Aryne–Imine–Aryne Coupling Reaction via [4+2] Cycloaddition between Aza‐o‐Quinone Methides and Arynes

Abstract

A direct method for the synthesis of acridane derivatives based upon a 2:1 coupling reaction between arynes and imines was developed. The reaction proceeds through formation of transient aza‐ortho‐quinone methides arising from a net [2+2] cycloaddition between arynes and imines. Introduction of steric bulk into an imine carbon has proven to be vital for this facile coupling.

Synthesis of Helical and Planar Extended‐Phenanthridinium Salts

“…Notably, the aryne‐imine‐aryne coupling reaction was described to proceed through the formation of aza‐ ortho ‐quinone methide intermediate III , which then reacts with the corresponding aryne to form a product possessing an acridane core ( Scheme 3, pathway a ) [7] . However, in the case of phenanthryne, the formation of 1,2‐dihydrobenzoazete II and intermediate III might be impacted by unfavorable steric interactions between the mesityl group and the extended system of the phenanthrene moiety.…”

“…Notably, the aryne-imine-aryne coupling reaction was described to proceed through the formation of aza-ortho-quinone methide intermediate III, which then reacts with the corresponding aryne to form a product possessing an acridane core (Scheme 3, pathway a). [7] However, in the case of phenanthryne, the formation of 1,2-dihydrobenzoazete II and intermediate III might be impacted by unfavorable steric interactions between the mesityl group and the extended system of the phenanthrene moiety. This increased steric bulk around the reactive center of phenanthryne could lead to a divergent mechanism of the aryne-imine-aryne coupling, which involves the nucleophilic attack of the imine at the phenanthryne, leading to the generation of intermediate I.…”

“…[1 -3] Acridinium salts are particularly useful scaffolds and have emerged as an efficient and sustainable class of cationic organic photocatalysts. [4][5][6] In recent studies, we investigated a short two-step route towards diverse acridinium salts involving the aryne-imine-aryne coupling reaction [7] followed by the subsequent oxidation of the acridane intermediate (Scheme 1,a). [8,9] Herein, we now disclose the utilization of this methodology for larger polycyclic aryne substrates.…”

The Versatility of the Aryne–Imine–Aryne Coupling for the ­Synthesis of Acridinium Photocatalysts

“…21 Notably, according to the proposed mechanism for the aryne–imine–aryne coupling, the imine requires sterically demanding substituents near the C-terminus to prevent an undesired 6π-electrocyclization. 20 Therefore, imines with di- ortho -substituted phenyl groups (R 2 ) were synthesized with the consequence that the acridinium salts prepared utilizing this two-step strategy possess a sterically shielded C9 position to further prevent photobleaching via nucleophilic attack or light-promoted radical–radical couplings. 22…”

Reactions of HDDA Benzynes with C,N‐Diarylimines (ArCH=NAr')

“…In 2017 H. Yoshida and co‐workers showed that trapping of the aza‐ ortho ‐quinone methide intermediate 3c to form 2:1 imine:benzyne adducts 4c was possible when benzyne was used in molar excess. This mode of reactivity prevails when the aryl moieties are more highly substituted, presumably slowing the electrocyclization of 3c to 3d .…”