Mendeleev Commun., 2011, 21, 194–195
Table 1 The optimization of reaction conditions for the electrocatalytic styrene oxide opening (C = 10 mmol dm–3, 3 mmol dm–3 Cp2TiCl2, E = –0.85 V vs.
Ag/AgCl/KCl, CH-donor = 15 mmol dm–3, CMe SiCl = 15 mmol dm–3).
3
Reaction products, yield (%)
Reagent for
Cl
OH
OH
Me
Ti–O bond
cleavage
H-donor
OH
OH
Cl
Ph
Ph
28
24
Ph
45
44
Ph
Collidine
Ph3CH
4
0
hydrochloride
12
0
Me3SiCl
Ph3CH
23
75
42
14
21
3
0
0
GCMS analysis of the reaction solutions obtained in the electro-
lysis of a mixture of styrene oxide and Cp2TiCl2 in the presence
of collidine hydrochloride revealed the formation of isomeric
chlorohydrins which are the products of the oxirane ring opening
with chloride anions (Table 1). The control experiments showed
that peak potential value for collidine hydrochloride reduction
is about –1.2 V vs. Ag/AgCl/KCl but at an applied potential of
–0.85 V a partial reduction of the protons in collidine hydro-
chloride occurs leading to the formation of some amount of chloride
anions. As it was already mentioned, the electrolysis was performed
in a two-compartment cell and the anodic oxidation of chloride
ions did not decrease their concentration in the cathodic compart-
ment. This enforced us to turn to trimethylsilane as the reagent
for Ti–O bond cleavage. The control experiments revealed that it
did not open oxirane ring under the reaction conditions (contrary
to Me3SiBr which gave bromohydrins). The silylated alcohol
formed at the final reaction step can be easily converted to the
corresponding primary alcohol by the treatment with the 2 m
HCl or KF. The optimization of the reaction conditions allowed
us to obtain 2-phenylethanol in 75% yield (Table 1). The regio-
selectivity of the reaction was proved using GCMS analysis: two
isomeric phenylethanols can be distinguished by their charac-
teristic fragmentations in mass spectra which are available in
‘Wiley275’ database.
It was of interest to investigate the applicability of the proposed
electrocatalytic ring opening to the substrates with easily reducible
functional groups, e.g., 4-nitrostyrene oxide. Electrochemical reduc-
tion of nitro group in this compound occurs at a potential of about
200 mV more cathodic as compared to the potential of titanocene
dichloride reduction (–1.05 V) and it constrains us to be especially
careful in the installation of the electrolysis potential value. To
prevent the direct reduction of nitro group at the electrode, the
electrolysis was performed at a potential of –0.75 V which is a
little less below the peak potential value for Cp2TiCl2. The reaction
protocol was the same as that used for unsubstituted styrene oxide,
KF was used for Si–O bond cleavage. As a result, 2-(4-nitro-
phenyl)ethanol was obtained in 81% yield.
determined by the stability of intermediate radical species and
steric interactions. The main advantage of the electrochemical
approach is a possibility of a precise adjustment of a reaction
potential value which allows one to avoid side reactions involving
easily reducible functional groups. This makes the approach
applicable to the substrates for which activation with organo-
metallic compounds is not valid.
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Thus, it was demonstrated that potentiostatic electrolysis at
a potential of Cp2TiCl2 electroreduction can be considered as a
convenient method for the electrocatalytic reductive opening of
the oxirane ring yielding primary alcohols. The regioselectivity
of the process is the opposite to that of SN2 reactions and it is
Received: 25th January 2011; Com. 11/3673
– 195 –