Indian Journal of Chemistry Vol. 50B, December 2011, pp 1774-1778 Europium triflate: An efficient, cost-effective and recyclable solid acid catalyst for highly chemoselective allylation of aldehydes Ravindra M Kumbhare* & M Sridhar Fluoroorganic Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India E-mail: rakumbhare@yahoo.com Received 18 October 2010; accepted (revised) 29 August 2011 Aldehydes undergo smooth nucleophilic addition with allyl-n-tributylstannane in the presence of europium triflate under extremely mild reaction conditions to afford the corresponding homoallylic alcohols in excellent yields with high chemoselectivity. The catalyst can be recovered by simple filtration and can be recycled in subsequent reactions. Keywords: Europium triflate, allyltributylstannane, aldehyde, homoallylic alcohols Eu (III) compounds are highly efficient catalysts for several important C–C bond forming reactions such as Mukaiyama reaction 1,2 , Michael addition 3-5 , Barbier- type reaction 6 , hetero-Diels-Alder cycloaddition 7 , Friedel-Crafts reaction 8 , Claisen rearrangement 9 , multi component synthesis of imidazoles 10 , oxidation of alkanes 11,12 and epoxidation of alkenes 13–15 . Herein is reported novel application of Eu(III) triflate as an efficient catalyst promoting allylation of aldehydes with allyl trialkylstannane under ambient conditions. Allylation of carbonyl compounds using allyltrialkyl- and allyl triarylstannanes under Lewis acid catalysis is an important reaction 16,17 . Study of this reaction using rare earth metal salts has received high importance in recent years. However, studies on this reaction using Eu (III) as a catalyst remained dormant in literature. As a part of the continued research interest in development of new synthetic methods using lanthanides as catalysts 18 , the scope of activation of the addition reaction of allylstannane to carbonyls under Eu triflate catalysis has been studied and these results are the subject of this paper (Scheme I). Results and Discussion In this study, a systematic evaluation of various solvents such as acetonitrile, dichloroethane, chloro- form, toluene and benzene has been carried out and also in the absence of solvent (Table I) for conducting allylation of benzaldehyde with allyl tri(n- butyl)tin (1 equiv) in the presence of europium triflate (5 mol % based on PhCHO) at RT. From this study, acetonitrile was found to be the solvent of choice for conducting this reaction as it gave the corresponding homoallylic alcohol in excellent yield. However, moderate yields were obtained when chloroform and 1,2-dichloroethane were used as solvents and also under solvent free conditions. No reaction could proceed in benzene and toluene. Thereafter, the experiment was carried out by varying the amount of catalyst (5-20 mol %). The use of just 5 mol % of europium triflate in acetonitrile was sufficient to drive the reaction to completion in 1 hr and in good yields. Further increasing the amount of catalyst did not improve the result to any great extent. After optimizing the reaction conditions, the procedure was extended to different aldehydes and the results are summarized in Table II. The reaction was carried out using 1 mmol of 4-nitrobenzalde- hyde, 1.05 mmol of allyltributylstannane and 5 mol % of europium triflate in acetonitrile (2 mL). The reaction mixture was stirred at RT for the appropriate amount of time (TLC). In this study, aromatic aldehydes (entries 1-10), aliphatic aldehydes (entries 11 and 12) and heterocyclic aldehydes (entries 13 and 14) gave corresponding homoallylic alcohols in 82- 95% yields under similar reaction conditions. This reaction, however, did not proceed with ketones such as cyclohexanone and acetophenone even after 140 min. A probable mechanistic pathway appears to proceed by initial coordination of europium triflate