Chemistry Reference
In-Depth Information
TABLE 8B.43. Ligand - Controlled Stereoselective R u - Catalyzed Allylic Alkylations
O
R
O
Ph
Ph
Ph
Ph
NaCR
1
(CO
2
Me)
2
(
S
)-
L53
(5 mol %),
THF, 20°C, 6 h
n
Ru
P
CR
1
(CO
2
Me)
2
MeCN
OCO
2
Et
Ph
2
MeCN
L53
Entry
Catalyst
R
n
R
1
Yield (%)
ee (%)
1
(
S
) -
L53a
Me
2
H
96
80 (
R
)
2
(
S
) -
L53b
Ph
2
H
91
91 (
S
)
3
(
S
) -
L53c
t
- Bu
2
H
97
96 (
S
)
4
(
S
) -
L53c
t
- Bu
3
H
14
10 (
S
)
5
(
S
) -
L53a
Me
2
Me
82
63 (
S
)
6
(
S
) -
L53c
t
- Bu
2
Me
75
82 (
R
)
taining a tethered diarylphosphine group (Table 8B.43) [258]. The selectivity and, inter-
estingly, also the direction of induction depends mainly on the substitution pattern on
the cyclopentadiene ring and the tether length. Dimethyl sodiomalonate was used as
pronucleophile for the evaluation of the ligands.
The methyl - substituted complex
L53a
induced formation of the substitution product
in excellent yield and 80% ee (Table 8B.43, entry 1). Increasing the steric size of the
substituent R did not only lead to an increase of the enantioselectivity (entries 2 and 3)
but also gave rise to the enantiomeric product. Increasing the size of the substituents on
the phosphorus had only a marginal effect. In contrast, the tether length (
n
) had a strong
infl uence not only on the selectivity but also on the yield. Prolonging the tether resulted
in a dramatic drop in both (entry 4), while nearly no reaction was observed if the tether
was removed completely. The yield also dropped if dimethyl sodiomalonate was replaced
by the sterically more demanding dimethyl sodio(methyl)malonate (entries 5 and 6).
Again, (
S
) -
L53a
and (
S
) -
L53c
gave rise to the opposite enantiomers, indicating that the
substituents at the 4-position of the cyclopentadienyl ring play an extremely important
role.
Later on, Onitsuka et al. used the same complexes for the kinetic resolution of sym-
metrically substituted allylic carbonates (Table 8B.44) [259]. The reaction rates and ees
were determined by HPLC showing a signifi cant difference in the reaction rates of the
two enantiomers (krel). Again catalyst
L53c
(entry 2) gave rise to the opposite enantio-
mer than catalyst
L53a
(entry 1). This was also true for other allylic substrates (entries
3 to 6). This different stereochemical outcome was explained via the formation of dia-
stereomeric π-allyl complexes with different confi guration at the metal center (Figure
8B.30 ).
It should be mentioned that chiral ligands other than those mentioned above have
been used in Ru-catalyzed allylic substitutions, for example, with phenols as pronucleo-
philes [260], but not with C-nucleophiles so far.
