Formation of hydridocobalt(iii) phthalocyanine by reaction of cobalt(ii) phthalocyanines with sodium borohydride and its reactions with antioxidant isoflavones

Article abstract of DOI:10.1039/c1nj20582j

The reduction of substituted isoflavones with sodium borohydride catalyzed by cobalt(ii) phthalocyanines has been achieved efficiently to yield cis- and trans-isoflavan-4-ols under mild conditions via the formation of a hydridocobalt(iii) phthalocyanine i

Full text of DOI:10.1039/c1nj20582j

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PAPER
Formation of hydridocobalt(III) phthalocyanine by reaction of cobalt(II)
phthalocyanines with sodium borohydride and its reactions with
antioxidant isoflavonesw
Poonam, Pratibha Kumari, Ritika Nagpal and Shive M. S. Chauhan*
Received (in Montpellier, France) 15th July 2011, Accepted 22nd August 2011
DOI: 10.1039/c1nj20582j
The reduction of substituted isoflavones with sodium borohydride catalyzed by cobalt(II)
phthalocyanines has been achieved efficiently to yield cis- and trans-isoflavan-4-ols under mild
conditions via the formation of a hydridocobalt(III) phthalocyanine intermediate. The hydroxy
isoflavones react with sodium borohydride to form the corresponding phenolate moiety and
thereby prevent the formation of a hydridocobalt(III) phthalocyanine intermediate which inhibits
its reduction to corresponding isoflavan-4-ols.
bis-tert-butylthioethane borane)14 and metal borohydrides
Introduction
(like NaBH4, LiBH4 and Li(t-BuO)3AlH),15 however, large
Isoflavones are very significant biologically active members of
excesses of the reducing agents are used in the above systems.
the flavonoid class of naturally occurring compounds and are
The slow reaction rate, decomposition of catalyst and low
exclusively produced by the Fabaceae/Leguminosae (bean)
yields of products are other drawbacks associated with these
family. Isoflavones and their reduced metabolites have been
reduction methods. Metallophthalocyanines (MPc) have been
widely studied due to their promising estrogenic,1 antifungal,2
widely used as a catalyst in various redox reactions.16 Recently,
and anticancer activity3 in humans. The antioxidant activities
we have reported the cobalt(II) phthalocyanine catalyzed reduction
of isoflavones contribute to their potential role in reducing the
of flavones using sodium borohydride in methanol and the
formation of certain types of cancers and some chronic
involvement of the Co(III) phthalocyanine hydride complex has
been demonstrated in the reduction process.17 Now, we report the
diseases such as cardiovascular disease, Alzheimer’s disease,
and osteoporosis.4 Moreover, the structural similarity of their
reaction of antioxidant isoflavones with sodium borohydride in
metabolites to human estrogens makes the study of their
the presence of cobalt(II) phthalocyanine under different
biochemical transformation important. These metabolites are
reaction conditions.
found in blood plasma,5 urine,6 and feces as a result of
biotransformation by intestinal microflora microorganisms.7
Results and discussion
Equol is the most effective reduced metabolite possessing
antitumor activity against human prostate cancer cells which is
produced from enzyme catalyzed sequential reduction reactions
of a natural isoflavone, 40,7-dihydroxyisoflavone (daidzein), via
the formation of 40,7-dihydroxyisoflavanone and then 40,7-
dihydroxyisoflavan-4-ol.7,8 In view of the useful information
on the mechanistic study of isoflavonoid biotransformation as
well as the significant biological properties of the reduced
metabolites, there is growing interest to develop an effective
reductive system for isoflavones.
The reaction of 7-methoxyisoflavone (1a) with NaBH4 in the
absence of cobalt(II) phthalocyanine [Co(II)Pc] (4a) did not
give any reduced product even after 12 h. However, the same
reaction in the presence of 4a gave cis (2a) and trans form of
7-methoxyisoflavan-4-ol (3a) in 68% and 30% yields, respectively
(Scheme 1, Table 1, entry 2) and no other byproduct was
observed. Isoflavone (1b) and substituted isoflavone (1c) were
also efficiently reduced to isoflavan-4-ol products (2b–c and
3b–c) (Table 1, entries 3 and 4). The reductive system failed to
reduce isoflavones bearing hydroxyl groups (1d–1h) even on
prolonging the reaction for 24 h (Scheme 2, Table 1, entry 5).
However, the reaction of 7-hydroxy isoflavone (1d) catalyzed
by 4a using excess of NaBH4 (30 equiv. w.r.t. 1) gave
7-hydroxy isoflavanone (7d) as a product in 40% yield
(see experimental section) along with other reduced products in
trace amounts. Recently, the stereospecific microbial reduction of
1d to 7-hydroxy isoflavanone (7d) has also been reported.18
Similar reactions of 1e–1h with excess of NaBH4 gave a
The reduction of isoflavones and isoflavanones to isoflavan-
4-ols has been achieved by using reagents such as nickel boride,9
Pt-H2 in acetic acid,10 Grignard reagent,11 Pd–charcoal–H2,12
sodium metabisulfite,13 boranes (such as borane-tetrahydrofuran,
Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: smschauhan@chemistry.du.ac.in; Fax: +91-11-27666845;
Tel: +91-11-27666845
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1nj20582j
c
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Scheme 2
Scheme 1
Fig. 1 Conformations of cis-isoflavan-4-ol (2).
mixture of reduced products in traces which were difficult to
isolate.
The cis and trans forms of isoflavan-4-ols (2 and 3) exist in
half-chair conformations as ascertained by comparing the
coupling constants in 1H-NMR spectra with reported values.19
Due to Ph-3 and OH-4 positions in half-chair conformations,
two conformers of cis- (Fig. 1, Form I and II) (2) and trans-
isoflavan-4-ols (3) (Fig. 2, diequatorial and diaxial) are possible.
In 1H-NMR spectra of cis-7-methoxyisoflavan-4-ol (2a),
Fig. 2 Conformations of trans-isoflavan-4-ol (3).
J3,4 value is 3.3 Hz which corresponds to a H–C3–C4–H dihedral
angle of about 501. As both conformers of cis-isoflavan-4-ol (2)
(I and II) have similar dihedral angles close to 501, the actual
conformation of cis-isoflavan-4-ol (2a) could not be explicated
on the basis of NMR data. Further, the observed J3,4 = 7.5 Hz
Scheme 3
1
in the H-NMR spectra of trans-7-methoxyisoflavan-4-ol (3a)
entries 6 and 7), suggesting that the present reductive system
initially reduces the conjugated double bond and then the
carbonyl group. The same reaction with high concentration of
NaBH4 (5 times w.r.t. 1a) results in complete reduction of 7a to
cis and trans forms of isoflavan-4-ol products (2a and 3a)
(Table 1, entry 2). The concentration of the active catalyst
present in the system also affects the yields of reduced products
(Table 1, entries 8 and 9). After the reductions, the catalysts
were quantitatively recovered by filtration and could be reused
several times without showing any loss in their catalytic activity.
The effect of the nature of the substituent attached to the
phenyl ring of catalyst 4 on the reduction process was examined
(Table 2, entries 1–3). The cobalt(II) phthalocyanine bearing an
suggests that two conformers, diaxial and diequatorial, of trans-
isoflavan-4-ol (3a) exist in equilibrium, preferring thrice more
diequatorial conformers (J3,4 of diequatorial conformer =
9.95 Hz and J3,4 of diaxial conformer = 1.66 Hz).20 A similar
equilibrium in the conformations of cis-2b–2c and trans-3b–3c
was observed on the basis of coupling constant values.
The mode of reduction of 1a by the present reductive system
was studied by performing the reaction of 1a with an equimolar
amount of NaBH4 (Scheme 3). Similar to flavones,17 the
formation of 7-methoxyisoflavanone (7a) along with a little
amount of 7-methoxyisoflavan-4-ol (2a and 3a) entails the
chemoselective approach of the reductive system (Table 1,
Table 1 Reductions of substituted isoflavones (1) catalyzed by 4a with NaBH4
Yieldb (%)
Conv.a (%)
cis-2
trans-3
Entry
System
Substrate : catalyst : NaBH4
Time/h
1
2
3
4
5
6
7
8
9
1a/NaBH4
1 : 0 : 16
12
1
1
1
24
1
1
4
4
0
100
88
0
68
59
61
0
14
47
62
40
0
30
25
37
0
1a/4a/NaBH4
1b/4a/NaBH4
1c/4a/NaBH4
1d–1h/4a/NaBH4
1a/4a/NaBH4
1a/4a/NaBH4
1a/4a/NaBH4
1a/4a/NaBH4
10 : 1 : 50
10 : 1 : 50
10 : 1 : 50
10 : 1 : 50
10 : 1 : 10
10 : 1 : 30
50 : 1 : 250
100 : 1 : 500
100
0c
64d
98e
87
8
25
23
22
64
a
b
Based on substrate 1. Isolated yields. Traces of reduced products were observed using excess of NaBH4 (30 equiv. vs. 1) after 24 h.
c
d
e
Isoflavanone (7a) was isolated in 22% yield. Isoflavanone (7a) was isolated in 42% yield.
c
2640 New J. Chem., 2011, 35, 2639–2646
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Table 2 Reduction of isoflavone (1a) with NaBH4 using different
catalystsa
entries 6–8). Many unidentified products were also observed
and the catalysts decomposed after the reductions and could
not be reused. These findings highlight the marked role of a Pc
ring in the reduction process along with the metal. The
reactions catalyzed by metal salts are known to proceed
through corresponding metal boride catalysts generated
in situ.25 Apparently, the reductions using MPc as catalyst in
place of metal salts take place through different pathways. The
present study provides an excellent opportunity to examine the
catalytic effect and mechanism of the reactions of cobalt(II)
phthalocyanine.
Yieldc (%)
7a
2a
3a
Entry
Catalyst
Time/h
Conv.b (%)
1
2
3
4
5
6
7
8
4b
4c
4d
5a
6a
CoCl2
d,f
NiCl2d,g
FeCl3
2
2
2
2
2
4
4
5
99
100
100
94
90
64
—
—
—
28
21
42
46
36
69
76
68
28
39
14
12
12
28
23
32
18
30
6
d,e
68
61
8
9
The reductions of isoflavone (1) have been carried out using
different reducing agents (Table 3). The use of sodium benzo-
phenone ketyl as a reducing agent in the reaction of 1a, 1b and
1c catalyzed by 4a gave reduced products in low yields
(Table 3, entries 1–3). In the case with isoflavone bearing a
hydroxyl group (1d–1h), reductions did not take place even on
prolonging the reaction time (Table 3, entry 4). The observations
justify that hydroxyl groups of isoflavones inhibit the reduction.
The reactions of 7-hydroxyisoflavones (1d) with hydrazine
hydrate gave corresponding additive products26 which
remained unaffected by the presence of Co(II)Pc 4a (Table 3,
entry 5). Other hydroxyl substituted isoflavones (1e–1h) react
with hydrazine hydrate in the same manner.
a
Reaction condition: substrate : catalyst : NaBH4
c
=
Based on substrate 1a. Isolated yields. The reaction mixture
10 : 1 : 50.
b
d
turned black immediately on addition of NaBH4 and many unidenti-
e
fied products were observed after the reaction. The black colour of
f
the reaction disappeared after 3 h. The black colour of the reaction
g
persisted even after completion of the reaction. The black colour of
the reaction disappeared after 90 min.
electron withdrawing nitro group (4c) exhibited better catalytic
activity than those bearing electron donating groups. The
variation in the redox potential of Co(II)Pc due to the presence
of electron withdrawing and electron donating substituents
governs the activity.21 Exceptionally high catalytic activity of
cobalt(II) phthalocyanine bearing a t-butyl group (4d) could be
attributed to non-aggregating property of 4d and other
functionalized phthalocyanines in solution.22 The catalyst 4d
could not be separated by direct filtration due to its good
solubility in methanol, limiting its applicability in reductions.
However, 4d could be recovered quantitatively by column
chromatography. The change of the central metal of MPc
from Co(II) to Ni(II) or Fe(III) resulted in poor yields of
isoflavan-4-ols (2a and 3a) along with the formation of 7a in
28% and 21% yields respectively (Table 2, entries 4 and 5).
The central metal changes the electronic state of the Pc ring,
thereby causing a change in the oxidation potential of MPc.
The difference in the redox potential of 4a and 5a may be the
driving force of the reaction.23 It has been well known that
interaction with groups above and/or below the Pc plane
produces significant changes in redox potentials and this
may be responsible for the least catalytic activity of
Fe(III)Cl-Pc (6a).24
The catalyst MPc becomes soluble in the reaction mixture of
1a–1c with a characteristic colour change corresponding to the
generation of an M(I)Pc intermediate (10) during the reduction
process, for instance, in the reactions catalyzed by 4a, 5a and
6a, colour of the reaction mixture changed to reddish brown,
dark green and brown, respectively.27 This could be ascribed
to the strong tendency of MPc, particularly Co(II)Pc, to attract
an electron.23 Further, the UV-vis spectra of 4a (Q band at
662 nm) on addition of NaBH4 exhibit the characteristic bands
in the region of 464 and 705 nm, suggesting the formation of
Co(I)Pc species (10) (Fig. 3) in the reduction reaction of
1(a–c).17 These bands were also observed when the UV-vis
spectral titrations of 4a with NaBH4 were carried out in the
presence of 1(a–c). Furthermore, a steady decrease in the
absorption intensity at 705 nm with the appearance of new
bands at 594 and 656 nm (Fig. 3) with a high extinction
coefficient suggested the transformation of Co(I)Pc species into
a transient hydridocobalt(III) phthalocyanine intermediate (11).28 It
is well known that Co(I)Pc species similar to other M(I)Pc species
forms alkyl cobalt(III) phthalocyanine complexes,16d,29 albeit no
hydridocobalt(III) phthalocyanine complexes (11) have been
isolated to date. The formation of transient hydridocobalt(III)
porphyrins30 and organocobalt(III) porphyrins complexes has
also been well documented in the literature.31 Furthermore, the
Another aim is an exploration of the relation between metal
and Pc ring in the reduction processes using NaBH4 as the
reducing agent. The reactions catalyzed by metal salts such as
CoCl2, NiCl2 and FeCl3 were sluggish giving 7a as the major
product in 42%, 46% and 36% yields respectively (Table 2,
Table 3 Reduction of isoflavone (1a–1h) catalyzed by 4a using different reducing agents
Yieldb (%)
Conv.a (%)
cis-2
trans-3
Entry
Reducing agent
Isoflavone
Time/h
1
2
3
4
5
Sodium benzophenone ketyl
Sodium benzophenone ketyl
Sodium benzophenone ketyl
Sodium benzophenone ketyl
Hydrazine hydrateb
1a
1b
1c
1d–1h
1d
4
4
4
24
3
45
40
55
0
24
20
28
—
—
18
17
25
—
—
100
a
b
Based on substrate 1. Reactions take place even in the absence of catalyst 4a.
c
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Scheme 5
NaBH4 preventing the conversion of Co(II)Pc into active
catalyst Co(I)Pc by NaBH4 (Scheme 5 Path A); (ii) preferential
reaction of NaBH4 with phenolic groups of isoflavones
(1d–1h) deterring the interaction of NaBH4 with Co(II)Pc to
generate Co(I)Pc species in the reaction (Scheme 5 Path B).
Various studies have been performed to elucidate the role of a
hydroxyl group in the failure of reduction.
Fig. 3 UV-vis spectra of 4a in the presence of sodium borohydride at
an interval of 1 min.
To examine the interaction of phenolic groups of isoflavones
(1d–1h) with the central cobalt metal of the catalyst, the UV-vis
spectral studies of 4d with 1d were carried out in the presence or
absence of NaBH4 (ESIw). No changes in the spectra of 4d were
observed on addition of 1d (Fig. S6, ESIw). However, the
addition of NaBH4 to the solution of 4d and 1d showed a
change in the spectra of 1d, though the bands for 4d remained
unchanged (Fig. S7, ESIw). The upshots ruled out the first
possibility for ineptitude of the present system and reinforced
the second prospect.
Scheme 4
UV-vis spectra of 4a (Q band at 656 nm) on addition of a
sodium benzophenone ketyl reducing agent gave bands corre-
sponding to the formation of Co(I) species which then converts
into methyl Co(III)Pc species (Q band at 660 nm) on addition of
methyl iodide29a (Scheme 4, Fig. 4).
The UV-vis spectral analysis of 4a in the presence of
hydroxyl isoflavones (1d–1h) did not exhibit the characteristic
bands for Co(I)Pc species (10). Moreover, no specific colour
change for Co(I)Pc species (10) was observed in the reactions
of 1d–1h. It justifies the preferable interaction of NaBH4 with
phenolic groups of isoflavones (1d–1h) which may prevent the
formation of active catalyst Co(I)Pc (10) from Co(II)Pc (4a) in
the reaction and thereby inactivate the system for the reductions.
Excess of NaBH4 could interact with cobalt metal of the catalyst
to generate the cobalt(III) hydride complex which would
reduce the a,b-unsaturated carbonyl moiety of 1d–1h to give
reduced products in trace amounts.
The UV-vis spectral studies of 1d on addition of NaBH4
were carried out to explore the interaction of hydroxyl groups
of isoflavones (1d–1h) with NaBH4. The UV-vis spectral
studies exhibited the gradual appearance of new bands at
263 nm and 338 nm with the disappearance of bands for 1d
at 242 nm and 299 nm (Fig. 5). Similar spectral changes were
observed on addition of NaOH to the solution of 1d in
methanol (Fig. S8, ESIw) which results in the formation of a
phenolate moiety by the abstraction of a proton from the –OH
group.32 The employment of NaBH4 in the formation of
phenolate of isoflavones (1d–1h) may prevent the transformation
of Co(II)Pc into Co(I)Pc species and thereby inhibit the
reduction of 1d–1h. Further, the interaction of the electron
rich phenolate moiety with electron deficient boron species
could generate an unusual boron–oxygen complex33
The reluctance of 1d–1h towards reduction by the present
system could be explicated by two prospects: (i) interaction of
hydroxyl groups of the isoflavones (1d–1h) with the central
cobalt metal of phthalocyanine catalyst in the presence of
Fig. 4 UV spectra of Co(II)Pc 4a (a), Co(I)Pc generated after the
addition of sodiumbenzophenone ketyl to Co(II)Pc (b), Co(III)CH3Pc
appeared after the addition of methyl iodide to Co(I)Pc (c).
Fig. 5 UV spectra of 7-hydroxyisoflavone 1d on addition of sodium
borohydride in methanol.
c
2642 New J. Chem., 2011, 35, 2639–2646
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(Scheme 5, Path B) which would make the system inactive by
preventing the interaction of NaBH4 with Co(II)Pc. Various
attempts were made to observe and isolate such type of species
under different conditions, but success could not be achieved.
Further, the use of a sodium benzophenone ketyl reducing
agent also failed to reduce 1d–1h, suggesting that there is no
boron–oxygen complex formation which could inhibit the
reduction. This supports that the formation of the phenolate
moiety is merely responsible for inactivation.
To substantiate that sodium phenolate formation by the
abstraction of a proton with NaBH4 prevents the reduction,
similar reactions of 1a–1c were carried out in the presence of
p-nitrophenol. Interestingly, as expected, no reduction was
observed even on prolonging the time. The interaction of
NaBH4 with the active hydroxyl group of p-nitrophenol
renders 1a–1c difficult to be reduced by the present reductive
system. The UV-vis spectral titrations of p-nitrophenol with
NaBH4 exhibited the appearance of a new band at 337 nm with
the disappearance of the band at 311 nm (Fig. S9, ESIw),
supporting the generation of the corresponding phenolate
moiety34 which then prevents the interaction of NaBH4 with
Co(II)Pc. It is noteworthy that the presence of p-tert-butyl phenol
in the reactions of 1a–1c failed to prevent reductions under
experimental conditions. Moreover, no phenolate generation
was observed when UV-vis spectral study of p-tert-butyl phenol
was carried out on addition of NaBH4. The results justify the
inhibitory effect of the phenolic group on reduction by the present
system which depends on the acidic nature of the –OH group.
The Co(II)Pc 4a catalyzed reduction of 1c with NaBD4 in
methanol was performed to understand the mode of reduction
by the present system (Scheme 6). The specific position of
deuterium in reduced products 8c and 9c supports that the
hydrogen at C-3 and C-4OH comes from methanol and the
source of hydrogen at C-2 and C-4 is NaBH4 in the reduction
reactions. Based on this observation, the reduction of 1a–1c is
believed to proceed through the active intermediate Co(I)Pc (10)
which takes up a proton from methanol to give hydridocobalt(III)
phthalocyanine (11) (Scheme 7). Hydrometallation of 11 with the
double bond of 1a–1c yields the corresponding organocobalt(III)
phthalocyanine intermediate (12).
Scheme 7
C–Co(III)Pc bond is reductively cleaved by NaBH4 to yield 7. In
the same way, 7 is reduced by 13 regioselectively to afford 2–3
(Scheme 7). This type of observation has also been observed in
reduction of flavanoids with NaBH4 catalyzed by cobalt(II)
phthalocyanines.15
Conclusions
In the reduction process, formation of hydridocobalt(III)
phthalocyanine by the reaction of cobalt(II) phthalocyanines
and sodium borohydride as well as other reducing agents has
been proposed. The reaction of hydridocobalt(III) phthalocyanine
with substituted isoflavones gives cis and trans forms of isoflavan-
4-ols exclusively. The role of hydroxy isoflavones in the inhibi-
tion of the formation of cobalt(III) hydride species provides
imperative information to study the mechanistic aspects of the
isoflavonoid biotransformation. Further, the present method
offers various advantages such as high yield of products,
shorter reaction time, milder conditions, simple work-up
procedure and reusability of the catalyst.
Experimental
All the melting points are uncorrected and were determined on
a Thomas Hoover Unimelt capillary melting point apparatus.
The IR spectra were recorded on a Perkin Elmer 1710 FTIR
spectrometer and the nmax are expressed in cmꢀ1. The electronic
spectra were recorded on a Perkin Elmer Lambda-35 UV/Vis
spectrophotometer and the lmax are expressed in nanometres.
A homolytic cleavage of the cobalt–hydrogen bond may
then generate a hydrogen radical which is trapped by the
conjugated double bond in the cage, leading to the corre-
sponding organocobalt(III) phthalocyanine (12). The stability
of organic radicals generated by addition of a hydrogen radical
to the double bond is proposed to determine the regioselectivity
of hydrometallation.35 Although the formation of the b-addition
radical is thermodynamically disfavored, the selective b-addition
may be ascribed to the steric effect of the phenyl substituent at
the C-3 position. This addition is in accordance with the catalysis
by metalloporphyrins,35,36 vitamin B12.37 Owing to the tendency
of alkylcobalt complexes to be dealkylated by nucleophiles,38 the
The H NMR, 13C NMR, spectra were recorded on Bruker
1
Avance-300 and Bruker Avance-400 spectrometers using TMS
as the internal standard (chemical shifts in ppm). The TOF
LC/MS spectra were recorded on a Hybrid Quadruple-TOF
LC/MS mass spectrometer 1011273/A using ion spray.
ESI-MS spectra were recorded on Micromass LCT KC 455
using electrospray positive ion mass spectra. The isoflavones
(1b–1h) were prepared according to the literature methods.39
7-Methoxyisoflavone (1a) was prepared by methylation of
7-hydroxyisoflavone (1d).40 Metallophthalocyanines (4–6)
were prepared according to the method reported by us.41
General procedure for the reduction of isoflavones (1a–1h) with
NaBH4 catalyzed by metallophthalocyanines (MPc) (4–6)
To the solution of isoflavone (1a–1h) (1.0 mmol) in dry
methanol (20 ml), MPc (4–6) (0.1 mmol) was added.
Scheme 6
c
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The solution was stirred at room temperature under a nitrogen
atmosphere and NaBH4 (5 mmol) was added in portions. The
progress of the reaction was checked by TLC. The reaction
mixture was filtered to remove the catalyst (4–6) and the
filtrate was concentrated under reduced pressure. The resulting
solid was chromatographed over silica gel (230–400 mesh) and
the mixture of cis- and trans-isoflavan-4-ols was separated by
column chromatography using ethyl acetate/petroleum ether
(varying from 1 : 99 to 10 : 90, v/v) as eluents to give isomers of
isoflavan-4-ols (2–3). The products were crystallized with
chloroform/petroleum (1 : 1, v/v). The recovered catalyst
was washed with methanol and reused to catalyze further
reduction reactions.
J = 11.8, 10.4 Hz, H-2b), 4.80 (1H, br s, H-4), 6.78 (1H, d,
J = 8.04 Hz, H-6), 7.04 (1H, d, J = 7.7 Hz, H-5), 7.20–7.41
(5H, m, H-20-50); dC (75 MHz, CDCl3) 11.6, 20.1, 44.2, 47.0,
64.1, 67.4, 120.9, 122.0, 127.2, 127.3, 128.0, 128.4, 128.8, 128.9,
138.2, 138.3, 152.0; MS (LC-MS) m/z calcd for C17H18O2:
254.1; found: 255.2 [M + H+].
trans-7,8-Dimethylisoflavan-4-ol (3c). Mp 87 1C; vmax(film)/
cmꢀ1 3361 (OH), 2922 (CH), 1495 and 1453 (CQC), 1103 and
1026 (C–O); dH (400 MHz, CDCl3) 1.91 (1H, d, J = 5.2 Hz,
OH,), 2.14 (3H, s, CH3-7), 2.27 (3H, s, CH3-8), 3.14 (1H, td,
J = 8.3, 7.9, 3.7 Hz, H-3), 4.26 (1H, dd, J = 11.2, 8.9 Hz,
H-2a), 4.40 (1H, dd, J = 11.2, 3.6 Hz, H-2b), 4.96 (1H, dd,
J = 7.6, 5.4 Hz, H-4), 6.80 (1H, d, J = 7.8 Hz, H-5), 7.20–7.36
(6H, m, H-6, H-20-50); dC (75 MHz, CDCl3) 11.7, 20.1, 46.9,
47.0, 67.9, 69.6, 121.8, 122.1, 124.0, 124.8, 127.3, 127.9, 128.8,
137.6, 138.9, 151.2; MS (LC-MS) m/z calcd for C17H18O2:
254.1; found: 278.1 [M + Na+ + H+].
cis-7-Methoxyisoflavan-4-ol (2a). Mp 144 1C (lit.,13 142 1C);
vmax(film)/cmꢀ1 3450 (OH), 2931 (CH), 1504 and 1466 (CQC),
1159 and 1031 (C–O); dH (400 MHz, CDCl3) 3.31 (1H, dt,
J = 11.9, 3.3 Hz, H-3), 3.78 (3H, s, OCH3,), 4.33 (1H, dd,
J = 10.4, 3.5, 1.3 Hz, H-2a), 4.60 (1H, dd, J = 11.6, 10.3 Hz,
H-2b), 4.77 (1H, br s, H-4), 6.45 (1H, d, J = 2.5 Hz, H-6), 6.54
(1H, dd, J = 8.6, 2.6 Hz, H-5) and 7.16–7.40 (6H, m, Ar H);
dC (75 MHz, CDCl3) 44.4, 55.5, 64.1, 66.9, 67.4, 101.5, 108.0,
117.9, 124.6, 127.7, 128.4, 128.9, 131.4, 137.9, 155.4 and 161.1;
MS (LC-MS) m/z calcd for C16H16O3: 256.1; found: 257.2
[M + H+].
Reduction of isoflavone (1c) with NaBD4 catalyzed by cobalt(II)
phthalocyanines (4a) in methanol
The reduction of 1c was carried out using NaBD4 following
the procedure as described above to give cis and trans products
(8c–9c) in 64 and 32% yields respectively.
cis-7,8-Dimethylisoflavan-4-ol (deuterated) (8c). Mp 134 1C;
trans-7-Methoxyisoflavan-4-ol (3a). Mp 130 1C (lit.,13 131 1C);
v
max(film)/cmꢀ1 3311 (OH), 2922 (CH), 1452 and 1421 (CQC),
v
max(film)/cmꢀ1 3435 (OH), 2929 (CH), 1502 and 1465 (CQC),
1103 and 1026 (C–O); dH (400 MHz, CDCl3) 1.85 (1H, s, OH),
2.21 (3H, s, CH3-7), 2.31 (3H, s, CH3-8), 3.30 (m, 1H, H-3),
4.39 (0.5H, d, J = 3.4 Hz, H-2a), 4.58 (0.5H, d, J = 12.1 Hz
H-2b), 6.81 (1H, d, J = 7.5 Hz, H-6), 7.05 (1H, d, J = 7.8 Hz,
H-5), 7.25–7.43 (5H, m, H-20-50); dC (75 MHz, CDCl3) 11.7,
20.1, 46.9, 47.0, 67.7, 67.9, 120.9, 121.8, 122.3, 124.2, 124.9,
127.5, 128.1, 129.0, 137.9, 139.1, 152.2; MS (LC-MS) m/z calcd
for C17H16O2D2: 256.1; found: 279.1 [M + Na+].
1161 and 1031 (C–O); dH (400 MHz, CDCl3) 3.16 (1H, td,
J = 8.3, 7.3, 3.9 Hz, H-3), 3.77 (3H, s, 7-OCH3), 4.29 (1H, dd,
J = 11.3, 8.3 Hz, H-2a), 4.38 (1H, dd, J = 11.3, 3.9 Hz, H-2b),
4.95 (1H, dd, J = 7.5, 5.4 Hz, H-4), 6.41 (1H, d, J = 2.4 Hz,
H-6), 6.56 (1H, dd, J = 8.8, 2.4 Hz, H-5), and 7.23–7.37 (6H, m,
Ar H,); dC (75 MHz, CDCl3) 47.2, 55.3, 64.0, 66.7, 67.0, 101.3,
107.9, 116.1, 127.5, 127.7, 128.4, 128.8, 131.2, 137.8, 155.4 and
160.7; MS (LC-MS) m/z calcd for C16H16O3: 256.1; found: 280.1
[M + Na+ + H+].
trans-7,8-Dimethylisoflavan-4-ol (deuterated) (9c). Mp 82 1C;
vmax(film)/cmꢀ1 3271 (OH), 2923 (CH), 1436 (CQC), 1109 and
1024 (C–O); dH (400 MHz, CDCl3) 1.88 (1H, s, OH), 2.13
(3H, s, CH3-7), 2.27 (3H, s, CH3-8), 3.13 (1H, m, H-3), 4.24
(0.5H, d, J = 8.9 Hz, H-2a), 4.39 (0.5H, m, H-2b), 6.79 (1H, d,
J = 7.8 Hz, H-5), 7.21–7.36 (6H, m, H-6, H-20-50); dC
(75 MHz, CDCl3) 11.6, 20.1, 44.0, 44.1, 64.0, 67.3, 120.9,
121.9, 121.9, 124.3, 127.2, 127.3, 128.5, 128.7, 138.2, 138.3,
152.0; MS (LC-MS) m/z calcd for C17H16O2D2: 256.1; found:
279.1 [M + Na+].
cis-Isoflavan-4-ol (2b). Mp 76 1C (lit.,42 75 1C); dH
(400 MHz, CDCl3) 3.35 (1H, dt, J = 11.7, 2.9 Hz, H-3),
4.30 (1H, ddd, J = 10.5, 2.9, 1.1 Hz, H-2a), 4.58 (1H, dd,
J = 11.7, 10.5 Hz, H-2b), 4.75 (1H, br s, H-4), 6.88–6.96
(2H, m, H-5 and 6), and 7.21–7.37 (7H, m, Ar H); dC (75 MHz,
CDCl3) 44.2, 64.1, 67.2, 116.9, 120.6, 123.5, 127.5, 128.4,
128.9, 129.9, 130.6, 137.7 and 154.2; MS (LC-MS) m/z calcd
for C15H14O2: 226.0; found: 227.0 [M + H+].
trans-Isoflavan-4-ol (3b). Mp 97 1C (lit.,42 98 1C); dH
(400 MHz, CDCl3) 3.19 (1H, td, J = 8.3, 7.3, 3.9 Hz, H-3),
4.27 (1H, dd, J = 11.2, 8.3 Hz, H-2a), 4.38 (1H, dd, J = 11.2,
3.9 Hz, H-2b), 4.99 (1H, m, H-4), 6.87–6.99 (2H, m, H-5 and 6),
and 7.20–7.52 (7H, m, Ar H,); dC (75 MHz, CDCl3) 47.2, 68.1,
69.4, 116.6, 120.9, 124.7, 127.6, 128.0, 128.2, 129.1, 129.3, 138.6
and 154.2; MS (LC-MS) m/z calcd for C15H14O2: 226.0; found:
227.0 [M + H+].
Reduction of isoflavone (1a) with an equimolar amount of
NaBH4 catalyzed by cobalt(II) phthalocyanines (4a) in methanol
To a mixture of 7-methoxy isoflavone (1a) (1 mmol) and 4a
(0.1 mmol), NaBH4 (1 mmol) was added in methanol under a
nitrogen atmosphere. The reaction mixture was stirred at
room temperature for 1 h. After removing the catalyst 4a by
filtration, the filtrate was concentrated under reduced pressure.
The resulting solid was chromatographed over silica gel
(60–120 mesh) and the reduced product 7-methoxy isoflavanone
7a (64% yield) was separated by column chromatography using
ethyl acetate/petroleum ether (varying from 1 : 99 to 10 : 90, v/v)
as eluents. The products were crystallized with chloroform/
petroleum (1: 1, v/v).
cis-7,8-Dimethylisoflavan-4-ol (2c). Mp 198 1C; vmax(film)/
cmꢀ1 3369 (OH), 2921 (CH), 1496 and 1423 (CQC), 1104 and
1016 (C–O); dH (400 MHz, CDCl3) 1.79 (1H, d, J = 3.2 Hz,
OH), 2.20 (3H, s, CH3-7), 2.31 (3H, s, CH3-8), 3.32 (1H, dt,
J = 11.9, 3.2 Hz, H-3), 4.40 (1H, m, H-2a), 4.62 (1H, dd,
c
2644 New J. Chem., 2011, 35, 2639–2646
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7-Methoxyisoflavanone (7a). Mp 91 1C (lit.,13 92–93 1C);
max(film)/cmꢀ1 2916 (CH), 1681 (CO), 1497 and 1463 (CQC),
over silica gel (60–120 mesh) and the product was separated by
column chromatography in 96% yield.
v
1115 and 1029 (C–O); dH (400 MHz, CDCl3) 3.79 (3H, s,
OCH3-7), 3.87 (1H, t, J = 6.9 Hz, H-3), 4.59 (2H, m, H-2),
6.39 (1H, d, J = 2.2 Hz, H-6), 6.56 (1H, dd, J = 8.8, 2.1 Hz,
H-8), 7.3 (5H, m, Ph), 7.85 (1H, d, J = 8.8 Hz, H-5); dC
(75 MHz, CDCl3): 52.5, 55.8, 102.1, 111.6, 118.4, 125.2, 127.8,
128.1, 128.4, 128.8, 129.0, 132.0, 165.6, 167.9, 190.2; MS
(LC-MS) m/z calcd for C16H14O3: 254.0; found: 277.0
[M + Na+].
4-(4-Phenyl-1H-pyrazol-3-yl) benzene-1,3-diol. Mp 186 1C;
v
max(KBr)/cmꢀ1 3266 (OH), 2925 (CH), 1604 (CQN), 1582
(NH), 1465 and 1410 (CQC); dH (400 MHz, DMSO-d6) 6.18
(1H, dd, J = 8.0, 5.8 Hz), 6.35 (1H, d, J = 2.2 Hz), 6.87
(1H, d, J = 8.8 Hz), 7.21–7.28 (5H, m), 7.8 (1H, s); dC
(75 MHz, CDCl3) 102.8, 106.5, 118.7, 125.6, 126.8, 128.3,
134.1, 156.6, 158.6.
Reduction of 7-hydroxy isoflavone (1d) with excess of NaBH4
catalyzed by cobalt(II) phthalocyanines (4a) in methanol
Acknowledgements
The authors are thankful to Department of Science and
Technology (DST), New Delhi, and the Council of Scientific
and Industrial Research (CSIR), New Delhi, for financial
support.
To a mixture of 7-hydroxy isoflavone (1d) (1 mmol) and
4a (0.1 mmol), NaBH4 (30 mmol) was added in methanol
under a nitrogen atmosphere. The reaction mixture was stirred
at room temperature for 6 h. The progress of the reaction was
checked by TLC. The catalyst 4a was separated by filtration
and the filtrate was concentrated under reduced pressure. The
reduced product 7-hydroxy isoflavanone (7d) was isolated
in 40% yield by column chromatography over silica gel
(60–120 mesh) using ethyl acetate/petroleum ether (6: 94, v/v)
as eluents.
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