Handbook of Functionalized Organometallics Applications in S

584
14 Polyfunctional Electrophilic Multihapto-Organometallics for Organic Synthesis
strong enough to send the nucleophile into a position substituted by a methyl
group [132]. In fact, in general, the larger period three elements tend to be x
directing. The highly substituted complex 27 (Scheme 14.11), has groups at both
ends of the p-system (and additional rings). Faced with this steric blockade, unusually,
MeLi adds a methyl group to the closed cyclopentadienyl ligand [133].
TfO
Bu
SiMe 3
Co +
Scheme 14.11
+
CoCp
28
27
MeLi
THF, -78 - 0 ºC, 59%
Ref. 113
Complex 28 formed in situ;
nBuLi, THF, -30 ºC, o/n, 82%
Ref. 142
Me3Si H
Co
O
Me
( +
- )
The effects discussed in cyclohexadienyliron complexes and their acyclic counterparts
are mostly reproduced in larger ring sizes, if complications from internal
(see Section 14.3.8) nucleophile addition are set aside. To minimize internal addition,
Fe(CO) 2 phosphine and phosphite complexes have been widely studied, but
even in the parent (g 5 +
-cycloheptadienyl)Fe(CO) 3 series with Knochel-type functionalized
organometallic nucleophiles, good yields (as high as 93%) of the
1,3-diene products can be obtained [134]. Because of the larger ring sizes, unsymmetrical
substitution in the saturated section of the cyclic ligand is possible, and
nucleophiles add under steric control (x addition) to the less hindered end of the
p system [135,136]. The ispo-directing effect of a C-1OMe group is also seen in the
cycloheptadienyliron series [137].
Ketones within the ring of cyclohexadienyl complexes interact with both ends
of the p system, though in the presence of this symmetrically imposed electronic
effect, the simple steric effects from both C-1and C-2 methyl groups seem strong,
and both direct x [138,139]. An A ring steroid complex of iridium provides
another example of a cyclohexadienylone ligand [140]. It is possible that steric distortions
of the ligand bias the strengths of interactions of the ketone with the two
ends of the haptyl section of the ligand, inducing an electronic effect. In the cycloheptadienyl
and cyclooctadienyl cases, the ketone substituent in the correspond-
Bu
Co
Bu