Chemoselective Preparation of 1 , 1-Diacetates from Aldehydes , Mediated by a Keggin Heteropolyacid Under Solvent Free Conditions at Room Temperature

A simple, general and efficient method has been developed for the conversion of aldehydes to 1,1-diacetates using acetic anhydride, a catalytic amount of non commercial Keggin heteropolyacid (H6 PAlMo11O40) (1% mol) in solvent free conditions at room temperature. Aromatic and aliphatic, simple and conjugated aldehydes were protected with excellent yields.


Introduction
Aldehydes are know to react with simple anhydrides to give acylals (1,1-Diacetates) in the presence of acid catalysts.1,1-Diacetates are synthetically useful for protecting aldehydes owing to their moderate stability in neutral or basic media and their easy formation as well as easy deprotection 1 .The 1,1-Diacetates of α,β-unsaturated aldehydes are important starting materials for the synthesis of valuable acetoxy dienes and vinyl acetates for Diels-Alder cicloaddition reactions 2 .They have also been applied as cross linking reagents for cellulose in cotton and useful as activators in the composition of bleaching mixtures used for the treatment of wine-stained fabrics 3 .In addition 1,1-diacetates are useful as substrates in nucleophilic substitution reactions 4 .
1,1-diacetates preparation from aldehydes and acetic anhydride using protonic or Lewis acids as catalysts has been known for a long time.Several catalysts have been employed for this reaction, e.g.sulfuric acid, methanesulfonic, or phosphoric acids 5 , FeCl 3 6 PCl 3 7 , InCl 3 8 Sc(OTf) 3 9 , Cu(OTf) 2 10 , ZrCl 4 11 , among others.However, some of these methods are not entirely satisfactory due to their drawbacks, low yield, long reaction time, environmental problems and tedious work-up.
Several catalysts have been used to improve yields, decrease reaction time and eliminate the unfavorable effects for this reaction, e.g.iodine 12 , expansive graphite 13 , montmorillonite clay 14 , solid superacid as nafion-H 15 , tungstosilicic acid and HZSM-5 16 , zirconium sulfophenyl phosphonate 17 , aluminium dodecatungstophosphate 18 , LiClO 4 19 , Zn(ClO 4 ) 2 20 , 12-molybdophosphoric acid 21 , NaHSO 4 22 silica sulfuric acid 23 , KHSO 4 24 and others.Heteropolycompounds are useful and versatile to a number of transformations because of their redox and superacidic properties.The Keggin primary structure presents the general formula [XM 12 O 40 ] (8-n)-, where M are addenda atoms, X is the heteroatom and n is the X valence.The oxygen atoms in this structure fall into four classes of symmetric-equivalent oxygen: X-O a -(M) 3 , M-O b -M, connecting two M 3 O 13 units by corner sharing; M-O c -M, connecting two M 3 O 13 units by edge sharing and terminal O d -M.
We have recently applied heteropolyacids with structure type Keggin for performing the tetrahydropyranylation of phenols and alcohols 25 and flavones preparation 26 .On the other hand, a heteropolyacid with a type Wells-Dawson structure was used for performing the tetrahydropyranylation-depyranylation of phenols and alcohols 27 , coumarins preparation 28 and 1,1-diacetates formation and the corresponding deprotection 29 .
As a part of an ongoing research project to develop environmentally friendly organic reactions, we report here a rapid preparation of 1,1-diacetates of aldehydes using a non commercially Keggin heteropolyacids (H 6 PAlMo 11 O 40 ) as catalyst, being tested as a homogeneous catalyst, at room temperature and in solvent free conditions.This Keggin heteropolyacid has been described in a previous paper 30 .

Experimental
All the products were well previously descript They were characterized by comparison (GLC, TLC, physical constant) with authentic samples prepared by the conventional method, using sulfuric acid as the catalyst as well as 1 H and 13 C-NMR All the yields were calculated from crystallized products.

Results and Discussion
The protection reaction is showed in Scheme 1, and it was studied using aldehydes 1a-1s as substrates.
The structures of aldehydes, 1,1-diacetates and the obtained results are shown in Table 1.
Benzaldehyde was chosen as the substrate for optimizing the reaction conditions: Temperature, time, concentration of the initial solutions and molar ratio of heteropolyacid to substrate were checked.
Various aldehydes reacted with Ac 2 O to afford the corresponding 1,1-diacetates in high yields.Reaction proceeds smoothly in 3 h at room temperature (20 °C).Both electronreleasing and electron-withdrawing substituted aromatic aldehydes were converted to their corresponding 1,1-diacetates in the same conditions; the nature of the substituents seems to have no effects on the reaction.
In the case of aliphatic aldehydes, high yields of products are obtained in the mentioned conditions.Acid-sensitive aldehydes, such as crotonaldehyde, furfural and cinnamaldehyde give very good yields, no decomposition or polimerization were observed Hydroxybenzaldehydes afford the corresponding triacetates in excellent yields.
Nevertheless, 4-dimethylaminobenzaldehyde failed to give the expected 1,1-diacetate even the mixture were being stirred for 16 h.We attempted the reaction of acetone, butanone and acetophenone with acetic anhydride; they have not reacted under the described experimental conditions.Besides, acetylation of a mixture of benzaldehyde and acethophenone gave only the 1,1-diacetate from the aldehyde (Scheme 2).

Conclusion
We report a new clean, mild and efficient procedure for preparing 1,1-diacetates from aldehydes in the presence of acetic anhydride at room temperature, mediated by a non commercial H 6 PAlMo 11 O 40 , a heteropolyacid with Keggin structure, under solvent free conditions.Some advantages of this procedure are the low cost with green methodology, short reaction time and the easy work up.

a
Compound 2j y 2k are the triacetates b Reaction time was 4 h.