Structure-Property Relationship of Aminonitrofluorenes Synthesized by Copper-Mediated Ullmann-Type C-N Bond Formation

Abstract

We describe the facile synthesis of a range of aminonitrofluorene derivatives by employing a copper-catalyzed Ullmanntype reaction protocol. The yields of the reactions compare fairly with the Buchwald-Hartwig coupling reaction. Structural evidence, spectroscopic data and theoretical calculations provide the structure-property relationship.Charge separation is one of the most fundamental processes involved in numerous chemical and biological transformations. 1-4 Electron-donor-acceptor (EDA) systems, which are also known as push-pull molecules, provide the testing ground for understanding the mechanism of the charge separation process. Fluorescent EDA molecules find use as dyes in laser applications 5 and as reporter molecules for micro-environments 6 in various organized assemblies. We have been interested in push-pull fluorene derivatives, in particular, the 2-amino-7-nitro derivatives, as these are known to be attractive candidates for studying solvation dynamics, 7 two-photon absorption phenomena 8 and also in medicinal applications. 9 Herein, we report the synthesis of a series of amine-terminated nitrofluorene derivatives by applying a C-N bond-formation reaction based on the copper-mediated Ullmann type condensation reaction. In all the fluorene derivatives synthesized, an amino group is the donor and a nitro group is the acceptor. Some of the systems for which single crystals could be obtained have been characterized by single crystal diffraction studies. The UV-vis absorption spectra of these pushpull systems have been correlated with the estimated C-N bond lengths and the ground state dipole moments of the systems, which have been estimated by semi-empirical AM1 (Austin Model 1) calculations.Carbon-nitrogen bond-formation reactions between aryl halides and amines have been extensively studied over the past several years. For the benzene derivatives, these reactions are achieved via an addition-elimination pathway when the ring is activated by a strong electron-withdrawing group, such as a nitro or a cyano group, and it requires high-pressure conditions. 10 The most versatile method of introducing an amino group into arenes is the BuchwaldHartwig reaction, which involves a palladium-catalyzed C-N coupling reaction of the halides to the haloarenes. 11-17 This reaction does not require an activating substituent in the ring, but it does require an efficient phosphine ligand in order to obtain the desired product. The coppercatalyzed Ullmann-Goldberg coupling reaction is also well known for introducing amine functionalities using aromatic halides. [18][19] However, for this reaction to happen under mild conditions, it often requires an external ligand. 20 For large systems, Pd-catalyzed reaction protocols are often superior to Cu-mediated reactions both in terms of the yield of the desired product and the milder reaction conditions required. For these reasons, amine-terminated push-pull fluorene derivatives have so far been synthesized based on the Pd-catalyzed BuchwaldHartwig protocol. 21 There is only one ...