Chemistry Reference
In-Depth Information
X
[M]
X
[M]
X
[M]
[M]X
Nu
Nu
Nu
[M]
Scheme 8B.2.
Representative catalytic cycle of a transition metal-catalyzed allylic substitution.
terion X. In most cases, a soft carbanion is the nucleophile, which typically adds directly
to a terminal carbon with inversion of confi guration rather than via the metal cation
with retention. The resultant olefi n complex dissociates to yield the product and the
regenerated catalyst.
Usually, but not always, the substitution step is irreversible and turnover determining.
However, there are examples where the formation of the olefi n complex or isomerization
among several π-complexes is slow. Rates of all these processes are dependent on ligands
at the metal, and therefore can be infl uenced by additives, for example, halide salts,
which introduce metal-coordinating species.
It is helpful for the understanding of substitutions at π - allyl complexes to consider
the fundamental types of substrates leading to chiral products as described in Scheme
8B.3. We are using here the classifi cation of Trost and Lee. The presentation is general
with respect to the metal as this classifi cation is very broadly applicable.
Type A:
meso
-Compounds are used as substrates. Chirality is introduced in the
oxidative addition step of the catalytic cycle. There are several structural
types; only one particular is shown for illustration.
Type B:
Chiral racemic allylic derivatives with two identical substituents in posi-
tions 1 and 3 are used as starting materials. If an achiral metal fragment
is present, the intermediary allyl complexes are achiral and the allylic
termini enantiotopic. With a chiral ligand at the metal, the termini are
diastereotopic and the rates of addition of a nucleophile are different.
Types C, D:
In this class, a racemic or prochiral substrate containing two identical
geminal substituents R
1
at one of the allylic termini reacts via a π - allyl
intermediate, which can isomerize by the well-established π - σ - π mecha-
nism (see below) [16]. Enantioselection can occur in the nucleophilic
addition step or in the ionization step leading to the allyl intermediate
(type D), depending on the relative rates of isomerization of allyl com-
plexes and their reactions with the nucleophile.
