A Remote ‘Imidazole’-Based Ruthenium(II) Para-Cymene Pre-catalyst for the Selective Oxidation Reaction of Alkyl Arenes and Alcohols

Article abstract of DOI:10.1002/asia.201901760

Herein we disclosed the use of a remote ‘imidazole’-based precatalyst [(para-cymene)Ru^II(L)Cl]⁺, C-1 where L=2-(4-substituted-phenyl)-1H-imidazo[4,5-f]^[1,10] phenanthroline) for the selective oxidation of a variety of alkyl arenes/heteroarenes and alcohols to their corresponding aldehydes or ketones in presence of tert-butyl hydroperoxide (TBHP). The remote ‘imidazole’ moiety present in the complex facilitates the activation of oxidant and subsequent generation of active species via the release of para-cymene from C-1, which in-turn was less effective without the ‘imidazole’ moiety. The mechanistic features of C-1 promoted oxidation of alkyl arenes were also assessed from spectroscopic, kinetic, and few control experiments. The substrate scope for C-1 promoted oxidation reaction was assessed based on the selective oxidation of 27-different alkyl arenes/heteroarenes and 25 different alcohols to their corresponding aldehydes/ketones in moderate to good yields.

Full text of DOI:10.1002/asia.201901760

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Accepted Article
Title: Remote ‘Imidazole’ Based Ruthenium(II) Para-Cymene Pre-
catalyst for Selective Oxidation Reaction of Alkyl Arenes and
Alcohols
Authors: Manali Dutta, Kusum K. Bania, and Sanjay Pratihar
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10.1002/asia.201901760
Chemistry - An Asian Journal
FULL PAPER
Remote ‘Imidazole’ Based Ruthenium(II) Para-Cymene Pre-catalyst
for Selective Oxidation Reaction of Alkyl Arenes and Alcohols
Manali Dutta,[a] Kusum K. Bania,[a] and Sanjay Pratihar*[a],[b]
Dedication ((optional))
Abstract: Herein we disclosed the utilization of remote ‘imidazole’
based precatalyst [(para-cymene)RuII(L)Cl]+, C-1 where L = 2-(4-
substituted-phenyl)-1H-imidazo[4,5-f][1,10] phenanthroline) for the
selective oxidation of variety of alkyl arenes/heteroarenes and
alcohols to their corresponding aldehydes or ketones in presence of
tert-butyl hydroperoxide (TBHP). The remote ‘imidazole’ moiety
present in the complex facilitate the activation of oxidant and
subsequent generation of active species via the release of para-
cymene from C-1, which in-turn was less effective without the
‘imidazole’ moiety. The mechanistic features of C-1 promoted
oxidation of alkyl arenes were also assessed from spectroscopic,
kinetic, and few other controlled experiments. The substrate scope for
C-1 promoted oxidation reaction was performed for the selective
oxidation of 27-different alkyl arenes/heteroarenes and 25 different
alcohols to their corresponding aldehydes/ketones with moderate to
good yields.
Very recently, it has been found that these intermediates could
also be generated under oxidizing environment via the oxidative
loss of Cp* from [Ir(Cp*)L] [7-9] and oxidative loss of arenes from
the ruthenium-based precatalysts [10-11]. Although these
oxidizing intermediates are known for their selective oxidative
transformation of variety of organic substrates including alkane,
alkene, and alkyne to their useful and functional oxygenated
products, but the selectivity and progress of the reaction depends
on various factors and could be tuned through the stereo-
electronic adjustment of the ligand [12-16]. Towards this, the
group of Mayer [17-18], Nam [19-20], Saik [21], Che [22],
Fukuzumi [23-25], and others [26-33] have developed variety of
ligand-bound well-defined high valent ruthenium-oxo/dioxo
intermediates and utilized as potential catalyst for variety of
oxidative transformation reaction. In continuation of our goal to
develop sustainable catalytic methods for the selective oxidation
reaction [11], [34], herein, we disclosed the utilization of remote
‘imidazole’ based ruthenium(II) ƞ6-p-cymene precatalyst for
selective oxidation of alkyl arenes/heteroarenes to their
corresponding aldehyde/ketones using tert-butyl hydroperoxide
(TBHP) as oxidant.
Introduction
Selective oxidation of alkyl arenes/heteroarenes to their valuable
building blocks such as aldehydes and ketones using transition
metal-based catalyst is one of the most challenging reactions [1].
In an early demonstration, Shing et al. first reported that
RuCl3·xH2O (7 mol%) in presence of NaIO4 was an effective
catalyst for the dihydroxylation of various alkenes in
EtOAc/MeCN/H2O solvent systems under controlled reaction
condition [2]. Later on, by adding Brønsted or Lewis acids with
lower amount of catalyst loading, Plietker and Niggemann
showed an improvement of the existing RuCl3·xH2O/NaIO4
system promoted reactions [3]. Waegell also demonstrated the
use of ruthenium in C-H bond activation with in-situ generated
RuO4 [4]. These potential high-valent oxidizing intermediates
could be generated from the reaction between low valent
transition metal complexes and sacrificial oxidant (such as
oxygen, tBuOOH, (NH4)2Ce(NO3)6, NaIO4 etc.) through (i)
reductive O2 activation, (ii) oxo transfer from peroxides, (iii)
proton-coupled electron-transfer (PCET) oxidation of a low-valent
aqua complex, and (iv) photo-induced homolytic/heterolytic
cleavage of the O-X bond [5-6].
Results and Discussion
The precatalysts (C-1 to C-5) were synthesized and charecterized
by following our earlier reported procedure [11]. Initially, to check
the reactivity of the complexes, reaction condition was optimized
from the screening of catalyst loading (entry 1-3, Table 1), solvent
(entry 8-13, Table 1), temperature (entry 3-5, Table 1), and time
(entry 1-7, Table 1) using oxidation of toluene to benzaldehyde as
model reaction. To check the effect of oxidizing agents (entry 2-9,
Table 1), C-1 promoted model reaction was performed in
acetonitrile in presence of various reagents such as; hydrogen
peroxide (H2O2), sodium metaperiodate (NaIO4), tert-butyl
hydroperoxide (TBHP), oxygen, and ceric ammonium nitrate
(CAN). Amongst all, TBHP was found to be effective for the
selective oxidation of toluene to benzaldehyde in 84% yield. The
precatalysts C-2 and C-3 promoted oxidation of toluene afforded
benzaldehyde with 72% and 74%, respectively after 3h (entry 18-
19, Table 1). The blank reactions without the catalyst or the
oxidant did not produce any desired product even after 6h (entry
5-6, Table 1). To check the effect of acid/base in the reaction,
model reaction was performed under optimized reaction condition
with addition of acid (trifluoro acetic acid, 1 eqv,) or base (triethyl
amine, 1 eqv.). However, the presence of acid with complex C-1
does not show any significant enhancement in its catalytic activity
(entry 14, Table 1). On the other hand, the presence of base
completely halt the model reaction. To check the effect of
imidazole moiety, model reaction was conducted with C-4 (does
not contain remote imidazole moiety) and C-5 (stable octahedral
[a]
Manali Dutta, Dr. Kusum K. Bania, Dr. Sanjay Pratihar
Department of Chemical Sciences
Tezpur University
Napaam, Assam-784028, India
E-mail: spratihar@csmcri.res.in or spratihar29@gmail.com
Dr. Sanjay Pratihar
[b]
Inorganic Materials and Catalysis Division
CSIR-Central Salt and Marine Chemical Research Institute
G. B. Marg, Bhavnagar, Gujarat-364002, India
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
1
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10.1002/asia.201901760
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geometry). C-4 and C-5 promoted model reaction afforded
benzaldehyde in 34% and 0% yield, respectively after 6h. The
inactivity of C-5 suggests that a vacant coordination site at the
Ru(II) center was required for the reaction and also rulled out the
possible involvemnet of ligand in the reaction, which encouraged
us to look for the reactivity with other Ru(II) complexes.
Interestingly, other Ru(II) complexes such as; [Ru(BPy)3]Cl2,
[Ru(BPy)2Cl2], [Ru(ƞ6-p-cymene)Cl2]2, and [Ru(ƞ6-p-cymene)Cl2]2
in combination with 2 equivalent AgPF6 were screened for the
model reaction. In all the cases, lower yield of the product was
achieved (entry 22-25, Table 1). At the same time, all of the tested
metal salts or complexes were also found to be less effective for
the reaction (entry 26-33, Table 1). Next, to check the stability of
the metal-ligand backbone in C-1 under oxidizing environment,
the reactivity of C-1 was compared with other reagent systems.
The significantly higher catalytic activity of C-1/TBHP for the
oxidation of toluene (84% yield of benzaldehyde) compared with
other systems (entry 2, Table 1) such as; RuCl3/NaIO4 (known to
generate RuO4, 8% yield of benzaldehyde, entry 32, Table 1) and
[Ru(ƞ6-p-cymene)Cl2]/TBHP (20% yield of benzaldehyde, entry
31, Table 1) and [Ru(ƞ6-p-cymene)Cl2]/NaIO4 (6% yield of
benzaldehyde, entry 34, Table 1) was observed. The above
mentioned comparative reactivity study suggested that complete
degradation of ligand backbone in C-1 to form “ligand-free” RuO4
was unlikely in the present system.
14
15
C-1
C-1
2
2
TBHP
TFA
+
+
6
6
MeCN
MeCN
82
0
TBHP
Et3N
16
17
18
19
20
21
22
23
24
25
C-1
2
TEMPO
DPPH
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
12
12
3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
15
10
74
72
34
0
C-1
1
C-2
2
C-3
2
3
C-4
2
6
C-5
5
6
[Ru(BPy)3]Cl2
Ru(BPy)2Cl2
[RuCl2(p-Cy)]2
5
6
0
5
6
30
15
26
5
6
[RuCl2(p-Cy)]2
+ AgPF6
5/5
6
26
27
28
29
30
31
32
CuCl2
5
5
5
5
5
5
5
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
NaIO4
6
6
6
6
6
6
6
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
20
21
32
0
CuCl
Cu(OAc)2
[Fe(Phen)3]Cl2
Cu(Phen)Cl2
RuCl3
Table 1. Screening of Catalyst for oxidation of toluene to benzaldehyde.
24
20
08
RuCl3
MeCN-
H2O
33
34
FeCl3
5
5
TBHP
NaIO4
6
6
MeCN
MeCN
12
06
[RuCl2(p-Cy)]2
#
Cat.
mol%
Oxidant
t, h
Solvent
Yield
(%)a
Reaction Condition: Toluene (1 mmol), TBHP (200 µL), Catalyst (2-5 mol%),
MeCN (5 mL), aThe yields were calculated based on gas chromatography
(GC) analysis using ortho-xylene as external standard. bReaction conducted
at 45 ºC.
1
2
3
4b
5
6
7
8
9
C-1
C-1
C-1
C-1
-
5
2
1
2
-
TBHP
TBHP
TBHP
TBHP
TBHP
-
1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
82
84
66
54
0
1
2
Mechanistic studies for C-1 promoted oxidation of toluene to
benzaldehyde
2
1
Next, to check the effect of remote “imidazole” in C-1, H NMR
6
titration was conducted with time for C-1 in CD3CN in presence of
TBHP at 60 °C. The 1H NMR spectrum of C-1 showed the
presence of ruthenium coordinated para-cymene, which gradually
release from C-1 during the generation of intermediate as was
evident from the regular appaearance of the peak due to free
para-cymene (Figure 1c). On the other hand, the 1H NMR
monitoring of C-1 in CD3CN in presence of substrate (toluene)
showed subsequent oxidation of toluene to benzaldehyde (Figure
S44). Next to check the invovement of para-cymene in the rate
determining step, C-1 promoted oxidation of toluene to
benzaldehyde was conducted in MeCN in presence of different
added arenes such as 1,3 dimethoxy benzene, para-cymene,
toluene, and 4-fluoro benzene (Figure 2).
C-1
C-1
C-1
C-1
2
2
2
2
6
0
H2O2
O2
4
25
0
12
4
NaIO4
MeCN/
H2O
20
10
11
C-1
C-1
2
2
CAN
4
2
MeCN
14
40
TBHP
Aceton
e
12
13
C-1
C-1
2
2
TBHP
TBHP
6
6
MeOH
DMSO
10
40
2
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However, we did not observe any change in the conversion of
benzaldehyde in presence of added arenes, which suggested that
the release of para-cymene from the precatalyst is not involved in
the rate determining step (Figure 2c). The complex C-1 showed
an intense absorption MLCT band centered at 405 nm in
acetonitrile. The reaction of C-1 with TBHP in acetonitrile at 60 C
produced a colour change (light yellow to green) along with the
generation of two new band centered at 580 nm and 704 nm due
the high valent ruthenium-oxo species [24], [35-39] in the solution
(Figure 1a).
cm-1 (Figure 1)assigned to the corresponding asyRuIV=O stretch
[6], [40]. Next, to identify the intermediate, C-1 was reacted with
TBHP (C-1 : TBHP = 1:3) in acetonitrile at 60 ºC for 1.5 h. Mass
spectral analysis of the corresponding reaction mixture showed
peaks at m/z = 531.03 due [(L)Ru(CH3CN)2=O-H]+ and m/z =
566.01 due to [(L)RuCl(CH3CN)2=O]+ apart from its parent
complex [ƞ6-p-cymene (L)Ru(Cl)]+ at m/z = 601.05 (Figure S9).
The presence of above mentioned fragments in ESI-MS study
further suggested the generation of high valent (L)RuIV=O from C-
1 under the oxidizing environment through the release of para-
cymene. Further, to understand the mechanism, kinetic orders of
dependency for precatalyst, TBHP, and substrate were
determined with model reaction by using initial rate methods
under optimized reaction condition (details supplied in the ESI).
The observed rate kinetics data showed first order rate
dependency with respect to precatalyst, TBHP, and substrate.
(Please see Figure S10 to S-12 in ESI). Next, initial rate kinetics
for C-1 promoted oxidation of four different para-substituted
toluene (p-Y-C6H4CH3; Y=OH, H, Br, NO2) to their corresponding
benzaldehyde was monitored and showed the order as
Y=NO2>Br>H>OH. The small positive Hammett reaction constant
(ρ=0.26) in case of C-1 indicates the generation of a negligible
negative charge at the transition state (TS) and consistent with
the addition of TBHP radical to the benzyl radical (Figure 2). Next,
we employed competition kinetic isotope effect (KIE) experiments
to get an idea about rate determining step of the reaction. An
inverse secondary intermolecular kH/kD = 0.86 ± 0.03 for toluene
to benzaldehyde was measured, which proposed rehybridization
of the C−H bond from sp2 to sp3 in the product determining
transition state. Further to confirm the involvement of radical, C-1
promoted oxidation of toluene to benzaldehyde was performed in
Figure 1. Monitoring of UV-visible spectra with time for the reaction between C-
1 with tert-butyl hydroperoxide (TBHP) in MeCN at 60 C (a), FT-IR spectra of
the intermediate obtained after the reaction between C-1 and tertbutyl
hydroperoxide in MeCN at 60 C (b), 1H NMR monitoring of C-1 in presence of
TBHP at 60 C in CD3CN (c).
presence of
tetramethylpiperidin-1-yl)oxyl
radical scavenger
such as; (2,2,6,6-
2,2-diphenyl-1-
(TEMPO),
picrylhydrazyl (DPPH) (entry 16-17, Table 1). In both the cases, a
substantial decrease in the yield of benzaldehyde was observed,
which justifies the involvement of radical species during the
reaction. Further, to check the life time and stability of the
intermediate under oxidizing environment, solid (sticky)
intermediate was isolated after the reaction of C-1 with TBHP (C-
1 : TBHP = 1 : 10) in MeCN for 1.5 h. The model reaction was
conducted with isolated intermediates (20 mol%). However, the
model reaction produced only 48% of benzaldehyde with isolated
intermediates. Based on the above-mentioned preliminary
experimental results and arguments, plausible mechanism have
been suggested for the present C-1 promoted C-H oxidation
reaction (Figure 2c). However, a detailed examination would be
further required to consider or nullify other possibilities [40].
Substrate scope for C-1 promoted oxidation of alkyl
arenes/heteroarenes
Next, the substrate scope of C-1 promoted oxidation reaction for
variety of aryl/heteroaryl alkanes is illustrated in Scheme 1 [41].
The oxidation of toluene derivatives attached with electron-
withdrawing groups at para-/ortho-position were effective and
produced their corresponding aldehyde selectively with very good
to excellent yield. C-1 promoted oxidation of 4-methoxy tolune
produced the corresponding 4-methoxy benzaldehyde in 74%
yield after 3h. The similar oxidation for 2-hydroxy or 4-hydroxy
toluene produced their corresponding benzaldehyde in 65% and
62% yield, respectively. Further, C-1 promoted oxidation reaction
is found to have excellent functional group tolerance and
Figure 2. Hammet plot for the C-1 promoted oxidation of four different para-
substituted toluenes to their corresponding aldehydes (a), C-1 promoted
oxidation of toluene in presence of different added arenes (b), and plausible
mechanism for C-1 promoted oxidation of toluene to benzaldehyde (C).
The FT-IR spectra of the reaction mixture (after the reaction of C-
1 and TBHP in MeCN at 60 C) suggest an intense band at 840
3
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produced their corresponding aldehyde in presence of functional
group such as; -NO2, -Cl, Br, -I, -OH, -OMe, -CHO, -CO2H, and
CO2Me (Scheme 1). However, reaction failed to produce any such
aldehyde in case of 4-methyl aniline, 2,6-dimethyl pyridine, 2-
amino,-6-methyl pyridine. Next, the oxidation reaction was tested
with alkyl-hetero-arenes such as; 2-methyl thiophene, 2-methyl
furan, and 3-methyl pyridine and observed their corresponding
aldehyde in 71% (2l), 70% (2m), and 65% (2n), respectively.
Notably, reaction proceeds smoothly in case of para-xylene and
meta-xylene afforded corresponding terephthalaldehyde (2o) and
isophthalaldehyde (2r) in 74% and 62% yield, respectively
(Scheme 1). However, similar type of oxidation failed in case of
ortho-xylene and 2,6-lutidine. Next, the methodology was also
found to be successful for the oxidation of alkyl benzene such as;
ethyl benzene, n-propyl benzene, n-butyl benzene and afforded
the corresponding ketones selectively in 80% (2w), 70% (2x), and
72% (2y) yields, respectively. Similarly, the oxidation of other
substrate such as; diphenylmethane, and 4-bromo acetophenone
were found to proceed smoothly and produced their
corresponding oxygenated products in 82% (2u) and 74% (2v)
yield. Next, to compare the reactivity between different alkyl
benzenes, initial rate kinetics were performed (Figure 3) for C-1
promoted oxidation of eight different substrates under optimized
reaction condition and suggested the following order as 1a > 1b
> 1c > 1d > 1e > 1f > 1g>1h (Table 2).
The observed reactivity trends also suggested the higher
oxidation tendency of secondary C-H bond as compared to
primary C-H bond (Table 2). Gratifyingly, C-1 promoted selective
oxidation 2-benzylisoindolin-1-one afforded 2-benzylisoindoline-
1,3-dione (3a) in 70% after 6h without hampering the benzyl
moiety
of
the
substrate.
Whereas
2-(3,5-
dimethylphenyl)isoindolin-1-one could also be succesfully
converted to 2-(3,5-dimethylphenyl)isoindoline-1,3-dione (3b) in
72% yield, without oxidizing the other alkyl substituent attached at
the aromatic moiety of the substrate (Scheme 1).
Figure 3. UV-Vis monitoring of intermediate of C-1 in presence of substarte.
Table 2. C-1 promoted oxidation of eight different substrate to their
corresponding aldehyde/ketone and their reactivity order.
Substrate
Rate (k ×10 min-1)
Diphenyl methane (1a)
Cumene (1b)
3.25
1.75
1.03
0.32
0.27
0.23
0.18
0.17
Ethyl benzene (1c)
4-nitro tolene (1d)
4-bromo toluene (1e)
4-iodo toluene (1f)
Toluene (1g)
4-hydroxy toluene (1h)
Substrate Scope for C-1 promoted oxidation of alcohols
During the course of our study, we realized that C-1/TBHP
reagent would be useful for the oxidation of alcohols to their
corresponding aldehyde or ketone. Thus, oxidation of variety of
primary and secondary benzyl alcohols were attempted and
illustrated in Scheme 2 [41]. Under optimized reaction condition,
very good to excellent yield were achieved for variety of primary
benzyl alcohols attaching with electron donating/withdrawing
Scheme 1. Substrate scope of C-1 promoted oxidation of alkyl
arenes/heteroarenes.
4
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substituent at ortho-/meta-/para- position. Whereas, the oxidation
of pyridin-2-ylmethanol produced the corresponding aldehyde in
40% yield. However, the oxidation of corresponding pyridin-4-
ylmethanol or thiophen-2-ylmethanol or furan-2-ylmethanol were
effective under similar reaction condition and produced their
corresponding aldehyde 4i, 4h, and 4f in 75, 68, and 70% yield,
respectively. The methodology was also effective for n-octanol to
produce the corresponding octanal selectively in 72% yield.
Furthermore, oxidation of cinamyl alcohol was also found to be
selective and afforded cinamaldehyde in 68% yield. The
methodology is not only limited for the oxidation of primary
alcohols, but it could be extended for the oxidation of secondary
alcohols to corresponding ketones. Thus, varieties of secondary
alcohols were screened under optimized reaction condition
(Scheme 2). Gratifyingly, selective oxidations of these alcohols
were found to be useful to produce their corresponding ketones
in good to excellent yield. It is noteworthy to mention that the
methodology was also effective for the selective oxidation of
alcohols to corresponding ketones for eight different (E)-1,3-di-
aryl-prop-2-en-1-ol derivatives with good to excellent yield.
Interestingly, double bond in these compounds was found to be
intact after the oxidation.
‘imidazole’ moiety present in the complex not only facilitates the
activation of oxidant and subsequent generation of active species
via release of para-cymene from the precatalyst but also stabilizes
the active species for the selective oxidation reaction, which in
turn was not effective without the ‘imidazole’ moiety. The
mechanistic evidences based on spectroscopic, kinetic and few
other controlled experiments suggested that rate determining step
(rds) of C-1 promoted oxidation of toluene involve the addition of
TBHP radical to the benzyl radical. The pre-catalyst C-1 showed
promising catalytic activity and good selectivity for the oxidation
of variety of substrates such as alkyl arenes/heteroarenes and
alcohols to their corresponding aldehydes/ketones.
Experimental Section
Experimental Details: For the synthesis of complexes, reactions were
performed under dry oxygen free argon atmosphere using standard
vacuum lines and Schlenk techniques. The solvents used for the synthesis
of complexes were dried and distilled by standard methods and previously
deoxygenated in the vacuum line. 1H (400/600 MHz) and 13C NMR
(100/150 MHz) spectra were recorded on 400/600 MHz spectrometers at
298 K. Electron spray Ionization mass spectra (ESI-MS) were recorded on
ESI-Q-TOF mass spectrophotometer.
General procedure for C-1 promoted oxidation of alkylarenes or
alcohols to aldehydes
In a Schlenk tube, substrate (alkyl arenes/alcohols, 1 mmol), catalyst (2
mol%) and TBHP (2.1 mmol) was taken in 5 mL acetonitrile. The reaction
mixture was stirred at 60 oC for the required time. The initial yellow/orange
colour of the solution turned yellowish green, which finally turned into deep
green. After the completion of the reaction (via TLC monitoring), water was
added to the reaction mixture and the product was extracted with ethyl
acetate (50 mL × 3), washed with brine solution (50 mL × 2) and dried over
anhydrous Na2SO4. The yield of the reaction was calculated from gas
chromatography (GC) analysis using ortho-xylene as external standard
and 1H NMR analysis using diphenyl methane as external standard. Some
of the isolated products were characterized via 1H/13C NMR analysis.
Acknowledgements ((optional))
Financial support of this work by Indian National Science
Academy, New Delhi and DBT-New Delhi (grant no. BMB/2015-
42 to SP) is gratefully acknowledged. MD is thankful to Tezpur
University (for institutional fellowship) and CSIR, New Delhi (for
SRF).
Keywords: Oxidation • Ruthenium • Toluene • Alcohol •
Aldehyde
[1]
[2]
C. L. Sun, B. J. Li, Z. J. Shi, Chem. Rev. 2011, 111, 1293-1394.
T. K. M. Shing, E. K. W. Tam, V. W. F. Tai, I. H. F. Chung, Q. Jiang,
Chem. Eur. J. 1996, 2, 50-57.
Scheme 2. Substrate scope of C-1 promoted oxidation of alcohols.
[3]
[4]
B. Plietker, M. Niggemann, J. Org. Chem. 2005, 70, 2402-2405.
A. Tenaglia, E. Terranova, B. Waegell, J. Org. Chem. 1992, 57, 5523-
5528.
Conclusion
[5]
[6]
J. M. Mayer, Acc. Chem. Res. 2011, 44, 36-46.
T. Ishizuka, H. Kotani, T. Kojima, Dalton Transactions 2016, 45, 16727-
16750.
In summary, we demonstrated the remote ‘imidazole’ based
precatalysts [(ƞ6-p-cymene)RuII(L)Cl]+, C-1 where L = 2-(4-
substituted-phenyl)-1H-imidazo[4,5-f][1,10] phenanthroline) for
the selective oxidation of alkyl arenes/heteroarenes and alcohols
to their corresponding aldehyde or ketones. The remote
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[40] Based on few experimental evidences, although we are considering the
active species as (L)RuIV=O, but the involvement of other active species
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The present work disclosed the dual role of remote ‘imidazole’ attached with the precatalyst [(ƞ6-p-cymene)RuII(L)Cl]+ (L = 2-(4-
substituted-phenyl)-1H-imidazo[4,5-f][1,10] phenanthroline) for the activation of oxidant (tert-butyl hydroperoxide, TBHP) and
generation of active catalyst via the release of para-cymene for the selective oxidation of 27-different alkyl arenes/heteroarenes and
25 different alcohols to their corresponding aldehydes/ketones with moderate to good yields.
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