Tetrahedron Letters
Oxidative coupling of formamides with b-dicarbonyl compounds
and the synthesis of 2-aminobenzothiazole using Cu(II)-
functionalized Fe3O4 nanoparticles
⇑
Kobra Azizi, Meghdad Karimi, Akbar Heydari
Chemistry Department, Tarbiat Modares University, PO Box 14155-4838, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2014
Revised 8 December 2014
Accepted 18 December 2014
Available online 26 December 2014
The Fe3O4@EDTA–Cu(II) nanoparticles catalyzed oxidative coupling of formamides with b-dicarbonyl
compounds is developed using tert-butyl hydroperoxide as an oxidant. In general, the enol carbamates
are synthesized in excellent yields (up to 92%) under the optimized reaction conditions. Fe3O4@EDTA–
Cu(II) has the advantage of being magnetically recoverable. Also, we have established a highly efficient
Fe3O4@EDTA–Cu(II)-catalyzed tandem reaction of 2-iodoanilines with isothiocyanates for the synthesis
of 2-aminobenzothiazoles.
This article is dedicated to memory of
Mostafa Chamran.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Enol carbamates
Dimethylformamide
2-Aminobenzothiazoles
Oxidation
Magnetic nanoparticles
Direct C–H functionalization for the formation of C–C and C–X
(X = O, S, N, P, etc.) bonds catalyzed by transition metals has
become a very useful tool in organic chemistry owing to its
remarkable potential for step economy, atom economy, and
environmental sustainability.1 Oxidative C(sp)–H and C(sp2)–H
cross-couplings for various C–C bond-forming reactions have
received significant attention and excellent progress has been
made.2 The direct C–H activation path has made the synthesis of
functionalized molecules more efficient by minimizing the number
of synthetic steps. Cross-dehydrogenative coupling protocols have
been employed to access a diverse range of C–X bonds (X = C,
heteroatom), by functionalizing C–H bonds of all types (sp, sp2,
sp3).3 Copper mediates different reaction types due to the ability
of the copper atom to act as a Lewis acid, a single-electron media-
tor, and a two-electron mediator. Also, Cu-X species generated as
reaction intermediates can act as either a nucleophile or an electro-
phile, depending on the reaction conditions and oxidation state of
the copper atom. These properties of copper species account for the
high utility of copper catalysts in C–X (X = C, N, O, etc.) bond for-
mation.4 Oxidative coupling combines two molecular entities
through an oxidative process, usually catalyzed by a transition
metal compound and involving dioxygen as the oxidant.5
In vivo data support the pharmacological efficacy of enol carba-
mates as promising anxiolytic therapeutics.6 Carbamates are
mixed ester–amides of carbonic acid. Their chemical behavior is
similar to that of carbonates. Dixneuf et al. have shown that enol
carbamates can be prepared by addition of carbamic acids to ter-
minal alkynes catalyzed by Ru3(CO)127 or Ru(Cl)3,8 where the yields
and selectivities of the carbamates are low for aliphatic acetylenes.
Also, Watanabe et al. have reported that a bis(g5-cyclooctadie-
nyl)ruthenium or Ru(COD)(CCT)-tertiary phosphine system cata-
lyzes the reaction of secondary amines with carbon dioxide and
terminal alkynes to give enol carbamates in good yields.9 Oxidative
C–O coupling by direct C–H bond activation of formamides has
been accomplished in the presence of CuBr2, but the main problem
of this reaction is that it suffers from the loss of the catalyst at the
end of the reaction.10 Therefore, the development of EDTA@Cu(II)
functionalized superparamagnetic nanoparticle-catalyzed C–O
cross-coupling methods would be of value. Herein, we report a
Cu-catalyzed dehydrogenative cross-coupling reaction for C–O
bond formation to construct enol carbamates using tert-butyl
hydroperoxide (TBHP) as the oxidant. We have previously reported
the preparation of EDTA@Cu(II) functionalized superparamagnetic
nanoparticles,11 and their application as a highly efficient and
⇑
Corresponding author. Tel.: +98 21 82883444; fax: +98 21 82883455.
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.