DOI: 10.1002/cctc.201501218
Communications
Copper–Manganese Spinel Oxide Catalyzed Synthesis of
Amides and Azobenzenes via Aminyl Radical Cations
Shaista Sultan, Manjeet Kumar, Shekaraiah Devari, Debaraj Mukherjee, and
Bhahwal Ali Shah*[a]
A highly efficient Cu–Mn-catalyzed process for the aminolysis
of esters was developed. Also, the catalyst promoted the self-
and cross-dehydrogenative coupling of anilines to generate
symmetrical and unsymmetrical azobenzenes, respectively. The
reactions were performed under neutral conditions with an in-
expensive catalyst, gave high yields, and offered wide function-
al group tolerance.
In this regard, we were intrigued to study the role of Cu–Mn
spinel oxides in the oxidative aminolysis of esters. Notably,
these Cu–Mn-based bimetallic catalysts have found application
in oxygen-storage materials, electrochemical systems, and
combined steam reforming of methanol, wherein they have
shown activity comparable to that of the commercial Cu–Zn–
Al catalyst. Recently, Cu–Mn spinel oxides were also used in
ligand-free [3+2] cycloaddition reactions[8a] and in the regiose-
lective halogenation of phenols and heteroarenes.[8b]
Unarguably, the transformation of amide linkages is one of the
most commonly executed reactions in synthetic and medicinal
chemistry programs owing to its pervasive presence in diverse
biological systems, including proteins, natural products, and
pharmaceuticals.[1] The synthesis of amides is general-
ly achieved by coupling carboxylic acids with
amines.[2] However, this method suffers from draw-
backs such as the generation of side products and
dry reaction conditions. To overcome these limita-
tions and in search of simpler protocols, alternative
substrates such as aldehydes,[3c–e] alcohols,[3a,f] and
halides[3b] have been explored to a great extent for
the direct amidation of amines. In this regard, ami-
Thus, in continuation of our efforts to develop new oxidative
amidation protocols,[9] we wish to report a novel method for
the aminolysis of esters leading to the synthesis of amides
(Scheme 1). This method encompasses the amidation of both
Scheme 1. The aminolysis of esters performed in this work.
nolysis of esters for the synthesis of amides repre-
sents a more attractive process. Although methods
are reported for which the use of bases such as NaOMe and
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) facilitates the aminol-
ysis of esters, they are still associated with disadvantages such
as long reaction times (up to 72 h) and the use of an excess
amount of base.[4] To obviate the use of an excess amount of
base and to reduce reaction times, some catalytic approaches
are known and they include the use of dearomatized rutheni-
um pincer complexes[5] of group IV metal alkoxides in combi-
primary and secondary amines. The yields obtained are high
and the reaction times are significantly lower than those for re-
ported methods. The reaction possibly proceeds via an aminyl
radical cation, and depending upon its stability, it undergoes
either addition to esters (with aliphatic amine) to give the cor-
responding amides or oxidative dehydrogenative coupling
(with anilines) to give the corresponding azobenzenes. The
idea of using Cu–Mn spinel oxides for the aminolysis of esters
stems from the fact that Cu–Mn spinel oxides can switch their
oxidation states between Cu+1/+2 and Mn+3/+4 in their active
form[10] and transition metals, such as copper, having two rela-
tively stable adjacent oxidation states are known to participate
in electron-transfer reactions for the generation of aminium
radicals.[11,12] Also, previous reports[13] show that Cu salts can
catalyze the generation of the superoxide radical (O2·À) by
changing its oxidation state from Cu+1 to Cu+2, which effec-
tively leads to the formation of the aminyl radical cation. As it
turns out, this work presents a new facet of the aminyl radical
cation through its addition to dienes, acetylenes, and alkenes,
both inter- and intramolecularly, to lead to the synthesis of
five- and six-membered heterocycles, including substituted,
fused, and bridged systems are well known.[14,15]
[6]
nation with additives such as Zr(OtBu)4 and lanthanum tri-
fluoromethanesulfonate.[7] The main factors limiting the use of
ruthenium and other rare-earth metals are their low availability
and high costs. Furthermore, the preparation of ruthenium
complexes and their characterization add to the demerits of
the use of rare-earth metals.
[a] S. Sultan, Dr. M. Kumar, S. Devari, Dr. D. Mukherjee, Dr. B. Ali Shah
Natural Product Microbes and ACSIR
CSIR-Indian Institute of Integrative Medicine
Canal Road, Jammu (India)
Supporting Information for this article is available on the WWW under
ChemCatChem 2016, 8, 703 – 707
703
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