Chemistry Reference
In-Depth Information
CO
2
Et
Cl
Pt(H
2
C=CH
2
)(PPh
3
)
2
+
Ph
OAc
EtO
2
C
Ph
O
O
9.95
9
9
6
9
9
Scheme 9.31
positive charge on the ligand. Nucleophilic attack may be directed towards the central carbon if the reaction is
frontier-orbital controlled. Some calculations have shown that this carbon has the highest LUMO coefficient.
Allyl complexes of other metals have also been found to exhibit this chemistry.
41
A novel furan synthe-
sis has been uncovered during this debate (Scheme 9.31); it involves both central and terminal attack on
chloroacetate
9.95
.
42
9.2.3 Stereoselectivity
The stereochemical results of these reactions have been carefully studied.
43
If a chiral acetate is employed,
then the product is found to have retention of stereochemistry. Hence, the
cis
acetoxy ester
9.98
gives the
cis
product
9.100
, while the
trans
acetoxy ester
9.101
gives the
trans
product
9.103
(Scheme 9.32).
44
Retention
3
-allyl complexes,
9.99
and
9.102
, and a
second inversion during attack by the nucleophile. In the case of the acetoxy esters
9.98
and
9.101
, a curious
observation can be made when the substrate is non-racemic. It is found that the product is racemic. This is
not a general observation. It occurs here, because the intermediate
is a result of two inversions - inversion during formation of the
3
-allyl complexes,
9.99
and
9.102
, each
have a plane of symmetry - they are
meso
compounds and the nucleophile is equally likely to attack either
terminus. Systems without the symmetrical intermediate do not show racemization (Scheme 9.33).
Inversion is not observed when an organometallic capable of transmetallation is used in place of the
nucleophile (Scheme 9.34). Treatment of the
cis
-acetoxy ester
9.98
with phenyl trimethyltin and a palladium
catalyst gives the
trans
isomer
9.108
,
45
while use of a malonate anion gives the
cis
product
9.100
(see Scheme
9.32 above). Both products are formed via the same
3
-allyl complex
9.99
, one by nucleophilic attack
trans
to Pd, one by transmetallation (presumably after acetate-chloride exchange).
The same stereochemical result has been observed with silicon,
46
boron
47
and aluminium reagents
(Scheme 9.35).
48
The lactone
9.110
reacts with the vinyl aluminium reagent
9.109
to give the
trans
acid
9.111
.
CO
2
Me
CO
2
Me
CO
2
Me
L
2
Pd(0)
oxidative
addition
(inversion)
Nuc
(inversion)
OAc
Nuc
PdL
2
9.99
meso-
9.98
9.100
CO
2
Me
CO
2
Me
CO
2
Me
L
2
Pd(0)
Nuc
OAc
Nuc
PdL
2
9
1
9
1
9
.
1
Scheme 9.32
