Oxidative rearrangement of internal alkynes to give one-carbon-shorter ketones via manganese porphyrins catalysis

Article abstract of DOI:10.1021/jo400209p

Oxidative rearrangement of internal alkynes catalyzed by manganese(III) porphyrin is described, which opens a new access to one-carbon-shorter ketones using molecular oxygen. Under the standard conditions, a variety of alkynes including diarylalkynes and arylalkylalkynes rearranged smoothly to the corresponding ketones in high yields. Based upon experimental observations, a plausible reaction mechanism is proposed.

Full text of DOI:10.1021/jo400209p

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pubs.acs.org/joc
Oxidative Rearrangement of Internal Alkynes To Give One-Carbon-
Shorter Ketones via Manganese Porphyrins Catalysis
Wen-Bing Sheng,†,‡ Qing Jiang,† Wei-Ping Luo,† and Can-Cheng Guo*,†
†College of Chemistry and Chemical Engineering, Hunan University, Changsha Hunan 410082, PR China
‡College of Pharmacy, Hunan University of Chinese Medicine, Changsha Hunan 410208, PR China
S
* Supporting Information
ABSTRACT: Oxidative rearrangement of internal alkynes catalyzed by manganese-
(III) porphyrin is described, which opens a new access to one-carbon-shorter ketones
using molecular oxygen. Under the standard conditions, a variety of alkynes including
diarylalkynes and arylalkylalkynes rearranged smoothly to the corresponding ketones in high yields. Based upon experimental
observations, a plausible reaction mechanism is proposed.
he oxidation of internal alkynes has received considerable
under 2 MPa of O2 for 4 h, benzophenone 2a, which was
Tattention for a long time because they can be converted to similar to the products of Wolff rearrangement,2d,12b was
obtained in 54% yield based on 1a (Table 1, entry 1). The
a wide variety of valuable compounds such as α,β-unsaturated
ketones,1 α-diketones,2 1,2-diols,3 or cleavage products of the
triple bond4−6 that occur ubiquitously in natural and synthetic
bioactive molecules. Over the past several decades, the
stoichiometric oxidation of alkynes using organic peracids,
thalium nitrate, ruthenium and osmium tetraoxides, permanga-
nate, peroxomonophosphoric acid peroxomolybdenum com-
plexes, and dioxiranes as oxidative reagents has been widely
developed.5,6 However, these are often stoichiometric, hazard-
ous, expensive, and difficult to remove from the reaction
solution. Recently, in the context of green and sustainable
chemistry, transition-metal-catalyzed oxyfunctionalization of
alkynes has emerged as an attractive alternative.7 Despite
considerable progress in the field, examples of catalytic methods
are still limited: these established methods require the use of
expensive transition-metal catalysts (often Pd and Ru).
Consequently, it is still highly desirable to develop novel
approaches for the oxyfunctionalization of alkynes.
a
Table 1. Optimization of Reaction Conditions
b
c
T
convn
(%)
yield
(%)
entry
catalyst
solvent
(°C)
oxidant
O2
1
2
TPPMnCl
TPPMnCl
TPPMnCl
TPPMnCl
TPPMnCl
TPPMnCl
TPPMnCl
TPPMnCl
none
n-hexane
n-hexane
DMF
110
150
150
150
150
150
150
150
150
150
60
94
<1
<1
<1
<1
<1
<1
<1
0
54
85
O2
3
O2
trace
trace
trace
trace
trace
trace
trace
0
4
MeCN
O2
5
benzene
n-hexane
n-hexane
n-hexane
n-hexane
n-hexane
O2
6
H2O2
PhIO
PhI(OAc)2
O2
7
8
9
10
TPPMnCl
N2
Hydrocarbon functionalization mediated by transition-metal
complexes based on porphyrins is of great interest in
biomimetic investigation and is potentially useful in organic
synthesis. The protocol of “metalloporphyrins/oxygen” used
for mono-oxygenation of saturated C−H and epoxidation of
olefin has been documented in many reports,8 and mono-
oxygenation of saturated C−H in alkane has been applied to
large-scale industrial processes.9 However, to the best of our
knowledge, metalloporphyrins/oxygen used for the oxidation of
internal alkynes has not been established. Herein, we report the
first example of the oxidative rearrangement of alkynes using
molecular oxygen as oxidant catalyzed by manganese(III)
porphyrin.
Inspired by recent reports that metalloporphyrin can be used
to catalyze epoxidation of alkenes with O2,8 we envision a
metalloporphyrin-based strategy could be applied to alkynes.
When the reaction of 1,2-diphenylacetylene 1a was carried out
in the presence of 10−6 mol m-tetraphenylporphyrin
manganese(III) chloride (TPPMnCl) in n-hexane at 110 °C
a
Unless otherwise noted, the reaction conditions were as follows: 1a
(1 mmol), catalyst (10−6 mol), solvent (5.0 mL), pressure (2 MPa),
reaction time (4 h). Determined by GC. Isolated yield based on 1a.
b
c
desired product 2a was obtained in 85% isolated yield when the
reaction temperature was increased to 150 °C (Table 1, entry
2). Changing the solvent to DMF, CH3CN, or benzene
afforded a trace amount of product (Table 1, entries 3−5),
which were consistent with the results that metalloporphyrins
could not activate O2 effectively in polar solvent.10 In addition,
some other oxidants, such as H2O2, PhIO, and PhI(OAc)2,
provided only a trace amount of the desired product (Table 1,
entries 6−8). The controlled experiment also showed that no
reaction occurred in the absence of either catalyst or O2, thus
suggesting that a metalloporphyrins/O2 combination is
Received: February 2, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo400209p | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
required for the success of the reaction (Table 1, entries 9 and
10). On the basis of these results, we decided to set heating the
alkynes at 150 °C in the presence of 10−6 mol TPPMnCl in n-
hexane under 2 MPa of O2 as our standard conditions.
Under the optimized reaction conditions, we then examined
the scope and limitations of the reaction, and the results are
summarized in Table 2. We found that the reaction worked
the rearranged product obtained, we introduced α-naphthyl
phenyl ketone, the proposed oxidative rearrangement inter-
mediate of α-naphthyl phenylacetylenes, to the controlled
experiment, and β-naphthyl phenyl ketone was obtained. This
showed that the β-naphthyl phenyl ketone, the reaction
product of α-naphthyl phenylacetylene, was probably generated
via the rearrangement of α-naphthyl phenyl ketone formed in
situ under the standard conditions. The results of aryl alkynes
were all favorable; the reactions of aliphatic alkynes were messy.
For example, the reaction of 4-octyne gave only a small amount
of rearrangement product (Table 2, entry 13). It was interesting
that one of the primary propyl groups that connected with the
C−C triple bond was changed to isopropyl connected on the
ketone, which was identified by GC−MS coinjection of
commercially available authentic samples. Compared with the
other ketones obtained, 2i and 2m were different from the
others. Moreover, no reaction occurred when 1-trimethylsilyl-1-
hexyne was subjected to the same reaction conditions (Table 2,
entry 12). Unfortunately, phenylacetylene, which is a terminal
alkyne, gave a complex mixture and only a trace amount of the
desired product was detected by GC−MS (Table 2, entry 11).
Though the exact mechanism is still not clear at present,
some information has been gathered: (1) When the reaction of
1a was carried out in the presence of 1.5 equiv of radical
inhibitor TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), the
desired product 2a was not detected in an appreciable amount,
thus suggesting that a radical mechanism could be involved. (2)
When the competitive reaction of 1e and 1n was conducted
under the same reaction conditions (Scheme 1, eq 1), only
rearrangement products 2e and 2n were detected by GC−MS
in addition to CO2, and no cross-rearrangement products were
detected. This result suggested that the rearrangement is strictly
an intramolecular process and one carbon atom in the triple
bond was oxidized to CO2. (3) Benzil 3 and 2-diphenylacetic
acid 4 seemed not to serve as intermediates in the reaction, as
these species were introduced into the standard conditions and
only a trace amount of 2a could be detected (Scheme 1, eqs 2
and 3).
Table 2. TPPMnCl-Catalyzed Oxidative Rearrangement of
a
Various Internal Alkynes
a
Reaction conditions: 1 (1 mmol), TPPMnCl (10−6 mol) in 5 mL of
b
n-hexane under 2 MPa of O2 at 150 °C for 4 h. Determined by GC.
c
d
Isolated yield based on 1. Detected by GC−MS.
very well for a wide variety of internal alkynes including
diarylalkynes and arylalkylalkynes, affording the desired ketones
in yields ranging from 49% to 85% (Table 2, entries 1−10).
Diarylalkynes bearing both electron-donating and electron-
withdrawing groups on the phenyl group provided good results
(Table 2, entries 6−8). The results indicated that the yield of
the reaction was not very sensitive to the electron density of the
phenyl group. The reaction of α- and β-naphthyl phenyl-
acetylenes also worked well, furnishing the same ketone, β-
naphthyl phenyl ketone 2i in moderate yields (Table 2, entries
9 and 10). It was noteworthy that the reaction of α-naphthyl
phenylacetylene gave β-naphthyl phenyl ketone, which was not
consistent with the above results. In order to understand how
In a metalloporphyrin/O2 system, the active species are
recognized to be high-valent metal-oxo species ([TPPMnIV
O]).11 On the basis of these results and related reports,2d,12
a
plausible mechanism is proposed in Scheme 2. Under the
reaction conditions, TPPMnCl reacted with O2 to give
intermediate [TPPMnIVO], which was then trapped by
substrate A to produce oxirene B,2d,12a,d its subsequent
rearrangement afforded C,12b,dand C was further oxidized to
give the desired ketone and released carbon dioxide.
Scheme 1. Investigation of Possible Reaction Intermediate
B
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Scheme 2. Proposed Reaction Mechanism for the Oxidation
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EXPERIMENTAL SECTION
■
General Comments. All reagents and solvent used were obtained
commercially and used without further purification unless indicated
otherwise. 1-chloro-4-(2-phenylethynyl)benzene (1f),13a 1-nitro-4-(2-
phenylethynyl)benzene (1g),13a 1-(2-(4-methoxyphenyl)ethynyl)-
benzene (1h),13a 1-(2-phenylethynyl)naphthalene (1i),13b and 2-(2-
phenylethynyl)naphthalene (1j),13b and 1-chloro-4-(1-propynyl)-
benzene (1n)13b were prepared according to literature procedures.
All products were characterized by IR, MS, H NMR, 13C NMR, and
1
elementary analysis. IR spectra were recorded on a FT-IR
spectrometer. Mass spectra were measured on a mass instrument
(EI). Analyses of the conversion of reagent were performed by gas
phase chromatography, using a RX-5 capillary column and a frame
ionization detector (FID). An elementary analyzer was used. 1H NMR
spectra were recorded on 400 MHz in CDCl3, and 13C NMR spectra
were recorded on 100 MHz in CDCl3 using TMS as internal standard.
Copies of 1H NMR and 13C NMR spectra are provided as Supporting
Information.
Typical Procedure for the Aerobic Oxidative Rearrange-
ment of Internal Alkynes Catalyzed by TPPMnCl. A 10-mL
homemade high-pressure kettle equipped with magnetic stirring bar
was charged with 1 mmol alkynes, 10−6 mol TPPMnCl, and n-hexane
(5 mL). The resulting mixture was stirring at 150 °C under 2 MPa of
oxygen. After being stirred for the time indicated, the mixture was
cooled to room temperature. Purification of the crude product by flash
chromatography on silica gel using the indicated solvent system
afforded the desired product.
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ASSOCIATED CONTENT
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S
* Supporting Information
1
Copies of H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-731-88821488. Fax: +86-731-88821488. E-mail:
ccguo@hnu.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (J1210040, J1103312) and the Youth
Found of Hunan University of Chinese Medicine (99820001).
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dx.doi.org/10.1021/jo400209p | J. Org. Chem. XXXX, XXX, XXX−XXX