Tetracyanoanthraquinodimethanes with a chiral amide group: Preparation, properties, and charge-transfer photochirogenetic reaction with 1,2-dianisylacenaphthene-1,2-diol

A series of butterfly-shaped tetracyanoanthraquinodimethanes (TCNAQs) with a chiral amide auxiliary 1a-f were prepared from the corresponding anthraquinones. They are stronger acceptors than the unsubstituted derivative and undergo one-wave two-electron reduction. They form weak electron-donor- acceptor (EDA) complexes with the title pinacol 2. Upon charge-transfer excitation of these complexes, dihydro-TCNAQs 3 and 1,8-dianisoylnaphthalene 4 were efficiently formed, the latter of which is the product of a retropinacol reaction via 2^+.. Partial enantiodifferentiation of rac-2 was realized during the photoreactions with 2-[(R)-1-phenylethylcarbamoyl]-TCNAQ 1a in CD₃CN. Thus, optically active (S,S)-(+)-pinacol 2 (12.3% ee at 54% conversion; 21.5% ee at 70% conversion) was recovered from the photolyzates. This reaction represents a new and rare example of the pseudokinetic resolution of ieri-alcohol accompanied by C-C bond fission. Significant differences in the association constants for the diastereomeric EDA complexes are responsible for the observed enantiodifferentiation.