ARKIVOC 2007 (xiv) 27-33
Gallium(III) chloride: an efficient catalyst for facile preparation of gem-diacetates from aldehydes
Sanjay Kumar, Anil Saini, and Jagir S. Sandhu* Department of Chemistry, Punjabi University, Patiala 147 002, Punjab, India E-mail: email@example.com
Abstract An efficient, facile preparation of gem-diacetates or diacetoxy acetals fromaldehydes in excellent yields, catalyzed by GaCl3, under solvent-free conditions, is described herein. Keywords: Gallium(III) chloride, aldehydes, gem-diacetates, acetic anhydride, solvent- free
Selective protection of carbonyl function as gem-diacetates (acylals) or gem-bis(acyloxy)alkanes is an important transformation in organic chemistry1 as an alternative to acetals because of theirstability under neutral and basic conditions1b as well as under critically controlled acidic conditions. Apart from mere protective groups acylals are important synthons and are useful precursors. The acylals derived from α-β unsaturated aldehydes are important starting materials for Diels-Alder reactions.2 These gem-diacetates have several synthetic as well as industrial applications. Inindustries, diactetates are utilized as cross linking reagents3 in cellulose and cotton industry, being also used as stain bleaching agents. As synthons, acylals have been exploited in well known reactions of organic chemistry, like Grignard,4 Barbier,4b and Prins5 reactions, condensation reactions of Knoevenagel,6a and benzoin6b type, and are also used in the synthesis of chrysanthemic acid,7a and thetotal syntheses of sphingofungins E and F.7b Because of their synthetic and industrial utility and unique properties as protective groups as well as important synthons, a search for efficient and facile preparation of acylals is of current interest. Apart from other methods, conventionally gem-diacetates are prepared from aldehyde, acetic anhydride and a catalyst viz strong Bronsted acids8 likeH2SO4, H3PO4, and super acids9 like Nafion-H and heteropolyacids. The use of strong Lewis acids10 like BF3, PCl3, ZnCl2, LiBF4, ZrCl4, Er(OTf)3, FeCl3, FeCl3/SiO2, Zn(ClO4)2.6H2O, etc have also been reported. In addition to these catalysts, graphite, zeolites, tungstosilicic acid and zirconium sulfophenyl phosphonate have also been employed in this protection process.11 Some of these methods stillsuffer from drawbacks like prolonged reaction time (viz. up to 120 h in case of 2-furyl aldehyde with PCl3),
ISSN 1424-6376 Page 27
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ARKIVOC 2007 (xiv) 27-33
low yields in the cases of 4-nitrobenzaldehyde (4 %) and cinnamaldehyde (30 %), when PCl3, is used and, in some cases, requirement of elevated temperature. Moreover, several of these catalysts areunsafe to handle, like metal perchlorates, BF3, etc. Consequently it seems desirable and necessary to develop a simple, safe, efficient and facile method for the preparation of these gem-diacetates. Though indium and gallium both are in same group i.e. IIIA, indium and its salts have been studied extensively and the results of this prolific exploration are reviewed from time to time,12 whilegallium and its salts remained almost ignored. Yet, the comparable ionization potentials ( Ga: FIP, 5.99 eV, Eo, Ga+3/Ga = - 0.56 V; In: FIP, 5.79 eV, Eo, In+3/In = -0.345 V ) indicate they should have equally attractive properties. Very recently, the use of gallium is reported in some major reactions of organic chemistry like Reformatsky,13 Barbier,14 Grignard,15 bromination of aromatics16a andallylation of indoles.16b The applications of gallium(III) halides are developing17 at a very fast pace, showing that its utility can match indium.18
Results and Discussion
In continuation of our own work19 on the use of gallium and its salts, in this communication we wish to report a gallium(III) chloride catalyzed preparation of gem-diacetates, from aldehyde and acetic anhydride (Scheme 1).