Chemistry Reference
In-Depth Information
a
b
Figure 8B.8.
Mechanistic aspects concerning phosphinooxazoline ligands. Left: X-ray crystal struc-
ture of a (
π
- allyl)Pd complex of
L14c
; right: (
π
- allyl)Pd complexes of Yamagishi ' s ligands
L14d,e
.
O
O
N
N
N
P
R
Pd
O
R'
Ph
Figure 8B.9.
Intermediary (
π
-allyl)Pd complex postulated by Landis et al.
coordination with a stereogenic phosphorus center. Remarkably, one of two possible
P-diastereomers is strongly preferred; furthermore, strong preference for
exo
- isomers
was found, which explains the high selectivity indicated by these ligands (cf. Fig. 8B.8)
[92] .
Gilbertson and Xie [93] introduced
PHOX
derived from proline (
L14f
), which gave
excellent results with small cyclic substrates (entries 8-10).
Phosphite/oxazoline ligands were fi rst used by Pfaltz
et al.
for Pd - catalyzed allylic
alkylations [95]. Pamies et al. developed a new series of such ligands forming a seven-
membered chelate ring upon coordination to palladium [96]. As in the case of the related
P,P-ligands presented above, the bulky tetra-
tert
- butyl - biphenyl unit is an essential
module of these P,N-ligands. Excellent results with respect to activity and selectivity
were achieved with carbocyclic substrates. Good results were also obtained with analo-
gous carbohydrate - based P,N - ligands [97] .
Clark and Landis introduced a series of putative P,N-ligands, which gave remarkably
high enantioselectivity with 1,3-dimethylallyl acetate as substrate [98]. The authors
proposed a P,N-chelate complex as intermediate (Fig. 8B.9). This proposal was mainly
based on the observation that the results are not dependent on the ratio ligand/Pd
(Table 8B.7 ).
P,S-ligands are interesting with respect to activity of the corresponding catalysts. A
number of such ligands have been developed. Early work of the Evans group still stands
out as an example of a well-planned and exhaustively documented project (Table 8B.8).
