Diphosphines: electronic effects

Electronic effects. To study the nature of the electronic effect in the rhodium diphosphine catalysed hydroformylation, a series of thixantphos 18 ligands with varying basicity was synthesized 25-30 (Figure 8.11). In this series of ligands, steric differences are minimal so purely electronic effects could be investigated. 

In order to investigate the electronic effects of equatorial diphosphines, electron-withdrawing substituents on the aryl rings were synthesized and tested [239]. The introduction of an electron-withdrawing group in the BISBI ligand increased the linear aldehyde selectivity in the hydroformylation of 1-hexene up to an l b ratio of 123 1. 

Electronic effects of diphosphine ligands have been reported by Unruh, Casey and Van Leeuwen [16-18]. Unruh studied the hydroformylation of 1-hexene using substituted ferrocene derived diphosphines Fe(C5H4PR2)2 (Fig. 6.4). 

The diphosphines 33 and 34 were tested as ligands in the rhodium-catalyzed hiphasic hydroformylation of propene. Both catalysts were foimd to exhibit higher activities and gave rise to higher l/b ratios [68] than TPPTS. Furthermore, it was shown that displacement of the biphenyl unit of 34 by a binaphthyl unit in 33 leads to an increase of the catalytic activity which was ascribed to electronic effects. 

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