摘要
氢键是调节各类催化反应体系中分子行为的重要作用力,能够有效活化分子、改变作用位置及控制反应途径,具有重要的科学意义和应用潜力.然而,氢键的利用过程存在着惰性效应难题,严重阻碍反应的顺利进行.如何有针对性地打破惰性的氢键作用力,探索氢键的构建与破坏机制,成为提高反应转化效率和选择性的重要途径,有利于实现高原子经济性并减少碳足迹.芳香烃氧化制羧酸是工业上重要的催化氧化过程,但存在高能耗、副产物多以及二氧化碳排放等问题.本文以甲苯液相氧气氧化制苯甲酸为模型反应,提出了旨在克服惰性氢键的"溶剂剪刀"策略.通过利用氢键受体溶剂(如醋酸、醋酸乙酯、氯乙酸乙酯或氯乙酸甲酯)作为破坏试剂,与强氢键供体溶剂六氟异丙醇(HFIP)形成氢键,从而将关键中间体苯甲醛从惰性的氢键结合状态中释放出来,将甲苯在温和条件下高效氧化为苯甲酸.使用NHPI/金属醋酸盐/HFIP-HOAc作为催化体系,在温和的反应条件(45 ℃,0.1 MPa O2)下反应4 h,甲苯转化率达到96.8％,苯甲酸选择性达98.7％.该方法不仅成功实现了甲苯到苯甲酸的高效转化,还可进一步拓展至邻(间,对)甲基甲苯等甲基芳烃,在25-45 ℃和常压氧气条件下高效氧化制备芳香羧酸.利用变浓度核磁共振实验,定量测定了氢键供体HFIP与不同"剪刀溶剂"形成氢键的标准吉布斯自由能变化(△G0).结果显示,-△G0顺序为醋酸＞醋酸乙酯＞氯乙酸乙酯＞氯乙酸甲酯,其中醋酸的△G0达到-4.319 kJ/mol,具有最强的氢键裁剪能力.实验显示,-△G0与周转次数(TON)值(促进苯甲醛氧化的能力)的顺序一致,表明氢键裁剪能力越强的"溶剂-剪刀"越有利于促进苯甲醛的氧化.综上,本文利用"溶剂-剪刀"重建了甲基芳烃氧化反应中的氢键平衡体系,切断了使苯甲醛惰性化的氢键,促进游离苯甲醛的释放,使甲基芳香烃能在温和条件下高效氧化为芳香酸.本研究为有机合成普遍存在的氢键惰性化效应提供了解决思路,有利于促进"氢键调控"在有机合成的应用.
Abstract
Hydrogen bonding is a fascinating interaction that controls the outcomes of chemical reactions.However,overcoming the strong deactivation arising from alterations in the polarity and electronic properties of the reactants and intermediates remains a challenge.Herein,we proposed a"sol-vent-scissors"strategy for overcoming the inert hydrogen bonding,enabling the efficient aerobic oxidation of methyl aromatics into aromatic acids under atmospheric oxygen at 25-45 ℃.The hy-drogen bonds between the key intermediate,benzaldehyde(PhCHO),and hexafluoroisopropanol(HFIP)were reconstructed using solvent-scissors(acetic acid(HOAc),ethyl acetate,ethyl chloro-acetate,and methyl chloroacetate),which promoted the release of free PhCHO from its inert hydro-gen-bonded state and enabled the one-step oxidation of toluene to benzoic acid under mild condi-tions.The standard Gibbs free energy changes(△G0)representing the proton acceptance capability of the solvent were of the same order of magnitude as the turnover number(TON)(capacity for promoting benzaldehyde oxidation).This approach affords remarkable benzoic acid selectivity(98.7％)with high toluene conversion(96.8％)at 45 ℃ within 4 h under 0.1 MPa O2 using NHPI/metal acetate/HFIP-HOAc.This strategy opens up a new avenue for regulating hydrogen bonding in a wider range of applications for the planning and development of synthesis protocols.
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