Chemistry Reference
In-Depth Information
Me
H
2
O
Ph
Me
5.222
Cp
2
ZrCl
2
, Me
3
Al
AlMe
2
Ph
Ph
Me
I
2
5.220
5.221
I
Ph
5.223
Scheme 5.63
Insertion of alkynes can also be used to form a carbon-carbon bond at the same time as the carbon-zirconium
bond.
69
This process, carbometallation, is useful for the stereospecific construction of trisubstituted alkenes
(Scheme 5.63). The active reagent is formed by mixing zirconocene dichloride and trimethylaluminium. Both
metals are required for the reaction to proceed. The vinyl organometallic
5.221
produced may be subjected
to protonolysis or halogenation. An application with iodination can be found in Scheme 8.121.
Transmetallation of the organic group from zirconium to another metal opens up possibilities. The
palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results
in transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition
to enones.
70
More economically, small amounts of copper can catalytically activate the zirconium com-
plex towards this kind of chemistry, although the precise mechanism is unclear.
71
Additions to enones can
also be achieved directly using nickel catalysis (Scheme 5.64).
72
Transmetallation to zinc has also been
demonstrated.
73
5.2.2 Alkene and Alkyne Complexes
The organometallic fragments titanocene, Cp
2
Ti, and zirconcene, Cp
2
Zr, have found an important role in
synthesis.
74
Unfortunately, being 14-electron species, they are highly unstable and tend to attack themselves.
To use them it is necessary to supply at least one other ligand to maintain some stability. 1-Butene is a
convenient ligand, as it is generated when the metallocene fragments are generated by treatment of the stable
and commercially available metallocene dichlorides
5.227
with butyl lithium (Scheme 5.65). Addition of
the substrate then results in displacement of the 1-butene. Alternatively, titanocene and zirconocene may be
generated by reduction of the dichlorides in the presence of the substrate, as long as the substrate is stable to
the reduction conditions.
75
Addition of a diene
5.231
to these reagents results in the formation of a metallacycle
5.232
(Scheme 5.66). The carbon-zirconium bonds can then be cleaved by electrophilic or oxidizing agents. If
the reaction is intramolecular, a reasonable degree of control of the new stereogenic centres is possible. The
insertion can, however, be reversible. The stereochemistry of the product therefore depends on the reaction
time and temperature, longer times yielding thermodynamically more stable complexes.
76,77
RO
O
RO
t
-Bu
O
Cp
2
ZrHCl
ZrClCp
2
Ni(acac)
2
, DIBAL
t
-Bu
t
-Bu
5.224
5
.
2
2
5
5
.
2
2
6
Scheme 5.64
