Oxidative olefination of secondary amines with carbon nucleophiles

Article abstract of DOI:10.1002/ejoc.201300368

An unprecedented olefination reaction of secondary amines with carbon nucleophiles has been developed through C-N/C-H functionalization under metal-free oxidative conditions. In the presence of a stoichiometric amount of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), a range of secondary N-alkylanilines smoothly underwent oxidative olefination with 2-alkylazaarenes, acetophenone, and malononitrile to give structurally diverse polysubstituted alkenes in moderate to excellent yields with excellent (E) selectivity. Preliminary mechanistic studies revealed that the oxidative olefination reaction proceeds through amine oxidation followed by imine olefination. A range of secondary N-alkylanilines smoothly underwent DDQ-promoted oxidative olefination with 2-alkylazaarenes, acetophenone, and malononitrile to give structurally diverse alkenes in moderate to excellent yields with excellent (E) selectivity. Mechanistically, the reaction proceeds through amine oxidation followed by imine olefination (DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone). Copyright

Full text of DOI:10.1002/ejoc.201300368

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300368
Oxidative Olefination of Secondary Amines with Carbon Nucleophiles
Yong-Gang Zhang,[a] Jing-Kun Xu,[a] Xi-Ming Li,[a] and Shi-Kai Tian*[a,b]
Keywords: Homogeneous catalysis / Olefination / Oxidation / Amines / Imines
An unprecedented olefination reaction of secondary amines
with carbon nucleophiles has been developed through C–N/
C–H functionalization under metal-free oxidative conditions.
In the presence of a stoichiometric amount of 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ), a range of secondary
N-alkylanilines smoothly underwent oxidative olefination
with 2-alkylazaarenes, acetophenone, and malononitrile to
give structurally diverse polysubstituted alkenes in moderate
to excellent yields with excellent (E) selectivity. Preliminary
mechanistic studies revealed that the oxidative olefination
reaction proceeds through amine oxidation followed by im-
ine olefination.
be further facilitated by the in situ generation of the N-aryl
imines. Although the condensation of aldehydes with pri-
mary amines is frequently used to prepare imines in situ,
the oxidation of secondary amines constitutes a powerful
alterative method that extends the scope of chemical reac-
tions involving the intermediacy of imines.[7] Thus, we
planned to develop an oxidative olefination reaction of sec-
ondary amines with 2-alkylazaarenes, wherein the oxidant
should be compatible with the acid catalyst and the alkene
product should avoid being consumed by possible side reac-
tions such as oxidation and addition. In the course of ex-
ploring new methods for alkene synthesis,[3d–3g,8] we devel-
oped an unprecedented (E)-selective olefination reaction of
secondary amines with 2-alkylazaarenes through C–N/C–
H functionalization under metal-free oxidative conditions
(Scheme 1). Moreover, the substrate scope was extended to
acetophenone and malononitrile. Mechanistically, the reac-
tion proceeds through amine oxidation followed by imine
olefination, and the byproduct generated in the step of
amine oxidation catalyzes the step of imine olefination.[9,10]
Introduction
The olefination of carbon electrophiles with carbon nu-
cleophiles constitutes one of the most important strategies
for the synthesis of alkenes, which are present in many func-
tional molecules. Alkenes also serve as a foundation for a
broad range of chemical transformations such as oxidation,
reduction, and addition.[1] Carbonyl compounds frequently
serve as the carbon electrophiles, and their olefination with
carbon nucleophiles has enjoyed widespread prominence
and recognition owing to their simplicity, positional selec-
tivity, and stereoselectivity.[2] In sharp contrast, much less
attention has been paid to the corresponding olefination
of imines with carbon nucleophiles. Nevertheless, sporadic
studies have demonstrated that imine olefination affords
better efficiency and/or stereoselectivity in certain cases for
alkene synthesis than carbonyl olefination.[2g,3–5]
Certain 2-alkylazaarenes are known to undergo C–H ole-
fination with N-aryl imines under strong basic condi-
tions.[4a–4e] By contrast, the corresponding olefination with
N-sulfonyl imines, which are more electrophilic than N-aryl
imines, has recently been reported to proceed in the pres-
ence of a metal Lewis acid and/or at high temperature.[4f–4g]
In this context, we envisioned that N-aryl imines could be
activated enough by certain acid catalysts to undergo ole-
fination with 2-alkylazaarenes under relatively mild condi-
tions.[6] Moreover, the proposed olefination reaction could
[a] Department of Chemistry, University of Science and
Technology of China,
Scheme 1. Oxidative olefination of secondary amines with carbon
nucleophiles.
Hefei, Anhui 230026, China
Fax: +86-551-63601592
E-mail: tiansk@ustc.edu.cn
Homepage: http://chem.ustc.edu.cn/szdw_16/bd/201210/
t20121023_142873.html
[b] Key Laboratory of Synthetic Chemistry of Natural Substances,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences,
Results and Discussion
Several commercially available oxidants were examined
in the reaction of secondary amine 1a with 2-methylquinol-
ine (2a) in DMF at 70 °C (Table 1, entries 1–7). Most of the
Shanghai 200032, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300368.
3648
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Eur. J. Org. Chem. 2013, 3648–3652

Oxidative Olefination of Secondary Amines
oxidants failed to promote the desired oxidative olefination tably, the reaction tolerated functional groups such as alk-
reaction; however, the use of 2,3-dichloro-5,6-dicyano-1,4- oxy, chloro, and nitrile. When the oxidative olefination reac-
benzoquinone (DDQ) led to the formation of alkene 3a in tion was extended to N-allylic aniline 1k or ethyl 2-(4-meth-
99% yield with Ͼ99:1 (E)/(Z) selectivity (Table 1, entry 1). oxyphenylamino)acetate (1l), a complex mixture was ob-
The reaction with DDQ also proceeded at room tempera- tained, and the desired alkene product was isolated in a
ture but gave a much lower yield (Table 1, entry 8). The moderate yield with Ͼ99:1 (E)/(Z) selectivity (Table 2, en-
yield was dramatically affected by the solvent, and much tries 11 and 12).[11]
lower yields were obtained when the reaction was per-
formed in a number of other common organic solvents
(Table 1, entries 9–17). It is noteworthy that hardly any de-
Table 2. Oxidative olefination of secondary amines with 2-methyl-
quinoline (2a).[a]
sired product was detected when the phenyl group on the
nitrogen atom of secondary amine 1a was replaced with a
hydrogen atom, a methyl group, a benzyl group, an acetyl
group, or a p-tolylsulfonyl (Ts) group (Table 1, entries 18–
22).
Table 1. Optimization of the reaction conditions.[a,b]
[a] Reaction conditions: secondary amine 1 (0.30 mmol), 2-methyl-
quinoline (2a, 0.25 mmol), DDQ (0.30 mmol), DMF (0.50 mL),
70 °C, 24 h. [b] Isolated yield. [c] Determined by 1H NMR spec-
troscopy. [d] The reaction was performed at 25 °C. [e] Substrate 1l
was ethyl 2-(4-methoxyphenylamino)acetate.
Under the standard reaction conditions, a range of sub-
stituted 2-methylquinolines were found to serve as suitable
substrates in the oxidative olefination reaction with second-
ary amine 1a, and the corresponding functionalized 2-alk-
enylquinolines were obtained in good to excellent yields
with excellent (E) selectivity (Table 3, entries 1–6). To our
delight, this chemistry was successfully extended to a wide
variety of 2-methylazaarenes such as 4-methylquinoline
(2h), 1-methylisoquinoline (2i), 2-methylquinoxaline (2j), 2-
[a] Reaction conditions: compound 1a–af (0.30 mmol), 2-methyl-
methylquinazoline (2k), and 2-methylbenzo[d]thiazole (2l),
quinoline (2a, 0.25 mmol), oxidant (0.30 mmol), solvent (0.50 mL),
and importantly, a range of structurally diverse β-azaaryl-
styrenes were obtained in moderate to good yields with
Ͼ99:1 (E)/(Z) selectivity (Table 3, entries 7–11).
25 or 70 °C, 12 h. [b] In all cases alkene 3a was obtained with Ͼ99:1
(E)/(Z) selectivity. [c] Isolated yield. [d] 70% Aqueous solution. Ts
= p-tolylsulfonyl, DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquin-
one, BQ = 1,4-benzoquinone, TBHP = tert-butyl hydroperoxide,
DTBP = di-tert-butyl peroxide, PIDA = iodobenzene diacetate,
DMF = N,N-dimethylformamide, DMSO = dimethyl sulfoxide,
THF = tetrahydrofuran, DCE = 1,2-dichloroethane.
The oxidative olefination reaction was further extended
to the synthesis of trisubstituted alkenes with very high
stereoselectivity. For example, the DDQ-promoted reaction
of secondary amine 1a with 2-ethylquinoline (2m) smoothly
In the presence of a stoichiometric amount of DDQ, a proceeded under the standard reaction conditions to give
range of N-benzylic anilines smoothly underwent oxidative trisubstituted alkene 3x in 85% yield with Ͼ99:1 (E)/(Z)
olefination with 2-methylquinoline (2a) to give various 2- selectivity (Scheme 2). In addition, the reaction worked well
alkenylquinolines in good to excellent yields with excellent with acetophenone (2n) and malononitrile (2o) and gave the
(E) selectivity (Table 2, entries 1–10). Both electron-with- corresponding electron-poor alkenes in good yields.
drawing and electron-donating groups were successfully in-
To gain insight into the reaction mechanism, we carried
troduced into the aromatic rings of the alkene products by out electrospray ionization (ESI) mass spectrometric analy-
employing secondary amines having such groups, and no- sis of the reaction mixture of secondary amine 1a with 2-
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Y.-G. Zhang, J.-K. Xu, X.-M. Li, S.-K. Tian
Table 4. High-resolution mass data.
SHORT COMMUNICATION
Table 3. Oxidative olefination of secondary amine 1a with 2-meth-
ylazaarenes.[a]
Entry Species
Formula
Mass (calcd.) Mass (found)
1
2
[4a + H]+
C13H11N+
C23H21N2
182.09643
325.16993
182.09583
325.16910
+
[5a + H]+
mote the olefination of imine 4a with 2-methylquinoline
(2a) led to the formation of alkene 3a in 99% yield with
Ͼ99:1 (E)/(Z) selectivity, and reducing the catalyst loading
to 5 mol-% afforded a lower but still good yield with ex-
tremely high (E) selectivity (Table 5, entries 2 and 3).
Table 5. Olefination of imine 4a with 2-methylquinoline (2a).[a]
[a] Reaction conditions: secondary amine 1a (0.30 mmol), 2-meth-
ylazaarene 2b–l (0.25 mmol), DDQ (0.30 mmol), DMF (0.50 mL),
70 °C, 24 h. [b] Isolated yield. [c] Determined by 1H NMR spec-
troscopy.
[a] Reaction conditions: imine 4a (0.30 mmol), 2-methylquinoline
(2a, 0.25 mmol), DDQH2 (0–120 mol-%), DMF (0.50 mL), 70 °C,
1
24 h. [b] Isolated yield. [c] Determined by H NMR spectroscopy.
DDQH2 = 4,5-dichloro-3,6-dihydroxyphthalonitrile.
On the basis of the above experimental results, we pro-
pose the following reaction pathway for the oxidative ole-
fination of secondary amines with carbon nucleophiles
(Scheme 3). Initially, secondary amine 1 is oxidized by
DDQ to give imine 4 as an intermediate and DDQH2 as an
acidic byproduct. Then, imine 4 and carbon nucleophile 2
are activated by DDQH2 through hydrogen-bonding inter-
actions and through isomerization to form a reactive spe-
cies (an enamine, an enol, or a ketenimine), and they un-
dergo Mannich-type reaction to give amine 5. Finally, elimi-
nation of aniline from amine 5 leads to the formation of
alkene 3. In the elimination step, secondary amine 1 and
the aniline byproduct serve as bases to remove the β-H of
amine 5, and the leaving ability of the phenylamino group
is enhanced by DDQH2 through hydrogen-bonding interac-
tions. For the synthesis of a 1,2-disubstituted alkene (R3 =
H), the stereoselectivity is determined by the elimination
Scheme 2. Oxidative olefination of secondary amine 1a with carbon
nucleophiles 2m–o.
methylquinoline (2a) in the presence of DDQ in DMF. As
shown by the results summarized in Table 4, we tentatively
assigned two intermediates, imine 4a and amine 5a, accord-
ing to the high-resolution mass data. Whereas imine 4a was
generated in a significant amount, as judged by thin-layer
chromatography (TLC) analysis, amine 5a could not be de-
tected by this method.
In the above oxidative olefination reaction, DDQ was
reduced by the secondary amine to yield 4,5-dichloro-3,6-
dihydroxyphthalonitrile (DDQH2) as an acidic byproduct.
As demonstrated by the results summarized in Table 5,
DDQH2 proved essential for the olefination of carbon nu-
cleophiles with N-arylimines, which were generated as inter-
mediates from secondary amines during the oxidative ole-
fination reaction. The use of 1.2 equiv. of DDQH2 to pro- Scheme 3. Proposed reaction pathway.
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Oxidative Olefination of Secondary Amines
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Conclusions
We have developed, for the first time, a highly (E)-selec-
tive olefination reaction of secondary amines with carbon
nucleophiles through C–N/C–H functionalization under
metal-free oxidative conditions. In the presence of a stoi-
chiometric amount of DDQ, a range of secondary N-alkyl-
anilines smoothly underwent oxidative olefination with 2-
alkylquinolines, 4-methylquinoline, 1-methylisoquinoline, 2-
methylquinoxaline, 2-methylquinazoline, 2-methylbenzo[d]-
thiazole, acetophenone, and malononitrile to give structur-
ally diverse polysubstituted alkenes in moderate to excellent
yields with excellent (E) selectivity. Preliminary mechanistic
studies revealed that the oxidative olefination reaction pro-
ceeds through amine oxidation followed by imine ole-
fination.
[4]
[5]
[6]
Experimental Section
General Procedure for the Oxidative Olefination of Secondary
Amines with Carbon Nucleophiles: To a solution of secondary
amine 1 (0.30 mmol) in DMF (0.50 mL) under an atmosphere of
nitrogen at room temperature was added DDQ (68.1 mg,
0.30 mmol). The mixture was stirred for 5 min and carbon nucleo-
phile 2 (0.25 mmol) was added. The mixture was heated at 70 °C
for 24 h, cooled to room temperature, and purified by silica gel
chromatography (ethyl acetate/petroleum ether, 1:25–1:3) to give
alkene 3.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, and copies of
1
the H NMR and 13C NMR spectra.
Acknowledgments
The authors are grateful for the financial support of the National
Natural Science Foundation of China (NSFC) (grant numbers
21232007 and 21172206), the National Basic Research Program of
China (973 Program 2010CB833300), and the Program for
Changjiang Scholars and Innovative Research Team in University
(IRT1189).
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SHORT COMMUNICATION
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Received: March 11, 2013
Published Online: May 6, 2013
3652
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Eur. J. Org. Chem. 2013, 3648–3652