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

Allyl ( -enyl) complexes
towards the metal; this is thought to be why the meso hydrogen also !ies below
the allyl plane. Such features are common to many 17-bonded ligands (p. 280).
Bulky substituents. at the meso position, for example, are often bent away from
the metal through steric effects.
Allylnickel halides are catalysts for the stereospecific polymerization of dienes.
Some methods of preparation and reactions are summarized in Fig. 8. 5.
In solution in polar solvents the complexes couple with primary and secondary
alkyl, aryl, vinyl and allyl bromides and iodides at or below room temperature.
This was exploited by Corey as early as 19 5 6 in his classic synthesis of a­
santalene. Coupling via the Grignard reagent gave low yields.
;&'
a
-sant alene
Under conditions of high dilution to prevent intermolecular reactions, two
allylic halide groups in the same molecule can be coupled to give a cyclic product.
Macrocyclic lactones have been made in this way. Coupling occurs only at the
primary centres. Note that the nickel reagents do not attack ester groups. Neither
do they react with acid chlorides, ethers, nitriles, olefins or alkyl. aryl or vinyl
chlorides (in contrast to bromides or iodides). Aldehydes and cyclic ketones are
attacked, however, above 40°C affording homoallylic alcohols.
OH
~ (79"/.:
The palladium analogue [Pd(I1 3 -C 3 H,)ClL is prepared from Na 2 PdCl 4 and allyl
chloride in the presence of a reducing agent such as CO, SnCl 2 or Zn. It is a yellow
crystalline substance, stable in air, which decomposes on heating only at its
melting point, 159°C. Its structure, determined by X-ray diffraction, shows
symmetrical 17 3 -allyl groups in which the C-C bond distances ( l. 36 Ă) are
equal and the Pd-Cl distances (about 2.41 Ă) are similar to those found in other
complexes in which an alkene is situated trans to Pd -CI. The plane of the three
allyl carbon atoms intersects the plane of the (PdCl) 2 system at an angle of
259