Chemistry Reference
In-Depth Information
X
R
X
X
X
X
U
H
X
A1
A2
A3
Figure 8B.2.
Types of
meso
-substrates that have been used in Pd-catalyzed allylic substitutions.
O
O
O
O
NH
HN
NH
HN
P
P
P
P
Ph
2
Ph
2
Ph
2
Ph
2
(
R
,
R
)-
DPPBA-1
(
L1a
)
(
R
,
R
)-
DPPBA-2
(
L1 b
)
Ph
Ph
O
O
O
O
NH
HN
NH
HN
P
P
P
P
Ph
2
Ph
2
Ph
2
Ph
2
(
R
,
R
)-
DPPBA-4
(
L1d
)
(
R
,
R
)-
DPPBA-3
(
L1 c
)
Figure 8B.3.
The most often used DPPBA ligands [38].
Diesters of types
A1
(U = O, (CH
2
)
n
) and
A2
(X = OAcyl) have mainly been used
by Trost and coworkers. The substitution products derived from substrates of type
A1
can be transformed into nucleosides and carbanucleosides. The work before 2000 has
been reviewed in detail by Trost and Lee [14]. In fact, Trost and coworkers' important
DPPBA
ligands (Fig. 8B.3) have originally been successful in this class of compounds
[35]. In the meantime, polymer-supported variants of these ligands have also been
developed [36,37] .
The confi gurational course of the
DPPBA
-controlled allylic substitutions can be
predicted with the help of a working model developed by Trost and coworkers (Fig.
8B.4) [39]. The situation is more complex than this model suggests because most of the
(allyl)(
DPPBA
)Pd intermediates seem to be oligomers [40], which makes elucidation
of reaction mechanism exceedingly diffi cult. Furthermore, π - allyl complexes with
C
2
-
symmetric ligands are
C
1
- symmetric (cf. Section 8B.2.2.2.1 ) [41] . Despite these problems,
the model has allowed to rationalize a large body of experiments coherently and to make
predictions.
8B.2.2.1.1. Alkylations of Esters of Allylic
meso
- 1,4 - Diols
A few benchmark examples
of the reactions of a
meso
- diester are described in Table 8B.1 and Scheme 8B.9 [37] . In
