Mendeleev
Communications
Mendeleev Commun., 2007, 17, 343–344
Catalytic reduction of an α,β-disubstituted alkene
with sodium borohydride in the presence of
tetra-tert-butylphthalocyanine complexes
Valery V. Kalashnikov and Larisa G. Tomilova*
Institute of Physiologically Active Compounds, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russian Federation. Fax: +7 495 939 0290; e-mail: tom@org.chem.msu.ru
DOI: 10.1016/j.mencom.2007.11.015
Cobalt tetra-tert-butylphthalocyanine was found an efficient catalyst for the catalytic reduction of 4-[(E)-2-phenylethenyl]-
pyridine to 4-(2-phenylethyl)pyridine with sodium borohydride.
The combination of a reducing agent, molecular oxygen and a
R
R
R
metal porphyrin is a potential catalytic system for the reductive
conversion of organic compounds. In 1979, Tabushi and Koga1
proposed the first catalytic system consisting of a substituted
manganese(III) porphyrin as the catalyst and sodium borohydride
as the reducing agent, which simulated the function of cyto-
chrome P-450. The major products of reductive hydroxylation
of cyclohexene included cyclohexanol and cyclohexen-2-ol.
It was assumed that cyclohexene oxide was formed as an inter-
mediate and was then reduced to cyclohexanol with sodium
borohydride in the presence of manganese tetraphenylpor-
phyrinate. The subsequent studies of this reaction2 cast doubt
that epoxides were formed in it. A mechanism was proposed for
the catalytic reductive hydroxylation of alkenes involving the
coordination of an alkene with manganese(II) porphyrin as
the initial stage, followed by incorporation of an oxygen
molecule into the carbon–metal bond. Presumably, a manganese
alkylporphyrin is the key intermediate in the catalytic hydroxy-
lation involving manganese porphyrinate. Aside from manganese
porphyrin, iron(III) tetraporphyrinate3 and cobalt(II) tetrapor-
phyrinate4 also showed catalytic activity in this reaction.
The structure of phthalocyanines is similar to that of por-
phyrins, but the former have been studied much less thoroughly
with respect to reductive hydroxylation. Reductive hydroxyla-
tion was studied using styrene and α-methylstyrene as model
substrates.4–6 It was reported4 that the reaction slowed down
with alkenes with higher substitution complexity; in many
cases, the yields of reductive hydroxylation decreased as well.
In view of this, it was interesting to study the conditions of
reductive hydroxylation for alkenes with higher substitution
complexity.
i
HO
+
N
N
N
1a,b
2a,b
3a,b
a R = H
b R = Br
t
Scheme 1 Reagents and conditions: i, NaBH4, Bu PcCo, Mg(ClO4)2, O2,
20 °C.
substituted (styrene and α-methylstyrene).5 The presence of
magnesium perchlorate increased the yield of reaction product
3a. The highest yield of compound 3a (78%) was achieved in
the THF–ethanol solvent system in the presence of an equivalent
amount of magnesium perchlorate in the reaction mixture† at
70 °C. In this case, no reductive hydroxylation product (com-
pound 2a) was found in the reaction mixture. Moreover, this
Synthetic procedure: Mg(ClO4)2 (111.6 mg, 0.5 mmol) and Bu PcCo
(8.0 mg, 0.01 mmol) were added to a solution of 90.6 mg (0.5 mmol)
of compound 1a (obtained according to a published procedure7) in a
mixture of anhydrous tetrahydrofuran (20 ml) and anhydrous ethanol
(10 ml). After the reagents dissolved, NaBH4 (56.7 mg, 1.5 mmol) was
added and the solution was kept at 70 °C for 9 h with stirring by an air
flow [after 3 and 6 h of heating, a portion of NaBH4 (56.7 mg, 1.5 mmol)
was added each time]. After cooling, toluene and water were added to
the mixture; the organic layer was separated while the aqueous layer was
extracted with toluene (2×20 ml). The solvents were evaporated in vacuo
and the precipitate was dissolved in 0.5 M HCl. The phthalocyanine
precipitated was filtered off, and a solution of NaOH was added to the
filtrate to adjust pH 10. The solution was extracted with dichloromethane,
and the extract was dried with K2CO3. The solvent was evaporated in vacuo,
and the crystalline residue was chromatographed with chloroform on silica
gel. The chloroformic solution was concentrated to give 71.5 mg (78%)
of compound 3a8,9 as colourless crystals, mp 69–70 °C. 1H NMR (200 MHz,
CDCl3) d: 2.91 (s, 4H, 2CH2), 7.02–7.39 (m, 7H, Ph and β-Py), 8.47 (d,
2H, α-Py). MS, m/z: 183 (M+).
t
†
We used 4-[(E)-2-phenylethenyl]pyridine, a heterocyclic
analogue of stilbene, as the substrate. Two reaction products were
obtained at room temperature in the presence of 1.5 mol% cobalt
t
tert-butylphthalocyanine (Bu PcCo) according to Scheme 1.
As expected, the reaction gave compound 2a, which resulted
from the reductive hydroxylation of alkene 1a. Unexpectedly,
4-(2-phenylethyl)pyridine 3a was also obtained, which is a
product of double bond hydrogenation in compound 1a. Note
that compounds 2a and 3a were formed in a ratio of 1:1. An
increase in the temperature to 70 °C resulted in a 1:3 ratio
between compounds 2a and 3a. If the reaction temperature
was increased to 120 °C (in 2-methoxyethanol), only compound
3a was obtained in 37% yield. There are examples of the hydro-
genation of alkenes on cobalt phthalocyanine, but less sterically
Similarly, 4-[2-(4-bromophenyl)ethyl]pyridine 3b was obtained from
compound 1b and isolated as a hydrochloride salt in 83% yield;
mp 154–155 °C. 1H NMR (200 MHz, [2H6]DMSO) d: 2.98 (t, 2H, CH2),
3.20 (t, 2H, CH2), 7.22 (d, 2H, 4-BrC6H4), 7.54 (d, 2H, 4-BrC6H4), 7.94
(d, 2H, β-C5H4N), 8.82 (d, 2H, α-C5H4N). MS, m/z: 262 (M+).
– 343 –
© 2007 Mendeleev Communications. All rights reserved.
Kalashnikov, Valery V.
