HOW ARE CARBANIONS STABILISED BY -SULPHUR
FUNCTIONAL GROUPS?
(1) PARTICIPATION OF VACANT SULPHUR d-ORBITALS?
S
C
_
S 3d
_
RS C
_
RS C
+ _
RS C
O
RS C
O
(2) HYPERCONJUGATION?

* (C--S )
_
S
C
sp3
C
(3) INDUCTIVE EFFECTS?
Group electronegativity:
RS- < RSO- < RSO2- < R2S+- < R2SO+-
•• _
O
••
••
+
••
S
C

* (C--S)
+S
C
_
O
APPLICATIONS OF SULPHUR-STABILISED CARBANIONS IN
SYNTHESIS.
(1) DITHIOACETALS
EQUIVALENTS
HS
R
C
O
+
H
(1,3-DITHIANES)
BF3 or HCl
AS
R
+ H2O
C
S
S
R
C
n - BuLi
R
H
S
pKa = ca. 30
[Compare CH3SCH3, pKa = ca. 48]
ANION
S
H
HS
ACYL
Li+
C
S
_
S
RCH=O
(i) HS (CH 2 )3 S H, BF 3
(ii) RLi
R1 X
S
S
S
C
R R1
S
CO 2
( ii )
H3O+
O
( ii ) H 3 O +
S
( i ) C6H5C
S
C
C(OH)R 1 2
R
HgCl2
H2 O
O
C
C6H5
HgCl2 / H 2 O
R
C
O
N
S
C
R
COOH
( ii ) H 3 O +
S
HgCl 2 / H 2 O
S
C
R
R C(OH)
R1
R1
RR 1 C=O
S
R
(i) R 1 2 C
HgCl2
H2O
_
C
(i)
C6 H5
C
O
R
C
OH
C
O
O
C
O
S
R
HgCl2
C
E
or
NBS
R
H2O
E
C
O
S
'Umpolung' of normal carbonyl reactivity:
R  
C O
H
R
H
C
O
- H+
- H+
R
R
_
E+
C O
_
C
O
R
C O
E
(i) E+
R
E
(ii) Raney Ni
CH2
(2) NUCLEOPHILIC
BEHAVIOUR
OF
-SULPHINYL
CARBANIONS - CONVERSION OF ESTERS INTO METHYL
KETONES
O
O
R
C
OR1
( i ) 2 CH3
S
_
CH2
( ii ) H2O, ( iii ) Al - Hg.
O
R
C
CH3
O
CH3
_
O
S CH2 +
OEt
C
O
O-
S
C
CH3
CH2
C6H5
OEt
- OEt-
C6H5
O
Al - Hg
CH3 C C6H5
O
O
S
C
CH3
CH2
C6H5
(A)
(3) NUCLEOPHILIC
CARBANIONS
SYNTHESIS
BEHAVIOUR
OF
-SULPHONYL
THE
RAMBERG-BAKLUND
ALKENE
O
RCH2
S
CHBrR1
KOH
- H+
O
O
O
S
CHR1
RHC
_
Br
- Br-
RCH2
CHR1
- SO2
Spontaneous
O
O
S
RHC
CHR1
(4) Nucleophilic Behaviour of -Sulphonium and -Sulphoxonium
Carbanions - Sulphur Ylides.
(4a) Ylide Functionalsiation
+
Ph2S
_
CHMe
+
Ph2S
MeI
RLi
CHMe2
_
RLi
+
Ph2S
_
CMe2
+
Ph2S
+
Ph2S
Cl
Cl
- Cl-
+
Ph2S
_
RLi
+
Me2S
Me3SiCl
+
CH2
Me2S
O
O
+
Ph2S
RLi
CH2SiR3
+
Me2S
O
MeSO2Cl
+
Me2S
O
CH2SO2Me
RLi
+
Me2S
O
CHSO2Me
CHSiR3
(4b) EPOXIDATION OF CARBONYL COMPOUNDS
O
O
+ [CH2]
C
C
+
(CH3)2S
_
CH2 +
_
O
O
R
CH2
+
(CH3)2S
C
H
O
H2C
C
R
+
C C
H2
H
R
(CH3)2S
H
+
(CH3)2S
O
_
CH2 + RCHO
O
H2C
CHR +
(CH3)2SO
STEREOCHEMICAL PREFERENCES OF SULPHONIUM AND
SULPHOXONIUM YLIDES:
O
O
Axial attack
O
Equatorial attack
+ _
(CH3)2S CH2
+ _
(CH3)2S CH2
O
Axial %
83.3
0.0
Equatorial %
16.7
100
How can this difference in stereochemical preference be explained?
To a first approximation the equilibrium positions of reactions reflect
the relative thermodynamic stabilities of reactants and product(s).
Consider the simple reaction shown below:
A + B
C
If the thermodynamic stabilities of A and B together are much greater
than that of C the equilibrium position will lie largely towards the left
hand (i.e. reactant) side.
If the stabilities of A and B taken together are broadly similar that of C
the equilibrium position will give rise to approximately equal
concentrations of reactants and products.
On the other hand if the thermodynamic stability of C is much greater
than that of A and B taken together the equilibrium position will lie
largely towards the rhs (i.e. product) side.
If the stability of C is overwhelmingly greater than that of A + B taken
together the thermodynamics of the system will bias the equilibrium
totally or near-totally in favour of product C. In this situation the
equilibrium concentration of the reactants A and B will be vanishingly
small or zero and we say that the reaction is essentially irreversible.
Consider, in this context, the reactions of a sulphonium and a
sulphoxonium ylides with a common carbonyl substrate as illustrated
on the following transparency: