Complexes of (η5-Cp*)Ir(iii) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-substituted benzimidazole synthesis

Article abstract of DOI:10.1039/c6dt04271f

The treatment of 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone [L1-L4] with [(η⁵-Cp*)IrCl(μ-Cl)]₂ at 25 °C followed by NH₄PF₆ results in [(η⁵-Cp*)Ir(L)Cl][PF₆

Full text of DOI:10.1039/c6dt04271f

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Complexes of (η5ꢀCp*)Ir(III) with 1ꢀbenzylꢀ3ꢀphenylthio /
selenomethylꢀ1,3ꢀdihydrobenzoimidazoleꢀ2ꢀthione/selenone:
catalyst for oxidation and 1,2ꢀsubstituted benzimidazole
synthesis
Received
Accepted
DOI: 10.1039/x0xx00000x
Alpesh K. Sharma, Hemant Joshi, Renu Bhaskar and Ajai K. Singh*
www.rsc.org/
The treatment of 1ꢀbenzylꢀ3ꢀphenylthio/selenomethylꢀ1,3ꢀdihydrobenzoimidazoleꢀ2ꢀthione/selenone [L1–L4] with [(η5ꢀ
Cp*)IrCl(µꢀCl)]2, at 25°C, followed by NH4PF6, results in [(η5ꢀCp*)Ir(L)Cl][PF6] (1–4 for L = L1 to L4) authenticated with
HRꢀMS and multi nuclei NMR (1H, 13C{1H} and 77Se{1H}). The structures of 1ꢀ4, established with single crystal Xꢀray
diffraction reveal geometry around Ir as “pianoꢀstool” type. The Ir–thio/selenoether (Ir−S/Ir−Se) bond distances (Å) are
2.347(18)ꢀ2.355(4)/ 2.4663(12)ꢀ2.4663(13)and Ir−thione/selenone (IrꢀS/IrꢀSe) ones, 2.4146(19)ꢀ2.417(2)/2.5141(16)ꢀ
2.5159(12). The reaction of 1,2ꢀphenylenediamine with benzylic alcohols and furfuryl alcohol under mild and ambient
condition, catalyzed efficiently with complexes 1–4 generates, in situ, bisimine. The cyclization and rearrangement via 1,3ꢀ
hydride shift triggered by its electrophilic activation with Ir(III) species finally result in 1,2ꢀdisubstituted benzimidazole.
The yield of the heterocycles in this one pot synthesis is excellent to good. The aldehydes generated in situ by aerial
oxidation of alcohols in the presence of 1–4 as catalysts, are precursor to the bisimine as the protocols of this heterocycle
synthesis carried out in the absence of 1,2ꢀphenylenediamine give them in excellent to good yield. The oxidation of
alcohols by hydrogen transfer to acetone was catalyzed efficiently with complexes 1–4 and resulted in aldehyde/ketone in
excellent to good yield. Each catalytic process is marginally more efficient with 1 than its counterparts.
acetone under mild reaction conditions.
Introduction
The continuous interest in 1,2ꢀdisubstituted benzimidazoles
is due to their broad spectrum biological and pharmacological
Metal complexes of organosulfur/selenium ligands as a
single source precursor for metal sulfide/selenide phases1aꢀc and
catalyst for various chemical transformations1dꢀf are of current
interest. The efficiency as a catalyst appears to be contributed
by strong donor properties of S/Se atom. Half sandwich
complexes of iridium(III) with ligands having combination of
hard donor(s), viz. N and O with one or more of S, Se and Te,
have been explored more than those of (E, E’) type ligands (E /
E’ = S or Se).2 No half sandwich complex of Ir(III) with a
bidentate ligand having a combination of thio/selenoether and
thione/selenone donor site is in our knowledge. Four such half
sandwich complexes of Ir(III) with (E, E’) ligands (E / E’ =
thione or selenone / thioether or selenoether) are reported
herein for the first time. They efficiently catalyze the reaction
between 1,2ꢀphenylenediamine and benzylic and furfuryl
alcohols, resulting in 1,2ꢀdisubstituted benzimidazole.,
oxidation of alcohols aerobic and via hydrogen transfer to
properties,3ꢀ5 which may result in clinical medicines. The
methods known for synthesis of benzimidazole skeleton
involve, reaction of a carboxylic acid and its derivatives with
1,2ꢀphenylendiamine under harsh dehydrating condition,6
combinatorial approach forming them on solidꢀphase,7
a
o
ꢀ
nitroaniline as intermediate and direct
benzimidazole.8 The protocols that involve
intermediate have resulted in these heterocycles on solid
support.9 The reactions of 1,2
phenylenediamine with
aldehydes catalyzed with acidic catalysts give
N
ꢀalkylation of
oꢀnitroaniline as an
ꢀ
benzimidazoles.10 However, many of these methods are poor in
selectivity due to several side reactions, resulting in low yield
(contributed by multistep workꢀup procedure also), inspite of
using exotic reagents, harsh organic solvents, and long reaction
time. The aldehyde used in the synthesis of 1,2ꢀdisubstituted
benzimidazole may be replaced with an alcohol, if a suitable
catalyst is available to promote both its selective oxidation and
subsequent condensation. Transitionꢀmetalꢀcatalyzed
Nꢀ
alkylation of an amine with alcohol, affords a short route for
C−N bond formation and involves in situ aldehyde formation.11ꢀ
13 Recently some reports have appeared on one pot synthesis of
pyrrole14aꢀb and pyridine14cꢀd from alcohol (via in situ generated
aldehyde). These reports strongly motivated us to explore
alcohol in the preparation of 1,2ꢀdisubstituted benzimidazole
Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi 110016,
India. *e-mail, aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com,
Fax: +91-01- 26581102; Tel: +91-011-26591379;
Electronic Supplementary Information (ESI) available: Spectral data and single
crystal data for complexes 1–4. CCDC 1474646 (1), 1474647 (2), 1499034 (3) and
1474648(4). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6dt01406b
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under ambient conditions and design a catalyst so that it’s in appear at a higher frequency (~0.5 to 6.5 ppm respectively)
situ oxidation (preferably aerobic) produces required aldehyde. relative to those of free L1 L4. It is probably due to
This has resulted in [(η5ꢀCp*) Ir(
)Cl][PF6] ( for L1 to
−
L
1
–
4
L =
L4) catalyzed synthesis of 1,2ꢀdisubstituted benzimidazole
from alcohol reported herein. The reactivity of O2 in its triplet
ground state is kinetically poor and transition metalꢀbased
catalysts are reported to activate it as an oxidant15 with and
without coꢀoxidant.15aꢀg,17 The examples of catalysts for aerobic
oxidation of alcohols without any coꢀoxidant are
Pd(OAc)2/l
i
PrꢀHBF4,15f [ImimꢀTEMPO][FeCl4]15g
,
Ru/HAP
(hydroxylapatite)15h, [η5ꢀCp*Ir(Cl)(bpy)]OTf and [η5ꢀCp*Ir(Cl)
(bpym)]OTf.16a The CuꢀMn oxide/CꢀTEMPO17a, Ru/quinine/
Co−salen complex17b and CuꢀNHCꢀTEMPO17c are examples of
efficient catalysts for aerobic oxidation with coꢀoxidant. The
catalysts based on Ir, are good16 for aerobic oxidation of
alcohols, but little explored and therefore present investigations
on complexes 1ꢀ4 for this purpose are worthwhile.
The aldehyde production by aerial oxidation with water as
sole byꢀproduct is environmentally benign in comparison to
oxidation with Nꢀmethylmorpholine Nꢀoxide (NMO), tertꢀbutyl
hydroperoxide, NaOCl, NaIO4 and H2O218 which are toxic and
often release toxic byꢀproduct(s). The complexes 1ꢀ4 are also
suitable catalysts for oxidation of alcohols by transfer of their
hydrogen to acetone19 which acts as a solvent cum oxidizing
agent. Its protocol is also environment friendly20 and
economical compared to conventional oxidants.21 Several
transition metal complexes of different ligands can catalyze
such transfer of hydrogen.22ꢀ23 The efficient ones for such
oxidation include Ru(II) complexes24 including Rh(I)−Ru(II)
Scheme 1. Synthesis of
1–4
coordination of ligands with Ir(III).27e The singlet due to η5
ꢀ
pentamethylcyclopentadienyl ring in both 1H and 13C{1H}
NMR of all four complexes is shifted to a lower frequency
(maximum shift 0.3 and 2.3 ppm respectively) relative to those
of [η5ꢀ(Cp*)IrCl2]2. This probably results due to substitution of
Cl with relatively less electronegative S or Se, which have
strong donor ability also.
type species and some complexes of
Nꢀheterocyclic carbene
(NHC) with η5ꢀCp*Ir(III) and η5ꢀCp*Rh(III).25
In mass spectra of complexes
1, 2, 3 and 4, the peaks are
Results and discussion
observed at m/z, 821.0281 773.0862, 773.0839 and 725.1392
respectively which correspond to their cations (M+) and
authenticate them.
The preparation of complexes [(η5ꢀCp*)Ir(
L
)Cl][PF6] (
L1 to L4) carried out at room temperature is shown in
Scheme 1. The L1 L4, were synthesized by reported
procedure 26
The were formed by chloro bridge cleavage of
[(η5ꢀCp*)IrCl(ꢁꢀCl)]2, followed by reaction with L1
L4, which
was facilitated by chloride extraction with NH4PF6. The 4,
1–4 for
L
=
−
Crystal structures
.
1ꢀ4
The slow evaporation of concentrated solutions of 1ꢀ4 made in
a methanol–acetonitrile mixture (1:4 v/v) resulted in their single
crystals, suitable for Xꢀray diffraction. In ESI (Tables S1 and
ꢀ
1−
moderately soluble in CHCl3, CH2Cl2, and CH3OH, have good
solubility in DMSO and CH3CN. They can be stored under
ambient conditions at 25°C for several months.
S2) single crystal data and refinement parameters for
given and molecular structures of their cations are shown in
Figs. with selected bond distances and angles. The H and
1ꢀ4 are
1ꢀ4
NMR and mass spectra
‾
PF6 anion are omitted in these structures for clarity. All ligands
form six membered chelate ring in all these complexes due to
their coordination with iridium through two S, (S, Se) or two
Se. Thus in the cation of all four halfꢀsandwich complexes,
donor atoms make pseudoꢀoctahedral “piano stool” type
geometry around Ir. The η5ꢀCp* ring occupies symmetrically
three octahedral sites. With S/Se of the ligand and chlorine six
coordination number is completed. The Ir−S bond lengths (Å)
1
The H, 13C{1H} and 77Se{1H} NMR and mass spectra of
1
−
4
(See ESI) are consistent with their molecular structures
(Scheme 1) supported by singleꢀcrystal Xꢀray diffraction. The
signals in 77Se{1H} NMR spectra of
1 and 2 due to selenoether
group are at a higher frequency by 7.5 and 11.5 ppm,
respectively, with respect to those of free L1 and L2. In
77Se{1H} NMR of
1 and 3 the signals of selenone are at a lower
frequency by 58.9 and 46.5 ppm, respectively, with respect to
those of free L1 and L3. This probably occurs due to
weakening of >C=Se double bond character. The signals of C5
of
[2.318(1)−2.3872(10)] in which such bond distances of [η5
Cp*Ir(CO)(ꢁꢀSTol)Pt(STol)(PPh3)],27a η5ꢀCp*Ir(η2ꢀppyꢀSꢀ
3 [2.355(4)] and 4 [2.3469(18)] fall in the range
ꢀ
ꢀ
[
p
and C13 (both methylene) in 13C{1H} NMR spectra of
1ꢀ4
tol)(H2O)][OTf]2,27b
[
η5ꢀCp*Ir(4,6ꢀdiꢀ
t
ꢀbutylꢀ(2ꢀmethylthioꢀ
phenylimino)ꢀ
o
ꢀbenzoquinone][PF6].CH2Cl2,27c and [η5ꢀCp*Ir
2 | J. Name., 2012, 00, 1-3
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27d
Cl{2ꢀ(phenylthiomethyl)pyridine}]PF6
have been reported.
The Ir−Se bond lengths in
1 and 2 [2.4663(12) and 2.4663(13)
Å] are similar to the values reported for [η5ꢀCp*IrCl{2ꢀ
(phenylselenomethyl)pyridine}]PF6 [2.4531(10) Å],27d
[ ꢀ
η5
Cp*IrCl(ꢁꢀSeCOC6H5)ꢀ(κ2ꢀSeCOC6H4−)Ir(η5ꢀCp*)]
[2.445(2)−2.495(1)Å],27e and [η5ꢀCp*Ir(ꢁ3ꢀSe)2{PtTol(PPh3)}2]
[2.416(1)−2.422(1) Å]27a but longer than the values reported for
[
η5ꢀCp*Irꢀ{Se2C2(CO2Me)2}] [2.3494(7) and 2.3520(7) Å].27f
͞
The PF6 is responsible for C−H···F secondary interactions in
all complexes 1ꢀ4 resulting in chains. In Fig. 5 it is shown for 1.
For 2ꢀ4 see ESI (Table S3 for C−H···F distances and Figs. S1,
S2 and S3 for the resulting framework).
Fig. 3 Molecular structure of
3. Bond lengths (Å): Ir1‒Se1,
2.5141(16), Ir1‒S1, 2.355(4), Ir1‒Cl1, 2.427(4), Se1‒C14,
1.844(15). Bond angles (º): Se1‒Ir1‒S1, 94.99(9), Se1‒Ir1‒Cl1,
90.85(12), S1‒Ir1‒Cl1, 79.62(14).
Fig. 1 Molecular structure of
1. Bond lengths (Å): Ir1‒Se1,
2.4663(12), Ir1‒Se2, 2.5159(12), Ir1‒Cl1, 2.439(3), Se2‒C14,
1.840(11). Bond angles (º): Se1‒Ir1‒Se2, 95.99(4),
Cl1‒Ir1‒Se2, 90.79(8). Cl1— Ir1— Se1, 78.31(8).
Fig. 4. Molecular structure of
4. Bond lengths (Å): Ir1‒S1,
2.3469(18), Ir1‒S2, 2.4146(19), Ir1‒Cl1, 2.4244(19), S2‒C14,
1.685(7). Bond angles (º): S1‒Ir1‒S2, 94.61(6), S1‒Ir1‒Cl1,
79.33(7), S2‒Ir1‒Cl1, 90.55(8)
Fig. 5. Nonꢀcovalent C–H⋯F interactions in 1.
Catalytic applications of complexes 1ꢀ4
Fig. 2 Molecular structure of
2, Bond lengths (Å): Molecular
structure of , Bond lengths (Å): Ir1‒S1, 2.417(2), Ir1‒Se1,
2
2.4663(13), Cl1‒Ir1, 2.436(3), C14‒S1, 1.692(10). Bond angle
(º): S1‒Ir1‒Se1, 95.41(7), Cl1‒Ir1‒Se1, 78.08(7), S1‒Ir1‒Cl1,
90.69(10)..
Synthesis of 1, 2ꢀdisubstituted benzimidazole
The complexes
1ꢀ4 have been applied to catalyze one pot
synthesis of 1,2ꢀdisubstituted benzimidazole from 1,2ꢀ
phenylenediamine and alcohol. Optimum conditions were first
established using the reaction between 1,2ꢀphenylenediamine
and benzyl alcohol (Scheme 2).
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Table 2: Control experimentsa
Base# Temperature
Entry Substrate
Catalyst*
Yield%c
nd
1
Benzyl
alcohol
ꢀ
80˚C
1
2
Benzyl
alcohol
KOtBu
KOtBu
KOtBu
RT
1
8
Scheme 2: Synthesis of 1,2ꢀdisubstituted benzimidazole
3
Benzyl
alcohol
80˚C
80˚C
80˚C
ꢀ
nd
nd
95
Table 1. Optimization of solvent, base and substrate: base
ratioa
4
Benzyl
alcohol
[Cp*IrCl2]2
Entry
A
B
C
5b
Bisimine KOtBu
1
Yield
[%]b
Yield
[%]b
Yield
[%]b
aReaction condition: Benzyl alcohol (2 mmol), 1,2ꢀphenylenediamine
(1 mmol), Toluene (2 ml), *0.1 mol %,#KOtBu (2 mmol), time 6 h.
Solvent
Base
Substrate/Base
bBisimine (1mmol), Toluene (2 ml), *0.1 mol %,#KO
time 6 h, cisolated yield; nd = not detected.
tBu (2 mmol),
1.
2.
3.
4.
5.
6.
DMSO
THF
13
nd
92
8
NaOH
KOH
14
27
79
nd
75
92
1:0.5
1: 1
1:2
10
60
92
92
Toluene
DMF
NaOtBu
Et3N
Table 3: Synthesis of 1,2ꢀdisubstituted benzimidazoles from
1,2ꢀphenylenediamine and benzylic and furfuryl alcohol
1:3
CH3CN
nd
K2CO3
KOtBu
aReaction conditions: 1,2ꢀphenylenediamine (1.0 mmol), benzyl
alcohol (2 mmol), catalyst
(0.1 mol%) in all cases, [A] KOtBu (2
1
mmol), solvent (2 mL), 80 ºC, 6 h. [B] Base (2 mmol), solvent:
toluene (2 mL), 80 ºC, 6 h, [C] KOtBu, toluene (2 mL), 80ºC, 6 h;
bisolated yield; nd = not detected.
Various organic solvents were screened to carry out the
reaction. In toluene the conversion was ~92% (Table 1, Entry
Aꢀ3) making it most suitable. In DMSO and DMF the
conversion to the desired product was low ~8ꢀ13 % (Table 1,
Entries Aꢀ1 and Aꢀ4). In THF and CH3CN no conversion was
detected (Table 1, Entries Aꢀ2 and Aꢀ5). Of the various bases
the presence of NaOtBu and KOtBu gave good conversion (79
and 92% respectively) to the desired product (Table 1, Entries
Bꢀ3 and Bꢀ6). With the use of NaOH and KOH conversion was
only ~14 and 27% respectively (Table 1, Entries Bꢀ1 and Bꢀ2).
In the presence of K2CO3 conversion was moderate but with
Et3N it was negligible (Table 1, Entries Bꢀ4 and Bꢀ5). Probably
the best results with KOtBu are due to its higher miscibility
with toluene than other bases. The influence of substrate: base
ratio was investigated to know the optimum amount of base
needed for maximum conversion. The results given in Table 1 [a] Reaction conditions: 1,2ꢀphenylenediamine (1.0 mmol),
(Entries Cꢀ1–Cꢀ4) suggest that optimum value of the ratio is benzylic alcohol (2 mmol), KOtBu (2 mmol), toluene (2 mL),
1:2.
catalyst (
isolated yield in % are below each structure.
1ꢀ4
) (0.1 mol%), temperature, 80oC, time, 6 h,
4 | J. Name., 2012, 00, 1-3
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The control experiments were carried in absence of each of the with oꢀphenylenediamine resulting bisimine (in good yield) as
1
three, base, catalyst and heating. The results given in Table 2 evidenced by H NMR spectra recorded after 30 and 90 min of
show that yield was insignificant after 6 h, when the reaction progress of the reaction (See ESI for the spectra Figures S31ꢀ
was carried out under optimum conditions in the absence of a S33). Of course formation of some bisimine by direct reaction
catalyst (Table 2, Entry 3). In the absence of base no is also possible. Thereafter electrophilic activation of nitrogen
conversion was observed (Table 2, Entry 1). At room of bisimine with Ir(III) of
A results in cyclization leading to
temperature only 8% conversion to the desired product was 1,2ꢀdisubstitutedbenzimidazole as proposed earlier for its
achieved (Table 2, Entry 2). On using [Cp*IrCl2]2 as a catalyst, synthesis promoted by electrophilic activation with hydrogen of
no conversion was observed. This reaffirms the role of ligand in fluorous alcohols.28
the catalytic activation by complexes
1ꢀ4 in the present
condensation reaction. The direct reaction of bisimine with
complex 1, gives 1,2ꢀdisubstituted benzimidazole in good yield
(Table 2).
Aerobic oxidation of alcohols
The complexes 1ꢀ4 catalyze aerobic oxidation of benzylic
alcohols to aldehyde if the protocol of 1,2ꢀdisubstituted
benzimidazole synthesis (Table 3) is executed in the absence of
the diamine. Therefore catalytic aerobic oxidation of benzylic
alcohol with 1ꢀ4 was studied in detail and found efficient.
Optimum conditions and effect of substituent on the ring of
aromatic alcohol are as in the case of benzimidazole synthesis.
The results are given in Table 4. The oxidation is promoted by
The scope of this catalytic reaction was studied, using a
series of benzylic alcohols and furfuryl alcohol. The optimum
reaction conditions were used in each case. 1,2ꢀDisubstituted
benzimidazoles with various functional groups resulted in
excellent to good yield as summarized in Table 3. No by
product was noticed with the formation of any 1,2ꢀdisubstituted
benzimidazole. The nature and position of the substituent
group(s) on the ring of benzylic alcohol affect yield of the
condensation product. The electron withdrawing substituent
groups like –NO2 gave better results than those of electron
donating groups such as, OCH3 and CH3 (Table 3, Entries 3bꢀc,
3g). The substitution at a para position gave better yield of the
desired product compared to ortho or meta (Table 3, Entries 3cꢀ
3e). The yield of furfural derivative (Table 3, Entries 3f) was
found good. The conversion (Table 3) was found high in the
case of benzyl alcohol (up to 92 %, Table 3, Entry 3a), with all
B
mentioned above
For aerobic oxidation of alcohols complexes 1ꢀ4 show good
catalytic conversion with 0.1 mol% loading without any coꢀ
oxidant/additive. This loading is lower compared to earlier
reported values (given in parentheses) for transition metal based
catalysts viz CuꢀMn oxide/CꢀTEMPO (0.5 mol%),17a Ru/
quinine/Co−salen complex (0.5mol%),17b CuꢀNHCꢀTEMPO (10
mol%),17c Pd(OAc)2/I PrꢀHBF4 (0.5 mol%),15f [ImimꢀTEMPO]
i
[FeCl4] (2.5 mol%)15g and Ru/HAP (hydroxyapatite)
(17mol%).15h Among the Ir species used for this purpose
[Cp*Ir(Cl)(bpy)]OTf and [Cp*Ir(Cl)(bpym)]OTf16a are worth
comparing with 1ꢀ4. However they are explored for few
substrates and TON values are low. The [Cp*IrCl2]2 at 2.5ꢀ5
mol%, loading gives poor conversion even after a reaction for
15 h.16b
the catalysts 1−4. Complex 1 (Ir bonded with two Se atoms) is
marginally more efficient as catalyst among all the four, as its
low loading (0.1%) is sufficient for good conversion.
For the reaction leading to 1,2ꢀdisubstituted benzimidazole
from 1,2ꢀphenylenediamine and benzylic and furfural alcohols
employing complexes 1ꢀ4 as preꢀcatalysts most probably
Table 4: Aerobic oxidation of benzylic alcohols
[Cp*IrIII(
L
)]2+
(
A
) is the real catalytic species. It is formed by
[Cp*IrIII( )Cl]+
into
L (A
)]2+ ) and Cl‾ and remains in equilibrium shown
dissociation of cation of 1ꢀ4
,
L
(B)
[Cp*IrIII(
below.
[Cp*IrIII(L)Cl]+
[Cp*IrIII(L)]2+
Clꢀ
+
(A)
(B)
O
O
O
In the presence of
a
base,
B
forms alkoxide
H
H
H
[Cp*IrIII(OCH2Ar)(
[Cp*Ir(H)(
L
)]+ which gives aldehyde along with
MeO
1
5b
5c
)]+. The formation of aldehyde is supported by H
5a
L
1=93% 2=88%
3=91% 4=85%
1=96% 2=91%
3=95% 4=89%
1=94% 2=87%
3=90% 4=83%
NMR of reaction mixture taken midway of the catalytic process
(See Figures S31ꢀ S33 in ESI). The iridium hydride (IrꢀH)
formation is supported by the presence of a signal at ꢀ11.41
ppm in 1H NMR spectrum (See Figs. S30 in ESI) of the reaction
O
O
O
H
H
H
O
OMe
OMe
mixture of [Cp*IrIII( )Cl]+ and benzyl alcohol, consistent with
L
5e
5d
5f
literature report.16a Iridium hydride species liberates H2 as
1=90% 2=81%
3=87% 4=83%
1=89% 2=78%
3=86% 4=82%
1=93% 2=84%
3=89% 4=81%
1
evidenced by a signal in H NMR spectra of reaction mixtures
at ~4.5 ppm and is converted back to [Cp*IrIII(
L
)]2+(
A
). The
[Cp*IrIIICl2]2 does not catalyze the synthesis of 1,2ꢀ
disubstitutedbenzimidazole. Thus there is a role of in the
catalysis. Most probably electrophilic activation of aldehyde
O
H
L
O2
N
5g
1=96% 2=88%
3=92% 4=82%
(oxygen of >C=O) by iridium(III) of species
A
makes it to react
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[a] Reaction conditions: benzylic alcohol (1 mmol), KOtBu (1
Journal Name
O
O
O
mmol), toluene (2 mL), catalyst (1ꢀ4) (0.1 mol%), temperature,
H
H
H
80oC, time, 6 h, isolated yield in % are given below each
structure.
O
5b
5c
5a
Oxidation alcohols via hydrogen transfer to acetone:
1=90% 2=85%
3=88% 4=85%
1=90% 2=85%
3=88% 4=85%
1=90% 2=85%
3=88% 4=85%
O
O
O
H
H
H
O
O
O
Scheme 3: Oxidation of alcohols H transfer to acetone
5e
5d
5f
1=87% 2=78%
3=85% 4=80%
1=82% 2=75%
3=83% 4=75%
1=90% 2=85%
3=88% 4=82%
The oxidation of alcohols via hydrogen transfer to acetone
(Scheme 3) catalyzed with 0.01 mol % of 1−4 was studied. To
O
O
O
optimize the reaction conditions benzyl alcohol and complex
1
H
were taken. Potassium tertꢀbutoxide was found the best base
among all species explored. In the presence of alternatives, viz.
K2CO3, KOH, Cs2CO3 or CH3ONa, yield was relatively low
under identical reaction conditions (Table 5).
O2N
5g
7b
7a
1=92% 2=88%
3=92% 4=80%
1=82% 2=78%
3=78% 4=73%
1=88% 2=85%
3=90% 4=83%
Table 5. Optimization of base for oxidation of alcohols via
hydrogen transfer to acetone [a]
O
O
C4H9
Entry
Base
KOH
Yield%
7c
7d
1
2
3
4
5
25
1=76% 2=73%
3=75% 4=70%
1=92% 2=85%
3=90% 4=84%
K2CO3
KOtBu
Cs2CO3
CH3ONa
40
90
[a] Reaction conditions: alcohol (1 mmol), KOtBu (1 mmol), catalyst
) (0.01 mol %), acetone (15 mL), 80o C, time 3h, isolated yield
8
12
(1ꢀ4
aReaction conditions: catalyst
1
(0.01 mol %), benzyl
in % are given below each structure.
alcohol (1 mmol), 15 mL of acetone, base (1 mmol), 80
°C, time 3 h.
The effect of nature and position of the substituent on the benzene
ring of benzylic alcohol on the yield in hydrogen transfer to acetone
is similar to that of synthesis of 1,2ꢀdisubstituted benzimidazole. The
presence of –NO2 gave somewhat better yield than –OCH3 or –CH3
(Table 6, entries 5b, 5c, 5g). The alcohols having substituent at a
para position gave better yield of the oxidized product compared to
ortho/meta substituted derivatives (Table 6, entries 5cꢀe). All the
four complexes are efficient as a catalyst for primary and secondary
aliphatic alcohols, as revealed by good yield in both the cases (Table
6, entries 7aꢀ7d). The yield of the product was also good with
heteroaromatic derivative (Table 6, entry 5f). In comparison to
aerobic oxidation, hydrogen transfer to acetone oxidizes alcohols
quickly i.e. maximum conversion occurs in nearly half of the time
required in the case of aerobic.
The substrates explored for oxidation of alcohols via hydrogen
transfer to acetone are summarized in Table 6. The signals in
77Se{1H} NMR spectra recorded when the reaction progressed in the
presence of
frequency, relative to those of free
ppm, observed in the H NMR spectra recorded during progress of
reaction, may be ascribed to the formation of metal hydride
species.25 These two observations indicate that in this oxidation
formation of metal alkoxide is involved, as reported earlier.25
1
ꢀ
3
as a catalyst, were noticed at a 21 ꢀ 28 ppm higher
. New signal at ≈ δ −11.36
1ꢀ3
1
Complex
1 gives somewhat high conversion in comparison to
complex 2ꢀ4 (Table 6).
Table 6 Oxidation of alcohols via hydrogen transfer to acetone
Conclusion
OH
O
Catalyst /3h
The four complexes of (η5ꢀCpMe5)Ir(III) with 1ꢀbenzylꢀ3ꢀ
phenylthio/selenomethylꢀ1,3ꢀdihydrobenzoimidazoleꢀ2ꢀthione
KOtBu/Acetone/reflux
R
R'
R
R'
6
5 aꢀg, 7 aꢀd
/selenone [
with NMR, HRꢀMS and singleꢀcrystal Xꢀray diffraction studies.
These are the first examples of Ir(III) complexes ( ) with
L = L1–L4] have been synthesized and characterized
1ꢀ4
unsymmetrical sulfurꢀselenium ligands having a unique combination
of thio/selenoether and thione/selenone donor sites. They are also
6 | J. Name., 2012, 00, 1-3
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ARTICLE
first examples of complexes of (E, E’) (E and E’ = S and Se) ligands Single crystals of 1ꢀ4 were grown by slow evaporation of their
with Ir(III), efficient to catalyze one pot synthesis of a series of 1,2ꢀ solutions made in a mixture of CH3OH and CH3CN (1:4).
disubstituted benzimidazoles from benzylic alcohol in excellent to
good yield under mild and aerobic conditions. The aerobic oxidation 1: Yield: 0.164g, (0.17mmol), 85% Anal. Calc. for
of benzylic alcohols catalyzed with 1ꢀ4 independently gave good to C31H33ClF6IrN2PSe2: C, 40.25; H, 3.09; N, 3.43. Found: C, 40.68; H,
1
excellent yield. Transfer of hydrogen from alcohols to acetone 3.32; N, 3.75. Mp: 200.0 °C. H NMR (DMSO, 25°C vs Me4Si): δ
catalyzed with 1ꢀ4 results in carbonyl compounds in good yield (ppm) 1.54 {(s, 15H, Me(Cp)}, 5.27 (s, 2H, CH2ꢀPh), 6.03 (s, 2H,
under mild reaction conditions. In comparison to aerobic oxidation, CH2ꢀSe), 7.86ꢀ7.84 (m,1H), 7.58ꢀ7.33 (m, 8H), 7.28ꢀ7.20 (m, 3H),
hydrogen transfer is fast, i.e. maximum conversion occurs in nearly 7.11ꢀ6.97 (m, 2H). 13C{1H}NMR (DMSO, 25°C vs Me4Si): 169.4,
half of the time required in case of aerobic.
135.9, 132.8, 131.2, 130.5, 129.1, 128.5, 128.3, 127.6, 127.3, 123.2,
122.8, 110.6, 109.9, 92.9 (C of Cp*), 48.7, 47.1, 8.1 {C of
Me(Cp*)}. 77Se{1H} NMR (DMSO, 25°C, Me2Se) δ (ppm): 396.75,
55.57. HRꢀMS (CH3CN) [C31H33ClIrN2Se2]+ (m/z)= 821.0281;
calculated value for [C31H33ClIrN2Se2]+ = 821.0276 (δ: ꢀ0.8 ppm).
Experimental
Physical measurements
The 1H, 13C{1H}, and 77Se{1H} NMR spectra were recorded at 2: Yield: 0.161g, (0.176mmol), 88%. Anal. Calc. for
300.13, 75.47, and 57.24 MHz respectively on a Bruker Spectrospin C31H33ClF6IrN2PSeS: C, 40.59; H, 3.63; N, 3.05. Found: C, 41.35;
1
DPXꢀ300 NMR. The C, H, and N analyses were carried out with a H, 3.47; N, 3.32. Mp: 190 °C. H NMR (DMSO, 25°C vs Me4Si): δ
Perkin–Elmer Series II C, H and N analyzer. Single crystal data were (ppm) 1.43 {(s, 15H, Me(Cp)}, 5.50 (S, 2H, CH2ꢀPh), 5.86 (S, 2H,
collected on Bruker AXS SMART Apex CCD diffractometer CH2ꢀSe), 7.51ꢀ7.43 (m, 3H), 7.40ꢀ7.28 (m, 8H), 7.23ꢀ7.17 (m, 3H).
diffractometer using Mo Kα (0.71073 Å) radiation at 298(2) K. The 13C{1H}NMR (DMSO, 25°C vs Me4Si): 167.1, 135.3, 133.8, 133.3,
software SADABS29 was used for absorption correction and 132.1, 129.8, 129.4, 129.1, 128.9, 128.3, 127.9, 126.0, 125.5, 112.2,
SHELXTL for space group, structure determination, and 111.3, 92.5 (C of Cp*) , 49.5, 47.8. 8.7 {C of Me(Cp*)}. 77Se{1H}
refinements.29 Hydrogen atoms were included in idealized positions NMR (DMSO, 25 °C, Me2Se) δ (ppm): 394.8. HRꢀMS (CH3CN)
with isotropic thermal parameters set at 1.2 times that of the carbon [C31H33ClIrN2SSe]+ (m/z)= 773.0862; calculated value for
atom to which they are attached in all cases. The leastꢀsquares [C31H33ClIrN2SSe]+ = 733.0833 (δ: ꢀ1.9 ppm).
refinement cycles on F2 were performed until the model converged.
High resolution mass spectral measurements were performed on 3: Yield: 0.148g, (0.162mmol), 81%.Anal. Calc. for
Bruker microtofꢀQ II with electron spray ionization (10 eV, 180 °C C31H33ClF6IrN2PSeS: C, 40.59; H, 3.63; N, 3.05. Found: C, 40.50;
source temperature, sodium formate as reference compound) taking H, 3.70; N, 3.24. Mp: 185.0 °C.1H NMR (DMSO, 25°C vs Me4Si): δ
sample in CH3CN. Nitrogen atmosphere, was created with (ppm) 1.46 {(s, 15H, Me(Cp)}, 5.66 (S, 2H, CH2ꢀPh), 6.00 (S, 2H,
commercial nitrogen purified by passing it successively through CH2ꢀS), 7.44ꢀ7.34 (m, 3H), 7.30ꢀ7.28 (m, 6H), 7.23ꢀ7.21 (m, 2H),
traps containing (as solutions/solid) alkaline anthraquinone, sodium 7.19 (m, 3H). 13C{1H}NMR (DMSO, 25°C vs Me4Si): 167.1, 135.6,
dithionite, alkaline pyrogallol, concentrated H2SO4 and KOH, using 133.0, 129.2, 129.1, 129.0, 128.5, 128.3, 128.2, 127.7, 127.3, 123.6,
Schlenk techniques. The glassware dried in an oven, were used for 123.2, 111.2, 110.5, 92.9 (C of Cp*) 53.8, 49.9, 8.3 {C of Me(Cp*)}.
various reactions under ambient conditions. The preparation of L1– 77Se{1H} NMR (DMSO, 25 °C, Me2Se) δ (ppm): 60.34. HRꢀMS
L4 was carried out in nitrogen atmosphere by reported procedure.26a
(CH3CN) [C31H33ClIrN2SSe]+ (m/z)= 773.0839; calculated value for
[C31H33ClIrN2SSe]+ = 733.0833 (δ: ꢀ0.8 ppm).
Chemicals and reagents
4: Yield: 0.156g, (0.18 mmol), 90% Anal. Calc. for
The reported methods were used for the synthesis of [{(η5ꢀ C31H33ClF6IrN2PS2: C, 47.78; H, 3.82; N, 3.22. Found: C, 46.14; H,
1
C5H5Me5)IrCl(ꢁꢀCl)}2]30 and ligands L1ꢀL426a
.
All the solvents 3.87.; N, 3.46. Mp: 205.0 °C. H NMR (DMSO, 25°C vs Me4Si): δ
were dried and distilled before use by standard procedures.31 The (ppm) 1.50 {(s, 15H, Me(Cp)} 5.58 (S, 2H, CH2ꢀPh), 5.95 (S, 2H,
common reagents and chemicals available commercially within the CH2ꢀS), 7.55ꢀ7.48 (m, 8H), 7.34ꢀ7.20 (m, 6H). 13C{1H} NMR
country were used.
(DMSO, 25°C vs Me4Si): 169.5, 135.9, 132.7, 132.2, 131.2, 129.1,
129.0, 128.5, 128.3, 128.1, 127.6, 127.3, 123.2, 122.8, 110.7, 109.9,
92.9 (C of Cp*) 48.7, 47.0, 8.1 {C of Me(Cp*)}. HRꢀMS (CH3CN)
[C31H33 ClIrN2S2]+ (m/z)= 725.1392; calculated value for [C31H33
Synthesis of complex [Ir{(η5ꢀCp*}Cl(L)]PF6 ( L = L1ꢀL4)
The solution of L1 (0.091 g, 0.2 mmol)/ L2 (0.082 g, 0.2 mmol)/ L3 ClIrN2S2]+ = 725.1388 (δ: ꢀ0.6ppm).
(0.082 g, 0.2 mmol)/ L4 (0.072 g, 0.2 mmol) made in CH3OH (5
mL) was mixed with a solution of [(η5ꢀC5(CH3)5IrCl(ꢁꢀCl)]2 (0.080 General procedure for the synthesis of 1,2ꢀdisubstituted
g, 0.1 mmol) in CH3OH (5 mL) and stirred for 8 h at 25 °C giving a benzimidazole:
yellow solution. It was filtered, volume of the filtrate reduced (~7
mL) with a rotary evaporator and mixed with solid NH4PF6 (0.032 g, An ovenꢀdried flask was charged with a benzylic alcohol /furfuryl
0.2 mmol). The resulting yellow coloured microcrystalline solid was alcohol(2 mmol), 1,2ꢀphenylenediamine (1 mmol), KOtBu (2 mmol),
filtered, washed with 5 mL of iceꢀcold CH3OH and dried in vacuo. catalyst 1ꢀ4 (0.1 mol%) and toluene (2 mL). The mixture was stirred
at 80°C for 6 h. Thereafter its solvent was evaporated off with a
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Journal Name
3
(a) S. Bhattacharya and P. Chaudhuri, Curr. Med. Chem.
2008 15 1762ꢀ1777 (b) D. A. Horton, G. T. Bourne and M.
Smythe, Chem. Rev. 2003 103 893ꢀ930; (c) M. Boiani
and M. Gonz MiniꢀRev. Med. Chem. 2005 409ꢀ424.
(a) P. N. Preston Chem. Rev. 1974, 74, 279ꢀ314; (b) H. Al. J.
Int. Med. Res., 1997, 25, 175ꢀ181; (c) L. J. Scott, C. J. Dunn,
G. Mallarkey, M. Sharp, Drugs 2002, 62, 1503ꢀ1538.
rotary evaporator and the residue extracted with ethyl acetate (3 × 50
mL). The organic layer was washed with water (3 × 50 mL) and
dried over anhydrous Na2SO4. The solvent of the extract was
removed with a rotary evaporator and the resulting residue was
purified by flash column chromatography on silica gel using ethyl
acetate:hexane mixture (2:98) as eluent. The isolated products
(yield between 72 and 92% ) were authenticated with 1H NMR (ESI,
Page no. 22).
,
,
;
L
.
,
,
, 5,
4
5
(a) Angiotensin II receptor antagonist: Y. Kohara, K. Kubo,
E. Imamiya, T. Wada, Y. Inada and T. Naka, J. Med. Chem
.
1996, 39, 5228ꢀ5235; (b) Antiviral: A. R. Porcari, R. V.
Devivar, L. S. Kucera, J. C. Drach and L. B. Townsend, J.
Med. Chem. 1998, 41, 1252ꢀ1262; (c) Neuropeptide Y Y1
receptor antagonist: H. Zarrinmayeh, A. M. Nunes, P. L.
Ornstein, D. M. Zimmerman, M. B. Arnold, D. A. Schober,
S. L. Gackenheimer, R. F. Bruns, P. A. Hipskind, T. C.
General procedure for the aerobic oxidation of substituted
alcohols:
An ovenꢀdried flask was charged with alcohol (1 mmol), KOtBu (1
mmol), catalyst 1ꢀ4 (0.1 mol%) and toluene (2 mL) .The reaction
mixture was stirred at 80°C for 6 h. After a work up similar to that of
synthesis of 1,2ꢀdisubstituted benzimidazole. Product were isolated
Britton, B. E. Cantrell and D. R. Gehlert, J. Med. Chem.
1998, 41, 2709ꢀ2719; (d) Factor Xa inhibitor: Z. S. Zhao, D.
O. Arnaiz, B. Griedel, S. Sakata, J. L. Dallas, M. Whitlow, L.
Trinh, J. Post, A. Liang, M. M. Morrissey and K. J. Shaw,
Bioorg. Med. Chem. Lett. 2000, 10, 963ꢀ966; (e) Antiallergic
5ꢀlipoxygenase inhibitor: H. Nakano, T. Inoue, N. Kawasaki,
H. Miyakata, H. Matsumoto, T. Taguch, N. Inagaki, H. Nagai
1
(yield between 78 and 96%) and authenticated with H NMR (ESI,
Page no. 23).
and T. Satog, Bioorg. Med. Chem. 2000, 8, 373ꢀ380; (f)
General procedure for catalytic hydrogen transfer from alcohols
to acetone
Antibacterial: H. Göker, C. Kuꢂ, D. W. Boykin, S. Yildiz and
N. Altanlar, Bioorg. Med. Chem. 2002, 10, 2589ꢀ2596; (g)
Antimicrobial: S. Özden, D. Atabey, S. Yildiz and H. Göker,
Bioorg. Med. Chem. 2005, 13, 1587ꢀ1597; (h) AntiꢀHepatitis
C: J. R. Hwu, R. Singha, S. C. Hong, Y. H. Chang, A. R. Das,
In 15 mL of acetone, catalyst 1ꢀ4 (0.01 mol %) was dissolved and
mixed with alcohol substrate (1 mmol) and KOtBu (1 mmol). The
mixture was refluxed for 3 h. After a work up similar to that of
synthesis of 1,2ꢀdisubstituted benzimidazole, The isolated products
(yield between 70 and 92% ) were authenticated with 1H NMR (ESI,
Page no. 23).
I. Vliegen and E. De Clercq, J. Antiviral Res. 2008, 77
162; (i) Burkitt’s lymphoma inhibitors: M. M. Ramla, M. A.
Omar, H. Tokuda and H. I. Elꢀ Diwani, Bioorg. Med. Chem
, 157ꢀ
.
2007, 15, 6489ꢀ6496; j) Antiparasitic: J. PérezꢀVillanueva, R.
Santos, A. HernándezꢀCampos, M. A. Giulianotti, R. Castillo
and J. L
. MedinaꢀFranco, Med. Chem. Commun. 2011, 2, 44ꢀ
49.
Acknowledgements
6
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(a) Z. Wu, P. Rea, G. Wickam, Tetrahedron Lett. 2000, 41,
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A.K.S. thanks the Council of Scientific and Industrial Research
(CSIR) of India and Department of Science ad Technology of India
for financial support. A. S. thanks the CSIR of India for SRF, H. J.
and R.B. thank the University Grants Commission (UGC) of India
for JRF/SRF. Dr. Nattamai bhuvanesh of Texas A&M University,
College Station (USA) is acknowledged for his help in the analyses
of crystallographic data.
7
8
Notes and references
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Complexes of (η5-Cp*)Ir(III) with 1-benzyl-3-phenylthio / selenomethyl-1,3-
dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-
substituted benzimidazole synthesis
Alpesh K. Sharma, Hemant Joshi, Renu Bhaskar and Ajai K. Singh*
(η5-Cp*)Ir(III)-1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone
complexes catalyze at 0.01-0.1 mol%, reactions of 1,2-phenylenediamine with benzylic/furfuryl alcohol
and oxidation of alcohols (conditions: mild and ambient).