Synthesis and chiroptical properties of inherently chiral ‘picket-fence’ catechol BOPHYs

Abstract

Chiral bis-N,N,O,O-boron chelated BOPHY dyes are produced in a one-pot reaction of (1E,2E)-1,2-bis((3,5-dimethyl-1H-pyrrol-2-yl)methylene)hydrazine, boron tribromide, and an unsymmetrically substituted catechol derivative. Remarkably, in the case of methyl 2,3-dihydroxybenzoate the chiral, racemic diastereoisomeric bis-boron chelate is formed with complete stereoselectivity. In contrast, the corresponding reaction with 3,5-di-tert-butylcatechol produces a 3:1 ratio of racemic and meso (achiral) diastereoisomers. These boron chelates display two strong (ε 33–41,000 M−1 cm−1) absorption maxima between 444 and 473 nm, typical of a BOPHY-like vibronic progression. The racemic isomers were resolved by chiral HPLC and display mirror image ECD spectra. The absorption dissymmetry factor (|gabs|) for the tert-butyl catechol chelate is 5–10 fold larger than that for the ester-substituted catechol derivative, which correlates with a highly twisted core chromophore revealed by the X-ray crystal structure of the tert-butyl derivative. Both catechol chelates display BOPHY-like emission but with very low quantum yield of fluorescence (φf ≤ 0.1%) presumably due to photoelectron transfer from the very electron-rich catechol system.