Metal free visible light driven oxidation of alcohols to carbonyl derivatives using 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) as catalyst

Article abstract of DOI:10.1039/c5ra18151h

3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalyzed oxidation of alcohols to the corresponding carbonyl compounds under visible light irradiation is described. This reaction occurs smoothly at room temperature and shows good tolerance of functional groups. It provides an alternative approach for the synthesis of alkyl and aryl aldehydes and ketones.

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Metal Free Visible Light Driven Oxidation of Alcohols to Carbonyl
Derivatives Using 3, 6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) as
Catalyst †
Received 00th January 20xx,
Accepted 00th January 20xx
Suvendu Samanta and Papu Biswas*
DOI: 10.1039/x0xx00000x
www.rsc.org/
3, 6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalyzed oxidation of light.2i,21 Due to the importance of this reaction, the
alcohols to corresponding carbonyl compounds under visible light development of efficient, mild, and metal free safe oxidation
irradiation is described. This reaction occurs smoothly at room processes are still highly desirable.
temperature and shows good tolerance of functional groups. It Recently, we have reported the visible light absorption and
provides an alternative approach for the synthesis of alkyl and aryl reversible one-electron reduction properties of the 3, 6-
aldehydes and ketones.
di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) molecule (Figure 1)2b
and its application to the photocatalytic synthesis of 2-
substituted benzimidazoles and benzothiazoles from different
aldehydes. s-Tetrazines (Tz) are known to have a strong
electron-deficient character and accept electrons and protons
Introduction
Recently, organic reactions driven by visible light at ambient
temperature in presence of dyes has attracted much interest
of synthetic chemists. Usually, polypyridine complexes of
ruthenium and iridium have been utilized as dyes in most of
these reactions.1 More recently, few organic dyes have also
been reported as visible light photoredox catalysts.2
Among organic transformations, the selective oxidation of
alcohols to the corresponding carbonyl groups is one of the
most important and widely utilized in the manufacture of fine
chemicals and intermediates.3 Consequently, several effective
reagents and methods have been developed, including
stoichiometric oxidizing agents such as hypochlorite,4
chromium based reagents,5 active manganese oxides and
permanganate,6,7 Dess-Martin periodinane,8 o-iodoxybenzoic
acid (IBX),9 ruthenium based reagents,10 osmium(VIII) oxide11
and activated DMSO in Swern oxidation.12 However, most of
these reagents are toxic, hazardous and sometimes generates
harmful wastes. Numerous homogenous as well as
heterogeneous transition metal13-19 and TEMPO20 based
catalysts have also been reported to be active for aerobic
oxidation of alcohols, as they are particularly advantageous
from environmental and economic perspective. However, they
require relatively expensive metal catalysts, a large amount of
additives, and higher temperature to obtain satisfactory
results. Few reports also appeared on photocatalytic
conversion of alcohols to aldehydes and ketones using visible
Fig. 1 3, 6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz).
simultaneously to form dihydrotetrazenes (H2Tz) via anion
radical (Scheme 1).22 This reducing nature of the s-tetrazines is
even more prominent for the first excited state, which
therefore has
a
relatively strong oxidizing power.22
Consequently, s-tetrazines interact with various electron
donor substrates at an excited state.
Scheme 1 Reduction of s-Tetrazine (Tz) to Dihydrotetrazine
(Tz) via Anion Radical
In our efforts to further explore the photocatalytic activity
of pytz and expand its synthetic utility, we report here metal-
free efficient oxidation of alcohols to the corresponding
carbonyl compounds under visible light irradiation using pytz
as catalyst (Scheme 2). During oxidation of alcohols, pytz is
converted to H2pytz. In order to reoxidize the H2pytz, an
efficient co-oxidant is required. We have utilized O2 or tert-
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butylnitrite/AcOH mixture as sacrificial co-oxidant to establish bient temperature. Use of 5 mol% of pytz catalyst was found
two alternative methods of oxidation.
to be optimum and any other substrate to catalyst ratio
furnished inferior yields. A Xenon lamp with a power of 300 W
equipped with a cut-off filter (λ> 420 nm) was used as a visible
light source (for schematic diagram of reaction set-up, see
Scheme S1†). Under nitrogen, only trace of the desired product
2a was obtained (Table 1, entry 1). Changing from oxygen-free
environment to aerobic condition resulted in a substantial
improvement in conversion (entry 2). Then we have
considered the use of pure molecular oxygen. Accordingly, the
conversion increased dramatically when reaction was carried
out under O2-atmosphere (entry 3). No noticeable over
oxidation to carboxylic acid was isolated or identified.
Performing the reaction in other solvents including water,
dioxane, THF, chloroform, or dichloromethane gave much
lower yields of 2a (entries 4−9). We have also invesꢀgated the
catalytic activity of the 3,6-diphenyl-1,2,4,5-tetrazine (phtz),
under identical conditions (entry 10). Phtz shows much inferior
activity. To establish that visible light is essential for the
reaction to proceed, a control reaction was carried out in the
dark in presence of pytz. No catalytic activity was observed in
the absence of visible light, which clearly demonstrates the
role of visible light in this reaction (entry 11). Similarly, a
control reaction in the absence of pytz did not proceed under
identical conditions (entry 12). Encouraged by these results,
we further carried out the reaction in presence of tert-
butylnitrite/acetic acid (AcOH) mixture as oxidant. Oxidation of
H2Tz by NaNO2/acid mixture is long known.22 In the presence
of air, these generate nitrogen dioxide, a sufficiently strong
oxidant to efficiently recycle the H2Tz. The effect of reaction
time and solvent on the reaction was again evaluated
thoroughly. Interestingly, the reaction proceeds within much
lower time in CH3CN (entry 13). No product was obtained
when a control reaction was done in presence of tert-
butylnitrite/AcOH mixture but without catalyst in presence of
visible light (entry 14). So, the optimum reactions were found
to involve 1a, pytz (5 mol%), and O2 in CH3CN at ambient
temperature under visible light irradiation (Method A, Scheme
3), and 1a, pytz (5 mol%), tert-butylnitrite (1.1 equiv), and
AcOH (1.1 equiv) in CH3CN at ambient temperature under
visible light irradiation (Method B, Scheme S2†).
Scheme 2 Oxidation of Alcohols to Carbonyls by Pytz Under
Visible Light Irradiation
Results and discussion
Our preliminary studies were carried out with benzyl alcohol
1a as a model substrate in presence of pytz in CH3CN. In a
typical experiment, benzyl alcohol 1a in CH3CN was irradiated
under visible light in presence of 5 mol% of pytz catalyst at am-
Table 1 Optimization of the Reaction Conditionsa
entry catalyst solvent time (h) yield (%)b
1c
2d
3
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
phtz
pytz
none
pytz
none
CH3CN
CH3CN
CH3CN
H2O
12
12
10
12
traces
13
85
4
26
5
dioxane 12
30
6
THF
12
12
12
12
10
12
12
4
42
7
CH3Cl
CH2Cl2
DMF
55
8
48
Having the optimum conditions in hands, the scope for the
oxidation was subsequently investigated with various primary
alcohols; reactions under oxygen environment (method A) are
shown for comparison. As shown in Scheme 3, high yields were
obtained for a wide range of alcohols. Under these reaction
conditions, various benzylic alcohols with different
substituents on the phenyl ring containing both electron-
withdrawing and electron-donating can be efficiently oxidized
to the corresponding aldehydes in high yields (Scheme 3, 2a-
9
38
10
11e
12f
13g
14g
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
35
traces
none
92
5
none
aReaction Condition (until otherwise specified): 2
mmol benzyl alcohol, solvent (5 mL solvents used
for all cases), pytz (5 mol%), O2-atmosphere,
room temperature, visible light. bIsolated yield.
cReaction done in inert atmosphere. dReaction
e
). Anthracen-9-ylmethanol (1f) and heterocyclic alcohols (1g
and 1h) were smoothly oxidized to give the aldehydes in good
yields (Scheme 3, 2f-h). In addition, primary allylic alcohol (1i
and -O-4 lignin model, 4-(hydroxymethyl)-2-methoxyphenol
1j), were also effective substrates, affording the
)
e
done in presence of air. Reaction carried out in
f
absence of visible-light (dark). Reaction carried
β
(
out without any catalyst but in presence of
g
visible-light. Reaction carried out in presence of
corresponding aldehydes in good yields (Scheme 3, 2i and 2j).
The present catalytic system is not efficient for aliphatic prima-
tert-butylnitrite (1.1 equiv), AcOH (1.1 equiv).
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Scheme 4 Oxidation of Secondary Alcohols
. Method A: 2 mmol
alcohol, acetonitrile (5 mL), pytz (5 mol%), O2-atmosphere,
Scheme 3 Oxidation of Primary Alcohols. Method A: 2 mmol room temperature, visible light; Method B: 2 mmol alcohol,
alcohol, acetonitrile (5 mL), pytz (5 mol%), O2-atmosphere, acetonitrile (5 mL), pytz (5 mol%), room temperature, visible
room temperature, visible light; Method B: 2 mmol alcohol, light, t-BuONO (2.2 mmol), AcOH (2.2 mmol). Isolated yields
acetonitrile (5 mL), pytz (5 mol%), room temperature, visible are shown.
light, t-BuNO2 (2.2 mmol), AcOH (2.2 mmol). Isolated yields are
shown.
electron to produce an anion radical which is very stable in
absence of any proton donor.22c,23 Most of the tetrazine
ry alcohols and shows relatively poor yields (
<
40%).
Secondary alcohols were also found to be effective derivatives can accept a second electron though this process is
substrates and generally more reactive than primary alcohols. not electrochemically reversible in standard conditions. The
Various secondary aryl alcohols were effectively oxidized to unstable dianion formed after transfer of second electron
ketones (Scheme 4, entries 4a-g
). A number of functionalities, reacts with proton donor to give dihydrotetrazine22c as shown
such as bromine, methyl, nitro, and diphenylmethanol are in Scheme 1. Fukuzumi et al. reported the reduction of 3,6-
compatible with the reaction conditions and furnished the diphenyl-s-tetrazine (PhTz) by 10-methyl-9,10-dihydroacridine
corresponding ketones in excellent yields (Scheme 4, 4b
(Anthracen-9-yl)ethan-1-ol (3f) and 9,10-dihydroanthracen-9- hydride transfer from AcrH2 or 10,10
-e
). 1- (AcrH2) promoted by scandium ion.24 In presence of Sc3+,
′
-dimethyl-9,9′-biacridine
ol (3g) underwent smooth oxidation to provide the ketones in [(AcrH)2] to PhTz occurs efficiently at room temperature to
good yield (Scheme 4, 4f and 4g). Furthermore, heterocyclic yield 10-methylacridinium ion (AcrH+). So, tetrazines can be
secondary alcohol, 1-(thiophen-3-yl)ethan-1-ol (3h) and alkyl utilized as two electron oxidising agent under suitable
secondary alcohol, cyclohexanol (3i), were also effectively conditions.
oxidised to corresponding ketones (Scheme 4, 4h and 4i) with
In order to elucidate the mechanism of the present direct
alcohol oxidation, the optical and electrochemical properties
71 and 79% yields, respectively by method B.
The visible light driven mild pytz-catalysed oxidation of of the pytz molecule were studied by UV–vis absorption
alcohols to carbonyls also proved easy to scale up, as spectroscopy and cyclic voltammetry in acetonitrile. Pytz
demonstrated by a gram scale experiment in which 20 mmol exhibits maximum absorption peaks at 532 nm (
of benzyl alcohol in 50 mL of acetonitrile (1a) was irradiated 1, Fig. 2a) and two quasi-reversible reduction peak at +0.31 and
under visible light in presence of t-BuONO/AcOH. The reaction –0.91 in acetonitrile (Fig. 2b and c). Pytz molecule
resulted in 2.06 g (19.4 mmol, 97% yield) of analytically pure successively accepts two electrons quasi-reversibly in
benzaldehyde in 7 h. acetonitrile and can be conveniently converted into H2pytz in
ε
= 190 M-1 cm-
V
To understand this visible light driven oxidation of alcohols presence of proton donor. Occurrence of first reduction
to carbonyls catalyzed by pytz, a detailed investigation on the potential at +ve potential indicates that pytz molecule is highly
mechanism was undertaken. Due to their electron deficient susceptible towards reduction and forms stable anion radical.
character, tetrazine molecules can be reversibly reduced in
organic solvents. Almost all Tetrazine molecules accept one
While a precise understanding of the reaction mechanism
awaits further study, a plausible catalytic pathway is proposed
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from activated photocatalysts to produce superoxide anion,
·− 2
O2 . As pytz molecule itself accepts electron in its excited
a
state to act as an oxidising agent, formation of superoxide is
not feasible under present reaction protocol. Hence, the
involvement of activated molecular O2 as the real oxidant is
ruled out.
Conclusion
In summary, we have developed a highly efficient metal free
oxidation of alcohols under visible light irradiation using pytz
as catalyst and molecular oxygen or tert-butylnitrite as co-
oxidant. Various primary and secondary alcohols can be
efficiently oxidized under mild conditions to the corresponding
aldehydes and ketones in high yields. The present procedure
shows the following favourable features: (1) the reaction does
not require any metal based oxidant; (2) the reaction proceeds
under neutral and mild conditions (neither acid nor base
required) and thus making it suitable with acid- or base
sensitive substrates; (3) the present method is effective for a
wide range of substrates bearing various functional groups; (4)
the reaction can also be carried out in presence of molecular
b
c
Fig. 2 (a) UV-vis absorption spectra and cyclic voltammogram oxygen further making it environmentally benign procedure;
(CV) of 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) in (5) our developed catalytic system should be suitable for large-
acetonitrile in the range (b) +1.0 to –0.5 V and (c) –0.5 to –1.0 scale synthesis of carbonyls from alcohols. Further
V. Scan rate for cyclic voltametic measurements were 50 mVs- development of other reaction processes with pytz and related
1
.
tetrazines is currently underway.
in Scheme 2. According to the previous studies,1a,b,21c,d we pro- Acknowledgements
posed that the reaction may proceed through the formation of S. S. is indebted to CSIR, India for his Senior Research
radical cation, ROH·+. In presence of visible light, pytz is Fellowship [08/003(0083)/2011-EMR-1]. P.B. acknowledges
converted to pytz*, which is transformed in to pytz radical CSIR-India for the Project (Sanction letter no.
anion through single electron transfer (SET) from alcohol to
01(2459)/11/EMR-II dated 16/05/2011). The authors also
form radical cation, ROH·+. The poor conversion of aliphatic acknowledge the Sophisticated Analytical Instruments Facility
primary alcohols supports the formation of radical cation as at North Eastern Hill University (SAIF-NEHU) for 1H NMR
aliphatic primary radical cations are not stable. Then, hydrogen analysis.
atom abstraction by stable pytz radical anion from the radical
cation ROH·+ followed by deprotonation gives the carbonyl References
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(eqn 1):
RCH2OH + pytz = RCHO + H2pytz
(1)
2
To prove the radical pathway, this reaction was also done
in the presence of a radical scavenger, TEMPO (TEMPO =
2,2,6,6-tetramethylpiperidinoxyl). Addition of TEMPO in a 1:1
molar proportion (with respect to benzyl alcohol) to the
reaction mixture, only trace yield can be identified by GC. To
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yield ∼ 80% benzaldehyde within 8 h. These results did not
support the involvement of activated molecular O2 as the real
3
oxidant. Moreover, molecular oxygen accepts one electron
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5RA18151H
Graphical Abstract
Metal Free Visible Light Driven Oxidation of Alcohols to Carbonyls Using 3, 6-
Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) as Catalyst†
Suvendu Samanta and Papu Biswas
A metal-free photocatalytic system for the transformation of alcohols to corresponding
carbonyls in high yields with visible-light irradiation has been achieved. Di(pyridin-2-yl)-
1,2,4,5-tetrazine (pytz) was found to be an efficient catalyst for this oxidation under photo
irradiation conditions.