Chemistry Reference
In-Depth Information
R
1
R
1
[Cp*IrHCl]
2
(5-10 mol%)
KO
t
Bu(3.3 equiv)
80 °C, 30 h
+
I
H
11 examples
16-72% yield
R
2
R
1
R
1
R
2
Ir(ppy)
3
(0.5-2.5 mol%)
h
+
X
H
,KO
t
Bu, DMSO
rt - 70 °C
ν
38 examples
35-85% yield
X= Br, I
Scheme 12.88 Direct C-H arylation of aryl halides with benzene.
ArX
-e
[Ir(ppy)
3
]
n+
*
Ar
ArX
H
[Ir(ppy)
3
]
(n+1)+
visible
light
H
base
[Ir(ppy)
3
]
n+
Ar
Ar
+e
-H
+
Ar
Figure 12.23 Proposed mechanism of photoredox homolytic aromatic substitution
(HAS).
to aryl iodides only, and the reaction was only poorly regioselective when
substituted arenes were used (Scheme 12.88, top). Recently, a similar type of
transformation was reported using a photoredox iridium-polybipyridyl
complex as the active catalyst (Scheme 12.88, bottom). In this protocol, aryl
bromides were also applied as successful substrates for C-H arylation and
the catalyst loading could be reduced to 0.5 mol% without any significant
decrease in yield.
157
Poor regioselectivity for substituted arenes with a mixture of ortho, meta and
para isomers suggests that a radical-based reaction mechanism is likely to be
operative. A recent breakthrough in the construction of biphenyl derivatives
mediated by strong base without any involvement of a transition metal,
popularly known as homolytic aromatic substitution (HAS), is likely to be
involved in these two cases.
158
Thermal or photolytic excitation of the catalyst
leads to the generation of an aryl halide radical anion through single-electron
transfer (SET), followed by the formation of an aryl radical, which attacks the
arene, resulting in a biaryl radical. Finally, after deprotonation in the presence
of a strong base, the aryl radical anion transfers one electron to the oxidized
metal catalyst to afford the biaryl product (Figure 12.23).
157
12.5.2 C-H Arylation With Regioselectivity
It was shown later that the regioselectivity problem could be overcome and
arylation of electron-rich heterocyclic arenes was performed selectively at the
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