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owing to their broad range of functional group compatibility, both chelation-
assisted and heteroatom-directed C-H bond arylation have been developed.
12.3.1 N-Atom-Directed C-H Arylation
Rh-catalyzed direct C-H arylation was carried out for the first time by Oi et al.
with the well-known Wilkinson's catalyst in 1998.
79
In the presence of tet-
raarylstannanes, ortho-arylated arenes were obtained in good to moderate
yields (Scheme 12.51). Mechanistically, it was proposed that the pyridine
nitrogen plays a pivotal role to bring Rh in close proximity to the C-H bond
of interest, subsequently facilitating ortho C-H activation.
In 2005, Miura and co-workers reported the N-atom directed ortho-
arylation of benzophenone imine using sodium tetraphenylborate.
80,81
The
scope of the protocol was limited owing to the formation of a hydrogenated
by-product of imine in a considerable amount (Scheme 12.52).
They subsequently resolved the problem in 2008 using ethyl chloroacetate
as hydrogen acceptor.
82
The new method was found to be more effective and
was successfully implemented for various phenylazoles and tetra-
phenylborates. It was also observed that the sterically hindered azole ring
promoted monoarylation over diarylation (Scheme 12.53).
In addition to the azole functionality as directing group, Miura and co-
workers applied these conditions for ortho-arylation of azobenzenes.
82
In
this case, [Rh(OMe)(COD)]
2
was found to be a more effective catalyst and
phenylboronic was the arylating agent of choice (Scheme 12.54).
A transmetallation process between Rh
I
X and arylboron species was pro-
posed as the first step of the cycle to generate active arylated Rh
I
complex,
which interacts with the N-atom of the substrate. This anchoring of the Rh
center facilitates the activation of the proximal ortho-C-H via an oxidative
addition. Subsequent reductive elimination generates the arylated product
Ar
Ar
Ar
[RhCl(PPh
3
)] (5 mol%)
DCE, 120 °C, 20h
+
+
Ar
4
Sn
N
N
N
Ar = Ph;
65%
20%
=4-OMe;36%
18%
Scheme 12.51 Direct ortho-arylation of 2-phenylpyridine with tetraarylstannanes.
Ph
[RhCl(cod)]
2
(1 mol%)
NH
4
Cl (1 equiv)
o-
xylene, 120 °C, 44h
NH
+
NaBPh
4
NH
+
NH
+
NH
2
Ph
Ph
1equiv
0.25 equiv
26%
20%
51%
Scheme 12.52 Rh-catalyzed C-H arylation of benzophenone imine.
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