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type of products without side reactions associated with arylation of
enolic forms.
Concerning the enolic forms of enolizable carbonyl compounds, both
cross-coupling and Heck reactions can be performed depending on the
particular type of derivative and conditions used. These two directions differ
in the products formed - the cross-coupling affords a-arylation. Ionic eno-
lates, silyl ethers and similar compounds have been developed in the last
decade as versatile nucleophiles suitable for the cross-coupling method-
ology, depending mainly on the design of special ligands (reviewed in
2009
94
).
On the other hand, enol ethers and similar compounds are well-known
targets for Heck reactions, both regular and oxidative, being considered as
electron-rich olefins subject to special Cabri-Larhed methodology using
bidentate phosphines for enforcing high arylation regioselectivity. This
method actually transforms aldehydes into ketones, thus effectively ex-
tending the classical Umpolung strategy of organic synthesis (acyl transfer
from aldehydes) to aromatic and unsaturated compounds.
Whereas the Cabri-Larhed approach uses preformed enol derivatives, new
protocols successfully employ raw aldehydes for acylation. Such a break-
through was reported by Xiao and co-workers
95,96
using organocatalysis
coupled with Heck arylation of enamines generated in situ from enolizable
aldehydes (Scheme 9.21). High a-regioselectivity of enamine arylation was
confirmed by reaction with a preformed enamine.
Aryl bromides are used with a typical catalytic system [Pd(dba)
2
, dppp,
pyrrolidine, 4A molecular sieves, DMF, 115 1C] for arylation of electron-rich
olefins.
95
Ruan and co-workers showed that the same goal can be achieved in
a simple ''phosphine-free'' system using palladium acetate as precatalyst
and Bu
4
NBr, the standard agent for stabilization of Pd(0) species
(Scheme 9.22). Palladium sol was detected in the reaction medium, which
apparently served as the actual precatalyst (see below). The system is re-
markable from many points of view, not only because of the apparent pre-
parative value, but also because it serves well for a wide range of aryl
bromides, including many electron-rich compounds, thus being one of the
ArP dX
R
N
H
O
N
N
R
R
XP d
Ar
H
H
H
XPdH
N
O
R
R
Ar
Ar
N
H
Scheme 9.21 Mechanism of
arylation
of
enolizable
aldehydes
using
organocatalysis.
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