Diels-Alder reactions trifluoromethanesulfonic acid

Trifluoromethanesulfonic acid (triflic acid) in toluene greatly activates the Diels-Alder reaction of benzaldehydes with dimethylated 1,3-butadienes [89] (Table 4.22). With mono-methylated 1,3-butadienes the reaction gives less... 

On the other hand, rare-earth trifluoromethanesulfonates (rare earth triflate, RE(OTf)3) have been found to work efficiently as Lewis acids even in aqueous media or in the presence of amines [4], A catalytic amount of RE(OTf)3 enables several synthetically useful reactions, for example aldol, Michael, allylation, Mannich, Diels-Alder reactions, etc., to proceed. It has also been demonstrated that a small amount of RE(OTf)3 is enough to complete the reactions and that RE(OTf)3 can easily be recovered from the reaction mixture and can be reused. A key to accomplishing the catalytic processes was assumed to be the equilibrium between Lewis acids and Lewis bases, for example water, carbonyl compounds, and amines, etc. A similar equilibrium was expected between Lewis adds and aromatic ketones, and, thus, RE(OTf)3-catalyzed Friedel-Crafts acylation was investigated [5]. 

Gassman first observed the intermediacy of allyl cations as dienophiles in connection with efforts to extend the aminium cation-radical catalyzed Diels-Alder reaction to intramolecular cases. It was found that the cyclization of tetraene (26) performed in the presence of 0.2 equiv. of tris(p-bromophe-nyl)aminium hexachloroantimonate [ArsN+ CleSb ] was in fact catalyzed by protic acid. - A better means of catalyzing the cyclization of (26) involves use of 4 mol % trifluoromethanesulfonic acid at -23 C for 6 min, which provides the trans-fused cycloadduct in 88% yield (98% isomeric purity). Similar treatment of (27), however, provided a mixture of cis- and trans-fused products. A subsequent report revealed that allylic alcohols and allylic ethers are useful precursors to the allyl cation dienophiles with... 

Aqueous aza Diels-Alder reactions were first described in cycloadditions of imini-um salts and dienes [38], Likewise, iminium salts derived from amino acids react in excellent yields in aqueous medium [39]. Such an aqueous aza Diels-Alder reaction was found to be catalyzed by lanthanide(III) trifluoromethanesulfonates [40]. 

More recently, Zhao illustrated the Diels-Alder reaction of 2-vinyl indoles with a,p-unsaturated aldehydes catalysed by prolinol Im and trifluoromethanesulfonic acid as cocatalyst (Scheme 7.24). ... 

Finally, it s worth emphasizing that trivalent bismuth salts such as the chloride, nitrate, and trifluoromethanesulfonate (triflate) are extremely attractive as Lewis acid catalysts. Applications include Friedel-Crafts acylations, Diels-Alder reactions, esterifications. 

RS Oakes, TJ HeppenstaU, N Shezad, AA Clifford, CM Rayner. Use of scandium tris(trifluoromethanesulfonate) as a Lewis acid catalyst in supercritical carbon dioxide efficient Diels-Alder reactions and pressure dependent enhancement of endo exo stereoselectivity. Chem Commun 1459-1460, 1999. 

Kaw cki used optically active sulfonamide 234 to perform an asymmetric aza-Diels-Alder reaction with Rawal diene 235. " The reaction was performed in the presence of a Lewis acid, trimethylsilyl trifluoromethanesulfonate (TMSOTf), to obtain dihydropyridinone 236 as the product with high enantiomeric excess (Scheme 40.51). 

Maruoka also utilized 3,3 -modified binaphthyl-derived chiral primary amine 21, in combination with trifluoromethanesulfonic acid (TfOH), as a catalyst for the Diels-Alder reaction of a-substituted a,(i-unsaturated aldehydes with cyclopenta-diene, which afforded the corresponding cycloadducts possessing an all-carbon quaternary stereocenter with good to high stereoselectivity (Scheme 7.34) [58]. 

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