2 Homometallic Alkoxides

Metal Aryloxides 563
can only arise via the 2,4-regioisomer. 351 Hence in solution a rapidly interconverting
isomeric mixture of metallacycles are present. The addition of donor ligands such as
PMe3 to these titanacyclopentanes produces titanacyclopropane and one equivalent of
olefin. The titana-bicycle produced from 1,7-octadiene initially has a cis configuration
but in solution will rapidly produce the trans species. At 100 Ž C this compound will
catalyse the cyclization of 1,7-octadiene to 2-methyl-methylenecyclohexane. 351
Although ˇ-hydrogen abstraction from five-membered titanacycle rings is slow,
expansion to a seven-membered ring containing ˇ-hydrogens can lead to facile
abstraction/elimination pathways. This principle underlies a number of highly selective
coupling and cross-coupling reactions that can be catalysed by titanium aryloxide
compounds. Hence, a mixture of 2,3-dimethylbutadiene and ˛-olefins can be selectively
dimerized by the titanacyclopent-3-ene compound [(ArO)2Ti(MCH2CMeDCMeCH2)]
to form 1,4-hexadienes. 383 A related coupling of 1,3-cyclo-octadiene produces 3-vinylcyclo-octenes.
358 Both reactions are overall 1,4-hydrovinylations of dienes and in the
case of 1,3-cyclo-octadiene the reaction has been shown to occur in a cis fashion.
The key to the selectivity hinges on initially formed 2-vinyl-titanacyclopentanes which
undergo facile ring expansion via 1,3-allylic shifts to produce titanacyclohept-3-ene
rings. Abstraction from the ˇ-position originating from the olefin produces the observed
products.
In the case of 1,3-cyclohexadiene, addition to any of the titanacyclopentane compounds
or to a mixture of f[⊲ArO⊳2TiCl2]/2n-BuLig leads to the rapid formation of a non-
Diels–Alder dimer. Following complete dimerization, subsequent isomerization via
metal mediated 1,5-hydrogen shifts occurs as well as coupling of dimers to higher
oligomers. 358 The regio- and in particular the exclusive threo stereochemistry of the
initially produced dimer is due to the coupling of 1,3-cyclohexadiene to produce a cisanti-cis-titanacycle.
An allylic shift followed by ˇ-hydrogen abstraction/elimination
process leads to the threo isomer. A logical extension of this work is the cross-coupling
of olefins with 1,3-cyclohexadiene. Again the products can be rationalized using the
sequence of coupling to a five-membered ring, expansion via 1,3 shifts and ˇ-abstraction/
elimination. 358
6.2.6 Group 5 Metal Aryloxides
For vanadium, the use of available lower valent halides has allowed the isolation of simple
aryloxides with the metal in a variety of formal oxidation states (Table 6.24). Important
mononuclear examples include [V(OC6H3Me2-2,6)3(py)2] 384 and square planar
[V(OC6H2Bu t 2 -2,6-Me-4)2(py)2]. 385 A variety of vanadium(V) aryloxides [(X)V(OAr)3]
have also been isolated containing oxo (X D O), 386,387 and imido (X D NR) 388–390
ligands.
Many early examples of niobium and tantalum aryloxide compounds were obtained
via the pentahalides by reaction with either the parent phenol or alkali metal aryloxides.
391–395 The homoleptic aryloxides, monomeric trigonal bipyramidal for bulky
ligands, e.g. [Nb(OC6H3Me2-2,6)5] 396 and edge-shared bi-octahedra for small ligands,
e.g. [Ta2( 2-OC6H4Me-4)2(OC6H4Me-4)8] 397 have been used as precursors for lower
valent derivatives. The mixed chloro, aryloxides [M(OAr)xCl5 x ] (Tables 6.24–6.25)
are an important group of starting materials. Structural studies typically show a square
pyramidal geometry for bulky aryloxides, e.g. [Ta(OC6H3Bu t 2 -2,6)xCl5 x ](x D 2, 3;
axial OAr), but the compounds [M( 2-Cl)(OC6H3Pr i 2 -2,6)2Cl2]2 (M D Nb, Ta) are