Azahelicenes from the Oxidative Photocyclization of Boron Hydroxamate Complexes

Abstract

Aromatic hydroxamic acids (Ar-CO-NOH-Ar') were used as bidentate chelating ligands to generate the corresponding boron hydroxamate complexes, which were subsequently transformed into nitrogen-containing helicenes (azahelicenes) using an oxidative photocyclization method that is frequently used for stilbene-type (Ar-CH=CH-Ar') precursors of carbohelicenes. The nitrogen atom of the hydroxamate linker was thus directly embedded into the helicene core without using nitrogen-containing aromatic rings in the stilbene-type precursors. In a batch photoreaction, aza[4]helicenes were readily and efficiently prepared, but aza[6]helicenes underwent severe decomposition upon irradiation. Alternatively, a continuous flow photoreactor was employed to furnish an amide-type aza[6]helicene.

One-Pot Preparation of (NH)-Phenanthridinones and Amide-Functionalized [7]Helicene-like Molecules from Biaryl Dicarboxylic Acids

“…We envisioned that amide-functionalized helicene-like molecules 9 and 14 could potentially serve as chiral-functionalized molecules, provided that their racemization barriers are sufficiently high to ensure configurational stability. We have already confirmed via density functional theory (DFT) calculations that 9 can be expected to exhibit a sufficiently high racemization barrier (Δ G ⧧ = 41.4 kcal/mol) to be configurationally stable at ambient temperature . In the present study, the racemization barrier of sulfur-functionalized 14 was determined to be 34.3 kcal/mol at 132 °C based on monitoring the decrease in enantiomeric excess in chlorobenzene under reflux conditions.…”

“…On the other hand, inspired by the [5]­helicene-like structure of 5 , amide-functionalized [7]­helicene-like molecule 9 was prepared via lactamization of in situ-generated biphenanthrene δ-amino acid derivative 8 in the racemic form (Scheme B) . This synthetic method is characterized by simple cyclization conditions without photocyclization of styrene-type substrates or transition-metal-catalyzed reactions, both of which are frequently employed as cyclization strategies to obtain helicenes and helicene-related derivatives.…”

Synthesis of Aza[n]helicenes (n = 4–7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

“…The proposed method of preparation of aza­[ n ]­helicenes ( n = 4–7) via a photocyclodehydrochlorination reaction of suitably substituted amides provides a straightforward approach to various π-extended aza­[ n ]­helicenes and a valuable alternative to known syntheses of 5-aza[4]­helicene ( 15a ) − or 7-aza[5]­helicene ( 15b ). , Although these compounds can be prepared in higher overall yields, the advantage of the new method lies in its universal applicability, as demonstrated in the preparation of other, previously unknown helicene derivatives, such as 8-aza[6]­helicene ( 15c ) and 8-aza[7]­helicene ( 15d ).…”

Gas phase synthesis of [4]-helicene

“…Here, we reveal a versatile route to form helicenes via a directed, vinylacetylene mediated gas phase chemistry. In contrast to the aforementioned routes following solution chemistry and often ionic reaction intermediates 28,35,40,41 , the innovative gas phase synthesis encompasses low-barrier reactions through targeted, stepwise ring expansion mechanisms involving free radical reaction intermediates. Exploiting the simplest helicene as a benchmark, we reveal the hitherto unknown gas phase chemistry synthesizing [4]-helicene (C 18 H 12 ; 228 amu) along with atomic hydrogen (1 amu) via the bimolecular reaction of the 4-phenanthrenyl radical ([C 14 H 9 ] • ; 177 amu) with vinylacetylene (C 4 H 4 ; 52 amu) (reaction (1)).…”