Remarkable Substituent Effects on the Oxidizing Ability of Tetraarylbismuthonium Tetrafluoroborates in Alcohol Oxidation
作者:Yoshihiro Matano、Takeshi Suzuki、Takaharu Iwata、Tomonori Shinokura、Hiroshi Imahori
DOI:10.1246/bcsj.81.1621
日期:2008.12.15
Substituent effects on the oxidizing ability of tetraarylbismuthonium tetrafluoroborates in alcohol oxidation are reported. Intermolecular and intramolecular competition experiments on geraniol oxidation by the combined use of tetraarylbismuthonium tetrafluoroborates and N,N,N′,N′-tetramethylguanidine (TMG) have revealed that the oxidizing ability of the bismuthonium salt increases by the introduction of methyl groups at the ortho position and an electron-withdrawing group at the para position of the aryl ligands. The intermolecular and intramolecular H/D kinetic isotope effects observed for the competitive oxidation of p-bromobenzyl alcohols have shown that the present oxidation reaction consists of fast pre-equilibrium leading to alkoxytetraarylbismuth(V) intermediates (first step) and α-hydrogen abstraction by the aryl ligand attached to the bismuth (second step). The experimental results demonstrate that the electron-deficient aryl ligands enhance the electrophilicity at the bismuth center to put forward the first step and that the bulky ligands destabilize the alkoxybismuth(V) intermediates to accelerate the second step. The newly explored mesityl- and 2,6-xylyltriarylbismuthonium salts have proven to convert primary and secondary alcohols to the corresponding carbonyl compounds with high efficiency under mild conditions. A remarkable steric effect of these oxidants has also been exhibited in the chemoselective oxidation between primary and secondary benzylic alcohols.
本文报道了四芳基铋鎓四氟硼酸盐在醇氧化中取代基效应对其氧化能力的影响。通过使用四芳基铋鎓四氟硼酸盐和N,N,N′,N′-四甲基胍(TMG)联合进行的香叶醇氧化分子间和分子内竞争实验揭示,当甲基引入芳基配体的邻位和吸电子基团引入对位时,铋鎓盐的氧化能力增强。在p-溴苯甲醇竞争氧化中观察到的分子间和分子内H/D动力学同位素效应表明,目前的氧化反应包括快速前置平衡,导致生成烷氧基四芳基铋(V)中间体(第一步)和与铋相连的芳基配体对α-氢的夺取(第二步)。实验结果表明,缺电子芳基配体增强了铋中心的亲电性,推动了第一步,而庞大的配体破坏了烷氧基铋(V)中间体的稳定性,加速了第二步。新探索的间甲苯基和2,6-二甲苯基三芳基铋鎓盐在温和条件下已被证明能高效地将伯醇和仲醇转化为相应的羰基化合物。这些氧化剂在伯醇和仲醇的化学选择性氧化中也表现出显著的立体效应。