Redox selective reactions of organo-silicon and -tin compounds

Article abstract of DOI:10.1039/a803343i

The C-Si and C-Sn σ orbitals are higher in energy than C-H or C-C σ orbitals, and therefore can interact with neighboring π systems, non-bonding orbitals of heteroatoms, and other σ systems such as those of C-Si and C-Sn. Such interactions cause an increase of the HOMO level which in turn favors electron transfer. On the basis of this effect various types of redox selective reactions of organo-silicon and -tin compounds have been developed.

Full text of DOI:10.1039/a803343i

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Redox selective reactions of organo-silicon and -tin compounds
Jun-ichi Yoshida* and Keiji Nishiwaki
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto, Japan
The C᎐Si and C᎐Sn ó orbitals are higher in energy than C᎐H
or C᎐C ó orbitals, and therefore can interact with neighboring
ð systems, non-bonding orbitals of heteroatoms, and other ó sys-
tems such as those of C᎐Si and C᎐Sn. Such interactions cause
an increase of the HOMO level which in turn favors electron
transfer. On the basis of this effect various types of redox sel-
ective reactions of organo-silicon and -tin compounds have been
developed.
the case of reduction, an electron moves from the electrode to
the LUMO (lowest unoccupied molecular orbital) of the sub-
strate molecule to produce an anion radical species which
undergoes subsequent reactions to give final products. Here-
after we only discuss oxidation because of the simplicity of the
discussion, but we should keep in mind that a similar discussion
can be applied to reduction, although the direction of the
electron transfer is reversed.
One of the major advantages of electrochemical reactions is
that the reactivity of the electrode, i.e. the position of the Fermi
level, can be easily adjusted by external tuning (i.e. tuning of the
dial of a potentiostat). When the electrode potential is adjusted
properly, the electron transfer from the HOMO of the substrate
molecule to the electrode takes place smoothly, although there
usually exists some energy barrier for this process. If there are
two kinds of substrate having different HOMO levels, selective
1
Introduction
In the past decades the chemistry of organo-silicon and -tin
compounds has witnessed a steady advance in progress, reveal-
ing their unique properties and the development of their syn-
1
thetic applications. In the last several years redox reactions of
organo-silicon and -tin compounds have received significant
2
research interest and the activity in this area has grown rapidly.
This account will provide a brief outline of redox selective
reactions of organo-silicon and -tin compounds using electro-
chemical methods, with special emphasis on their mechanistic
principles.
2
Redox Reactions on the Surface of an Electrode:
Molecular Orbital Considerations
Before discussing the redox reactions of organo-silicon and -tin
compounds, let us start off by analyzing the redox reactions on
3
the surface of the electrode from a view point of molecular
orbital theory (Fig. 1). There is an energy band of electrons in
the electrode, and this band is filled up to the Fermi level. In the
solution phase there exist substrate molecules and they have
molecular orbitals of discrete energies. The molecular orbitals
are filled up to the HOMO (highest occupied molecular
orbital). In the case of oxidation, an electron moves from the
HOMO of the substrate molecule to the electrode to produce a
cation radical species which undergoes subsequent reactions. In
Fig. 1 Schematic diagram of the electron transfer on the surface of the
electrode
After graduating from Kyoto
University in 1975, Jun-ichi
Yoshida became a research asso-
ciate at Kyoto Institute of Tech-
nology in 1979. He obtained his
Doctoral degree in 1981 at Kyoto
University. He moved to Osaka
City University in 1984 and
became an associate professor
there in 1992. In 1994 he joined
the Department of Synthetic
Chemistry and Biological Chem-
istry at Kyoto University as a
full professor. His research has
ranged from synthetic method-
ology, organosilicon chemistry,
organotin chemistry, electro-
organic chemistry, computa-
After receiving an M.S. from
Osaka City University in 1994,
Keiji Nishiwaki enrolled in a
doctoral course at Kyoto Uni-
versity. In 1997 he joined Kinki
University as a research asso-
ciate. His current research
interest is the development of
new electron donors based on
β-effects of Group 14 elements in
electron transfer.
Jun-ichi Yoshida
tional chemistry, and automated
synthesis.
Keiji Nishiwaki
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596
2589

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Table 1 Oxidation potential of allylsilane and related compounds
Compound
E /V (vs. Ag–AgCl)*
p
1
1
.30
.85
Si(CH3)3
(CH3)4Si
>2.5
*
Determined with cyclic voltammetry using a Pt working electrode in
LiClO –CH CN.
4
3
Fig. 2 Schematic diagram of the selective and non-selective electron
transfer on the surface of the electrode
oxidation can be accomplished by tuning the electrode poten-
tial appropriately as shown in Fig. 2. If the HOMO levels are
very close to each other, however, it is difficult to oxidize one
substrate without affecting the other. When the substrate that
we wish to oxidize has a lower HOMO level than that of the
other species, it is, in principle, impossible to accomplish select-
ive oxidation. However, we often use tricks to accomplish the
desired reaction in spite of unfavorable orbital situations.
For example, if the substrate that we wish to oxidize is
adsorbed onto the electrode selectively and the other species
are not, then electron transfer takes place selectively to accom-
plish the desired reaction. Therefore, the use of different
electrode material sometimes gives rise to different reaction
pathways.
Although such external control of electron transfer, i.e. con-
trol by electrode materials, solvent, current density, and so
on, is important, we have recently proposed the importance of
the internal control approach, and demonstrated that silicon
and tin are quite effective as controlling groups in redox reac-
tions. We call such a group ‘electroauxiliary’. Silicon and tin
promote the electron transfer from β-situated π systems, hetero-
atoms having non-bonding p orbitals, and some σ orbitals. The
following sections focus on the principle of the action of silicon
and tin as controlling groups and their applications to redox
selective reactions.
Fig. 3 Energy diagram of the interaction of the C᎐Si σ orbital and the
π orbital of the C᎐C bond of the allylsilane
᎐
3
Control of Redox Reaction by Orbital Interaction
3
.1 Electron transfer reactions of allylsilanes and benzylsilanes
(
ó,ð-interaction)
The C᎐Si bonding σ orbital is higher in energy than C᎐H or
C᎐C bonding σ orbitals, the energy match of the C᎐Si σ orbital
with the π orbital of a carbon–carbon double bond is better
than that with C᎐H or C᎐C bonds. Therefore, considerable
interaction between the C᎐Si σ orbital with the neighboring π
Fig. 4 Plots of HOMO level of H SiCH CH᎐CH vs. torsion angle of
3
2
᎐
2
Ϫ9
Si᎐C᎐C᎐C (MP2/LANL2DZ) (eV ≈ 1.602 × 10 J)
4
orbital is attained in allylsilanes and benzylsilanes (Fig. 3).
Such interaction produces two new molecular orbitals. The
energy level of the orbital produced by a bonding interaction is
5
lower than the original two and that of the orbital produced
by an antibonding interaction is higher than the original
two. As the original orbitals are both filled, the two new
orbitals should also be filled, and the orbital of higher energy
becomes the HOMO. Therefore, the interaction between the
C᎐Si σ orbital and the π orbital of the carbon–carbon double
bond causes a significant increase of the HOMO level which in
turn favors electron transfer. The magnitude of such an inter-
action depends upon the torsion angle Si᎐C᎐C᎐C. As shown in
Fig. 4 the HOMO level is maximum when the torsion angle is
9
0Њ, where the C᎐Si σ orbital and the π orbital of the carbon–
carbon double bond are in the same plane. Fig. 5 shows the
computer generated picture of the HOMO of the allylsilane
6
Fig. 5 The HOMO of H SiCH CH᎐CH (HYPERCHEM 5, HF/
3
2
᎐
2
(
torsion angle 90Њ) which exhibits the antibonding interaction
3-21G*)
2
590
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596

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–e
–'Si(CH3)3+"
Si(CH3)3
Si(CH3)3
–e
CH3OH
+
OCH3
OCH3
9% ( 68 : 32)
6
Scheme 1
Table 2 Oxidation potential of α-heteroatom-substituted heteroatom
compounds and related compounds
Compound
E /V (vs. Ag–AgCl)*
p
C7H15
OMe
>2.5
C7H15
OMe
1
1
.72
.20
SiMe3
C7H15
C8H17
SPh
SPh
1
.10
SiMe3
*
Determined with cyclic voltammetry using a Pt working electrode in
LiClO –CH CN.
4
3
between the C᎐Si σ orbital and the π orbital of the carbon–
carbon double bond.
Fig. 6 Energy diagram of the interaction of the C᎐Si σ orbital and the
non-bonding p orbital of oxygen
The oxidation potentials of allylsilanes are less positive
than those of the corresponding alkenes as shown in Table 1,
indicating that the silyl group at the allylic position acts as
7
an activating group for alkenes toward oxidation. The selec-
tive electrochemical oxidation of the carbon–carbon double
bond neighboring the C᎐Si bond in geranyltrimethylsilane is
remarkable (Scheme 1). The other carbon–carbon double bond
is not affected at all. It is also noteworthy that the C᎐Si bond is
8
cleaved selectively in the cation radical intermediate to give the
allyl radical which is further oxidized to the allyl cation. The
oxygen nucleophiles such as methanol are introduced to the
allyl cation intermediate to produce a mixture of two regio-
isomeric products.
Chemical oxidation and photoelectron transfer oxidation of
allylsilanes have also been studied extensively. Such reactions
proceed by closely related mechanisms to cleave the C᎐Si
9
bond selectively.
3
.2 Electron transfer from heteroatoms (ó,n-interactions)
Fig. 7 Plots of HOMO levels of H SiCH OH and H SiCH SH vs.
3
2
3
2
The introduction of a silyl group at the carbon adjacent to a
10
11
12
torsion angle of Si᎐C᎐O᎐H and Si᎐C᎐S᎐H, respectively (MP2/
heteroatom (e.g. oxygen, sulfur or nitrogen ) causes a sig-
nificant decrease of the oxidation potential (Table 2). In the
case of the ether, the effect of silicon is especially remarkable.
The oxidation potential decreases at least 0.8 V by the introduc-
tion of a silyl group on the carbon adjacent to the oxygen atom.
The decrease of the oxidation potential by silyl-substitution is
attributed to the interaction between the C᎐Si σ orbital and the
non-bonding p orbital of the heteroatom (i.e. oxygen) increas-
LANL2DZ)
13
ing the HOMO level as shown in Fig. 6. The important role
played by the orbital interaction is demonstrated by the geo-
metric dependence of the HOMO level. The plots of the
HOMO level of H SiCH OH (a model compound of silyl-
3
2
substituted ethers) obtained by ab initio molecular orbital cal-
culations vs. the torsion angle Si᎐C᎐O᎐H is shown in Fig. 7.
When the C᎐Si σ orbital and the non-bonding p orbital of the
oxygen atom are in the same plane (torsion angle 90Њ) the
HOMO level is at maximum. The HOMO shown in Fig. 8
Fig. 8 The HOMO of H SiCH OH (HYPERCHEM 5, HF/3-21G*)
3
2
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596
2591

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OCH3
Si(CH3)3
OCH3
Si(CH3)3
OCH3
OCH3
+
OCH3
–e
–e
CH3OH
(
a) C7H15
C7H15
C H
C H
7 15
C H
7
15
7
15
'Si(CH3)3+'
OCH3
OH
–
OH
OH
OH
OH
78%
Si(CH3)3
Si(CH3)3
OCH3
OCH3
(
b)
c)
H O
C7H15
–4e
CH3OH
2
C7H15
C7H15
OCH3
OH
OCH3
OH
OCH3
OH
O
9
2%
SPh
Si(CH3)3
OCH3
OCH3
SPh
OCH3
–2e
–2e
(
C7H15
C7H15
C7H15
CH OH
3
CH3OH
64%
Scheme 2
Table 3 Oxidation potential of acylsilane and related compounds
Compound
E /V (vs. Ag–AgCl)*
d
O
1
.23
C9H19
Si(CH3)3
O
>
>
2.00
2.00
C9H19
C6H13
H
O
CH3
*
Decomposition potential determined with rotating-disc voltammetry
using a Pt working electrode in LiClO –CH CN.
4
3
exhibits the antibonding interaction between the C᎐Si σ orbital
and the non-bonding p orbital. When they are in a perpendicu-
lar orientation (torsion angle 0 and 180Њ) the HOMO level is at
a minimum. It is noteworthy that we can design molecules hav-
Fig. 9 Energy diagram of the interaction of the C᎐Si σ orbital and the
non-bonding p orbital of carbonyl oxygen
14
ing specific oxidation potentials by controlling their geometry.
The magnitude of the orbital interaction strongly depends on
the nature of the heteroatom. In the case of sulfide the effect of
silyl-substitution is not so large. The molecular orbital calcu-
lations of a model compound, H SiCH SH (Fig. 7) are consist-
3
2
ent with the experimental oxidation potentials (Table 2). This is
probably because the energy match between the C᎐Si σ orbital
and the non-bonding p orbital of the sulfur is not good, thus
the effective interaction between the two orbitals cannot be
attained (see below).
The silyl group also controls the reaction pathway in prepar-
ative electrochemical oxidation. The oxidation of silyl-substi-
tuted heteroatom compounds leads to the selective cleavage
of the C᎐Si bond and the introduction of nucleophiles on the
carbon. Weakening of the C᎐Si bond by the interaction of the
C᎐Si σ orbital with the half-vacant p orbital of the cation
Fig. 10 The HOMO of H SiC(᎐O)H (HYPERCHEM 5, HF/3-21G*)
3
᎐
of the C᎐Si bond followed by the introduction of methanol
gives the O,S-acetal which is further oxidized under the same
conditions to give the acetal. In the example shown in Scheme
2(c), the carbon–carbon double bond is not affected at all
during the course of the transformation.
15
radical of the oxygen seems to be responsible for the facile
C᎐Si bond cleavage. Several examples of the use of silicon in
electrochemical oxidation of heteroatom compounds are shown
16
in Scheme 2. In the case of β-hydroxy-α-trimethylsilyl ether
Scheme 2(a)], the C᎐Si bond in the initially formed cation
Oxidation reactions of silicon-substituted heteroatom com-
pounds by chemical oxidizing agents and photoelectron trans-
[
17
radical is cleaved selectively without affecting other parts of
the molecule to give the carbon radical which is further
oxidized to the cationic intermediate. The attack of methanol
gives β-hydroxyacetal as the final product. The free hydroxy
group is not affected at all during the course of the reaction.
The anodic oxidation of compounds having two C᎐Si bonds
is interesting. Two C᎐Si bonds are cleaved successively to give
an ester probably via the hydrolysis of the initially formed
orthoester [Scheme 2(b)]. The electrochemical oxidation of
silyl-substituted sulfides also takes place smoothly. The cleavage
fer oxidation reactions have also been studied extensively.
Silicon also activates carbonyl compounds toward electron
transfer (Table 3). The introduction of a silyl group on the
carbonyl carbon (acylsilanes) causes a significant decrease in
18
the oxidation potential. The interaction of the C᎐Si σ orbital
with the non-bonding orbital of the oxygen is responsible and
this interaction is facilitated by the fixed geometry; the rotation
around the carbon–oxygen double bond is prohibited (Fig. 9).
This interaction is also responsible for other unique properties
19
of acylsilanes. The HOMO of H SiC(᎐O)H is shown in Fig.
3
2
592
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596

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Table 4 Oxidation potential of α-heteroatom-substituted organo-
silicon compounds
₁
Compound
E₂/V (vs. Ag–AgCl)*
�
OCH3
0
1
.97
.47
C7H15
C7H15
Sn(C4H9)3
OCO2CH3
Sn(C4H9)3
OCOCH3
Sn(C4H9)3
SPh
Sn(C4H9)3
1
0
.44
.74
C7H15
C8H17
(
C4H9)4Sn
1.52
Fig. 11 Plots of HOMO levels of H SnCH OH and H SnCH SH vs.
3
2
3
2
torsion angle of Sn᎐C᎐O᎐H and Sn᎐C᎐S᎐H, respectively (MP2/
*
Determined with rotating-disc voltammetry using a Pt working
LANL2DZ)
electrode in LiClO –CH CN.
4
3
O
O
O
–e
C10H21
Si(CH3)3
C10H21
Si(CH3)3 –'Si(CH ) '
3 3
C10H21
–e
O
O
OH
C10H21
O
2%
C10H21
9
Scheme 3
1
0 to demonstrate the antibonding interaction between the
C᎐Si σ orbital and the non-bonding p orbital of the carbonyl
oxygen. The electrochemical oxidation of silyl-substituted carb-
onyl compounds (acylsilanes) proceeds smoothly to cleave the
C᎐Si bond, and various nucleophiles such as alcohols, water
and amides can be introduced to the carbonyl carbon. In the
example shown in Scheme 3, allyl alcohol is effectively intro-
duced as a nucleophile to give the corresponding allyl ester. The
oxidation of acylsilanes by chemical oxidizing agents such as
Fig. 12 Energy diagram of the interaction of the C᎐Si σ orbital and
the C᎐Sn or orbital with the non-bonding p orbital of sulfur
Y
Y
Y
Nu–
–2e
20
–H+
R
R
Nu
Nu
H O also cleaves the C᎐Si bond.
R
R
H
R
R
2
2
Tin is also effective for the oxidation of heteroatom com-
pounds. The introduction of a stannyl group on the carbon
adjacent to the heteroatom decreases the oxidation potential
Y
Y
Y
Nu–
–2e
21
significantly (Table 4). The effect of tin is attributed to the
interaction between the C᎐Sn σ orbital and the non-bonding p
orbital of the heteroatom, increasing the HOMO level which in
turn favors electron transfer. The variation of the HOMO levels
of the model compounds H SnCH OH and H SnCH SH with
–H+
Sn
Y = heteroatom
Nu = carbon nucleophile
3
2
3
2
Scheme 4
torsion angle is shown in Fig. 11. Whereas silyl substitution
causes little decrease in the oxidation potentials of sulfides,
stannyl substitution causes a significant decrease. Although the
energy level of the p orbital of sulfur is much higher than the
C᎐Si σ orbital and they do not interact with each other effect-
ively, the energy level of the C᎐Sn σ orbital is similar to that of
the sulfur p orbital and they interact effectively to increase the
HOMO level (Fig. 12). Therefore, tin exhibits significant effects
for electron transfer from organosulfur compounds, whereas
silicon has little effect.
22
carbon bond formation. During oxidation of heteroatom
compounds the carbocation, stabilized by the adjacent hetero-
atom, is generated via proton elimination under suitable condi-
tions. The reaction of this carbocation with carbon nucle-
ophiles should lead to effective carbon–carbon bond formation
(Scheme 4). There are, however, two major problems. The first is
the oxidation of carbon nucleophiles. The oxidation potential
of carbon nucleophiles is often lower than that of the starting
heteroatom compounds, and therefore it is often difficult to
oxidize the heteroatom compounds without affecting the car-
bon nucleophiles. The second problem is overoxidation. Since
the carbon–carbon bond formation products are also hetero-
atom compounds and carbon is slightly more electron donating
than hydrogen, the oxidation potentials of the products are
The reaction pattern of the oxidation of stannyl-substituted
heteroatom compounds is quite similar to that of silyl-
21
substitutedheteroatomcompounds. Electrochemicaloxidation
leads to selective cleavage of the C᎐Sn bond and the introduc-
tion of nucleophiles on the carbon.
The effect of tin is especially important for oxidative carbon–
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596
2593

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OCO2CH3
Si(CH3)3
OCO2CH3
OCO2CH3
Sn(C4H9)3
OCO2CH3
Sn(C4H9)3
–e
(a)
–(C4H9)3Sn•
C7H15
C7H15
C7H15
C7H15
68%
(b)
–e
(CH3)3SiCN
O
O
Sn(C4H9)3
O
NC
84%
Sn(C4H9)3
(c)
O
BF4
–
O
–e
O
C7H15
C7H15
C7H15
F
83% (cis/trans = 2.8 : 1)
Scheme 5
Table
5
Oxidation potentials of 1,2-disilyl- and 1,2-distannyl-
substituted ethanes and related compounds
E /V (vs.
Ag–AgCl)* Compound
E /V (vs.
Ag–AgCl)*
d
d
Compound
Me3Si
2.19
2.20
1.74
>
2.5
Me3Si
Me3Si
Me3Si
SiMe3
SiMe3
SiMe3
1
1
0
.65
.41
.66
SiMe3
2.02
SiMe3
SiMe3
SiMe3
1
.45
Bu3Sn
SiMe3
1.22
SnBu3
Bu3Sn
Bu3Sn
*
Decomposition potential determined with rotating-disc electrode
voltammetry in LiClO –CH CN using a glassy carbon working elec-
4
3
trode and a Ag–AgCl reference electrode.
Fig. 13 Energy diagram of the interaction of two neighboring C᎐Si σ
orbitals
slightly less positive than those of the starting material. There-
fore, oxidation of the product is difficult to avoid.
The use of tin as an activating group (electroauxiliary) solves
these two problems. The introduction of tin causes a dramatic
decrease in the oxidation potentials of heteroatom compounds.
The oxidation potential of tin-substituted heteroatom com-
pounds is less positive than that of various carbon nucleo-
philes such as enol silyl ethers and allylsilanes. Therefore,
tin-substituted heteroatom compounds can be oxidized select-
ively in the presence of these carbon nucleophiles. The carbo-
cation intermediate generated by the cleavage of the C᎐Sn bond
reacts with carbon nucleophiles to give the product. Since the
product does not contain tin, its oxidation potential is higher
23
Fig. 14 The HOMO of H
1G*)
SiCH CH SiH (HYPERCHEM 5, HF/3-
3 2 2 3
than the starting tin-containing compounds. Therefore, there
is no longer a problem with overoxidation. Several examples
of electrooxidative carbon–carbon bond formation using tin-
substituted heteroatom compounds are shown in Scheme 5.
Chemical oxidation reactions of α-heteroatom-substituted
organotin compounds aimed at achieving carbon–carbon bond
2
than that of (trimethylsilyl)ethane which has only one C᎐Si
26
bond (Table 5). The lower oxidation potential of 1,2-bis(tri-
methylsilyl)ethane is attributed to the interaction between the
C᎐Si σ orbitals raising the HOMO level (Fig. 13). The HOMO
of H SiCH CH SiH is shown in Fig. 14, which indicates
24
formation have also been developed.
3
2
2
3
the antibonding interaction between two neighboring C᎐Si
σ orbitals. It is also noteworthy that the oxidation potential
of 1,2-bis(trimethylsilyl)ethane is much lower than those of
bis(trimethylsilyl)methane and 1,3-bis(trimethylsilyl)propane,
indicating the importance of the interaction of the two neigh-
boring orbitals. The geometric requirements of the present
3
.3 Electron transfer from ó systems (ó,ó-interaction)
The interaction of a carbon–metal (metal = Si or Sn) σ orbital
with a neighboring carbon–metal σ orbital is also effective in
raising the HOMO level. As a matter of fact, the oxidation
potential of 1,2-bis(trimethylsilyl)ethane is much less positive
25
2
594
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596

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Table 6 Oxidation potential of 1,2-diphenyl-1,2-bis(trimethylsilyl)-
ethane and related compounds
Compound
E /V (vs. Ag–AgCl)*
d
SiMe3
meso
DL
1.23
1.32
SiMe3
SiMe3
1
.39
Me3Si
SiMe3
1.74
*
Decomposition potential determined with rotating-disc electrode vol-
tammetry using a glassy carbon working electrode in LiClO –CH CN.
4
3
Fig. 15 Plots of HOMO level of H SiCH CH SiH vs. torsion angle
3
2
2
3
of Si᎐C᎐C᎐Si (MP2/LANL2DZ)
–
e
Si(CH3)3
Si(CH3)3
(3.0 F mol–1)
Si(CH3)3
Si(CH3)3
CH3OH
Si(CH3)3
Si(CH3)3
–e
96%
Scheme 6
Fig. 16 The HOMO of PhCH(SiMe )CH(SiMe )Ph (HYPERCHEM
3
3
5
, PM3)
effect are also interesting. The oxidation potential of cis-
exo,exo)-1,2-bis(trimethylsilyl)norbornane (Si᎐C᎐C᎐Si torsion
(
action system. The oxidation potential of this compound is less
positive than that of benzyltrimethylsilane (σ,π-interaction sys-
tem) and that of 1,2-bis(trimethylsilyl)ethane (σ,σ-interaction
system), indicating that the two systems interact with each
other effectively to raise the HOMO level (Table 6). The
molecular orbital calculations also indicate that the HOMO is
localized through two C᎐Si bonds and two benzene rings as
shown in Fig. 16. This fact demonstrates the potential of
the conjugation between the σ,σ-interaction systems and the
σ,π-interaction systems to construct various new electronic sys-
tems. Such effective conjugation provides a new concept in
material science.
angle ca. 1Њ) is less positive than that of trans-1,2-bis(trimethyl-
silyl)norbornane (torsion angle ca. 107Њ). This is consistent with
the molecular orbital calculations which indicate that the
HOMO is at a minimum when the torsion angle is 90Њ (Fig. 15).
The concept of σ–σ interaction is also applicable to organo-
27
26
tin compounds. The oxidation potential of 1,2-bis(tributyl-
stannyl)ethane is much lower than that of (tributylstannyl)-
ethane, indicating significant interaction between the two C᎐Sn
σ orbitals raising the HOMO level. It is also interesting that the
C᎐Si σ orbital and the C᎐Sn σ orbital interact with each other
less effectively, as demonstrated by the oxidation potential of
1
-tributylstannyl-2-(trimethylsilyl)ethane. Probably the energy
match between the two σ orbitals is not good for effective inter-
action because the energy level of the C᎐Sn σ orbital is much
higher than that of the C᎐Si σ orbital.
5
Conclusion
It is hoped that with the aid of molecular orbital calculations
and computer graphics this Perspective helps to demonstrate to
the reader that silicon and tin promote electron transfer from
β-situated π systems, heteroatoms and σ systems, and that this
effect is attributed to the interaction of C᎐Si or C᎐Sn σ orbitals
with neighboring p-type orbitals. Such σ–π interaction, σ–n
interaction and σ–σ interaction enjoy versatile applications in
the field of electron transfer. Several examples of the redox
selective reactions of organo-silicon and -tin compounds based
on these orbital interactions are demonstrated in this Perspec-
tive, although many other examples are omitted because of
space limitations. Further work is in progress to explore the full
scope of this concept and its application to synthetic transform-
ations.
The preparative electrochemical oxidation of 1,2-bis(tri-
methylsilyl)norbornane results in the cleavage of two C᎐Si
26
bonds to yield norbornene (Scheme 6). Presumably one of the
C᎐Si bonds is cleaved in the cation radical intermediate to gen-
erate the carbon radical which is further oxidized to give the
carbocation. The subsequent facile β-elimination of the silyl
group produces a carbon–carbon double bond.
4
Combination of the ó,ð-Interaction System and the
ó,ó-Interaction System
In the previous part of this account it was demonstrated
that the C᎐Si and C᎐Sn σ orbitals interact with neighboring
π-systems, n-systems and σ-systems to raise the HOMO level
which in turn favors electron transfer. It is interesting to exam-
ine whether such interaction systems interact with each other.
6
Acknowledgements
1
,2-Diphenyl-1,2-bis(trimethylsilyl)ethane is an example of a
We are pleased to acknowledge the Grant-in-Aid for Scientific
system which involves σ,π-interaction systems and a σ,σ-inter-
Research from Monbusho, Nagase Science and Technology
J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596
2595

View Article Online
Foundation, Yamada Science Foundation, Nihon Noyaku, and
Kanegafuchi Chemical Industry for partial financial support
of this work. We also thank Shin-etsu Chemical for the gifts of
some organosilicon compounds. We are also grateful to the
many dedicated co-workers, whose names appear in the
references.
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Received 5th May 1998; Paper 8/03343I
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J. Chem. Soc., Dalton Trans., 1998, Pages 2589–2596