钼催化芳香醛脱氧偶联反应机制的理论研究

doi:10.6023/cjoc202405018

摘要/Abstract

钼/邻醌配合物在适当还原剂存在下可以促进羰基脱氧并生成关键的钼卡宾中间体, 进而发生后续转化. 本文对钼催化芳香醛脱氧偶联反应机制进行了详细的理论研究. 计算结果表明, Mo(IV)配合物(含两个邻醌配体)比文献提出的Mo(II)配合物(含一个邻醌配体)更有利于促进芳香醛脱氧生成钼卡宾中间体, 提出了一种新颖的外球分步脱氧机制: 还原剂三苯基膦(PPh₃)与醛基氧结合生成P—O键, 随后醛基部分C—O键断裂生成关键的Mo(IV)卡宾中间体. 而文献中普遍认为的羰基直接在Mo(II)上发生氧化加成得到氧钼卡宾中间体并非有利路径. 生成Schrock型Mo(IV)卡宾后, 底物芳香醛可以与该中间体发生[2＋2]环加成, 进而通过复分解模式得到最终产物. 此外, 讨论了形成Mo(IV)卡宾的有利因素和烯烃产物顺反构型选择性产生的根源.

关键词: 钼催化, 脱氧偶联, 卡宾, 反应机制, 密度泛函理论计算

Mo/o-quinone complexes have shown great capability in promoting deoxygenation of carbonyl groups in the presence of appropriate reducing agents, which yields key Mo-carbene complexes and subsequently undergoes further transformations. However, the detailed mechanistic pathways for the deoxygenation of carbonyl groups with the assistance of additive remain unclear. Herein, a comprehensive density functional theory (DFT) study was performed to gain mechanistic insights into the Mo-catalyzed deoxygenative coupling of aromatic aldehydes to produce diaryl alkenes with the assistance of triphenylphosphine (PPh₃) as a reductant. Computational results suggest that the Mo(IV) complex (with two o-quinone ligands) is more efficient than the commonly proposed Mo(II) complex (with one o-quinone ligand) in mediating the deoxygenation of aromatic aldehyde to yield a critical Mo-carbene intermediate. An outer-sphere stepwise mechanistic pathway is suggested for the PPh₃ assisted deoxygenation of aromatic aldehyde, which proceeds through the generation of an adduct with aldehyde via P—O bond formation followed by the breaking of C—O bond of aldehyde to give the key Mo(IV)-carbene intermediate. The commonly proposed oxidative addition of carbonyl group onto Mo(II) to form an oxo-Mo-carbene intermediate might not be feasible. After the formation of the Schrock-type Mo(IV) carbene intermediate, a metathesis mechanistic pathway via a [2＋2] cycloaddition adduct is reasonable to afford the final product. The factors accounting for the formation of Mo(IV) carbene and the stereo-selectivity of the product are discussed.

Key words: Mo catalysis, deoxygenative coupling, carbene, reaction mechanism, density functional theory (DFT) calculation

引用此文

孙庆浩, 鲍晓光. 钼催化芳香醛脱氧偶联反应机制的理论研究[J]. 有机化学, 2024, 44(11): 3518-3525.

Qinghao Sun, Xiaoguang Bao. Computational Insights into the Mechanism of the Mo-Catalyzed Deoxygenative Coupling of Aromatic Aldehydes[J]. Chinese Journal of Organic Chemistry, 2024, 44(11): 3518-3525.

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图/表 9

Scheme 1. Two types of Mo carbene complexes

Scheme 2. Divergent transformations via Mo-carbene intermediates in the Mo/o-quinone catalyzed deoxygenation of carbonyl groups (a~d) and (e) possible mechanistic pathways in leading to the key Mo-carbene intermediate

Figure 1. Energy profiles (in kcal/mol) for the formation of Mo(II)-carbene complex through the proposed direct C=O bond breaking of 1 via paths I and II (bond lengths are shown in ?)

Figure 2. Energy profiles (in kcal/mol) for the formation of Mo(II)-carbene complex through the PPh3 assisted stepwise manner (path III) (bond lengths are shown in ?)

Figure 3. Energy profiles (in kcal/mol) for the formation of Mo(IV)-carbene complex through the proposed direct C=O bond breaking of 1 via paths I and II (bond lengths are shown in ?)

Figure 4. Energy profiles (in kcal/mol) for the formation of Mo(IV)-carbene complex through the proposed PPh3 assisted stepwise manner (path III) (bond lengths are shown in ?)

Figure 5. Energy profiles (in kcal/mol) for the formation of the product via Mo(IV)-carbene involving carbonyl olefination (bond lengths are shown in ?)

Figure 6. 3D structures of the Mo(II) and Mo(IV) stabilized adducts of PPh3 with aldehyde (INT10a and INT9b) and the respective Mo-carbene complexes (INT12a and INT11b) (the NBO charges for Mo and the carbon of carbene are also shown)

Figure 7. Proposed mechanistic pathway for the Mo-catalyzed deoxygenative coupling of aromatic aldehydes to yield diaryl alkenes with the assistance of PPh3

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