Introduction of oxygen and nitrogen into organic molecules is one of the fundamental strategies for their functionalization. Traditional approaches rely on the formation of an activated intermediate, which is then modified in a subsequent step. Here we report a simple direct oxidative α‐acetoxylation and azidation of α‐C−H bonds in Tröger's base analogues promoted by N‐bromosuccinimide (NBS) and palladium(II)acetate [Pd(OAc)2] under mild conditions. This approach is pivotal on NBS oxidizing the carbon atoms next to the nitrogen atoms in the presence of Pd(OAc)2 followed by a reaction with oxygen‐ or nitrogen‐centred nucleophiles such as KOAc or NaN3 to yield either the mono‐ and bisacetoxylated or azidated Tröger's base derivatives in a stereo‐ and chemoselective manner. The obtained products can be efficiently modified to access synthetically meaningful structures.magnified image

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.

A practical and scalable method for synthesis and resolution of 4,10-di-substituted Tröger's base derivatives is described. By combination of circular dichroism (CD) spectra and X-ray diffraction (XRD) analysis method, the absolute configuration on two sterogenic nitrogen atoms of the crucial intermediate (-)-diastereomer is assigned as 5S,11S.