Ytterbium triflate mechanism

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. 

The preparation of the quinoline system by pericyclic reactions is less important than the previous syntheses. Some cyclization/elimination reactions of iV-phenylimines 95 and 96 with alkynes, enol ethers or enamines are of preparative interest. Although they correspond to [4+2] cycloadditions, they are likely to proceed by SeAt mechanisms and are usually catalysed by Lewis acids. In the enolether cycloaddition to imines 95 ytterbium(III)triflate proved to be particularly effective [104]. 

A proposed mechanism for this reaction involves the following three steps to generate the nitronium ion. The trifluoromethanesulfonate (triflate) ions act as spectators. The ytterbium cation is believed to be hydrated by the water present in the aqueous nitric acid solution. Nitric acid binds strongly to the hydrated ytterbium cation, as shown in equation 1. A proton is generated, as shown in equation 2, by the strong polarizing effect of the metal. Nitronium ion is then formed by the process shown in equation 3. Although the nitronium ion may serve as the active electrophilic species, it is more likely that a nitronium carrier, such as the... 

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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