Published on the web May 12, 2012
585
Magnetically Separable CuFe2O4 Nanoparticle Catalyzed C-Se Cross Coupling
in Reusable PEG Medium
K. Harsha Vardhan Reddy, G. Satish, K. Ramesh, K. Karnakar, and Y. V. D. Nageswar*
Natural Product Chemistry, Indian Institute of Chemical Technology, Hyderabad-500 607, India
(Received February 13, 2012; CL-120121; E-mail: yvdnrao@iict.res.in)
A simple and efficient procedure for the synthesis of
unsymmetrical diaryl selenides has been developed by magneti-
cally separable CuFe2O4 nanoparticle catalyzed reactions of
organoboranes with PhSeBr in recyclable PEG-400 medium at
80 °C and Cs2CO3 as base. Using this protocol, a variety of
unsymmetrical selenides were obtained in good to excellent
yields. The copper ferrite nanoparticles were magnetically
separated, recycled, and reused up to four cycles.
copper-catalyzed cross-coupling reactions,21 we report herein
magnetically separable CuFe2O4 nanoparticle catalyzed reac-
tions of phenylboronic acids with phenylselenyl bromide
forming unsymmetrical diphenyl selenides under ligand-
free conditions in PEG, as a reusable solvent medium
(Scheme 1).
Initially, phenylselenyl bromide and phenylboronic acid
were used as model substrates to optimize the reaction
conditions such as bases, solvents, and reaction temperature
(Table 1). Among several bases screened, Cs2CO3 was found to
be an excellent base (Table 1, Entry 5). Among other bases
KOH also gave moderate yields (Table 1, Entry 1). In presence
of bases such as K2CO3, Na2CO3, and K3PO4, lesser amount of
the desired product was obtained (Table 1, Entries 2-4). The
effect of solvents was also investigated and the highest yield was
observed in PEG (Table 1, Entry 5), while reaction in solvents
such as DMSO and THF also resulted in moderate yields
(Table 1, Entries 7 and 8). A control experiment confirmed that
the reaction did not occur in the absence of the base (Table 1,
Entry 9). When the reaction was conducted at room temperature
Diaryl selenides act as versatile reagents and useful
synthons in organic synthesis and catalysis. These are also
known for their significant biological activities such as
antioxidant, antitumor, antiinfective, enzyme inhibition, gluta-
thione peroxidase mimicking, and immunomodulation.1 In
addition organic selenides are important products used in the
formation of selenonium salts, selenium dihalides, selenoxides,
and selenimines.2,3
Traditional methods for the preparation of diaryl selenides
involve the reaction of metal selenides or selenocyanates with
aryl halides, formation of selenides via diazonium intermediates,
cross coupling of aryl halides with aryl selenols4 and conversion
of aryl halides to diaryl selenides via the formation of
aryllithium, arylmagnesium, or aryltin compounds.5 Several of
these methods include multiple steps, often requiring harsh
reaction conditions, such as use of polar and toxic solvents such
as HMPA (hexamethylphosphoric triamide) and high reaction
temperatures.6 To overcome these limitations various metal salts
such as iron,7 copper,4,5,8 nickel,9 palladium,10 indium,11 and
lanthanum12 based catalytic systems were studied to assess their
efficacy in the synthesis of diaryl selenides.
Poly(ethylene glycol) (PEG) has emerged as a powerful
phase-transfer catalyst and is utilized in many useful organic
transformations under mild reaction conditions. Moreover, PEG
is inexpensive, recyclable,13 less flammable, less toxic, and
miscible with wide variety of organic solvents, rendering the
status of an eco-friendly alternative solvent for various organic
transformations such as coupling,14 substitution,15 oxidation,16
addition,17 and reduction reactions.18
In recent years, heterogeneous catalysts have attracted the
attention of researchers due to their economic and industrial
significance, and published reports indicate that they score over
homogeneous catalysts. Among these, nanoscale heterogeneous
catalysts are highly preferred as they offer high surface area and
low-coordinated sites, which are responsible for the higher
catalytic activity,19 having the advantage of easy product
purification, and reusability of the catalyst.
CuFe2O4, PEG-400
Se
X
RB(OH)2
Se
R
Cs2CO3, 80°C
R = aromatic,
X =Br,Cl
heteroaromatic
Scheme 1. CuFe2O4 nanoparticle catalyzed synthesis of un-
symmetrical diaryl selenides.
Table 1. Screening of CuFe2O4 nanoparticle catalyzed syn-
thesis of unsymmetrical diaryl selenidesa
CuFe2O4 (5.0 mol%)
PhSeBr + PhB(OH)2
PhSePh
solvent, base
Entry Catalyst
Base
Solvent Temp/°C Yield/%b
1
2
3
4
5
6
7
8
9
nano CuO
nano CuO
nano CuO
nano CuO
nano CuO
nano CuO
nano CuO
nano CuO
nano CuO
KOH
PEG
80
80
80
80
80
rt
80
80
80
80
80
80
80
80
62
40
20
34
90
trace
82
50
0
15
K2CO3 PEG
Na2CO3 PEG
K3PO4 PEG
Cs2CO3 PEG
Cs2CO3 PEG
Cs2CO3 DMSO
Cs2CO3 THF
®
PEG
10 nano Y2O3
11 nano NiO
12 nano Co3O4
13 nano NiFe2O4 Cs2CO3 PEG
14 nano CoFe2O4 Cs2CO3 PEG
Cs2CO3 PEG
Cs2CO3 PEG
Cs2CO3 PEG
trace
13
10
14
However, recently very few methods have been developed
using electrophilic substitution by ArSe+ using milder nucleo-
philes like aryl boronic acids for the synthesis of diaryl
selenides.20 In continuation of our interest in the field of
aReaction conditions: Phenylselenyl bromide (1.0 mmol),
phenylboronic acid (1.0 mmol), CuFe2O4 (5.0 mol %), base
b
(2.0 equiv), solvent (3.0 mL), 80 °C, 8 h. Isolated yield.
Chem. Lett. 2012, 41, 585-587
© 2012 The Chemical Society of Japan