Dual C-H functionalization of N-aryl amines: Synthesis of polycyclic amines via an oxidative povarov approach

Article abstract of DOI:10.1021/ol501073f

Iminium ions generated in situ via copper(I) bromide catalyzed oxidation of N-aryl amines readily undergo [4 + 2] cycloadditions with a range of dienophiles. This method involves the functionalization of both a C(sp ³)-H and a C(sp²)-H bond and enables the rapid construction of polycyclic amines under relatively mild conditions.

Full text of DOI:10.1021/ol501073f

Letter
pubs.acs.org/OrgLett
Dual C−H Functionalization of N‑Aryl Amines: Synthesis of Polycyclic
Amines via an Oxidative Povarov Approach
Chang Min, Abbas Sanchawala, and Daniel Seidel*
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United
States
S
* Supporting Information
ABSTRACT: Iminium ions generated in situ via copper(I) bromide
catalyzed oxidation of N-aryl amines readily undergo [4 + 2]
cycloadditions with a range of dienophiles. This method involves the
functionalization of both a C(sp3)−H and a C(sp2)−H bond and enables
the rapid construction of polycyclic amines under relatively mild
conditions.
irst reported in 1963,1 the Povarov reaction comprises a
formal [4 + 2] cycloaddition of an electron-rich dienophile
additional steps. These methods include intramolecular redox
transformations,5 oxidative couplings,6 C−N bond formation
via Pd-catalysis7 or benzyne intermediates,8 and Bischler−
Napieralski reactions followed by reduction.9 As part of our
continuing efforts to develop practical methods for the C−H
functionalization of amines,10 herein we report an alternate
Povarov approach to polycyclic tetrahydroquinolines 6 that
utilizes the in situ oxidation of readily available N-aryl amines 8.
The oxidative C−H functionalization of amines has a
venerable history and was greatly popularized by the pioneering
studies of the Murahashi11 and Li12 groups, who advanced the
applicability of catalytic approaches. Oxidative reactions in
which a C−H bond is replaced with a functional group are now
widely referred to as cross-dehydrogenative coupling reactions
(CDC reactions).13,14 A typical CDC reaction involves the
oxidation of a tertiary amine such as 8a to an iminium ion (e.g.,
9), followed by capture of 9 with a nucleophilic species (NuH)
to form product 10 (Scheme 2).15,16 The vast majority of these
F
and a 2-azadiene, typically an N-aryl imine or iminium ion.2
This transformation generally requires a Lewis or Brønsted acid
catalyst or promoter and provides an efficient route to
tetrahydroquinolines (Scheme 1).3 Povarov reactions are
Scheme 1. Variants of the Povarov Reaction
Scheme 2. Examples of Oxidative Amine C−H
Functionalizations
commonly performed as two-component transformations in
which a preformed imine 1 engages an electron-rich dienophile
2 to form tetrahydroquinoline product 3. Three-component
variants are also popular and involve the condensation of an
amine 4 with an aldehyde (or a ketone) and a dienophile 2.2
While the two-component method is limited to imines derived
from primary amines, the three-component approach is also
applicable to secondary N-aryl amines, including cyclic amines.4
The latter enables the formation of polycyclic products 5 in
which two rings are fused to the same aryl group. In contrast,
the classic Povarov reaction is not readily applicable to the
synthesis of polycyclic amines of type 6. A typical two-
component Povarov approach to these compounds would
require an aminoaldehyde 7, species that are not readily
accessible. Other approaches to polycyclic frameworks related
to 6 typically require ortho-functionalized N-aryl amines and/or
Received: April 13, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol501073f | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
transformations employ N-aryl tetrahydroisoquinolines and
lead to the monofunctionalization of the benzylic α-position of
these substrates. Few transformations of N-aryl amines have
been reported that, in addition to α-functionalization,
simultaneously result in the functionalization of an ortho aryl
C−H bond. An example of such a process is the oxidation of 11
to radical 12 which subsequently engages an electron-deficient
olefin to give product 14 via the oxidation of radical
intermediate 13.17 With regard to the proposed oxidative
Povarov reaction, to our knowledge such a process has only
been realized with N,N-dimethylanilines (e.g., 11),18 giving
relatively simple N-alkyl tetrahydroquinoline derivatives 16 (via
iminium ion 15), products that are also accessible via classic
Povarov reactions that utilize N-methyl aniline as the
substrate.2,4,19
Scheme 3. Scope of the Reaction
The title reaction was first evaluated using N-phenyl 1,2,3,4-
tetrahydroisoquinoline (8a) and 1-vinylpyrrolidin-2-one (17a)
as model substrates (Table 1). A preliminary survey of various
a
Table 1. Reaction Development
a
b
c
entry
catalyst
CuCl
solvent
time [h]
dr
yield (%)
1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
24
24
24
10
24
24
24
24
24
32
32
32
24
24
10:1
8.7:1
10:1
12:1
7:1
60
65
50
27
49
26
49
81
59
49
60
20
13
2
CuBr
CuBr2
CuCl2
CuI
a
3
Reactions were performed with 0.5 mmol of the amine, 0.55 mmol of
the dienophile, 0.05 mmol of CuBr, and 0.6 mmol of tBuOOH (5.5 M
in decane) in 2.5 mL of C2H4Cl2. All yields are combined isolated
yields of both diastereomers.
4
5
6
Cu(OTf)2
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
1:1
7
5:1
8
C2H4Cl2
dioxane
MeOH
CHCl3
PhMe
5.8:1
5.2:1
4.6:1
3:1
tetrahydroisoquinolines with different substituents on the N-
aryl ring readily underwent cycloaddition with 1-vinyl-
pyrrolidin-2-one (17a) to produce the corresponding products
18 in moderate to good yields. However, ortho-substituents on
the N-aryl ring proved problematic, presumably due to
developing A1,3-type strain in the transition state of this
reaction. As a consequence, product 18e was isolated in only a
26% yield. Although low-yielding, N-phenyl tetrahydroazepine
also engaged in a reaction with 17a to form polycyclic product
18g. To our knowledge, this represents the first example of a
tetrahydroazepine derivative undergoing a CDC-type reaction.
In order to explore the generality of the method, two acyclic
aniline derivatives were tested. N,N-Dimethylaniline, upon
reacting with 17a, provided the expected product 18h in 43%
yield. An N-phenylglycine ester was also found to be a suitable
substrate, giving cyclic amino acid derivative 18i, in 61% yield.
This illustrates the potential utility of this method in the direct
C−H functionalization of peptide derivatives.21 Finally, the
scope of the oxidative Povarov reaction was explored with
regard to the dienophile. Various acyclic and cyclic enol ethers
and enamides readily underwent the title reaction to provide
polycyclic products in moderate to good yields. The
diastereoselectivity for some of these reactions was rather poor.
An attempt to extend the scope of the oxidative Povarov
reaction to N-phenyl pyrrolidine (19) as the amine initially
only led to the formation of trace amounts of the expected
product 20 (Scheme 4). Instead, oxidation of 19 in the
presence of 17a resulted in the formation of 21 as a 1.3:1
mixture of diastereomers in 33% yield. The yield of 21
9
10
11
12
13
2.7:1
ND
6:1
−
d
e
14
C2H4Cl2
80 (62 )
a
Reactions were performed with 0.2 mmol of 8a, 0.4 mmol of 17a,
0.02 mmol of catalyst, and 0.24 mmol of tBuOOH (5.5 M in decane)
b
in 1 mL of solvent. The dr was determined by 1H NMR of the crude
c
1
reaction mixture. Yield was determined by H NMR with an internal
standard. 1.1 equiv of 17a was used. Isolated yield.
d
e
methods for amine oxidation (including aerobic conditions) led
to the identification of tert-butyl hydroperoxide (TBHP) as the
most promising terminal oxidant.20 Out of a number of copper
salts that were tested as catalysts in reactions performed in
acetonitrile solution, copper(I) bromide provided the best
results (entry 2). With regard to product yield, 1,2-dichloro-
ethane was found to be superior to acetonitrile and other
solvents such as tetrahydrofuran, dioxane, methanol, chloro-
form, and toluene. A reaction performed under neat conditions
resulted in a dramatically reduced yield of 18a (entry 13).
Decreasing the amount of 17a from two to 1.1 equiv had no
adverse effects on the reaction outcome (entry 14). Under
these optimized conditions, 18a was obtained in 62% isolated
yield and with a dr of 6:1 in favor of the endo-product.
The scope of the oxidative Povarov reaction was evaluated on
a set of amine/dienophile combinations (Scheme 3). A range of
B
dx.doi.org/10.1021/ol501073f | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Benohoud, M.; Dagousset, G. Chem. Soc. Rev. 2013, 42, 902. (e) Jiang,
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Scheme 4. Oxidative Functionalization of N-Phenyl
Pyrrolidine
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increased to 40% when the reaction was conducted in the
absence of 17a. This substrate dimerization (i.e., via 22) is
easily rationalized, as the iminium ion resulting from the
oxidation of 19 is expected to exist in equilibrium with its
corresponding enamine.22 As was observed previously with
cyclic enecarbamates, the diastereoselectivity of this process was
found to be low.
In order to obtain the desired product 20, a number of other
conditions were evaluated. Gratifyingly, 20 was obtained, albeit
in only 19% yield, in a reaction that was conducted in
methanol, using copper(II) chloride dihydrate as the catalyst
and air as the terminal oxidant. In this instance, dimerization
product 21 was obtained in less than 10% yield. This change in
product distribution may be rationalized on the basis that the
intermediate iminium ion can be captured by methanol to form
the corresponding N,O-acetal which in turn could act as a
reservoir for the iminium ion, thus reducing the concentration
of the N-phenyl pyrrolidine enamine.
In summary, we have reported oxidative Povarov reactions of
various N-aryl amines as a method to rapidly access polycyclic
amines. These reactions feature the dual functionalization of
both a C(sp3)−H and a C(sp2)−H bond and are likely
amenable to enantioselective catalysis.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data, including
an X-ray crystal structure of product 18k (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(9) Ohba, M.; Shinbo, Y.; Toda, M.; Fujii, T. Chem. Pharm. Bull.
1992, 40, 2543.
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2008, 130, 416. (b) Zhang, C.; Murarka, S.; Seidel, D. J. Org. Chem.
2009, 74, 419. (c) Murarka, S.; Zhang, C.; Konieczynska, M. D.;
Seidel, D. Org. Lett. 2009, 11, 129. (d) Murarka, S.; Deb, I.; Zhang, C.;
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Soc. 2010, 132, 1798. (g) Zhang, C.; Das, D.; Seidel, D. Chem. Sci.
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Chem. Soc. 2011, 133, 2100. (i) Deb, I.; Das, D.; Seidel, D. Org. Lett.
2011, 13, 812. (j) Deb, I.; Coiro, D. J.; Seidel, D. Chem. Commun.
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Lett. 2011, 13, 6584. (l) Zhang, C.; De, C. K.; Seidel, D. Org. Synth.
2012, 89, 274. (m) Ma, L.; Chen, W.; Seidel, D. J. Am. Chem. Soc.
2012, 134, 15305. (n) Das, D.; Sun, A. X.; Seidel, D. Angew. Chem., Int.
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Y.; Zhang, C.; Seidel, D.; Houk, K. N. J. Org. Chem. 2013, 78, 4132.
(p) Richers, M. T.; Deb, I.; Platonova, A. Y.; Zhang, C.; Seidel, D.
Synthesis 2013, 45, 1730. (q) Richers, M. T.; Zhao, C. F.; Seidel, D.
Beilstein J. Org. Chem. 2013, 9, 1194. (r) Das, D.; Seidel, D. Org. Lett.
2013, 15, 4358. (s) Chen, W.; Wilde, R. G.; Seidel, D. Org. Lett. 2014,
16, 730. (t) Seidel, D. Org. Chem. Front. 2014, 1, DOI: 10.1039/
C4QO00022F. (u) Chen, W.; Kang, Y.; Wilde, R. G.; Seidel, D. Angew.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: seidel@rutchem.rutgers.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NIH−NIGMS (Grant
R01GM101389-01) is gratefully acknowledged. We thank Dr.
Tom Emge (Rutgers University) for crystallographic analysis.
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