Chemistry Reference
In-Depth Information
CuX, O
2
HO
HO
OH
2.408
Scheme 2.127
I
Pd(Ph
3
)
4
,
LiCl
O
InX
2
O
O
In, CuI, LiC
l
N
EtO
Br
EtO
EtO
2.409
2.410
2.408
N
Scheme 2.128
MeO
F
F
Br
CF
3
Ge
(MeCN)
2
PdCl
2
,
P(
o
-tol)
3
CF
3
n
-C
8
F
17
+
OMe
2.411
2.412
2.413
CF
3
CF
3
Scheme 2.129
Pd(OAc)
2
, Ph
3
P,
K
3
PO
4
+
Bi
OEt
Br
S
S
3
OEt
2.414
2.416
2.415
Scheme 2.130
2.9 Other Metals
A range of other metals has been investigated as the source of one of the coupling fragments, such as indium
(Scheme 2.128),
152
germanium (Scheme 2.129)
153
and bismuth (Scheme 2.130).
154
Despite their potential,
none of these has yet gained widespread use.
2.10 Homocoupling
Homocoupling
155
can involve either two organic halides (or their equivalent) or two organometallic com-
pounds (Scheme 2.131). Palladium is often employed for this transformation. Neither reaction is of itself
catalytic, but each can be rendered catalytic by the inclusion of either a reducing agent or an oxidizing agent.
For the homocoupling of halides, there is a net oxidation of the metal. Either a stoichiometric amount
of a low-valent transition-metal complex must be used as a reagent, or a reducing agent must be included.
Zinc is frequently used,
156
and this can be promoted by ultrasound.
157
The reaction can also be achieved
electrochemically.
158
In some cases, it may not be entirely clear what is responsible for the reduction
(Scheme 2.132).
159
Distannanes compounds, such as hexa-
n
-butylditin, have also been used. In this case, it
is also possible that one molecule of halide couples with the distannane to give a tetraorganotin derivative,
which then undergoes a Stille coupling
in situ
.
