Handbook of Functionalized Organometallics Applications in S

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14 Polyfunctional Electrophilic Multihapto-Organometallics for Organic Synthesis
substantial selectivity for x a in preference to x s (4.6 : 1). Where nucleophile addition
favors x a, the nucleophile is adding to the less hindered of the two x positions,
so the outcome may be the result of steric effects [184]. In the cationic
Mo(CO) 2Cp series, large, functionalized, alkyl substituents again direct x, but in
this case, x s is the site of reaction [28].
Turning to the far more structurally diverse g 5 system, the alkyl groups direct x s
with a substantial degree of selectivity (9 : 1± 99 : 1), the minor pathway being ipso
[185,186]. These reactions afford trimethylenemethane products, and the preference
is strong enough to control nucleophile addition even to a position bearing
two Me groups. This is a good example of product-derived control (see Section
14.4). The ester directing group in 44, however, overcomes this effect and directs
x, but the x s : x a selectivity is only 2 : 1[184].
14.3.7
Conjugate Addition to Unsaturated Extensions of Electrophillic
Multihapto-Complexes
In all of the examples discussed so far, the haptyl portion of the ligand corresponds
to the whole of the conjugated p system. In cases where only part of the p
system is bound to the metal, the issue arises whether nucleophiles add to the
haptyl section, or to the uncomplexed section (i.e., ªlocallyº, or ªremotelyº relative
to the metal 3 [187]). The cationic Fe(CO) 2Cp complex 45 [188] provides a good
example (Scheme 14.18) of remote (ªconjugateº or ªMichaelº) addition in which
the methyl group is introduced to the alkene that is not bound to the metal.
Because the alkene is in conjugation with the cationic g 2 complex, it is activated as
an electrophile. The principle is general to all the sizes of hapticity discussed in
this chapter, and the issue of local versus remote reaction pathways can be examined
for the whole class of structures. Examples in the g 3 series tend to occur in
palladium-catalyzed processes [189±194] and are complicated by many selectivity
factors, but nonetheless can be interpreted in terms of local versus remote addition
[23]. A stoichiometric g 4 example has been examined. Reduction with
NaBH3CN gave a product consistent with initial remote addition of hydride to the
alkene [49]. In the g 5 +
series, using the classic Fe(CO) 3 complexes, a detailed study
has been made by my own group. With a simple ethenyl extension to the
g 5 cyclohexadienyl, selectivity for local/remote addition was found [195] to depend
on the nature of the nucleophile. Borohydride, cyanide and NaCH(CO2Me) 2 gave
exclusively products from local addition in which the CH=CH2 group directed x.
However, when organocuprates were used, remote addition was the major outcome.
The local addition pathway can be stopped by placing an additional directing
group on the cyclohexadienyl ligand, as shown by the example 46, where the
3) A recent paper by Trost ([187]) describes
these pathways as ªdirectº and ªS N2¢º, but
since palladium-catalyzed allylic substitution
can be discussed in terms of ªdirectº and
ªvia the metalº (ªindirectº) pathways, we pre-
fer the usage ªlocalº and ªremoteº to
describe reactions that take place in the haptyl
section of the ligand, and in the part that
is not bound to the metal.