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

Catalysis of reactions of alkenes by transition metal complexes
surplus ofpropene and a deficiency ofpolymerization grade ethene. Conversely a
plant opened in 19 8 5 in Channelview, Texas combines dimerization and
metathesis to convert ethene into propene. The Shell Higher Olefins Process
(p. 81) for generating C, 1 -C 16 olefins from higher and lower fractions also
involves a metathesis step. Neohexene. an intermediate in the manufacture of
musk perfumes for soap, is obtained by co-metathesis of commercial diisobutene
with ethene. The catalyst W0 3 on Si0 2 /Mg0 functions both for isomerization and
metathesis.
Analogous homogeneous systems. however, give very similar results. Studies
on the latter have thus shed much light on the mechanism of metathesis and
incidentally have revealed some intriguing navei chemistry. Homogeneous
metathesis catalysts resemble Ziegler-Natta systems in that they can be prepared
from a transition metal halide ( e.g. WC1 6 , MoCl 5 or ReCl 5 ) and a Main Group alkyl
(e.g. RLi, R 3 Al or R 4 Sn). A mechanism involving alkylidene intermediates, first
proposed by Chauvin in 19 70, is now well established. The more obvious but
rather vague four centre mechanism has been effectively disproved.
Alkylidene mechanism:
RCH=M
\"-..
R'CH=CHR"
RCH M
Il + Il
R'CH CHR"
metalle cyclobutane
Four-centre mechanism:
RCH CHR"
11 M Il
R'CH CHR"'
RCH---CHR"
1 ', ,' l
~ M :
R'CH~-~CHR"'
RCH=CHR"
M
R'CH=CHR"'
Cycloalkenes, such as cyclooctene, afford linear polymers on metathesis. This is in
accord with the carbene mechanism, but not with the four-centre mechanism
which predicts the formation of large rings (Fig. 12.11 ).
Cyclooctene is obtained by partial reduction of 1, 5-cyclooctadiene, which in
turn is available from butadiene (p. 369). Co-metathesis between the polyoctene
and cis-1, 4-polybutadiene gives a rubber which is very sta bie to heat, oxygen and
light, is easily moulded and is readily combined with other rubbers by
vulcanization.
375