Chemistry Reference
In-Depth Information
Zr aryloxides, reflecting the high electrophilicity of the metal centres. The addition
of alkenes or alkynes to the titanium species leads to cationic mono-insertion prod-
ucts in which chelation
via
the original benzyl phenyl ring occurs with displacement
of the boron anion. The corresponding titanium di-methyl substrates form cationic
species with [B(C
6
F
5
)
3
] which decompose to a mixture of [(ArO)
2
Ti(C
6
F
5
)Me] and
[MeB(C
6
F
5
)
2
].
361
In the presence of ethylene or propylene, polymerization by the
cationic methyl species occurs to yield polymers (low poly-dispersities) whose molec-
ular weights are strongly dependent on the nature of the aryloxide ligand. The poly-
propylene formed is atactic with predominantly vinylidene end groups (1,2-insertion
with
ˇ
-hydrogen elimination termination step).
361
Chelating diaryloxo derivatives of titanium have been applied to the polymerization
of olefins.
53-57
The nature of the bridging ligand has been shown to strongly affect the
activity of catalyst precursors [(di-ArO)TiX
2
] activated with MAO. For example the
highest activity for the syndiospecific polymerization of styrene was found to be for the
sulfur-bridged ligand.
376
Coordination of the sulfur atom is believed to be important
and was demonstrated in one case by crystallographic studies.
55
The derivatives [Cp
0
(ArO)TiX
2
](ArOD 2,6-dialkylphenoxide; X D Cl, Me,
O
3
SCF
3
)
377
and [Cp
Ł
Ti(OC
6
F
5
)X
2
]
378
have been shown to be active catalyst precursors
for the polymerization of olefins. Reaction of [Cp
Ł
TiMe
2
(OC
6
F
5
)] with the Lewis
acid [B(C
6
F
5
)
3
]leadsto[Cp
Ł
TiMe(OC
6
F
5
)(
-Me)B(C
6
F
5
)
3
] which is in equilibrium
with its ion pairs. In contrast [Cp
Ł
Ti(OC
6
F
5
)
2
][MeB(C
6
F
5
)
3
] exists in solution as
separated ion pairs.
378
In the case of the compounds [Cp(ArO)TiMe
2
], (ArO D 2,6-
diarylphenoxide) activation with [B(C
6
F
5
)
3
] again leads to thermally unstable cationic
species. In this case the use of chiral,
ortho
-(1-naphthyl)phenoxides has allowed
the molecular dynamics of these species to be studied in detail by low-temperature
NMR.
356
Decomposition in this case leads to elimination of methane and formation of
species [Cp
ArO
TifCH
2
B
C
6
F
5
2
g
C
6
F
5
].
The reduction of the dichlorides [(ArO)
2
TiCl
2
] in the presence of unsaturated organic
substrates can lead to a variety of stable metallacyclic compounds.
351
,
379 - 381
The reduc-
tion can be achieved using sodium amalgam or by alkylation with two equivalents
of
n
-BuLi. These are interesting molecules in their own right as well as being key
intermediates in a number of catalytic cycles. The addition of ketones or imines to
titanacyclopentadiene or titanacyclopentane species can each lead to simple ring expan-
sion. However, in a number of cases elimination of alkyne or olefin respectively can
occur. This indicates that metallacycle formation is a reversible process (see below).
The titanacyclopentadiene species generated from alkynes and di-ynes will carry out the
cyclotrimerization of alkynes to arenes as well as the selective catalytic (2 C 2 C 2)
cycloaddition of olefins with alkynes to produce the 1,3-cyclohexadiene nucleus.
382
The observed regio- and stereochemistry of the 1,3-cyclohexadienes is controlled by
the structure and isomerization of intermediate titana-norbornene compounds generated
by pseudo-Diels - Alder addition of olefin to titanacyclopentadiene rings. The titana-
cyclopentane rings formed by coupling of olefins exhibit much greater thermal stability
than their titanocene counterparts.
351
In solution the titanacyclopentane formed by
coupling of ethylene demonstrates fluxionality consistent with facile fragmentation
to a bis(ethylene) species on the NMR time scale. The structure of the metallacycle
formed from styrene has a
trans
-2,5-regiochemistry in the solid state. However, the
slow, catalytic dimerization of styrene by this species in solution produces a dimer that
