Radical‐Cation Dimerization Overwhelms Inclusion in [n]Pseudorotaxanes

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril ( CB7 ) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by ¹H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV ⁴⁺, which contains two viologen subunits, and HV ¹²⁺, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7 , that is, BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen ( MV ²⁺) and benzyl methyl viologen ( BMV ²⁺), as well as their [2]pseudorotaxane complexes with CB7 ( MV ²⁺? CB7 and BMV ²⁺? CB7 ) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆ led to host–guest decomplexation and release of the free threads BV ^{2( . +)} and HV ^{6( . +)}, respectively. ¹H DOSY NMR spectrometric and chronocoulometric measurements showed that BV ^{2( . +)} and HV ^{6( . +)} have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.     

Concise design and synthesis of pyridine-fused heterocycles via 6π-Azaelectrocyclization process of iminoalkyne derivatives

A concise and regioselective approach to the synthesis of pyridine-fused heterocycles and benzoxazepine derivatives was developed. Propargyl imines derived from aromatic aldehydes and propargyl amine underwent 6π-electrocyclization reactions at high temperatures in high yields to form pyridine-fused heterocycles. Application of the same methodology to aromatic imines having a hydroxyl group in the ortho position resulted in the formation of (benz)oxazepine derivatives. The formation mechanism of the products was discussed.