ca01 only detailed ToC 1..24

2.6.1 Metal–Carbene Complexes 99
of the first triple bond. [43] The nature of the coordination sphere on the carbene initiator is
of capital importance, as polymers containing both five- and six-membered rings that are
a consequence of an initial Æ- orâ-addition, respectively, have been obtained with other
initiators. [44]
Scheme 10 Alkyne Polymerization [42,43]
x R 1
[Mo] CHCMe2Ph toluene, rt
[Mo]
[Mo] =
OCMe(CF3)2
N Mo
OCMe(CF3) 2
; R1 =
TMS
x
EtO 2C
[Mo] =
24
CO 2Et
Bu t
[Mo] CHt-Bu
N Mo(O 2CCPh 3) 2
R 1
EtO2C
EtO2C [Mo]
CHCMe 2Ph
x
β-addition
; x = up to 150
CHBu t
PhCHO
PhCH
EtO 2C
[Mo]
R 1
x
23
CO2Et
CHCMe 2Ph
CHBut x
2,2-Diprop-2-ynylmalonic Acid, Diethyl Ester, Homopolymer (x = 20);
Typical Procedure: [43]
Polymerization stock solutions of diethyl 2,2-diprop-2-ynylmalonate (24; 0.406 M) and
[Mo(=CHt-Bu)(O 2CCPh 3) 2(=NC 6H 42-t-Bu)] (0.00676 M) in toluene that had been distilled
over sodium benzophenone ketyl, stored over molecular sieves (4 Š), and passed through
alumina, were prepared. To the catalyst stock soln (3.007 mL), toluene (3 mL) was added
and the monomer stock soln (1.0 mL) was squirted in. Within 30 s the mixture turned
deep red. After 6 h, benzaldehyde (16 ìL) was added. After stirring for a further 3 h, the
soln was concentrated in vacuo to about 1.5 mL. The polymer was precipitated in pentane
(60 mL), collected on a frit, and dried in vacuo to yield a red, powdery material. All operations
were carried out under a nitrogen atmosphere and the polymers were only briefly
exposed to air for sample preparation for GC analysis; yield: 90 mg (91%); 13 C{ 1 H} NMR
(CDCl 3, ä): 170.7 (CO 2Et), 54.5 (C quaternary).
2.6.1.5.3 Variation 3:
Ring-Closing Metathesis
Ring-closing metathesis (Scheme 11) affords cyclic products 25 in competition with intermolecular
metathesis processes to form polymeric materials. [45] The reaction is also complicated
by the possibility of ROMP (Section 2.6.1.5.1). Which products are obtained is the
result of an interplay of thermodynamic and kinetic parameters. Entropy (the generation
of two molecules from one) and evaporative subtraction of the volatile acyclic alkene
from solution provide the necessary driving force for the desired cyclization. The ringclosing
metathesis procedure can be applied to the construction of macrocyclic natural
products, although mixtures of E- and Z-isomers are often obtained in these cases (see
also Section 2.6.2.7). [46] The most widely used group 6 catalyst for this process is 26, although
ruthenium-based catalysts are generally preferred owing to their greater func-
for references see p 135