A Mild Benzannulation through Directed Cycloaddition Reactions

Abstract

Simple as ABC: Alkynyl borane cycloadditions can be substrate-directed to assemble aromatic difluoroboranes within an extremely mild and efficient reaction manifold compared to that of traditional methods (see scheme). The aromatic boranes are readily transformed into a range of useful products.

A Modular Class of Fluorescent Difluoroboranes: Synthesis, Structure, Optical Properties, Theoretical Calculations and Applications for Biological Imaging

“…Our cycloaddition chemistry offered the opportunity to prepare benzene and pyridine substituted difluoroboranes, however, looking ahead to applications of our fluorescent molecules for cell imaging, we chose the latter compounds as the polar heterocycle core offered the prospect of increased water solubility versus a benzene derived analogue. Moreover, in an effort to maximise our chances of obtaining a powerful fluorophore, we first set out to synthesise a target with an extended aromatic system.…”

“…Our cycloaddition chemistry offered the opportunity to prepare benzene [16] and pyridine [12] substituted difluoroboranes, however, looking ahead to applications of our fluorescent molecules for cell imaging, we chose the latter compounds as the polar heterocycle core offered the prospect of increased water solubility versusabenzene derived analogue.M oreover,i na n effort to maximise our chances of obtaining ap owerful fluorophore, we first set out to synthesise at arget with an extended aromatics ystem.A ccordingly,a midrazone 2a was prepared from the corresponding nitrile and hydrazine, and condensation with 9,10-phenanthrenequinone afforded triazine 3a (Scheme 2). Finally,c ycloaddition with potassium phenylethynyltrifluoroborate afforded the first BOBIPY 4a in good yield over the three steps.…”

N‐Heterocyclic Carbene‐Catalyzed Synthesis of Multi‐Substituted Benzenes from Enals and α‐Cyano‐β‐methylenones

“…Therefore, developing synthetic methods toward multi‐substituted benzenes is of great importance. Classically, electrophilic and nucleophilic aromatic substitution reactions are most employed strategies for the preparation of multi‐substituted benzenes . In modern organic chemistry, the metal‐catalyzed reactions, such as transition‐metal‐catalyzed cross‐coupling reactions, transition‐metal catalyzed arylation via C−H activation and benzannulation reactions, were powerful tools to establish the multi‐substituted benzene.…”

A Mechanistic Study of the Lewis Base-Directed Cycloaddition of 2-Pyrones and Alkynylboranes

“…We have developed a mild and regioselective synthesis of aromatic difluoroboranes via the cycloaddition of 2-pyrones with in situ generated alkynyldifluoroboranes. 9 As outlined in Scheme 1, our optimization studies highlighted some unexpected trends; the reaction required the use of 3 equivalents of both the alkyne substrate and the Lewis acid fluorophile for optimal yields. Moreover, alkynylated by-products were observed when the reaction was conducted under ambient conditions.…”