2 Homometallic Alkoxides

678 Alkoxo and Aryloxo Derivatives of Metals
complexes in which alkoxy groups play an important role in optimizing the catalytic
performance of the metal complex. Complexes of Mo(VI), W(VI), and Re(VII) with the
d 0 electronic configuration have proved to be most effective with OBu t ,OCMe2CF3,
OCMe(CF 3⊳2, OCH(CF) 3⊳2, andOC6H3R2-2,6 as the alkoxy groups.
The pseudo-tetrahedral tungsten alkylidene complexes W(CHR)(NAr)⊲OR 0 ⊳2 (Ar D
2,3-diPr i phenyl) are catalysts for olefin metathesis. 94 The activity of these molecules
is sensitive to electronic and steric factors of all three types of ligand. In the case
of the neopentylidene (R D Bu t ) complexes, W⊲CHBu t ⊳⊲NR⊳⊲OBu t ⊳2 does not react
with ordinary olefins but does react readily with more reactive substrates such as
norbornene. On the other hand W⊲NHBu t ⊳⊲NR⊳fOCMe(CF 3⊳2g2, containing the more
electronegative tertiary alkoxide, is an active metathesis catalyst for ordinary olefins
giving turnover rates of ca. 10 3 turnovers min 1 in a hydrocarbon solvent at 25 Ž C.
X-ray crystal structures of some of these complexes have been obtained. The course
of the reaction depends on the nature of the olefin RCHDCHR 0 and in some cases
tungstacyclobutane complexes are formed as pseudo-trigonal bipyramidal complexes
containing an axial arylimido ligand and an equatorial bent tungstacyclobutane ring
(see Table 4.11, p. 307). The corresponding molybdenum complexes are also active
towards olefins but less so than the tungsten complexes except towards the more
reactive olefins. Interestingly, although M⊲CHBu t ⊳⊲NR⊳⊲OBu t ⊳2 (M D Mo, W) does
not react readily with ordinary internal olefins such compounds do react extremely
rapidly with the strained double bonds in norbornenes and norbornadienes. 93 Ringopening
metathesis polymerization (ROMP) of norbornene occurs even at 80 Ž C with
W⊲CHBu t ⊳⊲NR⊳⊲OBu t ⊳2 giving a “living polymer” containing up to 500 monomer
units. The polymer can be released by addition of benzaldehyde to give the oxo
metal complex, or by chain transfer with styrene. The polymers produced are
essentially monodisperse. The molybdenum complex Mo⊲CHBu t ⊳⊲NR⊳⊲OBu t ⊳2 was
particularly effective as a catalyst for the ROMP of functionalized norbornadienes
giving monodisperse living polymers. 94 Alkynes (acetylenes) can be metathesized
(Eq. 7.5) using the metal alkylidyne complexes M(CR)⊲OR 0 ⊳3, where M D Mo or
W, R D Bu t ,andR 0 D 2,6-diisopropyl-C 6 H3. 92
2RC CR 0 ! RC CR C R 0 C CR 0
⊲7.5⊳
Although W⊲CBu t ⊳⊲OBu t ⊳3 readily metathesizes internal acetylenes the corresponding
molybdenum complex is inactive. However, with a more electronegative alkoxy
group such as OCMe⊲CF3⊳2 the Mo-complex will catalyse the metathesis of 3heptyne.
In the case of the more reactive tungsten complexes W⊲CBu t ⊳⊲OR 0 ⊳3, the
tungstacyclobutadiene complexes W⊲C3R3⊳⊲OR 0 ⊳3 were isolated from the metathesis
of RC CR. X-ray crystallography confirmed that the tungstacyclobutadiene complexes
have a distorted trigonal bipyramidal structure containing a planar WC3 ring. These
complexes are active in the metathesis of acetylenes. Some Re(VII) alkylidyne
complexes of the type Re(CR)(NAr)⊲OR 0 ⊳2 have also been shown to react with
acetylenes. 95 The neopentylidyne complex Re⊲CBu t ⊳⊲NR⊳fOCMe⊲CF3⊳2g2 reacted
rapidly with 3-hexyne at 25 Ž C giving Bu t C CEt and the rhenacyclobutadiene complex
Re⊲C3Et3⊳⊲NR⊳fOCMe⊲CF3⊳2g2. Acetylene has been polymerized using the tungsten
neopentylidene complex W⊲CHBu t ⊳⊲NR⊳⊲OBu t ⊳2. 96 Di-tertbutyl-capped oligomers
were obtained by carrying out the reaction in the presence of the base quinuclidine
and adding pivaldehyde to release the oligomer from the metal.