+•
At –2.2 V vs. Ag wire
Pt cathode, under N2
cleavage
anode
Si+•
Si
•
+
Mes2Si SiMes2
TMDS
Si Si
•
A
G
(Q = 1 F mol–1
)
Reactions with nucleophiles
and/or
Hydrogen atom abstraction
Mes
Si
Mes
Mes
Mes Mes
Mesitylene
F
F
F
Si
H
H
Si
H
F
Si
Si
F
F
Further anodic oxidation
and/or cleavage
Mes
Mes
2
Mes
4
Mes
1
7
8
+ PF6–(BF4– or HO–)
Products
+Si
A: 0.1 mol l–1 Bu4NPF6–THF
B: 0.1 mol l–1 Bu4NPF6–MeCN
+ H•
31%
25%
—
1
2
4
7
8
9%
H
18%
20%
47%
6%
+ PF6–(BF4– or HO–)
C
35%
9%
+ H•
H
Si F(OH)
Si F(OH)
•
Scheme 2 Results of cathodic reduction of TMDS.
D
B
anode
The authors are thankful to the Israel Science Foundation for
supporting this research through a grant.
+ PF6–(BF4– or HO–)
+Si F(OH)
(OH)F Si F(OH)
F
E
Scheme 1 Mechanism for the electrochemical oxidation of TMDS.
Notes and references
1 R. West, M. J. Fink and J. Michl, Science, 1981, 214, 1343.
2 R. Okazaki and R. West, Adv. Organomet. Chem., 1996, 39, 231.
3 G. Raabe and J. Michl, Chem. Rev., 1985, 85, 419; G. Raabe and J.
Michl in The Chemistry of Organic Silicon Compounds, ed. S. Patai and
Z. Rappoport, Wiley, New York, 1989, pp. 1015–1142.
4 R. West, Angew. Chem., Int. Ed. Engl., 1987, 26, 1201; R. West, in The
Chemistry of Inorganic Ring Systems, ed. R. Steudel, Elsevier,
Amsterdam, Netherlands, 1992, pp. 35–50.
A plausible mechanism which accounts for the formation of
all products in the electrochemical oxidation process is outlined
in the over simplified Scheme 1. The initially formed unstable
radical cation from TMDS is a highly reactive species and could
undergo a number of different reactions, i.e. reactions with
nucleophiles, abstraction of hydrogen atoms, cleavage, etc. For
example, the direct cleavage may lead to a radical cation A and
5 T. Tsumuraya, S. A. Batcheller and S. Masamune, Angew. Chem., Int.
Ed. Engl., 1991, 30, 902.
6 M. Weidenbruch, Coord. Chem. Rev., 1994, 130, 275.
2
silylene G. The radical cation A may undergo an attack by PF6
2
(BF4 or HO2) anion to form the radical B, or abstract a
7 B. D. Shepherd and R. West, Chem. Lett., 1988, 183.
hydrogen atom (from solvent or electrolyte) to generate a
silylenium cation C, followed by a chemical reaction with a
nucleophile to yield D (products 2 and 5). Intermediate B could
undergo further anodic oxidation to the cation E, or abstract a
hydrogen atom from its surroundings to generate D. Cation E
may react with PF62 (BF42 or HO2) anion to form F (products
1, 3 and 6). It is noteworthy that the formation of some of the
products outlined in Fig. 1, as well as 4, could also originate
from silylene G.
Results of the controlled potential electrochemical reduc-
tion12 of TMDS in both THF and acetonitrile are shown in
Scheme 2. A major product found was mesitylene. As in the
anodic process, the silicon products (except for compound 8)
contain only one silicon atom, probably due to fragmentation of
the initial electrochemically generated anion radical. The same
four products (1, 2, 7 and 8) were obtained both in THF and
MeCN. However, an additional product, 4, was detected in
THF.
Surprisingly, some of the products (1, 2 and 4) observed by
the electrochemical reduction process are identical to those
obtained by the anodic oxidation. Therefore, it is reasonable to
suggest that these products could stem from the same inter-
mediate, which might be generated in both types of reactions.
Attempts to trap such intermediate and characterize its nature
are underway,
The electrochemical reduction of TMDS in MeCN–Bu4-
NClO4 (0.1 mol l21) solution was also attempted. The GLC
chromatogram indicated the formation of a very complex
mixture of products ( > 20 peaks!), which has yet to be analysed,
among which Mes2SiH2 and Mes2Si(H)(OH) could be de-
tected.
8 H. B. Yokelson, A. J. Millevolte, G. R. Gillette and R. West, J. Am.
Chem. Soc., 1987, 109, 6865.
9 H. B. Yokelson, A. J. Millevolte, B. R. Adams and R. West, J. Am.
Chem. Soc., 1987, 109, 4116; K. L. McKillop, G. R. Gillette, D. R.
Powell and R. West, J. Am. Chem. Soc., 1992, 114, 5203.
10 H. Watanabe, K. Takeuchi, K. Nakajima, Y. Nagai and M. Goto, Chem.
Lett., 1988, 1343; A. J. Millevolte, D. R. Powell S. G. Johnson and R.
West, Organometallics, 1992, 11, 1091.
11 All experiments were carried out in a drybox ([H2O] < 1 ppm; [O2] <
1 ppm), employing controlled potential electrolysis (on Pt) in an ‘H’
type two-compartment cell and pulsing from 0 to 0.5 V (vs. Ag wire)
every 0.5 s. Typically, the working compartment contained 0.1–0.2
mmol of disilene dissolved in 25 ml solution. Electricity consumption is
ca. 1 F mol21. Solvents MeCN and THF were distilled over P2O5 and
benzophenone/Na, respectively. All electrolytes were dried under
vacuum (ca. 30 mmHg) at 105 °C for 48 h. HRMS results for MH+:
Found (calc. for M). 1: 305.1547 (305.1547 for C18H23F2Si); 2:
287.1620 (287.1640 for C18H24FSi); 3: 303.1590 (303.1595 for
C18H24FOSi); 4: 269.1733 (269.1745 for C18H25Si); 5: 285.1500
(285.1694 for C18H25OSi); 6: 301.1420 (301.1643 for C18H25O2Si); 7:
120.0910 (120.0948 for C9H12); 8: 470.2155 (470.2120 for
C
27H33F3Si2).
12 The chemical reduction of sterically congested disilenes by alkali metals
has not been studied. Formation of disilene anion radicals is reported to
result from reductions of the corresponding dichlorodisilenes by alkali
metals,13 but to the best of our knowledge there is no report on the nature
of products obtained by reaction/decomposition of this type of anion
radical.
13 M. Weidenbruch, K. Kramer, A. Schafer and J. K. Blum, Chem. Ber.,
1985, 118, 107; M. Weidenbruch, K. Kramer, K. Peters and H. G. von
Schnering, Z. Naturforsch., Teil B, 1985, 40, 601.
Communication 8/08198K
2720
Chem. Commun., 1998, 2719–2720