_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Allyl and diene complexes
The palladium catalysed substitution of an allylic acetate provides an example.
Pdl4
-Cot.
Nu­
-orNuH
Addition of the nucleophile can take place by two sterically different pathways.
In the first (path (a)), direct attack on the 1] 3 -allyl group occurs trans to palladium.
'!:his is by far ~he commone~ route and is followed by stabilized carbanions such as
CH(C0 2 R) 2 , CH(COR) 2 , PhCHCN and C 5 H;, and normally also with amines. Aryl
and alkyl carbanions (e.g. R 2 CuLi) or hydride however add initially to the metal
centre and then migra te to the allyl group (path (b) ). The stereochemistry of the
product depends on which mechanism is followed (v.i.).
frans attack, path (a) cis attack,path( b ).
Băckvall has devised a catalytic method for the conversion of 1. 3-dienes into
their 1. 4-diacetoxy-derivatives. Less than 3% 1. 2-addition occurs. The reaction
is carried out in acetic acid using manganese(IV) oxide, together with a small
quantity of benzoquinone, as oxidizing agent. The catalyst is palladium(II)
acetate in the presence of lithium acetate. Under these conditions, 1, 3-
cyclohexadiene affords trans-1, 4-diacetoxycyclohexene (path (b) ). If a little
lithium chloride is also added, coordination of the acetate ion to palladium is
blocked by chloride, the better of the two ligands. This suppresses the cismigration
pathway (b) so that cis-1,4-diacetoxycyclohexene (path (a)) now
becomes essentially the only product. If the concentration of lithium chloride is
increased still further, chloride replaces acetate as the second nucleophile.
o
Pd<nl/
-OAcr-YOAc
)0'.
[Pd]
poth {o)
direct ottock
y- lrans to Pd
poth!bl CrOAc (Y=OAc)
L----+AcO····
cis migrotion
from Pd
The chloro group in 1-acetoxy-4-chlorocyclohexene is more readily substituted
by nucleophiles than the acetoxy group. Direct SN2 attack of a
nucleophile leads to inversion of configuration at carbon. In the presence of
palladium catalyst, allylic substitution of chloride trans to the metal centre occurs,
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