The palladium-catalysed copper-free Sonogashira coupling of isoindoline nitroxides: a convenient route to robust profluorescent carbon–carbon frameworks?

Organic & Biomolecular Chemistry Pub Date: 2008-07-03 DOI: 10.1039/B806963H

Abstract

A series of novel acetylene-substituted isoindoline nitroxides were synthesised viapalladium-catalysed copper-free Sonogashira coupling. These results demonstrate that the Sonogashira reaction is suitable for the generation of a wide range of aryl nitroxides of expanded structural variety. The novel aryl-iodide containing nitroxide, 5-iodo-1,1,3,3-tetramethylisoindolin-2-yloxyl, 3, was a key intermediate for this coupling, giving acetylene-substituted isoindoline nitroxides in high yield. Subsequent reaction of the deprotected ethynyl nitroxide 12 with iodinated polyaromatics furnished novel aromatic nitroxides with extended-conjugation. Such nitroxides have been described as profluorescent, as their quantum yields are significantly lower than those of the corresponding diamagnetic derivatives. The quantum yields of the naphthyl- and phenanthryl-acetylene isoindoline nitroxides (13 and 14) were found to be ～200-fold and ～65-fold less than the non-radical methoxyamine derivatives (23 and 24). Ethyne- and butadiyne-linked nitroxide dimers could also be synthesised by this cross coupling methodology.

Graphical abstract: The palladium-catalysed copper-free Sonogashira coupling of isoindoline nitroxides: a convenient route to robust profluorescent carbon–carbon frameworks
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