Research On The Reducing Ability Of HfCl₄/Borohydride Systems

Borohydride complex system is one of the most important reducing systems of functional-group transformations. In recent years, various combinations of borohydrides (NaBH4, KBH4) and certain additives, such as metal halides, protonic acids, and halogens have been reported. However, most of the reported reducing systems suffer from various drawbacks, like lower chemoselectivity (e. g., AlCl3/NaBH4), weaker reducing ability (e. g., MgCl2/KBH4,H2SO4/NaBH4), etc.. In this dissertation, efficient and chemoselective reductions of some derivatives, like carboxylic acids and their derivatives, sulfoxides, disulfides, epoxides and so on by new borohydride complex systems (HfCl4/kBH4 and HfCl4/NaBH4 systems) are reported.Reduction of carboxylic acids and their derivatives by HfCl4/borohydride systems is firstly studied in this dissertation. Considering the effect of solvent, reaction temperature, and molar ratio of substrate/HfCl4/borohydride on the reaction, the experimental conditions of HfCl4/KBH4 and HfCl4/NaBH4 systems are optimized respectively. Then under optimum conditions, aliphatic, aromatic carboxylic acids and their derivatives can be reduced to the corresponding alcohols in excellent yields (up to 98%) at 40℃without any change to halogen-, and nitro-groups in substrates.HfCl4/KBH4 and HfCl4/NaBH4 systems are also attempted to reduce sulfoxides. The experimental conditions are optimized firstly. Then under optimized mild conditions, HfCl4/KBH4 and HfCl4/NaBH4 systems are found to be facile, efficient, and chemoselective reagents for the deoxygenation of dialkyl, diaryl, and aryl alkyl sulfoxides in excellent yields (up to 99%), especially for deoxygenation of diphenyl sulfoxide in yields of 93% and 98% respectively and dibenzyl sulfoxide in yields of 97% and 99%, which are difficult to reduce by other reagents. Besides, functional groups, such as halogen-, nitro-, and methoxy-groups are tolerated in the reduction process. Meanwhile, the microwave-assisted reduction of dibenzyl sulfoxide under solventless and environmental conditions by HfO2/NaBH4, nano-HfO2/NaBH4, HfCl4/nano-HfO2/NaBH4 or HfCl4/Hf02/NaBH4 system has been developed, but it is found that only the deoxygenation by nano-HfO2/NaBH4 system reaches a relatively high yield of 37%.Furthermore, the reduction and competitive reduction of some other substrates by HfCl4/borohydride systems are investigated. And the results are shown as follows. By using HfCl4/KBH4 system, m-CPBA is reduced to 73% of 3-chlorobenzyl alcohol, and disulfides can be rapidly reduced to the corresponding thiols or thiophenols. By using HfCl4/NaBH4 system, benzophenone is reduced to a mixture of diphenylmethanol and diphenylmethane (molar ratio= 73:27) at 165℃. However, the attempted reduction of sulfolane and diphenyl sulfone results in only the recovery of starting materials.p-Toluenesulfonic acid and p-toluenesulfonyl chloride can not be reduced to the corresponding thiophenols. Meanwhile, in research after the reductive ring-opening reaction of epoxides by HfCl4/KBH4 system, it is found that styrene oxide is regioselectively reduced to 78% of 2-phenylethanol without the formation of 1-phenylethanol. Besides, competitive reduction of styrene oxide in the presence of carboxylic acid, carboxylic ester, nitrile and amide is achieved at room temperature by HfCl4/KBH4 system, which features chemo-and regio-selective reduction of epoxide. Compared with catalystic hydrogenation and other chemical reduction methods, it is more facile, efficient, and chemoselective.