Cobalt(III) complex catalyzed aerobic oxidation of propargylic alcohols

Article abstract of DOI:10.1055/s-2007-990804

An o-phenylenebis(N′-methyloxamidate) cobalt(III) complex works as an efficient catalyst for the oxidation of propargylic alcohols to the corresponding α,β-acetylenic carbonyl compounds, in the presence of pivalaldehyde, under an atmospheric pressure of m

Full text of DOI:10.1055/s-2007-990804

PAPER
3329
Cobalt(III) Complex Catalyzed Aerobic Oxidation of Propargylic Alcohols
Catalytic
Aerobic
o
O
xidation of
n
P
ropargylic
z
A
lcohols alo Blay, Luz Cardona, Isabel Fernández, José R. Pedro*
Departament de Química Orgànica, Facultat de Química, Universitat de València, 46100 Burjassot (València), Spain
Fax +34(96)3544328; E-mail: jose.r.pedro@uv.es
Received 2 August 2007
sieves in the first system,15 and calcium phosphate–vana-
Abstract: An o-phenylenebis(N¢-methyloxamidate) cobalt(III)
complex works as an efficient catalyst for the oxidation of propar-
gylic alcohols to the corresponding a,b-acetylenic carbonyl com-
pounds, in the presence of pivalaldehyde, under an atmospheric
pressure of molecular oxygen.
date apatite in the second.16 Some isolated examples of
oxidation of propargylic alcohols with other catalytic sys-
tems (only one example of propargylic alcohol in each
case) have been reported with differing results: Ishii and
co-workers used Cu(acac)2-NHPI (NHPI = N-hydroxyph-
thalimide),17 Katsuki and co-workers employed (ni-
troso)(salen)Ru as catalysts,18 Sain and co-workers used a
cobalt phthalocyanine as catalyst19 and Toste and co-
workers employed an oxovanadium complex as catalyst
in the asymmetric oxidation of an acetylenic a-hydroxy
ester.20
Key words: alcohols, alkynes, cobalt complexes, oxidation, homo-
geneous catalysis, oxygen
a,b-Acetylenic carbonyl compounds (ynones) are ex-
tremely interesting substrates because of their versatility
for further synthetic transformations. They have been
used in the synthesis of a variety of biologically active
compounds, such as nucleosides,1 anti-cancer agents,2 sex
pheromones3 and alkaloids,4 among others.5 They have
also been employed as building blocks in the preparation
of aromatic compounds,6 including a full range of hetero-
cyclic derivatives.5,7
NMe4
O
O
N
Y
O
O
N
X
Co
A wide variety of synthetic approaches to the synthesis of
a,b-acetylenic carbonyl compounds have been reported.5
Among them, acylation of alkynyl organometallic re-
agents,8 cross-coupling between terminal alkynes and
acyl chlorides,9 carbonylation of terminal alkynes in the
presence of aryl halides,10 and alkynylation of aldehydes11
followed by oxidation of the resulting propargylic alco-
hols,12 are usually considered to be the methods of choice.
Ligand
Complex
X
Y
I
II
III
NMe
NMe
O
NMe
O
O
Me2opba
Meopba
opba
Figure 1 Structure of cobalt(III) complexes used in this study
Traditionally, oxidation of alcohols have been performed
with high-valent metal oxides or their mineral salts, nota-
bly of middle first-row transition metals.13 In most in-
stances, these inorganic oxidants are required in
stoichiometric amounts and are usually toxic, hazardous
or both. Moreover, purification of the reaction products is
often demanding and laborious. In recent years, different
catalytic methods using small amounts of metal deriva-
tives and clean oxidants have been developed.14 However,
to the best or our knowledge, only a few examples of tran-
sition-metal-catalyzed aerobic oxidation of propargylic
alcohols to the corresponding ynones have been pub-
lished. Uemura and co-workers have described two cata-
lytic systems that work effectively for this purpose with a
number of propargylic alcohols. In both systems, the oxi-
dation is carried out under an atmospheric pressure of mo-
lecular oxygen, using an oxovanadium acetylacetonate
complex as catalyst in the presence of 3 Å molecular
In this context, we have previously reported the prepara-
tion of square-planar cobalt(III) complexes with the o-
phenylenebis(N-methyloxamidate) ligand (Me2opba) and
its oxamate (Meopba) and bis-oxamate (opba) derivatives
(Figure 1), as well as their use as catalysts in the oxidation
of several kinds of organic compounds with oxygen and
pivalaldehyde21 (Mukaiyama’s conditions).22 Herein, we
report a new application of this catalytic system to the ox-
idation of propargylic alcohols (Scheme 1).
OH
O
Co(III) complex, O2
Me3CCHO, solvent
R1
R1
R2
R2
1
2
Scheme 1 Aerobic catalytic oxidation of propargylic alcohols to
ynones
First, the oxidation of 1,3-diphenyl-2-propyn-1-ol (1a)
was examined using the Me2opba cobalt(III) complex (I)
as catalyst and following the experimental protocol re-
ported previously by us.21 Thus, under an atmospheric
SYNTHESIS 2007, No. 21, pp 3329–3332
Advanced online publication: 21.09.2007
DOI: 10.1055/s-2007-990804; Art ID: P09607SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
7

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G. Blay et al.
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Table 2 Oxidation of Propargylic Alcohols 1 with Oxygen and
Pivalaldehyde Catalyzed by Complex I in Acetonitrile
Table 1 Oxidation of 1,3-Diphenyl-2-propyn-1-ol (1a) with
Oxygen and Pivalaldehyde To Give 2aa
Entry Product R1
R2
Time
(h)
Yield
(%)b
Entry
Catalyst
Solvent
MeCN
CHCl3
CH2Cl2
PhF
Time (h)
Yield (%)b
1
2
3
4
5
6
I
8
8
8
8
8
8
93
75
70
52
90
94
1
2
2a
2b
2c
2d
2e
2f
Ph
Ph
8
10
8
93
78
91
92
75
87
85
65
81
60
92
90
92
97
87
88
85
80
82
75
72
82
84
I
2-MeC6H4
3-MeC6H4
4-MeC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-MeOC6H4
3-MeOC6H4
4-MeOC6H4
a-naphthyl
b-naphthyl
a-thienyl
b-thienyl
Ph
Ph
I
3
Ph
I
4
Ph
8
II
III
MeCN
MeCN
5
Ph
10
8
6
Ph
a Reaction conditions: see experimental section.
b Yields refer to isolated pure products.
7
2g
2h
2i
Ph
8
8
Ph
10
8
pressure of molecular oxygen, in the presence of pivalal-
dehyde (aldehyde–propargylic alcohol, 3:1) in acetoni-
trile as solvent, complex I catalyzed the oxidation of the
propargylic alcohol at room temperature, to give 1,3-
diphenyl-2-propyn-1-one (2a) in good yield (Table 1, en-
try 1). When other solvents such as chloroform, dichlo-
romethane or fluorobenzene were used, the conversion of
1a to the corresponding ketone 2a was lower (entries 2–
4), probably due to the lower solubility of the cobalt com-
plexes in these solvents. Finally, we also tested the oxida-
tion of 1a using the Meopba cobalt(III) (II) and the opba
cobalt(III) (III) complexes as catalysts in order to evalu-
ate the effects of the catalyst ligand structure on the cata-
lytic activity. Under identical conditions, in acetonitrile at
room temperature, both complexes II and III gave similar
results to complex I (entries 5–6).
9
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2j
Ph
6
2k
2l
Ph
10
8
Ph
2m
2n
2o
2p
2q
2r
2s
Ph
8
Ph
10
8
PhCH2CH2
4-MeC6H4
4-FC6H4
n-Pr
PhCH2CH2
8
PhCH2CH2
8
Ph
Ph
Me
Me
H
24
24
36
36
10
10
Next, the oxidation of propargylic alcohols 1 was exam-
ined using complex I. Typical results are listed in Table 2.
Among the 1-aryl-3-phenyl-2-propyn-1-ols investigated,
those having a slightly electron-donating methyl group or
an electron-withdrawing chloro substituent at the meta or
para positions on the aryl group (1c, 1d, 1f, 1g) were ef-
ficiently oxidized to give the corresponding ketones in
high yields (entries 3, 4, 6, 7). Oxidation of the corre-
sponding ortho-substituted alcohols (1b, 1e) was slower
and afforded the oxidized products in somewhat lower
yields (entries 2, 5). These results reflect the relative im-
portance of steric effects compared with electronic ones.
However, the oxidation of substrates bearing a strong
electron-donating methoxy substituent on the aromatic
ring gave comparatively lower yields because of the for-
mation of over-oxidized by-products (entries 8–10). Pro-
pargylic alcohols having a naphthyl (1k–l) or thienyl
(1m–n) substituent also gave the corresponding ynones in
excellent yields under these conditions (entries 11–14).
The oxidation of propargylic alcohols having an alkyl
substituent at the 3-position, such as 1-aryl-5-phenyl-2-
pentyn-1-ols 1o–q, gave the corresponding ketones in
very good yields (entries 15–17), regardless of the type of
aromatic ring substituent.
c-Hex
2t
n-C9H19
2u
2v
2x
c-Hex
4-ClC6H4
b-naphthyl
H
a Reaction conditions: see experimental section.
b Yields refer to isolated pure products.
propyn-1-ols 1r–s (entries 18–19) or 1,3-dialkyl-2-pro-
pyn-1-ols 1t–u (entries 20–21), were oxidized to the cor-
responding ketones in good yields, although the reactions
were slower and required the addition of a second load of
pivalaldehyde (aldehyde–propargylic alcohols, 6:1). Fi-
nally, propargylic alcohols bearing a terminal acetylene
moiety, such as 1v–x, also gave good yields of the corre-
sponding ketones with our catalytic system (entries 22–
23).
In summary, we have described here an efficient oxida-
tion of propargylic alcohols to a,b-ynones with molecular
oxygen, using pivalaldehyde (Mukaiyama’s conditions)
and a low load (5 mol%) of a cobalt (III) complex with the
o-phenylene-bis(N¢-methyloxamidate) (Me2opba) ligand
as catalyst. The system works very efficiently with a
On the other hand, propargylic alcohols having an alkyl
substituent at the 1-position, such as 1-alkyl-3-phenyl-2-
Synthesis 2007, No. 21, 3329–3332 © Thieme Stuttgart · New York

PAPER
Catalytic Aerobic Oxidation of Propargylic Alcohols
3331
3-Phenyl-1-(3-thienyl)-2-propyn-1-one (2n)
Oil.
MS (EI): m/z (%) = 212 (100) [M+], 184 (86), 129 (56).
broad range of propargylic alcohols, affording good yields
of the expected ynones.
HRMS (EI): m/z calcd for C13H8OS: 212.0296; found: 212.0265.
1H NMR (CDCl3): d = 8.36 (d, J = 1.8 Hz, 1 H), 7.68–7.64 (m, 2 H),
7.48–7.34 (m, 5 H).
13C NMR (CDCl3): d = 171.4 (s), 142.9 (s), 135.5 (d), 133.0 (d),
130.7 (d), 128.7 (d), 126.80 (d), 126.77 (d), 120.0 (s), 91.3 (s), 87.3
(s).
All melting points are uncorrected. Column chromatography was
performed on silica gel (Merck, silica gel 60, 230–400 mesh). NMR
spectra were recorded on a Bruker Advance 300 DPX spectrometer
(300 MHz for 1H NMR and 75 MHz for 13C NMR) and referenced
to TMS as internal standard. The carbon type was determined by
DEPT experiments. Mass spectra (EI, 70 eV) were recorded on a
Fisons Instruments VG Autospec spectrometer. All starting propar-
gylic alcohols 1 were prepared according to literature proce-
dures.11,23 Data for compounds 2a,8 2b,9c,e 2c,9b 2d,9c,e,10b 2g,5,9c,e
2h,5 2i,5,10a 2j,9b,e 2k,10a,b 2m,9c,e,10b 2r,24 2s,25 2v16 and 2x15b were
consistent with those reported in the literature.
1,5-Diphenyl-2-pentyn-1-one (2o)
Oil.
MS (EI): m/z (%) = 234 (53) [M+], 206 (16), 129 (100).
HRMS (EI): m/z calcd for C17H14O: 234.1045; found: 234.1017.
1H NMR (CDCl3): d = 7.48 (d, J = 6.9 Hz, 2 H), 7.38–7.14 (m,
8 H), 2.95 (m, 4 H).
Catalytic Oxidation of Propargylic Alcohols 1; General Proce-
dure
A solution of propargylic alcohol 1 (0.22 mmol), NMe4⋅Co(III)-
Me2opba complex (5.3 mg, 0.013 mmol) and pivalaldehyde (74 mL,
0.66 mmol) in MeCN (1.5 mL) was stirred under an oxygen at-
mosphere (balloon) until consumption of the starting material was
complete (TLC). H2O (10 mL) was added and the mixture was extrac-
ted with Et2O (3 × 20 mL). The organic layer was washed with sat.
NaHCO3 (10 mL), brine (2 × 10 mL) and dried (MgSO4). The reac-
tion product 2 was obtained after flash chromatography on silica gel
(hexane–EtOAc). Yields are included in Table 2.
13C NMR (CDCl3): d = 186.9 (s), 140.2 (s), 133.0 (d), 130.7 (d),
128.59 (d), 128.52 (d), 128.3 (d), 126.3 (d), 119.8 (s), 91.2 (s), 87.7
(s), 46.9 (t), 29.9 (t).
1-(4-Methylphenyl)-5-phenyl-2-pentyn-1-one (2p)
Oil.
MS (EI): m/z (%) = 248 (31) [M+], 233 (17), 119 (28), 91 (100).
HRMS (EI): m/z calcd for C18H16O: 248.1201; found: 248.1209.
1H NMR (CDCl3): d = 7.80 (d, J = 7.8 Hz, 2 H), 7.26–7.11 (m,
7 H), 2.89 (t, J = 7.0 Hz, 2 H), 2.71 (t, J = 7.0 Hz, 2 H), 2.33 (s,
3 H).
13C NMR (CDCl3): d = 177.8 (s), 144.9 (s), 139.6 (s), 134.4 (s),
129.66 (d), 129.10 (d), 128.55 (d), 128.44 (d), 126.6 (d), 94.8 (s),
80.2 (s), 33.9 (t), 21.7 (q), 21.2 (t).
1-(2-Chlorophenyl)-3-phenyl-2-propyn-1-one (2e)
Oil.
MS (EI): m/z (%) = 242 (31), 241 (16), 240 (100) [M+], 141 (10),
139 (35), 77 (17).
HRMS (EI): m/z calcd for C15H9ClO: 240.0342; found: 240.0339.
1H NMR (CDCl3): d = 8.08 (d, J = 7.2 Hz, 1 H), 7.65 (dd, J = 8.7,
1.8 Hz, 2 H), 7.49–7.38 (m, 6 H).
13C NMR (CDCl3): d = 176.8 (s), 135.8 (s), 133.5 (s), 133.4 (d),
133.1 (d), 132.5 (d), 131.5 (d), 130.9 (d), 128.6 (d), 126.8 (d), 120.0
(s), 93.9 (s), 88.3 (s).
1-(4-Fluorophenyl)-5-phenyl-2-pentyn-1-one (2q)
Oil.
MS (EI): m/z (%) = 252 (19) [M+], 233 (3), 123 (13), 91 (100).
HRMS (EI): m/z calcd for C17H13FO: 252.0950; found: 252.0939.
1H NMR (CDCl3): d = 7.89 (d, J = 8.8 Hz, 1 H), 7.89 (d, J = 8.8 Hz,
1 H), 7.26–7.18 (m, 5 H), 6.99 (t, J = 8.7 Hz, 2 H), 2.90 (t, J = 7.1
Hz, 2 H), 2.74 (t, J = 7.1 Hz, 2 H).
13C NMR (CDCl3): d = 176.4 (s), 166.3 (d, JC–F = 254.9 Hz), 139.5
(s), 133.2 (d, JC–F = 2.9 Hz), 132.2 (d, JC–F = 9.8 Hz), 128.60 (d),
128.47 (d), 126.7 (d), 115.6 (d, JC–F = 21.5 Hz), 95.7 (s), 79.9 (s),
33.8 (t), 21.2 (t).
1-(3-Chlorophenyl)-3-phenyl-2-propyn-1-one (2f)
Mp 212–215 °C.
MS (EI): m/z (%) = 242 (15), 241 (7), 240 (41) [M+], 212 (46), 176
(15), 129 (100).
HRMS (EI): m/z calcd for C15H9ClO: 240.0342; found: 240.0335.
1H NMR (CDCl3): d = 8.17 (t, J = 1.8 Hz, 1 H), 8.11 (dt, J = 7.8, 1.2
Hz, 1 H), 7.71–7.68 (m, 2 H), 7.62–7.58 (m, 1 H), 7.51–7.41 (m,
4 H).
13C NMR (CDCl3): d = 176.5 (s), 138.4 (s), 135.0 (s), 134.0 (d),
133.2 (d), 131.1 (d), 130.0 (d), 129.4 (d), 128.7 (d), 127.7 (d), 119.8
(s), 93.9 (s), 86.5 (s).
1-(n-Nonyl)-2-butyn-1-one (2t)
Oil.
MS (EI): m/z (%) = 195 (1), 194 (0.8) [M+], 82 (100), 67 (71).
HRMS (EI): m/z calcd for C13H22O: 194.1671; found: 194.1664.
1H NMR (CDCl3): d = 2.50 (t, J = 7.5 Hz, 2 H), 2.00 (s, 3 H), 1.63
1-(2-Naphthyl)-3-phenyl-2-propyn-1-one (2l)
Mp 94–95 °C.
MS (EI): m/z (%) = 256 (71) [M+], 228 (100), 129 (74), 101 (16), 75
(17).
(m, 2 H), 1.28 (m, 12 H), 0.86 (t, 3 H).
13C NMR (CDCl3): d = 188.5 (s), 89.8 (s), 80.2 (s), 45.4 (t), 31.8 (t),
29.3 (t), 29.28 (t), 29.20 (d), 28.9 (t), 24.0 (t), 22.6 (t), 14.1 (q), 4.0
(q).
HRMS (EI): m/z calcd for C19H12O: 256.0888; found: 256.0870.
1H NMR (CDCl3): d = 8.70 (s, 1 H), 8.12 (dd, J = 8.7, 1.8 Hz, 1 H),
7.94 (d, J = 7.8 Hz, 1 H), 7.82 (t, J = 8.4 Hz, 2 H), 7.65 (dd, J = 8.1,
1.8 Hz, 2 H), 7.56–7.46 (m, 2 H), 7.42–7.32 (m, 3 H).
1-Cyclohexyl-2-butyn-1-one (2u)
Oil.
MS (EI): m/z (%) = 150 (3) [M+], 111 (15), 83 (66), 67 (34), 57
13C NMR (CDCl3): d = 177.9 (s), 136.1 (s), 134.4 (s), 133.0 (d),
132.6 (d), 132.4 (s), 130.8 (d), 129.9 (d), 129.0 (d), 128.7 (d), 128.5
(d), 127.9 (d), 126.9 (d), 123.9 (d), 120.2 (s), 93.0 (s), 87.0 (s).
(100).
HRMS (EI): m/z calcd for C10H14O: 150.1045; found: 150.1045.
Synthesis 2007, No. 21, 3329–3332 © Thieme Stuttgart · New York

3332
G. Blay et al.
PAPER
(11) Sakai, N.; Kanada, R.; Hirasawa, M.; Konakahara, T.
1H NMR (CDCl3): d = 2.36 (tt, J = 11.0, 3.5 Hz, 1 H), 2.02 (s, 3 H),
1.95 (m, 2 H), 1.80–1.61 (m, 4 H), 1.42–1.24 (m, 4 H).
Tetrahedron 2005, 61, 9298.
(12) (a) Chernick, E. T.; Eisler, S.; Tykwinski, R. R. Tetrahedron
Lett. 2001, 42, 8575. (b) Shun, A. L. K. S.; Chernick, E. T.;
Eisler, S.; Tykwinski, R. R. J. Org. Chem. 2003, 68, 1339.
(c) Lin, C.-F.; Lu, W.-D.; Wang, I.-W.; Wu, M.-J. Synlett
2003, 2057.
(13) Ley, S. V.; Madin, A. In Comprehensive Organic Synthesis,
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(14) (a) Frontiers in Benign Chemical Syntheses and Processes;
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13C NMR (CDCl3): d = 191.7 (s), 90.5 (s), 79.5 (s), 52.2 (d), 28.2 (t),
25.8 (t), 25.4 (t), 4.1 (q).
Acknowledgment
Financial support from Generalitat Valenciana (Grupos 03/168) and
from Universitat de València (Grant UV-AE-20060244) is acknow-
ledged.
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