可见光诱导Z型V_(2)O_(5)/g-C_(3)N_(4)异质结催化未活化烯烃串联反应

半导体光催化剂是一种极具前景的绿色催化剂,广泛用于污染物降解、水解制氢和有机合成等领域,有望利用太阳能来解决能源和环境问题,是当前的研究前沿和热点.然而,单组分半导体光催化剂的光生电子和空穴容易复合,导致量子效率差和光催化效率低.近年人们发现,将两种或多种催化材料结合,构建异质结光催化体系可有效促进光生电子-空穴分离.但传统的异质结体系中光生电子的还原性和光生空穴的氧化性通常在电荷转移后变弱,因此,很难同时具备高电荷转移效率和强氧化还原能力.研究发现,构建Z型异质结光催化体系不仅可以减少本体电子-空穴的复合,使其在不同半导体材料上实现空间分离,具有光谱响应宽、电荷分离效率高和稳定性高等优势,而且能保持良好的氧化还原能力.在半导体材料领域,石墨相氮化碳(g-C_(3)N_(4))作为一种无金属聚合物半导体,具有良好的热化学稳定性、电学和光学特性,但存在量子效率低和适用范围窄等局限性.而五氧化二钒(V_(2)O_(5))是一种重要的过渡金属氧化物半导体,由于具有良好的电学和光学性能被广泛用于锂离子电池、气敏传感器和光电器件.V_(2)O_(5)能带间隙(~2.19 e V)窄,具有合适的能量频带边缘(ECB=0.81 e V,EVB=3.0 e V),可以与g-C_(3)N_(4)(ECB=1.14 e V,EVB=1.59 e V)很好地匹配,形成稳定状态的Z型光催化体系,并提高光催化有机合成反应的效率.本文以三聚氰胺和偏钒酸铵为原料,采用热处理法分别制备g-C_(3)N_(4)和V_(2)O_(5),采用水热法制备Z型V_(2)O_(5)/g-C_(3)N_(4)二元复合材料.X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)和紫外-可见光吸收光谱(UV-Vis)等结果表明,成功制备了Z型V_(2)O_(5)/g-C_(3)N_(4).UV-Vis结果表明,V_(2)O_(5)/g-C_(3)N_(4)具有较宽的光吸收范围,从而提高了复合半导体材料的光学性能.在温和条件下,以未活化烯烃修饰的喹唑啉酮和芳基氧膦为反应物,V_(2)O_(5)/g-C_(3)N_(4)为多相光催化剂,进行膦酰化自由基偶联反应,制得一系列环合的膦酰化喹唑啉酮,收率为63%-83%.该反应具有原料易得、条件温和、底物范围广、产品收率及区域选择性良好等优点,同时催化剂循环使用性能良好.值得注意的是,不同吸电子取代基、供电子取代基修饰的喹唑啉酮和非对称结构的芳基氧膦均能兼容于该反应体系,并以中等至良好的收率得到了各种膦酰化喹唑啉酮化合物.本文采用的合成策略同样适用于三氟甲基化、二氟烷基化和芳基磺酰化等自由基串联环化反应,且具有良好的催化性能.机理研究结果表明,V_(2)O_(5)/g-C_(3)N_(4)被光激发后,V_(2)O_(5)导带(CB)上的光生电子与g-C_(3)N_(4)价带(VB)上的光生空穴迅速复合,导致g-C_(3)N_(4)的导带上无法与本体空穴复合的电子发生单电子转移(SET)过程,且与分子氧(空气中)反应生成超氧阴离子自由基(O_(2)·^(-)).V_(2)O_(5)价带上的空穴氧化芳基氧膦产生自由基阳离子,去质子化产生氧膦自由基,随后加成到未活化烯烃生成新的自由基物种,最后发生分子内环化反应,得到目标产物.V_(2)O_(5)/g-C_(3)N_(4)成本较低,且该光催化反应策略可实现克级制备,循环使用5次后催化活性保持不变.综上,本文可为光催化自由基串联环化反应,杂环化合物合成研究和Z型异质结的光催化应用提供参考.

Visible-light-initiated tandem synthesis of difluoromethylated oxindoles in 2-MeTHF under additive-,metal catalyst-,external photosensitizer-free and mild conditions

An efficient and eco-friendly protocol for synthesizing difluoromethylated oxindoles through a visiblelight induced one-pot tandem reaction of N-arylacrylamides,difluoroacetic acid and Ph I(OAc)_(2)was developed.This reaction proceeded in the absence of any additive,base,metal-catalyst and external photosensitizer,using cheap and easily available CHF_(2)CO_(2)H as the difluoromethylation reagent and bulk biomass-derived 2-Me THF as the sole solvent.26 Examples of N-arylacrylamide substrates were investigated,and all of them successfully underwent difluoromethylation to deliver the target products in good to excellent yields.

Practical and sustainable approach for clean preparation of 5-organylselanyl uracils

An eco-friendly,sustainable and practical method for the efficient preparation of 5-organylselanyl uracils through the electrochemical selenylation of uracils and dio rganyl diselenides at room temperature under oxidant-and external electrolvte-free conditions was developed.

Ultrasound-assisted tandem synthesis of tri-and tetra-substituted pyrrole-2-carbonitriles from alkenes,TMSCN and N,N-disubstituted formamides

An energy-saving and eco-friendly method for the efficient construction of various tri-and tetrasubstituted pyrrolecarbonitriles through ultrasound-assisted multicomponent tandem reaction of readily available alkenes,TMSCN and N,N-disubstituted formamides within 40 min under metal-,solvent-free and mild conditions was developed.The dual role of iodine(catalyst and oxidant)notably simplified the reaction conditions and reduced the chemical waste generated.