Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes ‐ Access to Red Emitters and Planar Chiral Stereochemical Traits

Abstract

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late‐stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron‐donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near‐infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG‐functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red‐shifted absorptions (Δλabs up to 25 nm) and emissions (Δλem up to 73 nm, ΦPL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in 1H NMR spectroscopy (218 K), N−CH2 protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol−1. This phenomenon is due to planar chiral conformations (Sp and Rp configurations), induced by the geometry of the alkyl (n‐propyl) side‐chains next to the acetylenic substituents. Ion pairing studies with Δ‐TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Triple Regioselective Functionalization of Cationic [4]Helicenes via Iridium‐Catalyzed Borylation and Suzuki Cross‐Coupling Reactivity

“…Using 6 b as model substrate, a Vilsmeier−Haack reaction was attempted; this formylation procedure being possible on unhindered cationic helicene 1 (82 % yield). [ 10b , 31 ] In this case, no reaction happened. Similarly, attempts at benzoylation of 6 b in positions 2 and 12 under strongly acidic media conditions failed in this instance.…”

“…Additionally, it was decided to test briefly whether previously‐developed LSF protocols (Scheme 1, A ) could still be applied on derivatives of type 6 , despite the steric encumbrance generated by the additional aryl groups at positions 3, 7 and 11. Using 6 b as model substrate, a Vilsmeier−Haack reaction was attempted; this formylation procedure being possible on unhindered cationic helicene 1 (82 % yield) [10b,31] . In this case, no reaction happened.…”

Chemo‐ and Regioselective Multiple C(sp2)−H Insertions of Malonate Metal Carbenes for Late‐Stage Functionalizations of Azahelicenes

“…However, when malonate FGs are introduced next to the propyl‐substituted N atom (compounds 4 aa and 6 ), strong interactions occur that (i) reduce the conformation freedom of the n ‐propyl and malonate groups (see below) and (ii) induce a puckering of the six‐membered aza cycle. In fact, as shown recently for alkynyl derivatives, [32] n ‐propyl chains are impacted strongly by neighboring substituents, in consequence, these alkyl residues adopt gauche conformations that fold above the helical core. Experimental characterization of this effect is observed in the solid‐state structures of r‐3 a and 4 aa (Figure S21).…”