Synthesis of Hypervalent Organobismuth-oxo Complexes and Application to Oxidation

在两种当量叔丁醇钾存在下，用过量水处理相应的三芳基二氯铋，制备了一系列系统的三芳基铋氧化物。我们揭示了这类化合物的结构、热稳定性和反应活性。铋的氧化物以二聚、水合或低聚形式存在，这取决于连接到铋中心的芳基配体的结构。研究发现，铋的氧化物是热不稳定的，很容易分解成三芳基铋。在不溶性铋衍生的副产物中，我们成功地用X射线结晶学表征了二芳基铋酸酐的结构。研究还发现，铋氧化合物的氧化能力远高于轻质硝酸铋的氧化能力，并通过在芳基配体上引入邻位取代基而显著增强。基于这一发现，我们开发了一种新的有效的方法，使用三芳基二氯化铋/DBU体系，在温和的条件下，伯醇和仲醇很容易被氧化成相应的羰基化合物。在该体系中，观察到显著的取代基对氧化能力的影响。因此，在对位引入吸电子取代基，在邻位引入甲基，提高了氧化速度。

A systematic series of triarylbismuthane oxides were prepared by treatment of the corresponding triarylbismuth dichlorides with excess water in the presence of two equivalents of potassium tert-butoxide. We disclosed the structures, thermal stability, and reactivity of this class of compounds. The bismuthane oxides exist as a dimeric, hydrated, or oligomeric form, depending on the structure of the aryl ligands attached to the bismuth center. It was found that the bismuthane oxides are thermally unstable and readily decompose to give triarylbismuthanes as the major products. Among the insoluble bismuth-derived byproducts, we successfully characterized the structure of a diarylbismuthinic anhydride by X-ray crystallography. It also was found that the oxidizing ability of the bismuthane oxides is much higher than those of lighter pnictogen counterparts, and is significantly enhanced by introducing ortho-substituents onto the aryl ligands. Based on this finding, we developed a new efficient method for alcohol oxidation using a triarylbismuth dichloride/DBU system, where primary and secondary alcohols were readily oxidized to the corresponding carbonyl compounds in good yields under mild conditions. In this system, the remarkably high substituent effects on the oxidizing ability were observed. Thus, introduction of an electron-withdrawing substituent at the para position and a methyl group at the ortho position enhances the rate of oxidation.