Chemistry Reference
In-Depth Information
Figure 8B.6.
Statistics of publications (SciFinder Scholar) on allylic alkylations according to Scheme
8B.16 (SciFinder analysis).
substrates such as derivatives of 1,3-dimethyl-allyl alcohol and a cyclic substrate (Scheme
8B.16 , R = Me,
n
= 1-3). A note of caution is necessary with respect to reactions of the
former substrate. It was found in our own work that in these reactions,
cis
isomers are
formed as by-products, which falsify the enantiomeric excess determination if they are
not properly taken into account.
8B.2.2.2.2. Catalysis with P d Complexes of
C
2
- Symmetric Ligands
The situation is
comparatively simple in the case of reactions promoted by a
C
2
- symmetric bidentate
ligand. In principle, three intermediary (π - allyl) - complexes are possible: a
syn
,
syn
- , a
syn
,
anti
- , and an
anti
,
anti
-complex [62]. In the case of a convex ligand the
syn
,
syn
-
complex is generally the most stable one (Scheme 8B.17). However, with concave ligands
and the dimethylallylic substrates (R = Me) the
anti
,
syn
-content can be high. The rate
of substitution at the
anti
- is higher than that at the
syn
- position [27] , and therefore, this
type of isomerism leads to a reduction of enantioselectivity.
R
R
*
L
a
X
Nu
L
Pd
[PdL*]
R
R
R
Nu
R
b
a
R
R
b
(
syn
,
syn
)
Nu
Scheme 8B.17.
Allylic substitution using a
C
2
- symmetric ligand.
Results obtained with
C
2
-symmetric phosphines as ligands are presented in Table
8B.3 . With Trost ' s ligands
DPPBA - 1
[63] and
DPPBA - 2
(entries 1-3), which belong to
the class b of Figure 8B.5 [64], enantioselectivities and yields were excellent in alkyl-
ations of small substrates, that is, cyclic substrates and methyl (3-penten-2-yl) carbonate.
