A Radical Mechanism for the Vanadium-Catalyzed Deoxydehydration of Glycols

We propose a novel mechanism for the deoxydehydration (DODH) reaction of glycols catalyzed by a [Bu4N]­[VO2(dipic)] complex (dipic = pyridine-2,6-dicarboxylate) using triphenylphosphine as a reducing agent. Using density functional theory, we have confirmed that the preferred sequence of reaction steps involves reduction of the V­(V) complex by phosphine, followed by condensation of the glycol into a [VO­(dipic)­(-O-CH2CH2-O-)] V­(III) complex (6), which then evolves to the alkene product, with recovery of the catalyst. In contrast to the usually invoked closed-shell mechanism for the latter steps, where 6 suffers a [3+2] retrocycloaddition, we have found that the homolytic cleavage of one of the C–O bonds in 6 is preferred by 12 kcal/mol. The resulting diradical intermediate then collapses to a metallacycle that evolves to the product through an aromatic [2+2] retrocycloaddition. We use this key change in the mechanism to propose ways to design better catalysts for this transformation. The analysis of the mechanisms in both singlet and triplet potential energy surfaces, together with the location of the MECPs between them, showcases this reaction as an interesting example of two-state reactivity.本研究提出了一种全新机理，用于由[Bu4N][VO2(dipic)]配合物（其中dipic代表吡啶-2,6-二羧酸根）催化、三苯基膦作为还原剂的二醇类化合物脱氧脱水（deoxydehydration, DODH）反应。本研究通过密度泛函理论（density functional theory, DFT）计算证实，该反应的最优步骤路径为：三苯基膦先将五价钒(V(V))配合物还原，随后二醇类化合物与还原后的配合物缩合生成[VO(dipic)(-O-CH2CH2-O-)]三价钒(V(III))配合物（中间体6），该中间体最终转化为烯烃产物，同时实现催化剂的再生。与后续步骤中通常援引的闭壳层机理（即中间体6发生[3+2]逆环加成反应）不同，本研究发现中间体6中任意一条C-O键的均裂路径的活化能比[3+2]逆环加成路径低12千卡/摩尔，为更优反应路径。生成的双自由基中间体随后坍塌为金属杂环，该金属杂环再通过芳香性[2+2]逆环加成反应转化为最终产物。本研究基于该机理中的关键变化，提出了优化该转化反应催化剂的设计思路。通过分析单重态与三重态势能面上的反应机理，并结合二者之间最低能量交叉点（minimum energy crossing points, MECPs）的位置，本研究证实该反应是双态反应性的典型案例。