Please do not adjust margins
RSC Advances
Page 3 of 5
DOI: 10.1039/C5RA25434E
Journal Name
ARTICLE
R2
g
-value
NH2
NH2
O2
2.0202.0162.0122.0082.0042.0001.9961.992
N
N
+
KOH
AN=15.1G
R1
R2
R1
g
= 2.0060
Y (%)b
Entry
Substrates
Product
T (°C)
NH2
90
81
1
2
3
N
N
P17
Cl
NH2
90
90
61
73
N
N
P18
P19
Cl
Cl
Cl
NH2
NH2
+
N
N
AH=3.1G
Cl
OMe
P20
3200
3210
3220
B [Gauss]
3230
3240
3250
NH2
90
90
52
96
N
N
4
5
MeO
MeO
Figure 1. Xꢀband EPR spectrum of radicals formed after heating a
0.6 M 4ꢀmethoxybenzylamine DMSO solution containing 1 mg of
PBN at 110 oC under 1 atm O2 for 1 h.
H
N
N
N
P17
N
H
a Reaction condition: 0.5 mmol of Substrate, 1.5 mmol of KOH,
1mL of DMSO, 1 atm oxygen. C = conversion of substrate , Y
= yield of isolated results after column chromatography.
The aforementioned preliminary mechanistic studies indicate a
radical chain pathway (Scheme 2) proposed to occur through (a)
the reaction of DMSO with O2 which leads to the formation of
DMSO cation and the very active superoxide radical; (b)
deprotonation of the newly formed DMSO cation, in which the
acidity of the C−H bond is stronger than that in DMSO, by the
Lewis base, which is benzylamine in this system, producing the
DMSO radical; (c) generation of aminomethyl radical by the
reaction of the superoxide radical O2¯ with the amine substrates
accompanied with the formation of H2O2 which can then oxidize
the DMSO solvent; (d) the key imine intermediate forms via the
reaction of the aminomethyl radical with DMSO radical; (e)
conversion of the imine intermediate to the final coupling
product. DMSO functions as solvent, radical initiator and
reductant simultaneously throughout the whole reaction process.
b
Conclusion
In this work, a simple but efficient protocol for aerobic oxidative
coupling of amines to form imines and azo compounds, just by
heating the mixture of amines and DMSO under aerobic conditions,
was reported. This green and lowꢀcost system using O2 as the sole
oxidant may find a wide range of application in green oxidation
chemistry. Detailed investigation on the mechanism and further
extension of this system to more practical reactions are currently
under investigation.
Scheme 2. Proposed radical chain mechanism.
Acknowledgement
This work was supported by National Science Foundation of
China (Grant. 21171012) and the 7th China Postdoctoral Science
Foundation Funded Project (2014T70009). M. Lin was also
supported in part by the Postdoctoral Fellowship of PekingꢀTsinghua
Center for Life Sciences.
Notes and references
1
For reviews on the formation of imines and typical reactions: (
Murahashi, Angew. Chem. Int. Ed. Engl. 1995, 34, 2443ꢀ2465; (
P. Adams, J. Chem. Soc. Perkin Trans. 1 2000, 125ꢀ139; ( ) J. S. M.
a
) S.ꢀI.
b
) J.
,
c
Samec, A. H. Éll and J.ꢀE. Bäckvall, Chem. Eur. J. 2005, 11, 2327ꢀ
2334.
2
3
4
M. T. Schümperli, C. Hammond and I. Hermans, ACS Catal. 2012, 2,
1108ꢀ1117.
F. Su, S. C. Mathew, L. Möhlmann, M. Antonietti, X. Wang and S.
Blechert, Angew. Chem. Int. Ed. 2011, 50, 657ꢀ660.
Inspired by the mechanistic insights of amine oxidation by
DMSO and oxygen, we also introduced aromatic amines into this
DMSO based system. However, no reaction occurred under the
standard conditions, possibly because the basicity of aromatic
amines acting as Lewis base is lower than that of benzylamines
and the process b of deprotonation in Scheme 2 could not happen.
Thus, KOH was added to take the deprotonation and azo
compounds could be achieved under similar conditions as shown
in Table 4. Proposed mechanism for azo compounds is shown in
Scheme S1 (see ESI), in which the function of DMSO is similar
as shown in Scheme 2.
(
a
) B. Zhu and R. J. Angelici, Chem. Commun. 2007, 2157ꢀ2159; (
B. Zhu, M. Lazar, B. G. Trewyn and R. J. Angelici, J. Catal. 2008,
260, 1ꢀ6; ( ) L. Aschwanden, B. Panella, P. Rossbach, B. Keller and
A. Baiker, ChemCatChem 2009, , 111ꢀ115; ( ) L. Aschwanden, T.
Mallat, F. Krumeich and A. Baiker, J. Mol. Catal. A: Chem. 2009,
309, 57ꢀ62; ( ) A. Grirrane, A. Corma and H. Garcia, J. Catal. 2009,
) M.ꢀH. So, Y. Liu, C.–M. Ho and C.–M. Che, Chem.
, 1551ꢀ1561; ( ) H. Miyamura, M. Morita, T. Inasaki
b)
c
1
d
e
264, 138ꢀ144; (
Asian. J. 2009,
f
4
g
and S. Kobayashi, Bull. Chem. Soc. Jpn. 2011, 84, 588ꢀ599; (h) S.
Naya, K. Kimura and H. Tada, ACS Catal. 2013, , 10ꢀ13.
) R. D. Patil and S. Adimurthy, Adv. Synth. Catal. 2011, 353, 1695ꢀ
1700; ( ) Z. Hu and F. M. Kerton, Org. Biomol. Chem. 2012, 10
3
5
(a
Table 4. Oxidative coupling reactions of aromatic amines.a
b
,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins