Chemistry Reference
In-Depth Information
Ph
Ph
Ph
Et
2
Zn, LiI, Ni(acac)
2
via
[Ni]
Br
2.113
2.114
2.115
Scheme 2.40
(
n
-C
5
H
11
)
2
Zn, Ni(acac)
2
F
3
C
S
S
S
S
2.117
I
2.116
2.118
Scheme 2.41
Br
+
Ni(COD)
2
, L
IZn
N
N
Ts
Ts
2.119
2.120
O
O
L =
N
N
N
s
-Bu
s
-Bu
2.121
Scheme 2.42
During studies of organozinc chemistry with nickel catalysis, it was found that the presence of a nearby
functional group capable of coordinating nickel could prevent
-hydride elimination and permit successful
coupling of alkyl halides (Scheme 2.40).
43
The functional groups involved include alkenes, both simple and
substituted, amides, ketones, and dithiolanes. The addition of electron-poor ligands improved the reaction,
particularly
p
-or
m
-trifluoromethylstyrene
2.117
,
44
or
p
-fluorostyrene (Scheme 2.41).
45
Secondary alkyl halides can also undergo coupling with zinc reagents using the bidentate pybox ligand
2.121
(Scheme 2.42).
46
2.4 Aluminium and Zirconium
Derivatives of these two metals are of interest as they can be generated by hydrometallation, a process
analogous to hydroboration. Treatment of alkynes with DIBAL gives a vinyl aluminium reagent
2.122
. These
can be coupled with organic halides in the presence of a palladium or nickel catalyst (Scheme 2.43). While
this works well for terminal alkynes, the coupling with derivatives of internal alkynes is slow due to steric
hindrance. The coupling reaction can be speeded up by employing a zinc halide additive, which acts as a
second metallic catalyst (Scheme 2.44). This has been termed “bimetallic” catalysis. In this case, the rate
acceleration and concomitant yield improvement is due to transmetallation: the vinyl group is transferred to
zinc and the resulting zinc reagent
2.126
is more reactive towards transmetallation.
Vinyl aluminium reagents
2.128
prepared by the hydroalumination of propargylic alcohols
2.127
can
also undergo coupling (Scheme 2.45).
47
This chemistry has been employed on a multigramme scale for the
