The Carbocation: A Forgotten Lewis Acid Catalyst

Abstract A Lewis acid that has received negligible attention as a catalyst is the carbocation. The carbocation is isoelectronic to boron and owes its Lewis acidity to a low‐lying empty p C orbital. In terms of reactivity and stability carbocations are very versatile Lewis acids, from the extremely unstable methylium cation to the water‐stable tris( N , N ‐dimethylaniline) methylium ion (crystal violet). Although the Lewis acid properties of carbocations have been extensively studied since the discovery of the tropolium ion more than 130 years ago there is only a handful examples on the application of carbocations as Lewis acid catalysts. Herein, the research on triarylmethylium (trityl)‐cation catalysis is summarized. In light of the reports the trityl ion emerges as a highly efficient and highly versatile Lewis acid catalyst capable of catalyzing different classes of reactions often with high selectivity and low catalyst loadings (for some reactions down to ppm levels).