Syntheses, Structures, and Redox Properties of 1,4-Bis(dimesitylphosphino)-2,3,5,6-tetrafluorobenzene and the Corresponding Bis(phosphoryl) and Bis(phosphonio) Derivatives
作者:Shigeru Sasaki、Yoshihiro Tanabe、Masaaki Yoshifuji
DOI:10.1246/bcsj.72.563
日期:1999.3
Sterically hindered 1,4-bis(dimesitylphosphino)-2,3,5,6-tetrafluorobenzene (1) was synthesized by the aromatic nucleophilic substitution of lithium dimesitylphosphide with hexafluorobenzene. Two phosphorus atoms of 1 were oxidized and methylated to give the corresponding bis(phosphoryl)benzene 2 and bis(phosphonio)benzene 32+·2TfO− (TfO− = trifluoromethanesulfonyl), respectively. On the other hand, reaction of 1 with butyllithium and phenyllithium gave more crowded 2,5-dibutyl-1,4-bis(dimesitylphosphino)-3,6-difluorobenzene (4) and 1,4-bis(dimesitylphosphino)-2,5-difluoro-3,6-diphenylbenzene (5), respectively. Structures of 1, 2, 32+·2TfO−, 4, and 5 were confirmed by conventional spectroscopic methods; particularly 19F NMR spectroscopy reflected their crowded structures. Molecular structures of 1, 2, and 32+·2TfO−, were further investigated by X-ray crystallography, where unusually large bond angles around phosphorus atoms were observed. Electrochemical measurements were carried out to investigate redox properties of 1, 32+·2TfO−, 4, and 5. Although the cyclic voltammogram of diphosphine 1 showed irreversible oxidation waves above 0.8 V, in spite of substitution at all the ortho positions to the phosphorus atoms in 1, displacement of the two fluorine atoms of 1 by the butyl or the phenyl groups lowered the oxidation potential and improved the stability of the corresponding radical cations. Particularly, 5 clearly showed two-step oxidation waves. On the other hand, bis(phosphonium salt) 32+·2TfO− showed two steps of quasi-reversible redox waves, which suggested reduction to the cation radical and the neutral species, although their stability was limited.
立体阻碍的1,4-双(二美克磷基)-2,3,5,6-四氟苯(1)是通过锂二美克磷化物与六氟苯的芳香族亲核取代反应合成的。1中的两个磷原子被氧化并甲基化,分别生成相应的双(磷酰)苯2和双(磷阳离子)苯32+·2TfO−(TfO− = 三氟甲烷磺酸根)。另一方面,1与丁基锂和苯基锂的反应分别生成更为拥挤的2,5-二丁基-1,4-双(二美克磷基)-3,6-二氟苯(4)和1,4-双(二美克磷基)-2,5-二氟-3,6-二苯基苯(5)。1、2、32+·2TfO−、4和5的结构通过常规光谱学方法得到了确认;尤其是19F NMR光谱反映了它们的拥挤结构。分子结构1、2和32+·2TfO−进一步通过X射线晶体学进行了研究,观察到磷原子周围异常大的键角。进行了电化学测量以研究1、32+·2TfO−、4和5的氧还原性质。尽管二膦化合物1的循环伏安图在0.8 V以上显示出不可逆的氧化波,但尽管在1中的磷原子所有的邻位均被取代,1中的两个氟原子被丁基或苯基取代后,氧化电位降低并改善了相应的自由基阳离子的稳定性。特别是,5清晰地显示出两步氧化波。另一方面,双(磷盐)32+·2TfO−显示出两个准可逆的氧还原波,这表明其还原到阳离子自由基和中性物种,尽管它们的稳定性有限。