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Me
Me
Ac
O
[Ph
2
I]BF
4
Pd(OAc)
2
-HOAc
N
Pd
N
Me
N
2
Scheme 12.18 Proposed mechanism of Pd-catalyzed ortho-arylation using [Ph
2
I]BF
4
.
Me
Me
N
Pd(OAc)
2
(5mol%)
oxone (2 equiv),
i
PrOH, 25 °C,17h
N
N
Me
86 %
F
Br
S
N
N
N
N
N
N
N
N
S
F
82 %
Br
67 %
44 %
41 %
Scheme 12.19 Pd-catalyzed cross-dehydrogenative dimerization.
APd
II
/Pd
IV
cycle has been proposed (Scheme 12.18). Several experiments
were performed which determined the first-order dependence on the
hypervalent iodine reagent [I
III
], a second-order dependence on [Pd] and an
inverse third-order dependence on [arylpyridine]. Hammett studies and la-
beling experiments showed that the C-H activation process occurs after the
turnover-limiting step, which is the oxidation of the cyclopalladated inter-
mediate with the I
III
reagent.
In 2006, Sanford and co-workers reported a Pd(OAc)
2
-catalyzed cross-
dehydrogenative dimerization of 2-arylpyridines (Scheme 12.19).
33
This lig-
and-free oxidative coupling was highly regioselective for 2-arylpyridine
derivatives.
The proposed mechanism included initial cyclopalladation with Pd
II
and
subsequent oxidation to Pd
IV
. Following another cyclopalladation at Pd
IV
,re-
ductive elimination gave the C-C coupled product, where oxone was presumed
to play the role of a two-electron oxidant for the Pd
II
/Pd
IV
cycle (Figure 12.7).
Amide-directed arylation, alkylation, vinylation and carbonylation reactions
were also achieved in simple aromatic ring systems (Scheme 12.20).
12
Dif-
ferent boron reagents were found to be suitable coupling partners in this case.
Interestingly, para-selective C-H arylation of monosubstituted arenes can
also be achieved by using an amide as the directing group (Scheme 12.21).
34
No prefunctionalization was required and para-selectivity over ortho- and
meta-arylation was fairly high.
35
Buchwald and co-workers reported the direct arylation of anisole deriva-
tives but the selectivity was not satisfactory as a mixture of regioisomers was
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