the R- and S-
configuration, respectively (see Supporting Information).
In summary, we have developed an efficient asymmetric
synthesis of 1,2-diamines bearing tertiary and quaternary centers
via rhodium-catalyzed DYKAT of racemic allylic trichloroaceti-
midates. This catalytic asymmetric system is tolerant of a variety
of substrates incorporated with a number of the fully protected β-
amino groups. We also discovered that nitrogen-containing
heterocycles can suppress competing vinyl aziridine formation
and enable the synthesis of 1,2-diamines possessing tertiary and
quaternary centers with high levels of asymmetric induction.
Furthermore, the method is amendable for the synthesis of 1,2-
diamines possessing two stereocenters with excellent diaster-
eoselectivity.
a−c
See Table 2.
nitrogen lone pairs of pyrrole, carbazole, and indole are involved
in the π-system of the aromatic ring and thus would not be
available to participate in vinyl aziridine formation. The
achievement of this goal would allow access to the important
class of bioactive targets incorporated with pyrrole, carbazole, and
indole motifs.20−23 In the area of transition-metal-catalyzed allylic
amination, methods for the constructions of the nitrogen-
heterocycle containing 1,2-diamines are limited to palladium-
catalyzed openings of vinyl aziridines with pyrroles24 and iridium-
catalyzed asymmetric allylic amination with indole25 and
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02256.
Experimental procedures and spectra data (PDF)
X-ray data for compound 55 (CIF)
X-ray crystallographic report for compound 55 (PDF)
X-ray data for compound 56 (CIF)
C
DOI: 10.1021/acs.orglett.7b02256
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
X-ray crystallographic report for compound 56 (PDF)
Letter
Commun. 2012, 48, 11531. (c) Arnold, J. S.; Nguyen, H. M. Synthesis
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Chem., Int. Ed. 2014, 53, 3688.
(9) For the most recent review on asymmetric allylic amination
reactions of allylic electrophiles, see: Grange, R. L.; Clizbe, E. A.; Evans,
P. A. Synthesis 2016, 48, 2911−2968.
(10) For selected examples of the transition-metal-catalyzed
enantioselective/enantiospecific amination of allylic electrophiles with
anilines, see: (a) Evans, P. A.; Robinson, J. E.; Moffett, K. K. Org. Lett.
2001, 3, 3269. (b) Takeuchi, R.; Ue, N.; Tanabe, Y.; Yamashita, K.; Shiga,
N. J. Am. Chem. Soc. 2001, 123, 9525. (c) Shu, C.; Leitner, A.; Hartwig, J.
F. Angew. Chem., Int. Ed. 2004, 43, 4797. (d) Satyanarayana, G.;
Pflasterer, D.; Helmchen, G. Eur. J. Org. Chem. 2011, 2011, 6877. (e) Cai,
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138, 14194.
(11) For selected examples of kinetic resolution of secondary allylic
electrophiles via transition metal-catalyzed amination reactions, see:
(a) Vrieze, D. C.; Hoge, G. S.; Hoerter, P. Z.; van Haitsma, J. T.; Samas,
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Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 8918. (c) Madrahimov, S. T.;
Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 8136.
(12) For selected examples of dynamic kinetic resolution of secondary
allylic electrophiles via transition metal-catalyzed amination, see:
(a) You, S. L.; Zhu, S. Z.; Luo, Y. M.; Hou, X. L.; Dai, L. X. J. Am.
Chem. Soc. 2001, 123, 7471. (b) Defieber, C.; Ariger, M. A.; Moriel, P.;
Carreira, E. M. Angew. Chem., Int. Ed. 2007, 46, 3139. (c) Roggen, M.;
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(13) Ibuka, T.; Mimura, N.; Aoyama, H.; Akaji, M.; Ohno, H.; Miwa, Y.;
Taga, T.; Nakai, K.; Tamamura, H.; Fujii, N.; Yamamoto, Y. J. Org. Chem.
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AUTHOR INFORMATION
■
Corresponding Author
* Email: hien-nguyen@uiowa.edu
ORCID
Hien M. Nguyen: 0000-0002-7626-8439
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by University of Iowa. We thank
Dr. Dale Swenson at University of Iowa for X-ray crystallographic
analysis.
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