Unprecedented transformation from cyclized zwitterionic oxazolidine derivatives to corresponding non-zwitterionic aromatic amides via Vilsmeier reagent in a one-pot reaction: optical property and crystallography†
Abstract
Tetracyanoquinodimethane (TCNQ) is known to react with primary/secondary/certain aromatic amines to generate mono-/di-substituted and cyclized derivatives manifesting optical and non-linear optical material attributes. Interestingly, further functionalization of mono-/di-substituted or cyclized TCNQ derivatives remains unexplored to date. Therefore, herein, we present the synthesis of two cyclized zwitterionic TCNQ derivatives (DDPM [1], DHDPM [2]) which on further reaction with oxalyl chloride ((COCl)2) in the presence of dimethylformamide (DMF) yielded non-zwitterionic aromatic amides, i.e. open-chain benzene derivatives (CDCB [3], CDCHB [4]). Remarkably [1] and [2] exhibited Vilsmeier-Haack type reaction by the combined use of oxalyl chloride and DMF; and the Vilsmeier reagent is produced in situ in a one-pot synthesis. Though the purpose of the said reagent is to introduce a formyl group on the phenyl ring, currently, it reacts with one of the cyano groups rather than attacking the phenyl group. While [1] (Φf = 3.8%) and [2] (Φf = 18.5%) exhibited enhanced fluorescence in the solid state, [3] and [4] displayed it in solution. Crystallographic study revealed a possible role of torsional angle, [1] ≈ 4.24° and [2] ≈ 24.19°, for enhanced fluorescence in the solid [2] compared with [1].