Metal triflates: On the question of Lewis versus Brønsted acidity in retinyl carbocation formation

Al(OTf)3 generates blue carbocations from retinyl acetate. The mechanism appears to be exclusively water-derived induced Brønsted acidity causing loss of acetic acid, not Lewis-mediated acetate abstraction. Dry DCM (1 ppm water) avoided water interference. The hindered base 2,6-di-tert-butylpyridine, used to distinguish between Lewis- and Brønsted acidity, was also probed. The results implicate a possible mechanism for similar transformations involving carbocations.

Retinyl acetate liberates intense blue carbocations with Brønsted acids. Al(OTf)3 also generates such carbocations apparently exclusively via water-derived induced Brønsted acidity (loss of acetic acid) as opposed to the Lewis acid acting as an acetate abstractor.Figure optionsDownload high-quality image (112 K)Download as PowerPoint slide