Chemistry Reference
In-Depth Information
Pd(0)
RX + Me
6
Sn
2
RSnMe
3
Scheme 2.57
1. DIBAL
2. TIPSCl, TMG
3. CrO
3
, acetone
1. NaCNBH
3
, TiCl
4
2. DCC, CuCl
CO
2
Et
CO
2
Et
AcO
AcO
O
t
-Bu
O
O
t
-Bu
2.167
1. L-selectride, Ph
2
NTf
2
2. Me
6
Sn
2
, (Ph
3
P)
4
Pd, LiCl
OTIPS
OTIPS
O
Me
3
Sn
O
t
-Bu
O
t
-Bu
2.168
2.169
Scheme 2.58
for their synthesis (Scheme 2.57). This last method was used to form vinyl stannane
2.169
for carbonylative
coupling, as part of a synthesis of strychnine (Scheme 2.58; see Scheme 4.12 and Scheme 9.53 for earlier
and later parts of this synthesis).
56
Tin has a valency of four, so there must be four “R” groups. Typically, only one R group transfers. Unless
the “R” group is something cheap and readily available, such as methyl, then three of the group must be
non-transferring dummies. Tri-
n
-butyltin derivatives,
n
-Bu
3
SnR, where R is aryl, vinyl or alkynyl, are often
used. The
n
-Bu group usually transfers more slowly than aryl, vinyl or alkynyl groups, although, sometimes,
the product is contaminated by butyl transfer products. Trimethyltin derivatives, Me
3
SnR, can also be used.
They are more reactive, but more expensive. The removal of the tin by-product can be a problem. While
trimethyltin halide by-products can be removed by aqueous extraction, tri-
n
-butyl tin by-products remain in
the organic phase and their removal can be troublesome. Treatment with potassium fluoride to convert them to
an insoluble polymer,
57
or the use of silica gel impregnated with either KF
58
or K
2
CO
3
,
59
is the best method
of removal. However they are removed, the toxicity of organotin compounds presents a serious problem, and,
if the products are being used in biological assays, any organotin contaminants can give erroneous results.
2.5.1 Vinyl Stannanes
The coupling of vinyl stannanes with vinyl halides is a useful route for making dienes, as, in general, the
stereochemistry of the double bonds of the coupling partners is retained. This property of the reaction was
exploited in a synthesis of coriolic acid (Scheme 2.59).
60
n
-Bu
3
Sn
n
-C
5
H
11
HO
2
C
+
I
OH
2.170
2.171
(MeCN)
2
PdCl
2
HO
2
C
2.172
HO
n
-C
5
H
11
Scheme 2.59
