Acta Chim. Slov. 2008, 55, 209–212
209
Short communication
Ruthenium(III) Chloride as an Efficient Catalyst
for Conversion of Aldehydes to 1,1-diacetates
Under Mild Conditions
Nafisehsadat Sheikhana, BiBi Fatemeh Mirjalilia,*, Abdolreza Hajipourb, Abdolhamid Bamoniric
a Department of Chemistry, College of Science, Yazd University, Yazd, Iran, P.O. Box 89195-741, phone: +98 351 8211674, fax: +98 351 8210644.
b Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan, Iran.
c Department of Chemistry, College of Science, Kashan University, Kashan, Iran.
* Corresponding author: E-mail: fmirjalili@yazduni.ac.ir
Received: 06-07-2007
Abstract
Ruthenium(III) chloride catalyzes the preparation of 1,1-diacetates from aldehydes under mild and in solvent or solvent-free conditions. This method has some advantages such as good to excellent yields, ease of operation and workup, short reaction time, high catalytic efficiency and high chemoselectivity.
Keywords: 1,1-diacetates, acylals, protecting group, ruthenium(III) chloride, aldehyde, solvent-free condition
1. Introduction
Geminal diacetates (acylals) are stable under neutral and basic conditions1 and they are frequently used as protecting groups for aldehydes.2 1,1-Diacetates are synthetically important precursors for the preparation of 1-acetoxy dienes for Diels–Alder reaction.3 The acyloxy groups in acylals can be converted into other useful functional groups by reaction with appropriate nucleophiles.4–5 Recently, palladium-catalyzed asymmetric allylic alkylation of acylals was applied for the total synthesis of saphingofungins E and F.6 In addition, indium-mediated allylations of gem-diacetates and palladium-catalyzed substitution reactions of geminal allylic diacetates have also been reported.7
Several reagents or catalysts such as InBr3,8 Cu(OT
f)2,9 NBS,10 Bi(NO3)3 · 5H2O,11 In(OTf)3,12 Bi(OTf)3 · x H2O,13 zirconium sulfophenyl phosphonate,14 H2NSO3H,15 LiBF4,16 zinc-Montmorillonite,17 Zn(OTf)2 · 6H2O,18 sul-fated zirconia19 and heteropolycompounds such as Wells–Dawson acid,20 P2O5/Al2O3,21 HClO4 · SiO2,22 erbium triflate23 and 12-molybdophosphoric acid24 have been employed for the synthesis of acylals from aldehydes using acetic anhydride.
Ruthenium(III) chloride has been used as a catalyst for oxidation of tertiary amines,25 alcohols26–28 and phosphorous acid.29 Recently, this catalyst was applied for the synthesis of 2-ethyl-3-methylquinolines from anilines and trialkylamine30 and for acylation of alcohols, phenols, thiols and amines.31
In this work, we wish to report a mild, facile and efficient method for conversion of aldehydes to corresponding acylals using acetic anhydride in the presence of catalytic amounts of ruthenium(III) chloride hydrate without solvent or in n-hexane at room temperature.
2. Experimental
General: Starting materials and ruthenium(III) chloride were purchased from Fluka, Merck and Aldrich. The diacetate products were characterized by their spectral (IR, 1H–NMR), TLC and physical data. IR spectra (KBr pellets) were recorded on Bruker Equinox 55 FTIR spectrometer. NMR spectra were obtained on a Bruker Avans 400 MHz or Hitachi-Perkin Elmer, R-24B, 60 MHz.
Typical procedure for preparation of 1,1-diace-toxy-1-(2,6-dichlorophenyl)methane from 2,6-dichloro-
Sheikhan et al.:   Ruthenium(III) Chloride as an Efficient Catalyst for Conversion of Aldehydes ...
Acta Chim. Slov. 2008, 55, 209–212 210
benzaldehyde: In a mortar, a mixture of 2,6-dichloroben-zaldehyde (0.525 g, 3 mmol), acetic anhydride (0.6 mL, 6 mmol), and RuCl3 · xH2O (0.018 g) was pulverized for 5 minutes. The reaction progress was followed by TLC (eluent: chloroform). After 5 minutes of pulverization, the conversion was above 99%. After completion of the reaction, silica gel (1.5 g) and chloroform (24 mL) were added to the mixture and filtered. The solvent was evaporated to give the pure product as a white solid. The solid was re-crystalized from a mixture of ethanol and water. Yield: 0.71 g (87%), m.p: 88–89 °C. [Lit 21 m.p: 88–89 °C].
3. Results and Discussion
At the begining, in order to evaluate various Lewis acids as potential catalysts in the acylal formation, we have investigated their efficiency in a model reaction between acetic anhydride (6 mmol) and 3-nitrobenzaldehyde (3 mmol) at room temperature under grinding conditions. Among the screened catalysts, RuCl3 · xH2O was found to be superior (Table 1, entry 14). It seems that not only the Lewis acidity of RuCl3 · xH2O is stronger than that of other catalysts used, but also for the conversion the necessary amount of RuCl3 · xH2O can be lesser than that of other catalysts. On the other hand, unfortunately, this very active catalyst is not reusable due to its high absorbency potential.
For a demonstration of the generality and ability of this catalyst, a variety of aliphatic and aromatic aldehydes were converted into acylals using catalytic amount of rut-henium(III) chloride hydrate (Scheme 1, Table 2). All products were known and characterized by their spectral data
Table 1: Conversion of 3–nitrobenzaldehyde to corresponding acylal by various catalysts.
entry	catalyst	condition/solvent	yield (%)
1	75% P2O5/SiO2 (0.45 g)32	r.t., grinding	75
2	Zr(HSO4)4 (4 mol %)33	r.t., grinding	86
3	Al(HSO4)3 (5 mol %)34	r.t., grinding	80
4	FeCl3 (40 mol %)	r.t., grinding	70
5	AlCl3 (40 mol %)	r.t., grinding	75
6	ZnCl2 (40 mol %)	r.t., grinding	64
7	SnCl4 (40 mol %)	r.t., grinding	72
8	SbCl5 (40 mol %)	r.t., grinding	75
9	BF3 · Et2O (40 mol %)	r.t., grinding	62
10	RuCl3 · H2O (0.03 g)	r.t., grinding	89
11	RuCl3 · H2O (0.025 g)	r.t., grinding	90
12	RuCl3 · H2O (0.022 g)	r.t., grinding	90
13	RuCl3 · H2O (0.02 g)	r.t., grinding	88
14	RuCl3 · H2O (0.018 g)	r.t., grinding	88
15	RuCl3 · H2O (0.015 g)	r.t., grinding	80
16	RuCl3 · H2O (0.013 g)	r.t., grinding	78
17	RuCl3 · H2O (0.01 g)	r.t., grinding	70
18	RuCl3 · H2O (0.018 g)	reflux/n-hexane	67
19	RuCl3 · H2O (0.018 g)	reflux/CH2Cl2	56
20	RuCl3 · H2O (0.018 g)	reflux/CH3CN	62
Sheikhan et al.:  Ruthenium(III) Chloride as an Efficient Catalyst for Conversion of Aldehydes ...
(IR, 1H–NMR and MS) and by comparison to the authentic samples. The results reveal that ruthenium(III) chloride catalyzed reactions generally result in good yields with aromatic aldehydes incorporating electron-withdrawing substituents. It is important to mention that other methods such as AlPW12O40-catalyzed reactions35 have provided poor yields in the presence of electron withdrawing sub-stituents.
Scheme 1
Cyclohexanone, 4-nitroacetophenone and 4-acetyl-benzaldehyde were also checked for the reactivity. The keto groups of these compounds have not reacted neither under room temperature nor reflux conditions. We suggest that the chemoselective protection of aldehydes in the presence of ketones can be achieved by this method (Scheme 2).
However, 4-(N,N-dimethylamino)benzaldehyde failed to give the expected acylal under grinding condition at room temperature or under reflux. The explanation for this result may be due to the strong electron donating properties of the dimethylamino group which will reduce the reactivity. It should be stressed that phenolic group was also protected as acetate in hydroxyl containing aromatic aldehyde (Table 2, entries 8 and 15) under such conditions.
Acta Chim. Slov. 2008, 55, 209–212
211
Xhi,
O       RuCI,.xM,0
Ac,0. r,l,            '       \;__fl
-    ,   ,.          H,C           -----           OAc
so I ven L- I ree
75%
OAc        AcO
H     +  H.C-
Arf )
AcO                       AcO
0%
NO,
CHO + NO,
RuCI,.xH,0               fi    \
CQCH3    A„n ,."   " N°2~(       T-CH(OAc), + NO,—(v       h— C(0/
AcjO, r.t solvent free
,,'¦"
ui,
ilO—Va        A-L'HO OCH, Scheme 2
suivent !rce           /
OCH,
63%
OCH,
unchanged
Table 2: Chemoselective conversion of carbonyl compounds to corresponding acylals by ruthenium(III) chloride hydrate as catalyst under solvent
free conditions.a,b
	substrate	mp (°C)	or bp (°C) (mmHg)	
entry		found	reported	time (min) / yieldc (%)
1	4-nitrobenzaldehyde	124-125	124–12521	5/90
2	3-nitrobenzaldehyde	65–66	65-6615	5/88
3	4–chlorobenzaldehyde	79-80	81-8221	7/87
4	benzaldehyde	42–43	44–4515	10d/76d
5	3-methoxybenzaldehydee	Oil	Oil12	5/55
6	2,6-dichlorobenzaldehyde	88-89	88-8921	5/87
7	4-cyanobenzaldehyde	97-98	98-10021	7/71
8	vanillinf	90-91	90-9112	5/63
9	furfural	49-50	52-5315	5/70
10	4-nitroacetophenone	-	-	15(60)f/0(0)e
11	3-phenylpropionaldehyde	Oil	Oil21	6/72
12	isobutyraldehyde	185 (740)	183 (630)21	5/50
13	cyclohexanone	-	-	10(30)f/0(0)e
14	4-(dimethylamino)benzaldehyde	-	-	15(30)f/0(0)e
15	4-hydroxybenzaldehydef	89-90	89-9017	60d/58
a) The ratio of substrate (mmol) / catalyst (g) / acetetic anhydride (mmol) is 3 / 0.018 / 6 b) All products were known and characterized from their spectral data (IR, 1H–NMR and MS) and comparison to authentic samples.c) Yields refer to isolated pure products. d) n–Hexane was used as solvent and reaction was carried out at r.t. e) The reaction was carried out under reflux condition in n–hexane as a solvent. f) The ratio of substrate (mmol) / catalyst (g) / acetetic anhydride (mmol) is 3 / 0.036 / 9
:0:
O     O
:0—RuCI,
A  *ua> ^:" :
RuCI,'*~~\
Nu-RuCI, +
Scheme 3
CH.-C-O-C-CH,           :0   \:Ç
¦ f\-y
Sheikhan et al.:   Ruthenium(III) Chloride as an Efficient Catalyst for Conversion of Aldehydes ...
212
Acta Chim. Slov. 2008, 55, 209–212
In our proposed mechanism, RuCl3 · xH2O activates the carbonyl group of aldehydes very fast (Scheme 3). On the other hand, wet or supported RuCl3, cannot catalyze acylal formation reaction.
4. Conclusion
We have established a mild and facile catalytic method for the synthesis of diacetates from aldehydes using ruthenium(III) chloride as an efficient heterogeneous inorganic catalyst at room temperature. Advantages of this method include high catalytic efficiency, the observed chemoselectivity, good to excellent yields, short reaction times and ease of operation and work-up.
5. Acknowledgement
Financial support for this work by the research affairs, Yazd University, Kashan University and Isfahan University of Technology are gratefully acknowledged.
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Povzetek
Rutenijev(III) klorid katalizira pripravo 1,1-diacetatov iz aldehidov pod milimi pogoji ob prisotnosti topila ali pa tudi pod pogoji brez uporabe topil. Opisana metoda ima kar nekaj prednosti, kot npr. odli~ne izkoristke, enostavnost izvedbe in izolacije, zahteva kratke reakcijske ~ase, ka`e visoko u~inkovitost katalizatorja in tudi visoko kemoselektivnost.
Sheikhan et al.:  Ruthenium(III) Chloride as an Efficient Catalyst for Conversion of Aldehydes ...