Development of an Unsymmetrical Cyclopropenimine‐Guanidine Platform for Accessing Strongly Basic Proton Sponges and Boron‐Difluoride Diaminonaphthalene Fluorophores

Abstract

An unsymmetrical guanidine-cyclopropenimine proton sponge DAGUN andt he relatedB F 2-chelate DAGBO are reported. Insight into the structural, electronic, bonding and photophysical properties of these two molecules are presented.J oint experimental and theoretical studies reveal the protonated form of DAGUN possessesa ni ntramolecular N•••HÀNh ydrogen bond which affords ah igh experimental pK BH + of 26.6 (computed = 26.3). Photophysical studies show that in solution DAGUN displays ag reen emission at 534 nm, with al arge Stokess hift of 235 nm (14,718 cm À1). In contrast, the conjugate acid DAGUN-H + is only weakly emis-sive due to attenuated intramolecular charge transfer.X-ray diffraction studies revealt hat DAGBO contains as table tetracoordinate boronium cation, reminiscent of the well-established BODIPY family of dyes. In solution, DAGBOe xhibits a strong blue emission at 450 nm coupled with al arge Stokes shift (Dl = 158 nm, Dn = 11,957 cm À1)a nd quantum yield of 62 %, upon excitation at 293 nm. DAGBO sets the stage as the first entry into an ew class of boron-difluoride diaminonaphthalenes (BOFDANs)t hat represent highlyf luorescent and tunable next-generation dyes with future promise for biosensing and bioimaging applications.

Push–Pull Effect on the Gas-Phase Basicity of Nitriles: Transmission of the Resonance Effects by Methylenecyclopropene and Cyclopropenimine π-Systems Substituted by Two Identical Strong Electron Donors

“…This choice does not imply a priori that the The cyclopropenimine (or cyclopropeneimine, less common name) skeleton (Z: N) has a special status among the heteroatomic analogs of methylenecyclopropene. It is present in many systems, which have been largely studied experimentally and theoretically, in particular in the last ten years, as platforms for developing superbases and organocatalysts [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. The imino bond carrying a nitrile group on the nitrogen, >C=N−CN, is present in molecules of biological interest, in particular the neonicotinoids [35][36][37], and in materials developed for their electronic applications [38][39][40].…”

Guanidine–Amide-Catalyzed Aza-Henry Reaction of Isatin-Derived Ketimines: Origin of Selectivity and New Catalyst Design

“…In addition, the alcohol moiety of the catalyst activated methyl acrylate via H-bonding, contributing to the high enantioselectivity results [57]. Feng and Liu found that the modified guanidine-amide with 1,2-diphenylethylcarbamoyl and the 2,6-difluorobenzenesulfonamide group was efficient in the asymmetric catalysis [58]. Inspired by the excellent performance of cyclopropenimine in Brønsted base catalysis, we modified the structure of G1 by replacing the guanidine moiety with cyclopropenimine.…”

“…According to synthesis investigation on guanidine derivatives with 1,2-diphenylethane-1,2-diamine groups by Feng and Liu [59], and guanidine-cyclopropenimine proton sponges by Dudding [58], we proposed a possible synthesis route for a new catalyst, G4 (Scheme 5). We expected that these results would be helpful for the development for the new catalytic system in the aza-Henry reaction of ketimine.…”

Trisaminocyclopropenium Cations as Small-Molecule Organic Fluorophores: Design Guidelines and Bioimaging Applications

“…In point of fact, Breslow in 1957 synthesized the first cyclopropenium ion, namely, triphenylcyclopropenium, while shortly thereafter Yoshida reported an amino-substituted derivative, thus giving rise to tris­(dialkylamino)­cyclopropenium (TDAC) ions or more generally trisaminocyclopropeniums (Figure A). Since, research into the synthesis and properties of amino-substituted cyclopropenium ions has been ongoing and they now find use as catalysts, superbases, ionic liquids, polyelectrolytes and nitrogen-based ligands, platforms for photoelectronic redox-active chemistries, and fluorophores as attested for by our fluorescent cyclopropenium-based proton sponge derivative Janus ( 1 ) , and a boron-containing analog coined DAGBO ( 2 ), among other derivatives (Figure A) . In reflecting upon these various developments, we posited that the coupling of excitation processes, viz, (1) intramolecular charge transfer (ICT, black dotted arrows) from a cyclopropenium core (electron donor, red sphere) to an appended group (electron acceptor, blue sphere), such as an arene ring system and (2) intramolecular through-space conjugation (ITSC, green arrows)a phenomenon observed in organic fluorophoreswould be feasible in trisaminocyclopropenium systems (Figure B).…”