Handbook of Functionalized Organometallics Applications in S

14.6
Conclusions
14.6 Conclusions
The examples given in Section 14.5 show that the general control effects elucidated
for each hapticity (see Section 14.3), taken as a whole, provide a uniform
pattern that has predictive value in the design of organic synthesis. The availability
of many types and sizes of electrophilic multihapto-complexes (Figs. 14.2 and
14.3) and generally applicable complete 6 control of stereochemistry (Fig. 14.1c) are
important benefits in this type of approach. In open ligand systems, terminal electron-donating
substituents direct ipso, and internally placed electron-donating
substituents direct x. When the electron-donating substituent is on a cyclic
(closed) ligand it directs b. Electron-withdrawing substituents at the end of an
open p system direct x, and when internally placed, they direct a (g 6 triene complexes
have not been examined). When located on cyclic (closed) ligands, electronwithdrawing
substituents also direct a. Terminally located aromatic substituents
when coplanar with the haptyl section of the ligand direct ipso, but when twisted
out of plane they direct x. When more than one substituent is considered, the
directing effects can be mutually reinforcing, or opposed. In the former case, prediction
of regiocontrol is now reliable, based on the considerations indicated
above, but with opposed groups, the relative powers of each group must be
assessed.
Neither time, nor indeed space in a multiauthor volume such as this, permits
in this chapter a fully comprehensive survey of the current state of knowledge of
regiodirecting effects of substituents on electrophilic multihapto-complexes,
though this will be presented in a later publication [23], but the examples given
here should suffice to illustrate the nature of the most clearly understood control
effects, and the general nomenclature for a universal description of this topic is
presented here (Section 14.2). If generally adopted, this will ultimately permit the
full classification of this chemistry, both from a conceptual point of view, and in
the laboratory in experiments that aim to complete the gaps in the state of knowledge
of the directions of control effects, and their relative powers when placed in
opposed combinations. The simple directing effects presented here, however, provide
enough examples to guide synthetic applications, and establish the frame-
6) Nucleophiles add to the face of the ligand
opposite to that which bears the metal when
reactions occur by a direct addition pathway
and under kinetic control ([2]). This stereocontrol
effect is very strong (100% diastereoselective),
so the planar chirality of the metal
complex can dominate other stereodirecting
influences. There are no conventional stoichiometric
control systems that can match
the generality, versatility and reliability of
this metal-mediated strategy for asymmetric
induction. In the case of stoichiometric-control
methods, it is essential that the multiple
use is made of the control group (otherwise
a catalytic approach would be more desirable),
and as the complexity of the molecule
increases, the diversity of competing control
influences also increases. For this reason, it
is necessary to have very powerful control
groups to systematically overcome all conventional
diastereodirecting effects. There is
a strong argument that nucleophile addition
to g 2 ±g 7 multihapto-electrophiles is the
method of choice for a general approach to
meeting these requirements in a wide variety
(Table 14.1) of target structures.
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