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

Allyl ( -enyi) complexes
tempera ture to give a mixture of two geometrica! isomers ( anti-methyl and synmethyl)
in the ratio 88:12 (kinetic control). This suggests that the diene adds
preferentially in the cisoid conformation which is 12 kJ moi- 1 higher in energy
than the transoid conformation (p. 26 3 ).
lnrll
[ l
+
~
H~L 3 -
The mixture of isomers is converted into the thermodynamically controlled
mixture syn:anti (95:5) by heating to 70°C.
Complexes with anti substituents are generally less stable than their syn
isomers. The anti position is the more congested one. Isomerization probably
occurs via alkene-hydride intermediates. HCo(C0) 4 adds to 1. 3-butadiene at
25°C to give a mixture of syn (35%) and anti (65%) isomers. but the Iatter
isomerizes to the former on heating to 80°C.
Co
(C0) 3
8.1.3 Allylpalladium complexes- intermediates in organic synthesis
Allylpalladium complexes find wide application in organic synthesis. With
carbon nucleophiles they form new C-C bonds, thus providing a method for
building carbon skeletons. Dimeric allylpalladium chlorides are not very susceptible
to attack by nucleophiles, but their reactivity can be enhanced by addition
of triphenylphosphine, which converts them into monomeric species. These
reactions, however, are stoichiometric in palladium, which is undesirable on
account of its high cost. Fortunately related catalytic procedures have been
discovered. Many allylic compounds react with Pd(PPh 1 ) 4 (p. 181) to form
cationic '1 3 -allyl complexes. These cations react with nucleophiles to form
substituted allylic derivatives. while the catalyst, effectively 'PdL 2 '. is
regenerated and reenters the cycle.
~X+'PdL2'-
\ ____ _
/Nu
-=!'-
+ 'Pdl2'
261