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

Catalysis of reactions of alkenes by transition metal complexes
u
vv
J Cope
rearrangement
~i-L) +2 ~--
~
L-Ni
1
~
l-Ni~"
gives VCH
(gives COD)
~
(gives DVCB)
Fig. 12.6 Catalytic conversion of 1, 3-butadiene into cyclic dimers. VCH = 4-
vinylcyclohexene: COD= cis-1. 5-cyclooctadiene: DVCB = cis-l. 2-divinylcyclobutane.
When L = P(OC 6 H 4 PH "), at 60C. COD= 94.5%: L =
PPhEt 1 • COD= 38.2%: DVCB = 12.6%: VCH = 46.0% of total products.
cis-Divinylcyclobutane is also formed with basic phosphines at low temperatures
( ~ 60 "C). At higher temperature a nickel catalysed Cope rearrangement to 1. 5-
COD occurs.
Whereas nickel complexes catalyse the trimerization or dimerization of
butadiene to cyclic products, palladium catalysts produce linear oligomers. When
bis(dibenzoylacetone)palladium is treated with butadiene at low temperature, a
yellow air sensitive complex ( IJ 1 , 1J '-dodecatrienediyl)palladium is formed initially.
It decomposes above -20 C in the absence of butadiene and in its presence
actively catalyses its trimerization to linear 1. 3. 6. 1 0-dodecatetraene. In contrast
to the corresponding nickel intermediate. the substituents on the allyl
groups are syn. Treatment with a diphosphine affords a metallacycle which
contains a huge 13-membered ring. If (A) or (B) is allowed to warm above -20 C
or is treated with ligands (Ph ,P or CO). dodecatetraene isomers are released
(Fig. 12.7).
In the presence of added ligand (Pd:L = 1: l ). l, 3, 7-octatriene isomers are
obtained (Fig. 12.8). The conformation ofthe Cs chain in the intermediate (C) bas
been determined in solution by n.m.r. spectroscopy and also in the solid state
(L = Me 3 P) by X-ray diffraction. While the analogous nickel intermediate rearranges
at room tempera ture. leading to cyclic products. the palladium compound
does not. If the dimerization is carried out in the presence of active hydrogen
369