Chemistry Reference
In-Depth Information
H
R
1
R
1
R
2
R
2
X
H
R
1
R
2
PdL
2
L
Nu
H +
X
L
Pd
0
Pd H
L
L
XH+ Nu
Nu
Allylic substitution
Hydrocarbonation
L
Pd
0
Nu
H
L
H
R
1
R
2
Nu
Scheme 8B.25.
Allylic alkylation and hydrocarbonation.
acid/NEt
3
, was possible. To get stereoselection in the addition of the nucleophile to the
(π-allyl)Pd complex, monosubstituted allenes had to be used to get equilibrating inter-
mediates. Good results were achieved with benzyloxy-allene, using
DPPBA - 1
as the
chiral ligand (Table 8B.13) [116,117]. In addition to Meldrum's acid derivatives, azlac-
tones and a wide range of symmetric 1,3-dicarbonyl compounds have been used as
addition partners.
8B.2.2.3.2. Allylic Alkylation via Trisubstituted ( π - Allyl) Complexes
Reactions pro-
ceeding via trisubstituted allyl complexes (Scheme 8B.20, R ≠ H) have been studied
already in the 1980s in lieu of success with the monosubstituted allylic substrates. With
diphenyl-substituted compounds (Scheme 8B.26), both electronic and steric effects guar-
antee a high degree of regioselectivity in favor of the chiral product. Up to 86% ee were
achieved even with
CHIRAPHOS
[118] and Sparteine [119] as chiral ligands. This work
is covered well in all previous reviews. Standard
PHOX
ligands such as
L14a
induced
superior enantioselectivity; however, the reactions were generally slow (Scheme 8B.26)
[120]. Higher degrees of reactivity were obtained with the very bulky ligands
L22a
(aminations) [121] and
L22b
(alkylations) [122] .
A result with interesting mechanistic connotations shall be added. The reaction of the
triphenylallyl acetate described Scheme 8B.26 must proceed via addition of the nucleo-
phile at the secondary allylic carbon of the
endo
,
trans
(
nt
) or
exo
,
cis
(
xc
) isomer to yield
the observed product (cf. Scheme 8B.27). A Pd complex was prepared and found to be
the
exo
,
trans
(
xt
) isomer in the crystal and in solution [89]. As the presumably reactive
complex is the
xc
isomer, reacting at the allylic carbon
trans
to phosphorus, it was con-
cluded that its reaction is faster by a factor >10.000 than the reaction of the observed
complex
xt
, yielding the minor enantiomer of the alkylation product via reaction at the
carbon
trans
to N. This example illustrates once more the importance of the Curtin-
Hammett principle.
