Computational Insights into the Mechanism of the Mo-Catalyzed Deoxygenative Coupling of Aromatic Aldehydes

doi:10.6023/cjoc202405018

Abstract

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

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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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Fig. & Tab. 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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