such physical properties have been exploited to date, the
examination of electrides as chemical reagents in synthetic
organic chemistry remains to be carried out. Although various
reducing reagents, such as metal, metal hydrides, and
organometallic compounds, are already employed in synthetic
organic reactions,5 there have been no reports on the
application of electrons in an electride to synthetic organic
reactions. We adopted pinacol coupling of aromatic alde-
hydes as the first probe of the utility of C12A7:e- in synthetic
organic chemistry, because the reaction is induced by the
transfer of electrons from the chemical reagent to the
aldehyde. Electrochemically induced pinacol coupling in-
volves an initial transfer of electrons from the electrode to
the organic substrate molecules on the surface of the
electrode.6 In this paper, we report the pinacol coupling of
aromatic aldehydes promoted efficiently by the electride.
When benzaldehyde (0.094 mmol) and C12A7:e- (196
mg)7 are mixed in water (5.0 mL), the dark brown electride
gradually dissolves and the mixture slowly becomes a white
suspension. Hydrolysis of the white gel by aqueous HCl (1
M), followed by Et2O extraction produces 1,2-diphenyleth-
yleneglycol, indicating that the pinacol coupling reaction
shown by eq 1 took place. The resulting solution showed a
pH of 11.6 (26 °C). 1H NMR analysis of the reaction mixture
confirmed the formation of 1,2-diphenylethyleneglycol.
Table 1. Pinacol Coupling of Aldehydes (RC6H4CHO) by
C12A7:e- a
electrons/
temp/ time/ yieldc/
run
R
aldehydeb solvent
°C
h
%
dl:mesod
1
H
H
H
H
H
2.9
1
H2O
H2O
H2O
H2O
rt
3
71
21
39
0
55:45
38:62
52:48
2
rt
rt
rt
3
24
24
18
3e
4f
5
20
2.9
2.9
H2O:THF rt
69
50:50
67:33
(4:1)
H2O:THF rt
(1:1)
THF
H2O
H2O
H2O
H2O
6
H
18
33
7
8
9
H
CH3
(CH3)3C
2.9
2.9
2.9
2.9
2.9
rt
24
12
12
24
24
0
35
11
33
11
100
100
100
100
40:60
41:59
53:47
g
10 Cl
11 CH3CO
a Reaction conditions: [aldehyde] ) 0.094 mmol, solvent ) 5.0 mL.
b Molar ratio of electrons in C12A7:e- to aldehyde. c Determined by 1H
NMR with 4-methoxybiphenyl as an internal standard. d Ratios of the
stereoisomers of the products determined by 1H NMR. e Polycrystalline
C12A7:e- was employed (solvent ) 50 mL). f C12A7 powder was used in
the same amount as the electride in run 1. g Not determined due to severe
overlap of the signals due to the stereoisomers.
trons in the C12A7:e- are responsible for promoting the
reaction. A mixed solvent system of tetrahydrofuran (THF)
and water reduced the saturation yield to 69% for H2O:THF
of 4:1 (run 5) and 33% for 1:1 (run 6), and finally, no
reaction was observed in pure THF (run 7), suggesting that
water is essential for the release of electrons from the C12A7:
e-.
The results of run 1 indicate that 30% of the electrons in
the C12A7:e- are utilized in the reaction at maximum. Olga
reported that the thermodynamic potential of the electron
within C12A7:e- is high enough to promote the reduction
of water.8 Actually, Na intercalated in silica gel, which is
effective for the reduction of organic compounds,5b prefer-
entially produces hydrogen gas in the presence of water.9
However, the reactions with C12A7:e- are accompanied by
some reduction of water, but the electrons are consumed
effectively for the reduction of aldehyde rather than the
reduction of water. This observation implies that some
reaction medium produced from C12A7:e- acts as a reaction
selector.
The C12A7:e- in the single crystalline form (2 × 1021
electrons cm-3)2 effectively promotes the pinacol coupling
of benzaldehyde to yield the corresponding 1,2-diol (71%),
when the electride used contains a large excess amount of
electrons relative to the aldehyde (Table 1, run 1). The use
of polycrystalline C12A7:e- (1019-20 electrons cm-3) leads
to a much lower yield of the product (39%) even after
reacting for 24 h (run 3). Furthermore, neither stoichiometric
C12A7 powder (run 4) nor Ca(OH)2 caused any pinacol
coupling. These observations clearly indicate that the elec-
Table 1 shows the reactions of other substituted aldehydes
in water (runs 8-11), and indicates that they proceeded at
100 °C to form the corresponding diols as the products with
moderate yields. The reactivity of the C12A7:e- system
changes apparently depending on the substituents of the
aromatic ring among these four aldehydes, and no direct
relationship is observed between the electron-withdrawing
(or releasing) properties of the substituents and the product
yield. The aldehyde group of p-acetylbenzaldehyde was
selectively reduced to pinacol, whereas the carbonyl group
in the ketone moiety remained unreacted (run 11).
The XRD analysis of the residual powder after the
completion of the reaction confirmed that the reaction is
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