Selective oxidation of acetylenic 1,4-diols with dioxiranes in comparison with the methyltrioxorhenium-hydrogen peroxide oxidant

Article abstract of DOI:10.1016/j.tetlet.2004.09.110

Dimethyldioxirane (DMD) and its trifluoro analog (TFD) were employed to achieve selectively the direct transformation of diol 3a and of diol 3b into the corresponding carbonyls. The results are compared with oxidations using methyltrioxorhenium (MTO)/85%

Full text of DOI:10.1016/j.tetlet.2004.09.110

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 8575–8578
Selective oxidation of acetylenic 1,4-diols with dioxiranes
in comparison with the methyltrioxorhenium–hydrogen
peroxide oxidant
Lucia D’Accolti, Michele Fiorentino, Caterina Fusco, Pasquale Crupi and Ruggero Curci*
`
Dipartimento Chimica, Universita di Bari, v. Amendola 173, I-70126 Bari, Italy
C.N.R.—Istituto di Chimica dei Composti Organometallici (ICCOM), Bari Section, Italy
Received 6 July 2004; revised 13 September 2004; accepted 15 September 2004
Available online 2 October 2004
Dedicated to Professor Giorgio Modena (University of Padova, Italy) on the occasion of his 80th birthday
Abstract—Dimethyldioxirane (1a) and its trifluoro analog (1b) were employed to achieve selectively the direct transformation of
hex-3-yne-2,5-diol 3a and 1,4-diphenyl-but-2yne-1,4-diol 3b (two representative acetylenic 1,4-diols) into the corresponding carbon-
yls, leaving the carbon–carbon triple bond moiety untouched. The results are compared with those recorded in the analogous oxi-
dation using the methyltrioxorhenium (MTO)/85% H2O2 homogeneous system. The powerful methyl(trifluoromethyl)dioxirane (1b)
is the reagent of choice to achieve optimum yields of the target alkyne-1,4-diones, which are extremely versatile synthons.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Simple acetylenic 1,4-diones are versatile building blocks
in cycloaddition reactions,1 for the synthesis of various
heterocyclic compounds,2 as well as for the preparation
of other compounds of interest.3 Thus, considerable
effort has been devoted to the preparation of conjugated
acetylenic diketones based on a wide variety of synthetic
approaches4 including multistep synthesis.5 Besides
these methods, particularly attractive is the one-step
selective oxidation of acetylenic 1,4-diols since these
substrates are readily available.6 This transformation
has been attempted previously with varying success.
For instance, Li et al. have reported the direct radical
a-oxidation of alkynes using Cu2+/t-BuOOH under oxy-
gen; they found that, in order to obtain acetylenic 1,4-
diketones (e.g., hex-3-yne-2,5-dione) in fair yield (ca.
15%), the corresponding acetylenic diols have to be
employed as the starting material.7 As part of a contin-
uing effort to devise selective oxidations of key organic
compounds, we decided to apply two novel oxidants,
namely dioxiranes 18 and the methyltrioxorhenium–
probe into the direct transformation of representative
acetylenic diols into the corresponding carbonyls.
The catalytic cycle for oxidations with CH3ReO3[MTO]/
H2O2 involves, besides the monoperoxo complex 2a, the
rhenium(VII) diperoxocomplex 2b as the active spe-
cies;9,10 the latter became well characterized crystallo-
graphically.10a
Both dioxiranes and the MTO/H2O2 system have been
the focus of much interest as oxidation catalyst in recent
years. Numerous transformations have been reported
for these versatile, highly reactive, and yet selective oxi-
dants;8–10 these include the recently reported oxidation
of 1,2-diols to the corresponding 1,2-diketones by
MTO/H2O2.10e The ever increasing number of their
applications in synthesis has spurred intensive mechanistic
studies of the related oxygen-transfer processes.8–10 Akin
to peracids11 and metal peroxides,12 both dioxiranes13
9
hydrogen peroxide reagent, CH3ReO3(MTO)/H2O2 to
Keywords:
Dioxiranes;
Dimethyldioxirane;
Methyl(trifluoro-
methyl)dioxirane; Methyltrioxorhenium; Hydrogen peroxide; Acetyl-
enic 1,4-diols; Oxidation.
*
Corresponding author. Tel.: +39 080 544 2070; fax: +39 080 544
2924; e-mail: csmirc17@area.ba.cnr.it
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.09.110

8576
L. D’Accolti et al. / Tetrahedron Letters 45 (2004) 8575–8578
14
and MTO/H2O2
have been found to allow the
majority of cases). While substrate conversion is only
35% during 12h with in situ DMD (entry 4), this can
be considerably improved using caroate/1,1,1-trifluoro-
propanone (in situ TFD), yielding acetylenic ketol 5a
in 90% isolated yield, with 75% substrate conversion
occurring in just 1.2h (entry 9).
conversion of isolated alkynes into a-diketones and/or
a,b-unsatured carbonyls, via intermediate a-ketocarb-
enes.11–14
We have now applied both MTO/H2O2 and dioxiranes
(either isolated or generated in situ)8 to the direct oxida-
tion of two representative acetylenic diols, namely the
commercially available hex-3-yne-2,5-diol 3a and 1,4-
diphenyl-but-2yne-1,4-diol 3b; an authentic sample of
the latter could be synthesized by following literature
procedures (Scheme 1).15
High substrate conversions with short reaction times
(0.3–6h) are also generally accomplished using isolated
dioxiranes as stoichiometric oxidants (entries 7, 8 and
10–12). Using DMD 1a, both the acetylenic ketol 5
and the diketone 4 are produced (entries 6–8). While
the ketol 5 is still the largely prevalent product using
DMD in CH2Cl2 (entry 6), the balance can be changed
to favor formation of the diketone (its sequential oxida-
tion product) upon changing the solvent from CH2Cl2
to acetone (entries 7 and 8). The yield in the desired di-
ketones 4a,b climbs to a remarkable 90–96% using the
highly reactive TFD in acetone/TFP (entries 11 and
12); substrate conversion is practically total in a matter
of minutes, rather than hours. It is interesting to note
that, when the oxidation of acetylenic diol 3a is carried
out with TFD in CH2Cl2, besides the diketone 4a as the
major product, a sizable amount (ca. 18%) of diketol 6a
Dimethyldioxirane (DMD) (1a)16 and methyl(trifluoro-
methyl)dioxirane (TFD)17 (1b) solutions [0.08–0.1M in
acetone, and 0.7–0.8M in 1,1,1-trifluoropropanone
(TFP), respectively] were prepared as already reported
in detail.
For the MTO catalyzed oxidation of the substrates at
hand, we adopted the established homogeneous system
using 85% H2O2 as bulk oxidant in methylene chlo-
ride.18 It is well recognized that under the conditions
adopted herein (i.e., with over a 50-fold excess of the
oxygen donor H2O2 relative to the rhenium catalyst),
the diperoxo complex 2b is largely prevalent as the
active oxygen-transfer agent, and practically no
MTO is present in solution.18 This was confirmed by
our own 1H and 13C NMR control experiments in
CD2Cl2.19
Data collected in Table 1 are representative of the oxida-
tion of acetylenic diols 3a,b using dioxiranes and MTO/
H2O2.20 Inspection of these reveals that either the MTO/
H2O2 (entry 1–3) and the ketone/caroate system that is
in situ dioxirane (entry 4 and 9) are capable to afford
catalytically the acetylenic ketols in good to excellent
yield with reasonable reaction times (12–30h, in the
Scheme 1.
Table 1. Oxidation of two representative acetylenic 1,4-diols with methyltrioxo-rhenium(VII)/H2O2 and with in situ generated or isolated dioxiranes
No.
Oxidant
Methoda
Substrate
t, ꢁC
Ratio Ox/Sb
Reaction time (h)
Convn %
Product (yield %)
Diketone
Ketol
1
MTO/H2O2
A
3a
3a
3b
3a
3a
3a
3a
3b
3a
3a
3a
3b
20
20
20
20
0
6
20
20
10
1
20
30
30
12
3
90
95
60
35
60
95
95
95
75
95
95
95
—
—
5a (88)c
5a (80)c
5b (90)c
5a (90)c
5a (98)c
5a (95)d
5a (20)d
5b (30)d
5a (90)d
—
2
A
3
A
A0
—
—
4
DMD (1a)
TFD (1b)
5
B
—
6
B
0
3
6
4a (3)c
4a (65)d
4b (70)d
—
7
B00
B00
A
0
3
6
8
0
3
6
9
0
10
3
1.2
0.5
0.3
0.3
10
11
12
B00
B000
B000
0
4a (74)d,e
4a (90)d
4b (96)d
0
3
—
—
0
3
a A: Methyldiperoxorhenium(VII) oxidant 2b generated in situ from 85% H2O2/MTO (from 60:1 to 100:1), solvent CH2Cl2; A0: DMD in situ from
acetone/caroate 3:1; buffered aqueous solution, pH7.5–8.0 [Ref. 16]. A00: TFD in situ from 1,1,1-trifluoropropanone (TFP)/caroate 2:1; buffered
aqueous solution, pH7.5–8.0 [Ref. 17d]. B: DMD in isolated form [Refs. 8 and 16] solvent CH2Cl2/acetone 50:50. B0: DMD in isolated form [Refs. 8
and 16] solvent acetone. B00: TFD in isolated form [Refs. 8 and 17] solvent CH2Cl2/TFP 1:1. B000: TFD in isolated form, solvent acetone/TFP 2:1.
b Ratio of bulk oxidant (H2O2, caroate, or isolated dioxirane) to substrate.
c As determined ( 3%) by GC (ZB-1, 0.25lm film thickness, 30m · 0.25mm ID, capillary column).
d Isolated yield ( 5%).
e Byproduct 2,5-diidrossi-esan-3,4-dione (6a) was also isolated (yield 18%).

L. D’Accolti et al. / Tetrahedron Letters 45 (2004) 8575–8578
8577
Supplementary data
Experimental details and supplemental characterization
1
data for compounds 4a, 5a, 5b, and 6a (four pages). H
NMR and {1H}13C NMR spectra recording the forma-
tion of peroxocomplexes 2b from MTO and 85% H2O2
in CD2Cl2 (two pages). Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2004.09.110.
Scheme 2.
is also formed. The latter also represents a useful
synthon.21
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best suited if one wishes to obtain the target alkyne-
1,4-diones in excellent yields.
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Partial support of this work by the Ministry of Univer-
sity and Scientific and Research of Italy (COFIN
National Project) and by CNR (National Research
Council of Italy) is gratefully acknowledged.

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should be noted that, in carrying out oxidations of
unsatured substrates by the MTO/H2O2 reagent, it is
common practice to employ additives such as pyridine or
urea; apparently, this serves to avoid undesirable side
reactions (e.g., the hydrolytic opening of epoxides ring,
ensuing pinacol-type rearrangements and C–C bond cleav-
age).9,10 However, pyridine and other electron donors can
bind to the metal center and might trigger MTO decompo-
sition to HRO4 and MeOH;10d also, the catalytically active
rhenium species can no longer be defined with clarity.
19. We find that the {1H}13C NMR (125.759MHz) spectrum
of CH3ReO3 (MTO) in CD2Cl2 presents a sharp C3
resonance at 19.8ppm; upon addition of 1equiv of 85%
H2O2, along with a residual MTO peak, two new
resonances appear at 33.0 and 30.3ppm (rel intensity ca.
6:1), pertaining to the diperoxo complex 2b and to the
monoperoxo complex 2a, respectively. Then, after addi-
tion of excess 85% H2O2 (6equiv), the mentioned 13C
NMR MTO and 2a peaks disappear, and just the diperoxo
complex 2b resonance is present (cf., Supporting Info).
This feature is confirmed by similar 1H NMR experiments,
in comparison with the CH3 NMR shifts reported for
MTO, 2b, and 2a in CD3NO2 [Ref. 10d]. Also, notably
absent are 13C or 1H NMR resonances that could be
attributed to ÔfreeÕ or metal-bound CH3OH in CD2Cl2.
20. General procedures for oxidations with MTO/H2 O2, with
dioxiranes in situ, and with isolated dioxiranes are
reported in the Supplementary data.
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