A Copper(II)-Catalyzed Aza-Friedel-Crafts Reaction of N-(2-Pyridyl)sulfonyl Aldimines: Synthesis of Unsymmetrical Diaryl Amines and Triaryl Methanes

摘要

The Lewis acid-catalyzed addition of electron-rich aromatic compounds to aldehydes, ketones and imines, the latter of which is known as the aza-Friedel-Crafts reaction (AFCR), is a synthetically outstanding CC bond-forming process that leads to functionalized alcohols and amines in a completely atom-economical way.[1] However, despite its significance, a number of unsolved aspects related to functional-group tolerance, either with regard to the arene nucleophile or the electrophilic substrate, still remain for this transformation. For instance, the mono Friedel-Crafts reaction of carbonyl compounds and imines seems to be restricted to highly electrophilic substrates[2] such as glyoxylates,[3] chloral (trichloroacetaldehyde),[4] pyruvates,[5] trifluoromethyl imines,[6] or -imino esters.[3c], [7] In contrast, aromatic aldehydes[8] and their imines,[3c], [9] generally evolve according to a double Friedel-Crafts reaction to give symmetrical triaryl methanes due to the intrinsic instability of the intermediate benzylic alcohol (or amine derivative) under the acidic reaction conditions (Scheme 1 a). Triaryl methanes display varied and interesting properties and have received a great deal of attention as leuco dyes,[10] photochromic agents,[11] suitable building blocks for generating dendrimers,[12] and as substrates for theoretical[13] and biological[14] studies. While many methods have been reported for the preparation of symmetrical triaryl methanes,[15] the synthesis of unsymmetrical derivatives is much less developed.[16], [17]