1008
Chemistry Letters Vol.37, No.9 (2008)
Electro-initiated Coupling Reactions of N-Acyliminium Ion Pools
with Arylthiomethylsilanes and Aryloxymethylsilanes
Seiji Suga, Ikuo Shimizu, Yosuke Ashikari, Yusuke Mizuno,
Tomokazu Maruyama, and Jun-ichi Yoshidaꢀ
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510
(Received June 20, 2008; CL-080629; E-mail: yoshida@sbchem.kyoto-u.ac.jp)
Electro-initiated coupling reactions of N-acyliminium ion
with arylthiomethylsilanes and aryloxymethylsilanes were de-
veloped. Pulse electrolyses with intervals were found to be quite
effective for the initiation. A chain mechanism involving cation,
radical cation, and radical intermediates has been proposed.
H3Si
H
H
H
More stable
H3Si
vs.
vs.
H
S
O
S
O
CH3
CH3
4av
More stable
CF3
4ah
H3Si
H
H
H
H3Si
H
S
CF3
S
Electro-initiated chain reactions constitute an interesting
class of reactions. Only a catalytic amount of electricity is
required for completion of the reaction, because electrolysis is
used for generation of a reactive species that initiates a chain
process. For example, oxygenation involving a radical inter-
mediate,1 [2 + 2] cycloaddition,2 and olefin metathesis3 via
radical ion intermediates, and a cation chain reaction4 have been
reported in the literature. We report herein another example of
electro-initiated chain reactions, which involves cation, radical,
and radical cation intermediates.
4dv
4dh
Figure 1. Structures of the model radical cations 4a and 4d
obtained by DFT calculations (B3LYP/6-31G(d)).
Table 1. Electro-initiated reactions of N-acyliminium ion 1
with arylthiomethylsilanesa
Oxidation
potential
/V
Arylthiomethyl-
silane
Electricity
/F molꢁ1
Yield
/%
Product
The present work stems from our earlier observations that
the reactions of N-acyliminium ion pools with benzylsilanes pro-
ceed by a radical/radical–cation/cation crossover mechanism.5
We examined a similar reaction of N-acyliminium ion pools
with arylthiomethylsilanes because it is well established that
the anodic oxidation of arylthiomethylsilane leads to facile
cleavage of the C–Si bond (eq 1).6
b
2a
2b
2c
2d
1.18
1.41
1.43
1.58
0
0:003 ꢂ 3
0
3a
3a
3b
3b
3c
3c
3d
3d
3d
—
—
b
14
46
27
69
45
72
90
0:003 ꢂ 3
0
0:003 ꢂ 3
0
0.01
Electrochemical
0:003 ꢂ 3
Me3Si
S
S
initiation
N
N
aReactions were carried out at 0 ꢃC for 1 h. bThe expected product 3a
was not detected.
1
+
CH2Cl2
CO2Me
CO2Me
R
R
ð1Þ
2a: R = OMe
2b: R = H
3a: R = OMe
3b: R = H
product 3b without electrochemical initiation, although the yield
was low (Table 1). The introduction of an electron-withdrawing
group such as F (2c) and CF3 (2d) improved the yield of the
coupling products 3c and 3d.
2c: R = F
3c: R = F
2d: R = CF3
3d: R = CF3
Thus, N-acyliminium ion 1 was generated from the corre-
sponding silyl-substituted carbamate by low-temperature elec-
trolysis using Bu4NBF4 as a supporting electrolyte in CH2Cl2,
and a solution of 1 was allowed to react with arylthiomethyl-
silane 2a without electrochemical initiation.7 The expected
coupling product 3a was not obtained, and most of 2a remained
unchanged. The result was surprising because the oxidation
potential of 2a seems to be low enough for the initial electron
transfer (Table 1). In fact, the benzylsilanes of higher oxidation
potentials reacted with 1 to give the coupling product.5 Presum-
ably, the C–Si bond in the radical cation is difficult to cleave.
In fact, DFT calculations of a model radical cation 4a indicate
that the conformation in which the C–Si bond can interact with
the p orbital of sulfur (4av) is less stable than the conformation
in which the C–Si bond is perpendicular to the p orbital of
sulfur (4ah), although the energy difference is small (ꢀE ¼
0:97 kJ/mol) (Figure 1).
DFT calculations indicate that 4dv is more stable than 4dh
(ꢀE ¼ 3:70 kJ/mol). The interaction with the C–Si bond stabil-
izes the radical cation, although such stabilization is not neces-
sary for radical cations having a strong electron-donating group
like 4a. Such interaction weakens the C–Si bond and facilitates
C–Si bond cleavage by nucleophilic attack on the silicon atom.
The present reaction seems to proceed by a chain mecha-
nism shown in Figure 2. The initial single-electron transfer from
2 to N-acyliminium ion 1 gives radical cation 4 and radical 5.
The C–Si bond in 4 is cleaved to generate radical 6.8 This
process is presumably assisted by nucleophilic attack of BF4
ꢁ
,
which is the counter anion of 1. Radical 6 thus generated adds
to N-acyliminium ion 1 to give radical cation 7. We have already
reported that an alkyl radical adds N-acyliminium ion pools very
rapidly.9 Radical cation 7 undergoes a single-electron transfer
reaction with 2 to give the coupling product 3 and radical cation
4, which collapses to radical 6.
The reaction with 2b gave the corresponding coupling
Copyright Ó 2008 The Chemical Society of Japan
Suga, Seiji
