Efficient Selenium-Catalyzed Selective C(sp3)?H Oxidation of Benzylpyridines with Molecular Oxygen

Article abstract of DOI:10.1002/adsc.201601065

An efficient selenium-catalyzed direct oxidation of benzylpyridines in aqueous DMSO has been successfully developed by using molecular oxygen as the oxidant. A variety of benzoylpyridines with broad functional group tolerance were obtained in modest to excellent yields and with exclusive chemoselectivity. (Figure presented.).

Full text of DOI:10.1002/adsc.201601065

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DOI: 10.1002/adsc.201601065
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Efficient Selenium-Catalyzed Selective C(sp3)ÀH Oxidation of
Benzylpyridines with Molecular Oxygen
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Weiwei Jin,a Poonnapa Zheng,a Wing-Tak Wong,a,* and Ga-Lai Lawa,*
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State Key Laboratory of Chirosciences and Department of Applied Biology and Chemical Technology, The Hong Kong
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Polytechnic University, Hung Hum, Kowloon, Hong Kong
E-mail: wing-tak.wong@polyu.edu.hk; ga-lai.law@polyu.edu.hk
Received: September 25, 2016; Revised: January 17, 2017; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201601065
group also mentioned one special substrate without
Abstract: An efficient selenium-catalyzed direct
oxidation of benzylpyridines in aqueous DMSO has
been successfully developed by using molecular
activators in their work on copper-catalyzed a–meth-
ylenation of benzylpyridines in air.[12] Recently, Gao
and co-workers disclosed the first nonmetallic cata-
oxygen as the oxidant. A variety of benzoylpyr-
lytic oxidation of the sp3 CÀH bonds of benzylpyr-
idines with broad functional group tolerance were
obtained in modest to excellent yields and with
idines with the synergistic H4NIÀAcOH catalyst and
molecular oxygen (Scheme 1c).[13] However, although
exclusive chemoselectivity.
there are several impressive achievements in this area,
there is still a need to explore and develop more
diverse catalytic systems to establish new catalysts to
enhance molecular constructions.
Keywords: selenium catalysis; heterocycles; CÀH
oxidation; molecular oxygen
31 Direct oxidation of inert sp3 CÀH bonds to the
32 corresponding carbonyl groups is one of the most
33 prevailing and reliable approaches for the construction
34 of ketone building blocks.[1] Benzoylpyridines are a
35 class of significant architecture in light of their
36 potential applications in the pharmaceutical indus-
37 try,[2,3] organic transformations[4] and coordination
38 chemistry.[5] Many traditional synthetic methods for
39 these compounds have been established with the aid
40 of transition metal catalysts[6] and stoichiometric
41 quantities of toxic oxidants.[7] In catalytic asymmetric
42 hydrogenation of N-heteroaromatic compounds,
43 brønsted acids and organic halides were introduced as
44 activators to interact with N-containing substrates in
45 order to destroy the aromaticity partially and activate
46 the substrates.[8] The use of molecular oxygen as an
47 oxidant has always been a long-term goal in organic
48 synthesis because molecular oxygen is one of the most
49 inexpensive, available and eco-friendly oxidants.[9] In
50 recent years, some impressive progress with molecular
51 oxygen as oxidants has been made in the direct
52 oxidation of methylene sp3 CÀH bonds.[10–13] In 2012,
53 Maes’ group reported a Cu and Fe-catalyzed oxidation
54 protocol for benzylpyridines based on AcOH as the
Scheme 1. Versatile oxidation routes of benzylpyridines with
molecular oxygen to benzoylpyridines.
Over the past few years, organoselenium catalysis
55 promoter and O2 as the stoichiometric oxidant has emerged as a new and helpful alternative in the
56 (Scheme 1a).[10a] Lei et al. extended a similar idea to methodological development of non-metallic catalytic
57 chloroacetate as the activator (Scheme 1b).[11] Miura’s organic reactions.[14] For example, some reports on the
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Table 1. Optimization of the reaction conditions.[a]
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synthesis of N- and O-heterocyclic compounds via
selenium-catalyzed direct CÀH amination and acylox-
ylation have been well documented separately in
Zhao’s[15] and Breder’s[16] groups. Meanwhile, organo-
selenium chemistry has also received increasing
attention from applied chemists because of its signifi-
cance for sustainable chemistry and industrial poten-
tial.[17] A representative green oxidation procedure
using the polymer resin-supported hexavalent Se
Entry [Se]/
( mol%)
Additive
(equiv.)
Solvent
Conv.
(%)[b,c]
1
2
N-PSP (10) AcOH (1)
DMSO
DMSO
81
89
PhSeCl
(10)
AcOH (1)
10 species as the recyclable heterogeneous catalyst has
11 been successfully established very recently by Yu
12 et al.[17a] As part of our continued effort on CÀH
13 activation and selenium chemistry,[18] we report herein
14 our preliminary studies on selenium-catalyzed direct
15 oxidation of methylene sp3 CÀH bonds to form
16 ketones with molecular oxygen as the oxidant
17 (Scheme 1).
3
PhSeBr
(10)
AcOH (1)
DMSO
100
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5
(PhSe)2 (5) AcOH (1)
DMSO
DMSO
25
21
PhSeH
(10)
AcOH (1)
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PhSeBr (5) AcOH (1)
PhSeBr (3) AcOH (1)
PhSeBr (5) AcOH (1)
PhSeBr (5) TFA (1)
PhSeBr (5) TsOH·H2O
(1)
DMSO
DMSO
DMSO
DMSO
DMSO
100
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18
100
100
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8[d]
9
18
Initially, we selected AcOH as the additive to
19 activate the sp3 CÀH bond of the model substrate 2-
20 benzylpyridine (1a) in the presence of 1 atm O2 as the 10
21 oxidant. It was found that various organic selenium
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PhSeBr (5) PivOH (1)
PhSeBr (5) 1-AdOH (1) DMSO
PhSeBr (5) AcOH (0.5) DMSO
PhSeBr (5) AcOH (1)
PhSeBr (5) AcOH (1)
PhSeBr (5) AcOH (1)
PhSeBr (5) AcOH (1)
PhSeBr (5) AcOH (1)
DMSO
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53
85
9
22 catalysts could participate in this reaction in different
23 levels (Table 1, entries 1–5), but only PhSeBr pro-
24 duced the desired product 2a in quantitative con-
25 version. No adverse effect was observed to the
26 conversion when decreasing the amount of catalyst to
27 5 mol% (Table 1, entry 6). Further lowering the cata-
28 lyst loading to 3 mol% or reducing the reaction
29 temperature to 808C resulted in an obvious detrimen-
dioxane
toluene
DMF
H2O
CF3CH2
OH
30 tal influence on the catalytic activity (Table 1, entries 7
31 and 8). A systematic screening of other Brønsted acids
32 showed that TFA and TsOH·H2O can also catalyze
33 the model reaction in completely different ways
34 (Table 1, entries 9–12). Considering its low price and
35 easy handling, we fixed the acetic acid as the best
36 activating agent. The optimal loading of AcOH was 1
37 equivalent (Table 1, entry 13). Intriguingly, 1a showed
38 quite good reactivity in pure water to yield the
39 corresponding product 2a in 85% conversion (Table 1,
40 entries 14–18). By carefully regulating the ratio of
41 DMSO/H2O to 1:1 as the mixed solvents, we got the
42 best conditions and prepared the target product 2a in
19
PhSeBr (5) AcOH (1)
DMSO/H2 100 (91)
O
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PhSeBr (0) AcOH (1)
PhSeBr (5) AcOH (0)
PhSeBr (0) AcOH (0)
DMSO
DMSO
DMSO
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[a] Reaction conditions: 1a (0.5 mmol), [Se] catalyst, additive,
solvent (0.5 mL), O2 balloon, 100oC, 22 h.
[b] Determined by H NMR analysis.
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[c] Isolated yields in parentheses.
[d] 80oC.
43 91% isolated yield (Table 1, entry 19 and Table S1, either electron-donating (Me, Et, MeO and MeS) or
44 entries 33–37). Blank experiments showed that both electron-withdrawing groups (F, Cl, CN, CF3, CHO,
45 the catalyst PhSeBr and the activator AcOH were COOMe and NO2) at the aromatic rings were
46 indispensable in achieving higher conversions (Ta- introduced to give the desired product 2 in modest to
47 ble 1, entries 20–22). Based on literature reports,[8,11] good yields (2a–o, 2r–t and 2u–w).
48 we also tested the possibility of using organic halides
The electronic effect of the functional groups on
49 as the activator. Although comparative conversions the phenyl ring had notable influence on the reaction
50 could be achieved, only modest isolated yields were efficiency [Eqs. (1) -(3)]. For example, in order to get
51 obtained, probably due to the strong covalent bonds the satisfactory yields of products 2d–f, 10 mol%
52 between the activating groups and substrates (see PhSeBr was necessary. Starting materials that contain
53 Supporting Information, Table S2).
two or three different methylene reaction sites (2b-d)
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With the optimized conditions in hand, the scope were involved in this reaction, and no aldehyde or
55 of the substrates was then investigated to probe the diketone by-products were detected; this highlights
56 generality of this methodology (Table 2). A wide the exclusive chemoselectivity of this method. The
57 variety of 2 or 4-substituted benzylpyridines bearing introduction of a methyl group on the pyridine moiety
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Table 2. Substrate scope of the benzylpyridines.[a,b]
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were executed. A 1:1 mixture of 1a and 1b reacted
with O2 under typical conditions to afford the
oxidative products 2a and 2b in a 53:47 M ratio
[Eq. (1)]. When using 1a and 1h as the starting
materials, the products 2a and 2h were obtained in a
33:67 M ratio [Eq. (2)]. A similar trend was observed
in Eq. (3). These results showed that the electron-
withdrawing group (EWG) in phenyl rings apparently
facilitated the oxidation process by contributing to the
formation of the stable free radical intermediates
during the catalytic cycle. To elucidate the details of
the mechanism, a variety of control experiments were
performed. The model reaction was apparently sup-
pressed at various levels when two equivalents of
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), BHT
(2,6-di-tertbutyl-4-methylphenol) or 1,1-diphenylethy-
lene were added as the radical scavengers, demonstrat-
ing that an organic radical process might be involved
in the catalytic cycle (Scheme 2a). Furthermore, no
simple interaction between TEMPO and PhSeBr was
detected by TLC or GC-MS analysis. Under standard
conditions, the substrate phenyl(pyridin-2-yl) metha-
nol (3a) was converted to the ketone 2a in 87%
isolated yield, showing that the benzyl alcohol might
be an intermediate during this reaction (Scheme 2b).
In an attempt to elucidate this further, we attempted
to make the substrate 2-(phenyl(phenylselanyl)
methyl) pyridine 3b. However, despite having fol-
lowed several synthetic routes from literature, we
were unsuccessful.[19] Hence, the formation of this
intermediate as a possible precursor for the reaction
cannot be excluded completely, although there are no
known literature reports to date. Upon using diphenyl
diselenide as a catalyst, no product was evidenced
even under visible light irradiation (Scheme 2c).[20]
However, when diphenyl diselenide was combined
with catalytic amounts of HBr or Br2 as the additives,
it formed the desired product 2a in almost quantita-
tive conversions (Scheme 2d). We believe this was due
to the function of Br2 or HBr, which caused the easy
transformation of (PhSe)2 to PhSeBr.[21] As mentioned
above, iodide can catalyze the oxidation of sp3 CÀH
bond of benzylpyridines.[13] Therefore, control experi-
ments were conducted in the presence of Br2 or HBr
as the catalysts, where very low conversions (7–20%)
of the product resulted. (Scheme 2e). These results
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[a] Reaction conditions: 1a (0.5 mmol), PhSeBr (5 mol%),
AcOH (0.5 mmol), DMSO/H2O (v/v=1:1, 0.5 mL), O2
balloon, 1008C, 22 h.
[b] Isolated yield.
[c] 10 mol% PhSeBr was used.
47 also allowed the oxidation reaction to smoothly further demonstrated that PhSeBr is the real catalyst
48 furnish the products 2r–t in 58–77% yields. Surpris- behind this catalytic system. Our control experiments
49 ingly, this protocol was able to be extended to 2- using (PhSe)2, Br2 or HBr support the inefficiency of
50 (furan-2-ylmethyl) pyridine 1p and 2-(thiophen-2- these as catalysts in the reaction (Scheme 2c–e).
51 ylmethyl) pyridine 1q, forming the corresponding
Finally, we investigated the possibility of using
52 products 2p and 2q in 80% and 84% yields, respec- PhSe(O)OH, which can be formed by hydration
53 tively. In particular, the structures of 2g, 2m and 2w through moisture and oxidation by O2, as a catalyst.
54 were confirmed by single X-ray structure analysis (see Our studies showed that it was a good oxidant but not
55 Supporting Information).
In order to investigate the reactivity and electronic Considering the fact that only 5 mol% PhSeBr was
57 effect of the substrates, three competition experiments used as the catalyst in our method, these experiments
a good catalyst for the model reaction (Scheme 2f).
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Isotope labeling technique was also used to trace
the origin of the oxygen atom in the final product to
support our mechanism. The model reaction was
carried out under standard conditions in the presence
of 1 atm 17O2 or H2O18. The desired 17O and 18O-
labeled products were not detected by HRMS or GC-
MS analysis, thus manifesting that a rapid isotope
exchange may occur during the MS conditions, which
would obscure the true origin of the oxygen atom
(Scheme 3a–b). An intermolecular secondary kinetic
isotope effect (KIE) of 1.2 was obtained, thus
manifesting that the cleavage of the methylene sp3
CÀH bond might not be involved in the rate-determin-
ing step in this oxidation reaction (Scheme 3c–d).
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Scheme 3. Isotope labeling experiments and kinetic isotope
effect study.
A plausible mechanism is proposed based upon
our above mentioned experimental results and related
literature reports[8,11–12,14] (Scheme 4). We infer that the
reaction is initiated by the formation of 2-benzylpyr-
idinium salt A through the interaction of 2-benzylpyr-
idine 1a with HOAc. Then, A reacts with the PhSe
radical, which comes from the homolytic cleavage of
PhSeBr, to form the key free radical intermediate B.
Oxidation of B with molecular oxygen leads to
another peroxy radical intermediate C followed by
protonation to produce the hydroperoxidate inter-
mediate D. Finally, the release of the HOAc gives the
Scheme 2. Control experiments.
52 ruled out the possibility of PhSe(O)OH as an active target product 2a as well as a molecule of water. The
[17c]
53 catalytic species in the mechanism.
Our studies intermediate D may also be transformed into the
54 also showed that molecular oxygen was critical in the benzyl alcohol derivate E, which undergoes further
55 system to ensure a smooth proceeding of the oxidation oxidation to the ketone 2a.[23]
56 reaction, by contrast, the oxidative ability of H2O2, air
57 and DMSO was insufficient (Scheme 2g–j).[22}
Consequently, we have developed an efficient
selenium-catalyzed direct oxidation of methylene sp3
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Scheme 4. A proposed mechanism.
20 CÀH bonds to form the ketones with molecular
21 oxygen as the oxidant. HOAc was used as the
22 promoter to activate the starting materials. A number
23 of benzoylpyridines with broad functional groups
24 compatibility were obtained in good to excellent yields
25 with exclusive chemoselectivity. A tentative organic
26 free radical procedure was also proposed as the
27 mechanism. Further studies are now in progress in our
28 laboratory to expand this strategy to other useful
29 heterocyclic compounds.
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Experimental Section
33 Typical procedure for the synthesis of 3and 4:
34 synthesis of 3a
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Acknowledgements
The authors acknowledge the financial support from the
Hong Kong Polytechnic University (1BD07) (PolyU 153014/
15P) (PolyU 5096/13P)
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