实现醇作烷基化试剂的光催化反应基础

Catalytic transformation of alcohols as alkylating reagents is one of the most important approaches to greenly build C-C and C-N bonds and cyclize either carbocyclic or heterocyclic rings. However, it is a long-standing challenge to realize this transformation by traditional noble-metal catalysis. In this research, we propose to use surface-modified and bulk-phase-doped TiO2 catalyst to realize this alkylation by photocatalysis. The main strategy is to confine the traditional unselective properties of direct activation of C-H, C-C, C-X and molecule oxygen by valence-band holes（Ti-O• and Ti-O•-Ti）and conductive-band electrons. And we would convert the valence-band holes into surface- and bulk- modified secondary redox species and tunnels such as Fe（IV）=O, Ti-N• and Ti-N•-Ti to abstract hydrogen from alcohols and coordinate to the hydrogen-transfer pathway by conductive-band Ti-H species to realize the cleavage of alcohol C-O bond and insert to the desired C-H, C-C and N-H bond and rings. Based on the successful pre-study of using ethanol as alkylating reagents to transform N-benzyl imine into tetrahydroimidazole compounds by modified-TiO2 photocatalyst, we would systematically study the pathways and mechanism of this high-yielding and highly-selective method of alcohol as alkylating reagents by surface- and bulk-phase-modified TiO2 photocatalyst, develop the wide scope of alcohols as alkylating reagents and study the influence of reaction conditions such as atmosphere, water and solvent polarity to the photocatalytic reaction. These studies would provide novel alternative and green approaches for the synthesis of molecules with bio-activities and pharmacological activities.

催化转化醇作烷基化试剂是今后绿色拼接C-C、C-N以及关环反应的重要途径之一，但传统的贵金属催化难以实现。本项目提出利用表面修饰、体相掺杂等改性的二氧化钛光催化策略完成该烷基化反应，主要通过构建限制传统二氧化钛光催化中价带空穴（Ti-O•和Ti-O•-Ti）和导带电子直接活化C-H、C-C、C-X和分子氧的无选择性特性的方法，而转化为表面、体相修饰的次级氧化还原物种如Fe（IV）=O或Ti-N•和Ti-N•-Ti反应通道对醇抽氢，耦合导带的Ti-H物种还原转移H步骤，完成醇的C-O键断裂结合烷基到需要的C-H、C-C、N-H或关环反应上。在预研改性TiO2光催化成功实现乙醇烷基化转化亚胺为四氢咪唑化合物的基础上，系统研究表面、体相修饰二氧化钛光催化高产率高选择性实现醇作为烷基化试剂的反应途径机理，验证醇作为烷基化试剂的范围。为今后合成具有不同生物活性和药物分子提供新的绿色途径。

本项目在温和条件下建立了TiO2化异相光催化体系，实现了利用醇类化合物进行烷基化反应，完成了无贵金属的C-烷基化和N-烷基化，并获得TiO2 基光催化剂与醇做底物进行亚胺烷基化反应之间的构效关系，发展了咪唑烷天然产物和药物分子光催化绿色催化合成方法。主要完成了三类重要的转化反应：亚胺和醇通过叶立德合成咪唑烷环结构、α，β-不饱和酮C=C双键的选择性还原、TiO2/Ni协同光催化体系的卤代芳烃脱卤氢化和卤代芳烃的烷氧基化反应，重点考察脂肪醇在TiO2价带空穴氧化以及脂肪醇作为最终的H给体和空穴受体在TiO2光催化剂价带实现选择性转化反应的控制因素，寻找避免活性氧物种和次级的碳中心自由基造成有机转化反应的低选择性的方法。