For Preparation of Large Bite Angle Metal Complexes

4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene (1), as bidentate phosphine ligand, forms stable complexes with transition metals such as palladium and rhodium. These complexes have a large bite angle, and they are used in organic synthetic reactions as highly selective catalysts.

For example, Kranenburg and co-workers carried out an alkylation reaction between 2-hexenyl acetate and the carbanin of diethyl malonate in the presence of various bidentate phosphine ligands and palladium, and made a comparative study on the reactivity of the various products derived. According to this research, the palladium complex catalyst in which 1 is used as the ligand has the largest bite angle and its selectivity is very high. Moreover, they reported that only diethyl 2-(2-hexane-1-yl)-2-methyl malonate (3) was produced, with no diethyl 2-(1-hexane-3-yl)-2-methyl malonate (4) by-product.¹⁾ In addition, Kranenburg and co-workers reported that n-aldehyde is formed with high selectivity in the hydroformylation reaction of 1-olefin in which the complex composed of 1 and rhodium is used as the catalyst.²⁾

On the other hand, Sandee and co-workers used 4,6-bis(diphenylphosphino)phenoxazine (2) as a starting material to synthesize the silica gel-supported rhodium complex catalyst 5. This catalyst 5 is also utilized for highly selective hydroformylation reactions since it too has a large bite angle nearly equal to that of the complex catalyst composed of 1 and rhodium, and it is easy to recover and reuse.³⁾