Lanthanide triflates mechanism

Another example of the use of Lewis acids in organic reactions in water is the lan-thanide(III) triflate catalysed aza-Diels-Alder reaction, exemplified in Scheme 14. In this reaction the hetero-dienophile is formed in situ from a primary ammonium hydrochloride and a carbonyl compound followed by the actual Diels-Alder reaction288,289. This type of reaction proceeds readily in aqueous media290-296, and a dramatic increase in the yield upon addition of lanthanide triflates was observed288,289. The exact role of the catalyst, however, is not entirely clear. Although it was suggested that the catalyst binds to the dienophile, other mechanisms, such as simple proton catalysis, are also plausible. Moreover, these reactions are further complicated since they are often heterogeneous. 

Jia et al. [396] have recently disclosed an example of a solid-phase application of their ytterbium] 111) triflate-mediated cyclization of glyoxylate-derived unsaturated imines. Instead of the expected imino-ene products, the authors observed the sole formation of y-lactones, in good to high yields and with good trans stereoselectivity. An explanation of both the mechanism and the observed preferred trans selectivity was presented. The importance of using the lanthanide triflate in its hydrated form in order to fadhtate a catalytic process is also in accordance with the proposed mechanism, in which water plays a role in attacking the oxonium ion intermediate. 

Thus, a new type of Lewis acid, lanthanide triflates, is quite effective for the catalytic activation of imines, and has achieved imino Diels-Alder reactions of imines with dienes or alkenes. The unique reactivities of imines which work as both dienophiles and azadienes under certain conditions were also revealed. Three-component coupling reactions between aldehydes, amines, and dienes or alkenes were successfully carried out by using Ln(OTf)3 as catalysts to afford pyridine and quinoline derivatives in high yields. The triflates were stable and kept their activity even in the presence of water and amines. According to these reactions, many substituted pyridines and quinolines can be prepared directly from aldehydes, amines, and dienes or alkenes. A stepwise reaction mechanism in these reactions was suggested from the experimental results. 

A possible mechanism of the present reaction is accompanied by imine formation, and successive addition of a vinyl ether proceeds smoothly in aqueous solution. Use of lanthanide triflates, water-tolerant Lewis acids, is of key importance and essential in this reaction. 

Dzudza, A. and Marks, T.J., Efficient intramolecular hydroalkoxylation of unactivated alkenols mediated by recyclable lanthanide triflate ionic liquids scope and mechanism, Chem. Eur. J. 16, 3403-3422 (2010). 

Lanthanum is the first element of the sixth-period transition metals. Its properties are close to those of the other rare earth elements and, as a consequence, the catalytic behavior of the lanthanum triflate is also very close to that of the entire series. An important example concerning the stability of these compounds in water is that indicating the capability of metal triflates, such as Yb(OTf)3, Eu(OTf)3, Sc(OTf)3 or La(OTf)3, to catalyze the hydration of alkynes to the corresponding ketones [56]. However, the interest in water-compatible lanthanide triflate-based catalysts is much larger and mainly includes the carbon-carbon bond-forming reactions. To increase the utility of these catalysts understanding of their aqueous mechanism is very important. For such a purpose, dynamic measurements of the water... 

The reaction is believed to proceed via a mechanism analogous to hydroamina-tion and hydrophosphination. There is experimental evidence for a rate-determining insertion step (Fig. 23). The high oxophilicity of the lanthanide ion results in a high barrier for the olefin insertion and therefore, diminished reactivity of alkenyl alcohols. Rare-earth metal triflates are also capable to catalyze cyclization of alkenyl alcohols in ionic liquids [193], although the mechanism is unlikely to be similar to the o-bond metathesis mechanism discussed above. 

Inspection of the proposed nitration mechanism (Scheme 1) reveals that the mononitrate dipositive lanthanide species [Ln(H20)x(N03)](0Tf)2 (1) is the key intermediate. An independent preparation and characterisation of such a species enables possible indentification of 1 directly in situ in the reaction mixture. Additionally, spectroscopic examination of these salts may provide some evidence for our working model. We have developed a novel preparation of these mixed salts by simple metathesis of lanthanide chlorides with the requisite quantities of silver nitrate and silver triflate in water (Scheme 3).17 The resulting hydrated salts were white or lightly coloured (pink, green or yellow) solids which were found to be stable indefinitely at room temperature in the solid state. 

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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