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Abstract

This study was aimed at investigating the suitability of 2-alkyl/alkenylbenzo-1,3,2-diazaborolane compounds as potential nucleophilic coupling partners in the Suzuki-Miyaura cross-coupling reaction. A range of aryl bromides and iodides bearing electron-donating as well as electron-withdrawing substituents were reacted with 2-alkyl/alkenylbenzo-1,3,2-diazaborolane compounds under the influence of Pd(OAc)2/PCy3 combination. The cross coupling reaction afforded the desired products in yields ranging from 35% to 89% in less than 20 minutes. The catalytic system was found to be versatile and general tolerating a variety of functional groups including OMe, NO2, OH, COOMe and COMe2, thus demonstrating the suitability of 2-alkyl/alkenylbenzo-1,3,2-diazaborolane as coupling partners in the Suzuki-Miyaura cross-coupling methodology. The results from this study have been accepted for publication, full reference: (Sithebe, S., Hadebe, S. W., Robinson, R. S. Tetrahedron, 2011, 67, 4277.)
Encouraged by the successful application of 2-alkyl/alkenylbenzo-1,3,2-diazaborolanes as coupling partners in the Suzuki-Miyaura (SM) cross-coupling reaction, we then extended our studies to investigate the synthesis and subsequent application of 2-arylbenzo-1,3,2-diazaborole analogues as potential coupling partners under the Suzuki-Miyaura cross-coupling reactions. The cyclocondensation of arylboronic acids with the corresponding 1,2-phenylenediamine afforded 2-arylbenzo-1,3,2-diazaboroles in yields ranging from 43% to 93%. The cross-coupling reaction of 2-arylbenzo-1,3,2-diazaboroles with the range of aryl bromides afforded the desired biaryl products in moderate to excellent yields ranging from 62% to 96%. Substrates bearing electron-withdrawing substituents were shown to be more reactive under these reaction conditions affording biphenyls in excellent isolated yields ranging from 83% to 96%. While our yields are comparable with the yields reported in literature, our reactions take only 10 minutes (!) compared to many hours of reflux as reported in the literature.
This project was also aimed at investigating the spectroscopic characteristics of 2-arylbenzo-1,3,2-diazaborole compounds by acquiring and studying their absorption and emission spectra. The data obtained revealed the lack of significant
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solvatochromism for all the compounds in the ground state which is indicative of the presence of low dipole moments. These values were confirmed computationally which showed low calculated dipole in a range 0.1379-2.2773 D. In the excited state, all chromophores are influenced by the polarity of the solvent used pointing to the presence of solvatochromism. The introduction of a donor group such as thioether (MeS) and the introduction of bromine atom, on the π-system, have proven beneficial for the emission maxima of the species investigated.
The extension of π-conjugation length at the 2-position of these species and the methylation at the backbone of 1,3,2-benzodiazaborolyl group leads to bathochromic shifts of the emission maxima, which in turn lead to large Stokes shifts of up to 11000 cm-1. Alternatively, the formal insertion of the phenyl spacer between the naphthyl ring and the 1,3,2-benzodiazaborolyl group does not have any influence on the photophysical properties of these compounds.
The HOMOs solvatochromism for all the compounds in the ground state which is indicative of the presence of low dipole moments. These values were confirmed computationally which showed low calculated dipole in a range 0.1379-2.2773 D. In the excited state, all chromophores are influenced by the polarity of the solvent used pointing to the presence of solvatochromism. The introduction of a donor group such as thioether (MeS) and the introduction of bromine atom, on the π-system, have proven beneficial for the emission maxima of the species investigated.
The extension of π-conjugation length at the 2-position of these species and the methylation at the backbone of 1,3,2-benzodiazaborolyl group leads to bathochromic shifts of the emission maxima, which in turn lead to large Stokes shifts of up to 11000 cm-1. Alternatively, the formal insertion of the phenyl spacer between the naphthyl ring and the 1,3,2-benzodiazaborolyl group does not have any influence on the photophysical properties of these compounds.
The HOMOs of all the chromophore are purely represented by the 1,3,2-benzodiazaborolyl group except for anthracenyl-functionalised benzo-1,3,2-diazaborolane compounds in which the HOMO are located on the π-system with no contribution of the vacant 2pz-orbital of the boron atom. The large Stokes shifts and significant solvatochromism displayed by these compounds are suggestive of the potential application in organic light emitting diodes (OLED) as emitters. The results from this study have been drafted for publication in Dalton Transition.