Diquats with Robust Chirality: Facile Resolution, Synthesis of Chiral Dyes, and Application as Selectors in Chiral Analysis.

Diquats with extremely high racemization barriers with ΔG≠theor of 233 kJ mol-1 at 180 °C are described. Reported configurational robustness is due to a combination of two structural features: the rigid o-xylylene tether connecting the nitrogen atoms and the presence of two substituents in the bay region of the bipyridinium scaffold. The straightforward synthesis of diquats, plus facile resolution and derivatization make them attractive for chiral application studies. This is demonstrated by: 1) synthesis of the first non-racemic diquat dyes with pronounced chiroptical properties, and 2) capability of diquats to interact stereospecifically with chiral molecules. This suggests potential for diquat derivatives to be used as chiral selectors in separation methods.

Synthesis and Photophysical Properties of Aza[n]helicenes.

Synthesis and study of aza[7]helicene and aza[9]helicene is presented in this paper. Photo-dehydrocyclization of the 3,6-bis-styryl derivative of carbazole leading to sterically less demanding aza[7]helicene resulted in smooth reaction, and only the desired angular-angular product was detected. However, in the case of aza[9]helicene, along with the expected angular-angular cyclization, three other products involving linear mode of cyclization were also isolated and characterized. In this case, the helical compound aza[9]helicene was predominantly formed at lower concentration while the other isomers were obtained at higher concentration. All of the compounds formed by angular-angular, angular-linear, and linear-linear modes of cyclization were fully characterized, and their photophysical properties were investigated.

Synthesis of enantiomerically pure helicene like bis-oxazines from atropisomeric 7,7'-dihydroxy BINOL: Preliminary measurements of the circularly polarized luminescence.

Racemic sample of 2,2',7,7'-tetrahydroxy-1,1'-binaphthyl was resolved with (S)-proline and the separated enantiomers were independently converted to atropisomeric bis-oxazines by aromatic Mannich reaction. These chirally pure oxazines were then converted to the helicene like cyclic ethers. The Circularly Polarized Luminescence (CPL) profile was consistent with the isolation of the targeted helical-like molecules in optically pure form, prepared from the achiral primary amines. The compounds of interest displayed active and opposite CPL activities for each sets of the helicene like isomers (P)-/(M)-3 and (P)-/(M)-5.

Synthesis of chiral bis-oxazines: a preliminary assessment of helical conformational framework.

A series of novel naphthalene attached bis-oxazines were synthesized and characterized. The bis-oxazines were studied by VT-NMR analysis to assess the possibility of conformational twist. The bis-oxazine prepared from (l)-methylvalinate show a helical conformational twist in the single crystal X-ray analysis. Three isomers of bis-oxazines were prepared from chiral α-methylbenzyl amines, the meso isomer showed small optical rotation probably indicating the helical conformational twist in the molecule.

Synthesis of chiral helical 1,3-oxazines.

A series of novel 1,3-oxazines were prepared to construct a helical framework. The 1,3-oxazine attached to the phenanthrene unit showed a small bite angle θ (∼12°), while the units attached to [4]helicene showed a larger θ (∼35°) and exhibited helical isomers at ambient conditions. The diastereomers of the third type of helicene-like bis-oxazine attached to binaphthyl were easily separable and showed good thermal stability. All four diastereomers of bis-helicene were synthesized, and their absolute configuration was established.