Ruthenium-catalyzed facile aerobic oxidation of α-hydroxycarbonyls to 1,2-dicarbonyl compounds

Article abstract of DOI:10.1081/SCC-120003622

The aerobic oxidation of α-hydroxycarbonyl compounds was efficiently carried out with a ruthenium catalyst [RuCl₂(p-cymene)]₂ in the presence of a catalytic amount of base to afford 1,2-diketones or α-ketoesters in good to excellent yields.

Full text of DOI:10.1081/SCC-120003622

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RUTHENIUM-CATALYZED FACILE AEROBIC OXIDATION
OF α-HYDROXYCARBONYLS TO 1,2-DICARBONYL
COMPOUNDS
Sukbok Chang a , Myunghee Lee a , Sangwon Ko a & Phil Ho Lee b
a Department of Chemistry , Ewha Womans University , Seoul, 120-750, Korea
b Department of Chemistry , Kangwon National University , Chuncheon, 200–701, Korea
Published online: 16 Aug 2006.
To cite this article: Sukbok Chang , Myunghee Lee , Sangwon Ko & Phil Ho Lee (2002) RUTHENIUM-CATALYZED FACILE AEROBIC
OXIDATION OF α-HYDROXYCARBONYLS TO 1,2-DICARBONYL COMPOUNDS, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 32:8, 1279-1284, DOI: 10.1081/SCC-120003622
To link to this article: http://dx.doi.org/10.1081/SCC-120003622
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SYNTHETIC COMMUNICATIONS, 32(8), 1279–1284 (2002)
RUTHENIUM-CATALYZED FACILE
AEROBIC OXIDATION OF
a-HYDROXYCARBONYLS TO
1,2-DICARBONYL COMPOUNDS
Sukbok Chang,1, Myunghee Lee,1 Sangwon Ko,1
*
and Phil Ho Lee2
1Department of Chemistry, Ewha Womans University,
Seoul 120-750, Korea
2Department of Chemistry, Kangwon National
University, Chuncheon 200-701, Korea
ABSTRACT
The aerobic oxidation of a-hydroxycarbonyl compounds
was efficiently carried out with
a ruthenium catalyst
[RuCl2( p-cymene)]2 in the presence of a catalytic amount
of base to afford 1,2-diketones or a-ketoesters in good to
excellent yields.
1,2-Dicarbonyl compounds have been frequently utilized as impor-
tant intermediates in synthetic organic chemistry.1 Direct oxidation of
a-hydroxycarbonyl compounds would provide the corresponding a-dicarbo-
nyls and several reagents have been developed for this transformation.2 How-
ever, many of the available reagents have to be employed in stoichiometric,
*Corresponding author.
1279
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

1280
CHANG ET AL.
and they are highly corrosive or toxic and produce undesirable waste
materials that persist in the environment. There has been great interest, there-
fore, in the development of a more practical and catalytic version of this
oxidation with the use of a transition metal catalyst in combination with
benign oxidants.3 Among these oxidants, molecular oxygen is highly attrac-
tive from an economical and environmental point of view.4 During the course
of our recent investigation on aerobic oxidation,5 we found that a-hydroxy-
carbonyl compounds could serve as good substrates under relatively mild
reaction conditions to afford the corresponding 1,2-dicarbonyls in good to
excellent yields, which is described in this communication (Eq. 1).
With the use of a dimeric ruthenium catalyst, [RuCl2( p-cymene)]2
(1), a variety of benzoin derivatives were efficiently oxidized under an
atmospheric oxygen (Table 1). As demonstrated in our previous study,5
additives exhibited significant effects on the efficiency of the reactions, and
among several additives tested, catalytic amounts of Cs2CO3 provided the
highest rate enhancement and the best selectivity. The requisite amounts of
the base could be reduced to as little as 1 mol% without affecting the
oxidation efficiency. It should be noted that while the aerobic oxidation
of benzylic- or allylic alcohols was necessary to carry out at higher
than 100ꢀC (1 : 5 mol%, Cs2CO3: 10 mol%) to get acceptable yields of
ketones or aldehydes,5 the aerobic oxidation of a-hydroxycarbonyls to
a-dicarbonyls could be performed under rather milder conditions (80ꢀC,
1 : 5 mol%, Cs2CO3: 1 mol%). Aromatic solvents such as benzene or
toluene were media of choice for this reaction. Efficiency of the oxidation
was not changed upon the introduction of various substituents on the
phenyl ring,6 and unsymmetrical 1,2-diketones could also be obtained in
good yields (Entries 5–7).
It should be noted that oxidation of an aliphatic a-hydroxyketone also
proceeded to give a a-diketone with reasonable reaction rate and yield
(Entry 7).7 For some substrates, tertiary amine bases such as triethylamine
or 1-methylpiperidine were marginally more effective than cesium carbonate
(compare Entries 8 and 9).8 The same situation was also observed for
the oxidation of a-hydroxyesters, by which a-oxoesters were obtained in

OXIDATION OF a-HYDROXYCARBONYLS
1281
Table 1. Ruthenium-Catalyzed Aerobic Oxidation of a-Hydroxycarbonyl
Compoundsa
Yieldb
(%)
Entry a-Hydroxyl Carbonyl Base (equiv.) t/h
Product
1
Cs2CO3 (0.01)
3
83
2
Cs2CO3 (0.01)
3
85
3
4
5
Cs2CO3 (0.01)
Cs2CO3 (0.01)
Cs2CO3 (0.01)
3
3
3
74
83
94
6
7
Cs2CO3 (0.01)
Cs2CO3 (0.01)
4
4
79
65
8
9
Cs2CO3 (0.1) 14
77
80
90
Et3N (0.1)
Et3N (0.3)
7
4
10
11
Et3N (0.3)
11
73
aAll reactions were carried out in 0.25 M concentration under an oxygen atmos-
phere (using an O2 balloon); bRefer to isolated yields after silica gel column
chromatography.

1282
CHANG ET AL.
good yields in the presence of catalytic amounts of triethylamine (Entries 10
and 11).
During the course of the present study, we found that in some cases
mono-carbonyl compounds were also produced as a by-product from the
aerobic oxidation of a-hydroxyketones although in minor amounts (3–5%).
Because no decarbonylation occurs by subjection of 1,2-diketones to the
employed reaction conditions, we surmised that mono-ketones are formed
by initial competitive decarbonylation of a-ketols followed by subsequent
oxidation of the decarbonylated secondary alcohols (Eq. 2).9
In summary, a simple and efficient aerobic oxidation of various
a-hydroxycarbonyl compounds has been achieved to afford 1,2-dicarbonyls
in good to excellent yields using a ruthenium catalyst [RuCl2( p-cymene)]2 in
the presence of a catalytic amount of base. Further study concerning
mechanistic aspects of the presented aerobic oxidation and decarbonylation
is now underway.
EXPERIMENTAL SECTION
a-Hydroxycarbonyl compounds were prepared according to the
reported procedure.6
1
(2-Hydroxy-2-phenyl)cyclohexyl ketone (Table 1, Entry 6): H NMR
(250 MHz, CDCl3) d 7.39–7.28 (5H, m), 5.19 (1H, d, J ¼ 4.6 Hz), 4.40 (1H,
d, J ¼ 4.7 Hz), 2.42 (1H, m), 1.85–1.70 (4H, m), 1.37–1.17 (6H, m); 13C NMR
(63 MHz, CDCl3) d 212.9, 138.3, 130.3, 129.3, 128.0, 46.4, 29.9, 28.3, 26.1,
25.4; HRMS (EI) calcd for C14H18O2 [M]þ 218.1307, found 218.1303.
1
(2-Hydroxy-2-cyclohexyl)phenyl ketone (Table 1, Entry 7): H NMR
(250 MHz, CDCl3) d 7.94–7.91 (2H, m), 7.68–7.63 (1H, m), 7.56–7.50
(2H, m), 4.97 (1H, dd, J ¼ 6.4, 1.9 Hz), 3.64 (1H, d, J ¼ 6.6 Hz), 1.84–1.55
(6H, m), 1.22–1.05 (5H, m); 13C NMR (63 MHz, CDCl3) d 202.6, 134.6,
134.3, 129.3, 128.9, 42.9, 30.7, 26.9, 26.2, 25.1; HRMS (EI) calcd for
C14H18O2 [M]þ 218.1307, found 218.1306.

OXIDATION OF a-HYDROXYCARBONYLS
1283
Representative aerobic oxidation procedure: To a solution of benzoin
(212 mg, 1.0 mmol) in benzene (4 ml) was added [RuCl2( p-cymene)]2
(1, 31 mg, 0.05 mmol) and Cs2CO3 (3.3 mg, 0.01 mmol). The atmosphere
of the reaction flask was evacuated and filled with oxygen gas. The reaction
mixture was stirred at 80ꢀC for 3 h under atmospheric oxygen (using an O2
balloon). After cooling to room temperature, the reaction mixture was
purified by column chromatography on silica gel (ethyl acetate/
hexane ¼ 1 : 10) to afford benzil (175 mg, 83%).
1
Cyclohexylphenylethanedione (Table 1, Entry 6): H NMR (250 MHz,
CDCl3) d 7.96–7.93 (2H, m), 7.69–7.64 (1H, m), 7.55–7.49 (2H, m), 3.17–
3.08 (1H, m), 1.98–1.70 (4H, m), 1.47–1.23 (6H, m); 13C NMR (63 MHz,
CDCl3) d 206.6, 194.7, 135.0, 132.9, 130.3, 129.3, 46.3, 30.1, 26.2, 25.7;
HRMS (EI) calcd for C14H16O2 [M]þ 216.1151, found 216.1150.
Ethyl (4-bromobenzoyl)formate (Table 1, Entry 11): 1H NMR
(250 MHz, CDCl3) d 7.92–7.89 (2H, m), 7.68–7.65 (2H, m), 4.45 (2H, q,
J ¼ 7.2 Hz), 1.43 (3H, t, J ¼ 7.2 Hz); 185.5, 163.6, 132.7, 131.9, 131.7,
130.9, 63.0, 14.5; HRMS (EI) calcd for C10H9BrO3 [M]þ 255.9735, found
255.9742.
ACKNOWLEDGMENT
This work was financially supported by Korea Research Foundation
Grant (KRF-2000-015-DP0272).
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1284
CHANG ET AL.
6. Starting materials of a-hydroxycarbonyl compounds were prepared
according to the reported procedure: Furniss, B.S.; Hannaford, A.J.;
Smith, P.W.G.; Tatchell, A.S. Vogel’s Textbook of Practical Organic
Chemistry. Longman Scientific & Technical: Essex, UK, 1989; pp. 1045.
7. Although complete conversion was also observed within 4 h for this
substrate, the moderate isolated yield for the product was partly
due to competitive decarbonylation of the substrate under the reaction
conditions.
8. The aerobic oxidation did not proceed at any significant rates in the
presence of secondary amines such as piperidine or pyrroridine instead
of tertiary amines under otherwise identical conditions.
9. For a previous example of the metal-catalyzed oxidative cleavage of
a-hydroxyketones, see: Kirihara, M.; Takizawa, S.; Momose, T.
J. Chem. Soc., Perkin Trans. 1 1998, 7.
Received in the UKMarch 26, 2001