Visible-Light-Promoted Metal-Free Aerobic Oxidation of Primary Amines to Acids and Lactones

Article abstract of DOI:10.1002/chem.201604440

A unique metal-free aerobic oxidation of primary amines via visible light photocatalytic double carbon–carbon bonds cleavage and multi carbon–hydrogen bonds oxidation was observed. Aerobic oxidation of primary amines could be controlled to afford acids by using dioxane with 18 W CFL, and lactones by using DMF with 8 W green LEDs, respectively. A plausible mechanism was proposed based on control experiments. This observation showed direct evidences for the fragmentation in the aerobic oxidation of aliphatic primary amines.

Full text of DOI:10.1002/chem.201604440

A Journal of
Accepted Article
Title: Visible-Light-Promoted Metal-free Aerobic Oxidation of Primary Amines to
Acids and Lactones
Authors: Xiaokai Cheng; Bo Yang; Xingen Hu; Qing Xu; Zhan Lu
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Visible-Light-Promoted Metal-Free Aerobic Oxidation of Primary
Amines to Acids and Lactones
Xiaokai Cheng,[a,b] Bo Yang,[a] Xingen Hu,[b] Qing Xu,[b] and Zhan Lu*[a]
Dedication ((optional))
Abstract: A unique metal-free aerobic oxidation of primary amines
via visible light photocatalytic double carbon-carbon bonds cleavage
and multi carbon-hydrogen bonds oxidation was observed. Aerobic
oxidation of primary amines could be controlled to afford acids by
using dioxane with 18W CFL, and lactones by using DMF with 8W
green LEDs, respectively. A plausible mechanism was proposed
based on control experiments. This observation showed direct
evidences for the fragmentation in the aerobic oxidation of aliphatic
primary amines.
Selective oxidation is one of the most fundamental reactions in
organic synthesis, as well as one of the most critical challenges
facing the chemical industry.[1] In most cases, oxidizing agents
are required in stoichiometric amounts which are usually toxic or
hazardous. Owing to increasing demand on environmental
Scheme 1. Diverse products from aerobic oxidation of aliphatic primary
protection and decreasing pollution, dioxygen is regarded as an
amines.
ideally clean and green terminal oxidant for catalytic oxidations.
[2]
2a was observed in 18% yield. It should be noted that the
reaction should undergo C-C bond oxidative cleavage to lose
one carbon atom from an amine to an acid. Compared to aerobic
oxidation of oxygen-containing cyclic compounds[10] to provide
the corresponding ring-opening products without losing any
carbon atom, this aerobic oxidation of primary amines is unique.
Aerobic oxidations of primary amines[3] under thermal and
photosensitized conditions have been proven as powerful
methods for synthesis of diverse products, such as imines,[4]
nitriles,[5] amides,[6] and oximes[7] (Scheme 1). However, to the
best of our knowledge, the unselective auto-oxidation of aliphatic
primary amines with dioxygen to complicated fragmentation
products has still not been well explored and is a remaining
problem. We are particularly interested in visible-light-promoted
Various photosensitizers such as [Ir(ppy)2(dtbbpy)](PF6),
Ir(ppy)3, Eosin Y, Mes-acridine and Rose Bengal (RB) were
screened (entries 2-6) among which Rose Bengal[11] could
catalyze the reaction to afford 2a in 49% yield. When 2.0 equiv of
HCOOH and 2,6-lutidine was added in the reaction no significant
increase in yields were observed (entries 7 and 8). Increasing the
reaction to 24 h, the yield was up to 58% (entry 9). Among
different solvents, such as MeOH, THF, DCM, CH3CN, acetone
and dioxane (entries 11-16), dioxane was the best solvent to give
2a in 72% yield. Control experiments without O2 or light did not
afford any desired products (entries 17and 18).
nitrogen radical chemistry.[8] Here, we developed
a
visible-light-promoted metal-free aerobic oxidation of amines to
acids or lactones via double C-C bonds cleavage and multi
carbon-hydrogen bonds oxidation.
The primary amine 1a was chosen as a model substrate which
was proposed to undergo intramolecular C-N bond formation via
one electron oxidation of amine. The reaction of 1a in the
[9]
presence of Ru(bpy)3(PF6)2 in a solution of DMF with an
oxygen balloon under the irradiation of 18W CFL did not afford
any C-N bond formation product, however, an unexpected acid
With standard conditions in hands, the scope was shown in
Table 2. Various substrates with electron-rich and electron-poor
substituents on the phenyl ring were suitable to give the
corresponding acids 2b-j in 50~71% yields. The reaction of
anti-1d could also occur to afford 2d in a slightly low yield.[12] The
TBS-protected phenol and benzyl alcohol could undergo the
oxidation reaction to afford 2k and 2l in 56% and 63% yield,
respectively. The free benzyl alcohol was tolerated to afford 2m
in 46% yield. The diol-protected or unprotected ketones were
also survived to deliver 2n and 2o in 55 and 54% yield. Other
polycyclic rings and heterocycles such as 2-naphenyl (2p),
1,3-benzodioxole (2q) could be converted in 45~64% yields. The
reaction of 2-(5’-indyl)cycloamine (1r) was slow under standard
conditions, however, affored 2r in 36% yield under 8W green
LEDs probably due to the power density of light sources. The
[a]
[b]
X. Cheng, B. Yang, Prof. Z. Lu
Department of Chemistry, Zhejiang University
Hangzhou, Zhejiang 310058, China
E-mail: luzhan@zju.edu.cn
X. Cheng, Prof. X. Hu, Prof. Q. Xu
College of Chemistry and Materials Engineering, Wenzhou
University
Wenzhou 325035, P. R. China
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Optimization of Aerobic Oxidation of Primary Amines to Acids[a]
Table 2. Aerobic oxidation of amines to acids[a]
Entry
Photocatalyst
Solvent
t (h)
Yield[b] of
2a (%)
1
Ru(bpy)3(PF6)2
[Ir(ppy)2(dtbbpy)]PF6
Ir(ppy)3
DMF
12
12
12
12
12
12
12
12
24
36
24
24
24
24
24
24
24
24
18
0
2
DMF
3
DMF
9
4
Eosin Y
DMF
8
5
Mes-acridine
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
DMF
0
6
DMF
49[c]
42
45
58[c]
53[c]
0
7[d
8[e]
9
DMF
DMF
DMF
10
11
12
13
14
15
16
17[f]
18[g]
DMF
MeOH
THF
51[c]
18
32
16
72[c]
0
CH2Cl2
CH3CN
acetone
dioxane
dioxane
dioxane
[a] Standard conditions A: using 1 (0.4 mmol) and Rose Bengal (2 mol%)
in dry dioxane (8 mL) with O2 balloon under the irradiation of 18W CFL for
24 h at 25 oC, average yields run twice. [b] For 60 h. [c] For 48 h. [d] With
8W green LEDs for 24 h.
0
Table 3. Aerobic oxidation of amines to acids[a]
[a] Unless otherwise noted, all reactions were conducted using 1 (0.4 mmol)
and photocatalyst (2 mol%) in dry solvent (8 mL) with an O2 balloon under the
irradiation of a 18 W fluorescent lamp. [b] Yields determined by 1H NMR using
TMS-Ph as an internal standard. [c] Isolated yields. [d] HCOOH (2.0 equiv) was
added. [e] 2,6-lutidine (2.0 equiv) was added. [f] Under an atmosphere of N2.
[g] Without light.
2-phenyl cyclohexylamine 1s with ethyl substitution on the linker
could participate to give 2s in 50% yield.
[a] Standard conditions B: using 1 (0.4 mmol) and Rose Bengal (2
mol%) in dry DMF (8 mL) with O2 balloon under the irradiation of 8W
green LEDs at 47 oC for 48 h.
To our surprise, the reaction of 1a using DMF as a solvent
under the irradiation of 8W green LEDs afforded the lactone 3a in
42% yield. Although the yield of this transformation was a lightly
low, this both solvent and light source-controlled[13] phenomenon
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(see Table S1 in SI) is unexpected and interesting to lead to a
significant increase in diversity.
Under the conditions B, the acid 2i could be converted to the
lactone 3i in 32% yield (eq.5, also see Table S2).[16]
The scope of the oxidation to lactones under conditions B
was shown in Table 3. The reaction of cyclohexyl amines with
both electron-rich and electron-poor substitutions on the phenyl
ring gave the corresponding lactones 3a-i in 27~56% yields. The
TBS-protected phenol gave the deprotected lactones 3k` in 24%
yield. The TBS-protected benzyl alcohol and diol-protected
ketone could be tolerated to the corresponding lactones 3l and
3n in 29% and 32% yield, respectively. Amines with heterocycles
such as 2-naphthalene (1p), 1,3-benzodioxole (1q) and indole
(1r) could be delivered to lactones in 17-40% yields.
Based on above results, the plausible mechanism was
proposed in Figure 1. Under irradiation of visible light, RB could
absorb the visible light to form photoexcited state RB* which
could undergo single electron transfer (SET) reaction to oxidize
the amine, generating nitrogen radical cation A and radical ion
RB-.[17] The radical ion RB- could be oxidized by dioxygen to
regenerate RB and produce dioxygen radical anion. The nitrogen
radical cation A underwent a radical ring-open fragmentation
process[18] to generate a distonic ε-radical iminium cation
intermediate B which was quenched with dioxygen radical anion,
forming a pair of tautomers C and D.[19] A [2+2] cycloaddition of D
with singlet dioxygen which was generated through energy
transfer (ET) from photoexcited state RB*[20] could lead to
dioxetane E. Another plausible pathway was that D could be
oxidized by photoexcited state RB* and then trapped by dioxygen
radical anion to form the dioxetane E. The fragmentation reaction
of E gave the aldehyde F which could be oxidized to the acid.[21]
The acid was isomerized to enol configuration which could be
photo-peroxidized by singlet dioxygen to the peroxide A’. In this
plausible pathway, the amine might accelerate the isomerization
and oxidation reactions via formation of imine/enamine
intermediates. An intramolecular nucleophilic substitution of the
peroxide A’ led to the lactone and released the side product
hydroperoxide. The difference between 18 W CFL and green
LEDs might be the light intensity which might affect the efficiency
of activating the photosensitizer to undergo single electron
transfer and energy transfer processes.
Using hydroxyl, carbonyl or diol-protected carbonyl groups
instead of amino group, no reaction occurred which indicated that
these oxygen-containing substrates could not be oxidized under
this mild conditions (Scheme 2).
Scheme 2. The unreacted oxygen-containing substrates
To demonstrate the mechanism of both novel transformations,
several control experiments were carried out. The reaction of
tertiary N,N-dimethylamine 7 could undergo smoothly to give 2a
in 16% yield (eq.1). This suggested that the initial step of the
reaction might be single electron transfer (SET) oxidation of
amine to nitrogen radical cation.[14] The reaction in the presence
of a radical trapper TEMPO did not occur with 78% recovery of
1a (eq.2). This was also a good evidence for single election
oxidation of amine. The reaction of aldehyde 8 with piperdine
afforded benzylic acid (eq.3). The similar transformation has
been reported through a visible-light promoted C-C cleavage by
forming imine/enamine intermediates.[15] In the presence of
cyclohexyl amine, lactone could be obtained from 8 in which the
primary amine might accelerate the formation of the
imine/enamine intermediates that could be easily oxidized (eq.4).
Figure 1. Proposed mechanism for aerobic photooxidation of primary amines
to acids and lactones
In summary, we developed
a
visible-light-promoted
metal-free aerobic oxidation of primary amines to acids and
lactones via double carbon-carbon bonds cleavage and multi
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carbon-hydrogen bonds oxidation. Aerobic oxidation of primary
amines could be controlled to afford acids by using dioxane with
18W CFL, and lactones by using DMF with 8W green LEDs,
respectively. A primary mechanism was proposed based on
control experiments. This observation would give some
fundamental evidences for systematical studies of amine
oxidation, particularly for fragmentation processes.
Experimental Section
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Standard conditions A: To an overdried reaction tube equipped with a
magnetic stir bar under O2 atmosphere was added Rose Bengal (0.0082
g, 0.008 mmol, 2 mol%), 1 (0.400 mmol, 1.0 equiv) and dioxane (8 mL,
0.05 M). The reaction was stirred under irridation of 18 W CFL in a
distance of 10 cm at room temperature for 24 h. Upon completion, the
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100/50/1) to afford the corresponding acids.
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Acknowledgements
[12] The oxidation potentials of 1d and anti-1d vs SCE are 0.69 V and 0.84 V
by Cyclic Voltammetry using Bu4NClO4 as supporting electrolyte,
Ferrocene as an internal standard and DMF as solvent, also se Table
S3 in supporting information. The reaction of anti-1d under 8W green
LEDs afforded 2d in 44% yield.
Financial support was provided by the National 973 Program
(2015CB856600) and NSFC (21472162, 21573161). We also
thank Mr. Yejun Lin for the initial screen.
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Keywords: aerobic oxidation • amines • carbon-carbon bond
cleavage • visible light • acids
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Entry for the Table of Contents (Please choose one layout)
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Xiaokai Cheng,a,b Bo Yang,a Xingen Hu,b
Qing Xu,b and Zhan Lua*
Page No. – Page No.
Visible-Light-Promoted Metal-Free
Aerobic Oxidation of Primary Amines
to Acids and Lactones
Light controlled: A unique metal-free aerobic oxidation of primary amines via
visible light photocatalytic double carbon-carbon bonds cleavage and multi
carbon-hydrogen bonds oxidation was observed. Aerobic oxidation of primary
amines could be controlled to afford acids by using dioxane with 18W CFL, and
lactones by using DMF with 8W green LEDs, respectively. A plausible mechanism
was proposed based on control experiments. This observation showed direct
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