Chemistry Reference
In-Depth Information
As reported by Trost et al., a complete chirality transfer can be obtained if chiral
allylic substrates are submitted to Ru-catalyzed allylation (Scheme 8B.61) [255]. The
reaction occurs regioselectively at the branched position with excellent yield. The
nucleophile attacks the
-allyl complex from the face opposite the Ru [256], comparable
to the Pd-catalyzed reactions. The perfect chirality transfer indicates that the nucleo-
philic attack is much faster than the equilibration of the
π
π
- allyl - ruthenium intermediates,
although a terminal
π
-allyl complex is formed. Similar results were obtained with phenols
as nucleophiles.
OCO
2
t
-Bu
CH(CO
2
Me)
2
Na CH(CO
2
Me)
2
[(Cp*Ru(NCMe)
3
]PF
6
,
DMF, rt
94% ee
94% ee (98%)
NHBoc
MeO
2
C
CO
2
Me
OCO
2
t
-Bu
NaC(NHBoc)(CO
2
Me)
2
[(Cp*Ru(NCMe)
3
]PF
6
,
DMF, rt
99% ee
99% ee (93%)
Scheme 8B.61.
Regio - and stereoselective Ru - catalyzed allylic alkylations (Cp * = pentamethylcy-
clopentadienyl).
Burger and Tunge investigated the decarboxylative allylation of nonstabilized ketone
enolates, formed from chiral
-keto allylic esters [257]. With a Ru-bipy complex as cata-
lyst, excellent regioselectivities and a high chirality transfer were obtained (Scheme
8B.62). The not absolute perfect chirality transfer was explained by a slow isomerization
of the branched, chiral allylic ester into the achiral linear allylic ester.
β
O
O
O
O
(Cp*RuCl)
4
,bipy,
CH
2
Cl
2
,rt
Ph
Ph
83% cee (86%)
Scheme 8B.62.
Regio- and stereoselective Ru-catalyzed decarboxylative allylation. cee, conserva-
tion of enantiomeric excess.
The fi rst and so far only report on asymmetric allylic alkylations in the presence of
chiral ligands came from the group of Takahashi et al. They obtained excellent yields
and enantioselectivities in the presence of planar-chiral ruthenium complexes
53
con-
