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

Catalysis of reactions of alkenes by transition metal complexes
A related tungsten complex (R'0) 2 W(=NR)(=CHBut) is an exceptionally
active catalyst for alkene metathesis. Its activity is unchanged over periods of at
least 1 day and the turnover is in hundreds of conversions per minute. A trigonal
bipyramidal tungstenacyclobutane intermediate is again involved.
',','
R'O OR'
Some alkene metathesis catalysts, notably WCl 6 activated by Me 4 Sn, are
tolerant ofmany functional groups such as -C0 2 R, -COR, -CN, -halogen or
-OSiMe 3 • Fischer carbenes such as Ph(MeO)C= W( C0) 5 in the presence ofLewis
acids are also useful. This has been exploited in the synthesis of long chain
dicarboxylic esters from, for example, methyl oleate.
CH,(CH 2 ),CH=CH(CH 2 ),C0 2 Me CH,(CH 2 ) 7 CH=CH(CH 2 ) 7 CH,
IlO C
+ Me0 2 C(CH 2 ),CH=CH(CH 2 ) 7 C0 2 Me
Dicarboxylic esters are used to prepare polyesters and polyamides. The residual
double bonds permit cross-linking of the polymer chains. Cross metathesis of
methyl oleate with excess ethene affords methyl 9-decenoate, a key intermediate
in the synthesis of 'queen bee substance', a honey bee pheromone.
CH,(CH 2 ) 7CH=CH(CH 2 ),C0 2 Me~H 2 C=CH(CH 2 ) 7 CH,
+H 2 C=CH 2 H 2 C=CH(CH 2 ),C0 2 Me--->
+
---> MeCO(CH 2 ),CH=CHC0 2 Me
The compositions of classical Ziegler-Natta and metathesis catalysts are very
similar. It has therefore been suggested by Green and also by Schrock that Ziegler­
Natta polymerization might occur via alkylidene intermediates. The tantalum
complex Ta(CHCMe,)H(PMe 3 ) 3I 2 , for example, in toluene converts ethene into a
living polymer.
H H
1 1 a-H transfer
M-C-R
1 1
CH2 CH 2
tr
etc.
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