Chemistry Reference
In-Depth Information
BnPd(PPh
3
)
2
Cl, ZnCl
2
Br
9.54
+
Sn
n
-Bu
3
+
9.52
9.53
9.55
Scheme 9.19
Pd(OAc)
2
P(2-Fu)
3
OAc
CO
2
Et
+
BrZn
CO
2
Et
9.56
9.57
9.58
Scheme 9.20
9.2.1 Regioselectivity
3
-allyl complex is the key intermediate that is attacked by the
nucleophile. In principle, the nucleophile can attack either of the two termini. In practice, it is found that this
depends on the catalyst. With the commonly used palladium-phosphine systems, the less-hindered terminus
is attacked. A synthetic consequence is that either of the two isomeric allyl derivatives may be used, but will
give the same product (Scheme 9.21). The two isomeric acetates
9.59
and
9.60
gave the same mixture of
products
9.62
and
9.63
, with nucleophilic attack on the common intermediate
An important feature of the reaction is that an
3
-allyl complex
9.61
to the
less bulky methyl group being substantially favoured.
30
On the other hand, molybdenum catalysis results in predominant attack at the more-hindered position when
non-bulky nucleophiles are employed (Schemes 9.22 and 9.23). While the use of the palladium-phosphine
system gives the product under steric control, with the molybdenum carbonyl system, attack is now under
charge control. The carbon better able to support positive charge, i.e. the more substituted carbon, is the
site of attack. More-hindered nucleophiles, however, again favour attack at the less hindered terminus,
31
OAc
OAc
Me
Ph
Me
Ph
9.59
9.60
L
n
Pd(0)
P
dL
2
Me
Ph
9.61
CO
2
Et
CO
2
Et
EtO
2
C
CO
2
Et
EtO
2
C
CO
2
Et
92:8
Me
Ph
Me
Ph
9.62
9.63
Scheme 9.21
