4
R. Das, D. Chakraborty / Tetrahedron Letters xxx (2012) xxx–xxx
R
1
R
2
ronic acid, C–S coupling of thiocresol and phenylboronic acid and
N
N
C–S coupling of thiocresol and p-cyanophenylboronic acid. High
pressure liquid chromatography (HPLC) was used to determine
the various starting materials and products present as a function
of time. Concentration versus time plot shows the disappearance
of iodide and the appearance of the corresponding product with
time (Please see the figure in Supplementary data).
Ag
R
1
X
I
N
N
R
1
Ag
R
2
IV
Van’t Hoff differential method was used to calculate the order
(
n) and rate constant (k). At different concentrations, the rate of
Y
B
Y
II
N
N
the reaction was calculated by estimating the slope of the tangent
at each point on the concentration curve. From this data, log10(rate)
versus log10(concentration) was plotted (Fig 3) and the order (n)
and rate constant (k) were obtained from the slope of the line
and its intercept on the log10(rate) axis. From these figures it is
clear that reaction proceeds with second order kinetics (n = 2)
K
But-O
O-tBu
R
Ag
X
1
Transmetallation
III
KO-tBu
Y
N
N
KX
À4
À1 À1
Y
KO-tBu
and the rate constant is 1.94 Â 10 L mol
s
. Similarly, for the
R
2
B
R
2
B
Y
O-tBu
R
Ag
O-tBu
1
À4
Y
other three reactions, the rate constants are 1.01 Â 10
,
À4
À4
À1 À1
2
.50 Â 10 and 1.58 Â 10 L mol
s
respectively.
Y= OH
In conclusion, we have developed a convenient Ag(I) catalyzed
Figure 2. Proposed mechanism of C–C coupling.
carbon-carbon and carbon–sulfur bond forming reaction to give
biphenyl and thioethers. The methodology tolerates many substit-
uents of a wide range of functional groups. The C–C bond formation
reaction goes well with AgOAc as catalyst and trans-1,2-diamino-
cyclohexane as ligand in DMSO at 150 °C. All the biphenyls are
formed in good yield. We have observed the formation of the
homocoupling product from boronic acids in the case of C–C cou-
pling but no side product formation was observed in C–S coupling
when performed under a nitrogen atmosphere. The C–S bond
formation reaction also goes very well with AgOTf as catalyst in
DMF. It tolerates all substituents on thiol and boronic acid giving
good to excellent yield. Kinetic studies yielded interesting
results.
Y = - 3.71 + 2.19 X
R = 0.99904
-
-
-
-
-
-
4.2
4.4
4.6
4.8
5.0
5.2
Acknowledgments
This work was supported by the Department of Science and
Technology New Delhi. R.D. thanks the Council of Scientific and
Industrial Research, New Delhi for a research fellowship.
-
1.3
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
log 10
C
Supplementary data
Figure 3. Van’t Hoff differential plot for the coupling reaction of p-iodotoluene and
phenylboronic acid to give p-methyl biphenyl catalyzed by AgOAc.
yield whereas electron donating group in thiol (Table 3, entry 7) in-
creases the yield with high reaction rate. The trend is reversed in
the case of electron withdrawing substrates (Table 3, entries 14,
References and notes
1
(
5 vs entry 9). A challenging substrate such as napthyl boronic acid
Table 3, entry 16) also proved to be a good coupling partner with
thiocresol.
We have used AgOAc and AgOTf (Aldrich catalogue No. 204374
1.
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0
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