Synthesis and Conformational Studies of Urobilin Difluoroboron Complexes. Unprecedented Solvent-Dependent Chiroptical Properties of the BF₂ Chelate of an Urobilinoid Analogue¹

Abstract

Optically active difluoroboron complexes of both a conventional urobilin derivative (3) and a nonracemizable urobilin analogue (4) have been synthesized for the first time, and their structures have been determined, in solution, by means of 1H{1H}NOE difference experiments. Whereas the BF2 chelate 5 occurs in a “stretched” 9Z,4(5)-ac, 5(6)-sc,10(11)sp,14(15)- sc,15(16)-ac conformation in both dimethylformamide and methylene chloride solutions, the preferred conformation (6 or 7) of the difluoroboron complex of 4 is dependent on the solvent, thus giving rise to opposite CD curves in the aforementioned solvents. As revealed by X-ray diffraction analysis, however, only conformer 6 is present in the solid state, in which extended intermolecular hydrogen bonding involving the lactam groups takes place, independently of the solvent used for crystallization. Conformer 7, on the contrary, is stabilized, in solution, by intramolecular NH···F bonds, as evidenced by measurements of “through-space” 1H−19F spin−spin coupling.

Experimental Evidence for Intramolecular Attractive Nonbonded C–F...H–C Interactions in 2′,3′-Dideoxy-4′-(fluoromethyl)nucleosides – Through-Space JCF and JHF NMR Coupling Constants, Correlation with Empirical Parameters of Solvent Polarity and Single-Crystal X-ray Structures

“…Only those signals that are affected by heteronuclear FϪH spin-spin be discussed later. [24] The data reported in Tables 1, 2 and 3 fit measured, independent of solvent polarity. The α anomer of compounds 1, 2 and 3 show a large through-space FϪH coupling constant, sumed that any variation in J can be ascribed to two main factors: [16] (i) electronic changes due to mutual solute-sol-whilst the corresponding values for 4 and 5 could not be obtained due to the intrinsic large width of the signal re-vent interaction; (ii) solvent-induced conformational transitions, such as internal rotation and ring interconversions.…”

Circularly Polarized Luminescence from Simple Organic Molecules

“…546 nm). 34 Therefore, an energy-transfer process is involved in the CPL activity of ( R , R )-9 . Interestingly, a favored C 2 -symmetric conformation involving intramolecular NH-F bonding was demonstrated to exist in ( R , R )-9 , which must favor the chiral perturbation of the emitting achiral BODIPY.…”

Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study

“…In the two enantiomeric pairs of atropisomeric dimers, the chirality is brought about by the chirality axis coincident with the bond connecting the two BODIPY monomers. This aspect strongly differentiates our system from the design of previously reported chiral BODIPYs . Compounds 1 and 2 are quasi‐isomers differing in the position of the aryl–aryl junction, namely C3/C3’ (ring α atoms) in 1 , and C1/C1’ (ring β’ atoms) in 2 (Scheme ).…”

“…Moreover it is possible to finely tune their spectroscopic properties modifying the substituents on the pyrrole rings or on the boron atom. These features make them suitable to be employed in OLEDs or as light‐emitting probes and bioprobes, and prompted some research groups to develop chiral BODIPYs (or analogues) capable of emitting circularly polarized light . In the present work, we studied the circularly polarized emission properties of BODIPY “DYEmers” 1 and 2 (Scheme ).…”