Praseodymium triflate

The use of Ln(OTf)3 in the activation of aldehydes other than formaldehyde was also investigated [18]. The model reaction of 1-trimethylsiloxycyclohexene (2) with benzaldehyde under the influence of a catalytic amount of Yb(OTf)3 (10 mol%) was examined. The reaction proceeded smoothly in H20-THF (1 4), but the yields were low when water or THF was used alone. Among several Ln(OTf)3 screened, neodymium triflate (Nd(OTf)3), gadolinium triflate (Gd(OTf)3), Yb(OTf)3, and lutetium triflate (Lu(OTf)3) were quite effective, while the yield of the desired aldol adduct was lower in the presence of lanthanum triflate (La(OTf)3), praseodymium triflate (Pr(OTf)3) or thulium triflate (Tm(OTf)3) (Table 2). 

Wang reported three-component coupling reaction of aldehydes, amines, and dienes by lanthanide triflate catalyzed aza-Diels-Alder reaction in water [35, 36]. Among lanthanide triflates screened, lanthanum, praseodymium, neodymium, and ytterbium triflate were found to be effective (Table 13.16). It is worth noting that aqueous formaldehyde could be employed as substrate. These reaction conditions were applied in the synthesis of aza-sugars. 

The same researchers found that lanthanide triflates could catalyze aldol reactions and allylations in aqueous media [10]. However, Wang et al. exploited the potential of lanthanide triflates to act as stable Lewis-acid catalysts in the aqueous imino Diels-Alder reaction [11]. This variant of the aqueous heterocycloaddition protocol also expands the scope of such reactions. The use of higher aldehydes in such reactions generally met with limited success under the conventional protocol. For example, under the standard conditions, the reaction of hexanal and benzylamine hydrochloride with cyclopentadiene in water is sluggish and affords only 4% of the Diels-Alder adducts 26 and 27 in a 2.7/1 ratio. In sharp contrast, the addition of various lanthanide(lll) triflates (0.25 M) to this reaction results in substantial increases in both the rate and yield of Diels-Alder adduct formation. In particular, use of praseodymium(iii) triflate results in a 68% yield of adducts 26 and 27 ... 

Among the seven lanthanides screened, praseodymium(iii), ytterbium(iii) and neodymium(//7) triflates were shown to be most effective. Magnesium chloride and lithium chloride showed no substantial influence on these reactions, thereby ruling out the salt effect for explaining the rate and yield enhancements of lanthanides. The details of the catalytic mechanism of this intriguing lanthanide-mediated effect are under investigation. 

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