Multifunctionalization of Unactivated Cyclic Ketones via Synergistic Catalysis of Copper and Diarylamine: Access to Cyclic α-Enaminone

Article abstract of DOI:10.1021/acs.orglett.8b00125

A multifunctionalization of unactivated cyclic ketones via synergistic catalysis of copper and diarylamine for the direct synthesis of cyclic α-enaminone is reported for the first time. This reaction goes through oxidative α-amination, followed by a desaturation, and features mild reaction conditions, a broad substrate scope, and great functional group tolerance.

Full text of DOI:10.1021/acs.orglett.8b00125

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Multifunctionalization of Unactivated Cyclic Ketones via Synergistic
Catalysis of Copper and Diarylamine: Access to Cyclic α‑Enaminone
Yang Li,† Ran Zhang,† Xihe Bi,*,†,‡ and Junkai Fu*,†,§
†Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis, Department of Chemistry, Northeast Normal
University, Changchun 130024, China
‡State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
§Key Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate
School, Shenzhen 518055, China
S
* Supporting Information
ABSTRACT: A multifunctionalization of unactivated cyclic
ketones via synergistic catalysis of copper and diarylamine for the
direct synthesis of cyclic α-enaminone is reported for the first time.
This reaction goes through oxidative α-amination, followed by a
desaturation, and features mild reaction conditions, a broad
substrate scope, and great functional group tolerance.
n the past few years, cooperative catalysis of combining
Itransition metals with organocatalysts has attracted increasing
attention. In this synthetic strategy, separate catalytic cycles are
simultaneously promoted by two different catalysts to achieve an
overall transformation.1 This concept has greatly pushed the
innovation in functionalization of unactivated cyclic ketones, and
a series of practical or enantioselective methods have emerged to
afford different types of functionalized ketones in strong base-
free or environmentally friendly pathways (Figure 1a). Dong and
co-workers successively utilized synergistic catalysis of Rh(I) and
bifunctional arylamine to realize the α-alkylation, α-alkenylation,
and α-arylation of cyclopentanone.2 α-Allylic alkylation of cyclic
ketones has been achieved by several groups employing
cooperative Pd/secondary amine catalysis.3 Later, Wang’s
group4 and Wu’s group5 designed elegant combinations of
metal Lewis acids and bifunctional amines to promote the
asymmetric aldol reaction of cyclic ketones. Recently, Jia and
Dixon independently developed highly enantioselective palla-
dium or silver/amine-co-catalyzed intramolecular desymmetriza-
tion of cyclohexanones.6 In addition, significant breakthroughs
have been achieved recently in β-functionalization of unactivated
cyclic ketones by the MacMillan’s group using cooperative
organocatalysis and photoredox catalysis through a β-enaminyl
radical intermediate.7 Despite these great achievements in
Figure 1. Different strategies for the functionalization of carbonyl
compounds.
monofunctionalization of unactivated cyclic ketones,8 direct
multifunctionalization via synergistic catalysis, which means the
Cu(I)-promoted electrophilic amination of ketene silyl acetals
with O-benzoylhydroxylamines to obtain α-amino esters.10a
Later, Wang and co-workers described α-amination of esters and
amides through the reaction of zinc enolates and hydroxyl-
amines, which was ineffective for α-substituted esters and
amides.10b However, the direct α-amination of carbonyl
introdution of several functional groups into simple cyclic
ketones in one single step, has been rarely studied. Such
investigations, if realized, would allow for rapid construction of
synthetically significant and structurally complex motifs.
O-Benzoylhydroxylamines, which are the most widely used
electrophilic aminating reagents, have attracted much attention
in recent years.9 These reagents are easily prepared, fairly stable,
and compatible with a series of metals including Pd, Cu, Rh, Ru,
Ni, and Fe complexes (Figure 1b). Hirano and Miura reported
Received: January 11, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00125
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Letter
compounds using O-benzoylhydroxylamines as nitrogen sources
has not yet been explored.11 Herein, we report the first
multifunctionalization of unactivated cyclic ketones by employ-
ing O-benzoylhydroxylamines as both amination reagents and
oxidants via cooperative Cu and organocatalyst to generate α-
enaminone, which is a type of versatile synthon showing “dual
electronic attitude”,12 but is not easily obtained13 (Figure 1c). To
realize this strategy, a few challenges would need to be overcome:
(1) the nucleophilic amine should be compatible with electro-
philic hydroxylamines in the presence of metal; and (2) the
reaction of Cu catalyst with hydroxylamines would generate high
valent Cu complex, and this intermediate should not be inhibited
by the aminocatalyst.
when Sc(OTf)3 was employed as the additive, the desired
product 3aa could be isolated in 68% yield. The frequently used
ligands (for instance, 1,10-phenanthroline) would slightly
decrease the yield (see entry 13). Later, control experiments
were carried out (entries 14 and 15). Without an organocatalyst,
only 9% yield of 3aa was obtained, while no desired product
could be detected in the absence of CuCl, showing the important
roles of organocatalyst and transition metal. It was noteworthy
that when 1.0 equiv hydroxylamine was used, 3aa was isolated in
only 20% yield, together with some unidentified impurities.
With the optimized reaction conditions secured, we explored
the substrate scope with various cyclic ketones. As shown in
Scheme 1, the cyclopentanones with methyl or isopropyl group
Our work was initiated by the reaction of cyclopentanone 1a
with O-benzoylhydroxylamine 2a in the presence of CuCl and
TsOH (see Table 1). Various aliphatic amines, including
a
Scheme 1. Substrate Scope of Cyclic Ketones
a
Table 1. Optimization of the Reaction Conditions
b
yield
entry
metal salt
amine
pyrrolidine
additive
(%)
1
2
CuCl
CuCl
CuCl
CuI
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
0
0
piperidine
3
diphenylamine
diphenylamine
diphenylamine
diphenylamine
diphenylamine
48
13
0
4
5
CuCN
CuCl2
FeCl2
6
26
0
7
8
Pd(OAc)2 diphenylamine
0
9
CuCl
CuCl
aniline
42
56
10
bis(4-bromophenyl) TsOH
amine
11
12
CuCl
CuCl
CuCl
bis(4-bromophenyl) Zn(OTf)2
amine
31
68
57
bis(4-bromophenyl) Sc(OTf)3
amine
c
13
bis(4-bromophenyl) Sc(OTf)3 + 1,10-
amine
phenanthroline
14
15
CuCl
none
none
Sc(OTf)3
9
0
bis(4-bromophenyl) Sc(OTf)3
amine
a
All reactions were carried out in 0.1 M CH3CN with 10 mol % metal
salt, 30 mol % amine, 10 mol % additive, and 2.5 equiv O-
b
c
benzoylhydroxylamine under N2 atmosphere. Isolated yields. With
10 mol % 1,10-phenanthroline.
a
Reaction conditions: 1 (0.20 mmol), 2a (0.50 mmol), CuCl (10
mol %), bis(4-bromophenyl)amine (30 mol %), and Sc(OTf)3 (10
mol %) in CH3CN (2.0 mL) at room temperature (25 °C) for 36 h.
pyrrolidine and piperidine, all failed to give any desired product
(entries 1 and 2). Arylamines are softer aminocatalysts,
compared to aliphatic amines, and prove to be ideal partners
when combined with a hard metal catalyst.14 On the other hand,
we speculate that the low nucleophilicity of arylamines would
enable the compatibility with electrophilic hydroxylamines.
Luckily, when diphenylamine was utilized as an aminocatalyst,
the reaction smoothly produced α-enaminone 3aa in 48% yield
(entry 3). Other metal salts either gave unsatisfied yields or made
no effects on the desired reaction (entries 4−8). We then tested
different aryl amines. The aniline gave a similar result as
diphenylamine (entry 9), while bis(4-bromophenyl)amine could
improve the yield up to 56% (entry 10). The Lewis acids could
also efficiently promote the condensation of amines and ketone
substrates to generate enamine intermediates.15 So we tried
several different Lewis acids (entries 11 and 12). To our delight,
c
bThe reaction was run at 50 °C for 12 h. Cu(NO3)2·3H2O (10 mol
%) was used as a metal salt.
at the C3 position could all be converted to corresponding α-
enaminone 3ba and 3ca with excellent chemoselectivity and
regioselectivity at the ketone C5 position. A wide range of
different functional groups were tolerated under the oxidative
amination conditions. Nitro, amine, and tert-butyldimethyl silyl
(TBS) ether all proved to be compatible to give 3da−3fa in good
yields. Carbonyl groups, such as ethyl or benzyl ester, could all be
well-tolerated (3ga, 3ha). Moreover, the reactive terminal alkene
could survive to give 3ia, which would allow for further
manipulations. Delightedly, the cyclopentanones bearing
quaternary carbon centers could be smoothly transformed to
the corresponding products 3ja−3la in good yields. Later, the
B
DOI: 10.1021/acs.orglett.8b00125
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Letter
transformation was found to proceed efficiently with a variety of
aryl groups at the C3 position, including phenyl, naphthyl,
thienyl, and various substituted phenyl groups (3ma−3sa). The
cyclohexanone could also undergo the desired reaction pathway
to offer α-enaminone 3ta, albeit in a low yield of 31%, probably
due to the overoxidation of the formed α-enaminone.16 When
3,3- or 4,4-dimethylcyclohexanone were empolyed as the
substrates, 3ua and 3va were obtained in acceptable yields.
Unfortunately, the cyclopentanones bearing substituents at the
C2 position, such as methyl or n-butyl groups, failed under the
optimized reaction conditions.
Scheme 3. Representative Derivatizations of α-Enaminone
Next, various hydroxylamines were subjected to the oxidative
amination reaction of cyclopentanone (Scheme 2). The numbers
a
Scheme 2. Substrate Scope of Hydroxylamines
simple hydrolysis into corresponding 2-hydroxy cyclopentenone
4 from 3ma in an acidic medium, the α-enaminone 3aa could
react with N-bromosuccinimide to install a bromo atom on the β
position to produce α-amino-β-bromocyclopentenone (5).17
The vinyl bromide moiety allows for further Suzuki coupling
reactions of 5 with different aryl boric acids to deliver a sequence
of complex cyclopentenone derivatives 6a−6c.
Finally, we performed extensive experiments to elucidate the
reaction mechanism (Scheme 4). The addition of TEMPO or
Scheme 4. Mechanistic Investigations
butylated hydroxytoluene (BHT) into the reaction mixture
would totally suppress the desired reaction. Treatment of known
α-amino cyclopentanone 7 with the standard reaction conditions
generated α-enaminone 3aa in 79% yield, which indicates that
the reaction goes through α-amino intermediate. When
hydroxylamine 8 was employed as the partner, aminooxygena-
tion products (10) were obtained, along with a trace amount of
the desired product (9), suggesting that the aminyl radical might
be generated in the reaction process.18
On the basis of above experimental results, a plausible
mechanism is illustrated in Figure 2. Initially, the condensation of
organocatalyst with ketone in the presence of Lewis acid would
produce enamine A. The enamine intermediate A then
underwent single electron transfer (SET) with Cu(III) species
B, which was generated from the oxidative addition of O-
benzoylhydroxylamine (2) and Cu(I), to deliver radical cation C
and Cu(II) species D.19 Recombination of radical cation C and
Cu(II) D would give the iminium ion E and regenerate Cu(I)
catalyst through either radical coupling or reductive elimination
of Cu(III) complex.9e,18b Hydrolysis of iminium E into ketone,
followed by further oxidation with O-benzoylhydroxylamine,
resulted in the formation of α-enaminone 3aa.20
a
Reaction conditions: 1a (0.20 mmol), 2 (0.50 mmol), CuCl (10
mol %), bis(4-bromophenyl)amine (30 mol %), and Sc(OTf)3 (10
mol %) in CH3CN (2.0 mL) at room temperature (25 °C) for 36 h.
of O-benzoylhydroxylamines, derived from cyclic amines such as
morpholine, N-protected piperazines, and functionalized piper-
idines, could all be transformed to corresponding α-enaminones
3ab−3ak in good yields. Later, acyclic hydroxylamines were
proven to be suitable partners for the reaction. Hydroxylamines
prepared from N-benzyl-N-methyl and N,N-dibenzyl amine
could provide products 3al and 3an in 71% and 75% yields,
respectively, while N-benzyl-N-butyl amine-based hydroxyl-
amine just resulted in the formation of 3am in a yield of 60%.
Functionalized O-benzoylhydroxylamines bearing a fluorine
atom or allyl, propargyl, or naphthyl moieties all readily
participated in the reaction to generate 3ao−3ar, which offered
opportunities for further transformations. Furthermore, the
reaction was compatible with O-benzoylhydroxylamines derived
from primary amines, and α-enaminones 3as−3au were isolated
in moderate yields, showing the broad scope of hydroxylamines.
The α-enaminones are versatile building blocks and could be
further utilitied in various transformations (Scheme 3). Besides
In summary, a multifunctionalization of unactivated cyclic
ketones by simultaneously introducing double bond and α-
amino group is developed via synergistic catalysis of copper and
diarylamine. The O-benzoylhydroxylamines are employed as
C
DOI: 10.1021/acs.orglett.8b00125
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Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00125.
■
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Experimental procedures and spectra copies (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: bixh507@nenu.edu.cn (X. Bi).
*E-mail: fujk109@nenu.edu.cn (J. Fu).
ORCID
Xihe Bi: 0000-0002-6694-6742
Junkai Fu: 0000-0002-7714-8818
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the NSFC (Nos. 21502017,
21522202, 21702027), the Ministry of Education of the People’s
Republic of China (No. NCET-13-0714), and Fundamental
Research Funds for the Central Universities (No.
2412017QD010).
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