Modelling of Transition State in Grignard Reaction of Rigid N-(Aryl)imino-Acenapthenone (Ar-BIAO): A Combined Experimental and Computational Study

We present a combined synthetic and computational study on the addition of Grignard reagents RMgBr/RMgI (R = Me, Et) to various sterically rigid N-(aryl)imino-acenapthenone (Ar-BIAO) (Ar = 2,6-iPr2C6H3 (1), 2,6-Me2C6H3 (2), and 2,4,6-Me3C6H2 (3) ligands). In the experimental method, when compounds 1–3 were treated with RMgBr (R = Me, Et) at room temperature, the corresponding racemic N-(aryl)imino-acenapthylene-1-ol (Ar-BIAOH) compounds (Ar = 2,6- iPr2C6H3, R = Me (1a), Et (1b); Ar = 2,6-Me2C6H3, R = Me (2a), Et (2b); and Ar = 2,4,6-Me3C6H2, R = Me (3a), Et (3b)) were obtained in yields up to 82 %. The Ar-BIAOH compounds were characterized by spectroscopic and combustion analyses. The solid state structures of compounds 1a–3a were established by single-crystal X-ray diffraction analysis. To model the transition state of the Grignard reaction with asymmetrical and sterically rigid Ar-BIAO ligands having three fused rings containing exo-cyclic carbonyl and imine functionalities, we carried out computational analysis. During our study, we have considered the gas phase addition of CH3MgBr to 2 and the model system of 2-(methylimino)pentanone (2′). We have carried out ab initio (HF/3–21G*) and density functional theory calculations with the hybrid density functional B3LYP/6–311+G(2d,p) to probe two major aspects: (1) the stability of an intra-molecular chelation involving magnesium, carbonyl oxygen, and imine nitrogen and (2) to suggest a probable transition state and a mechanistic pathway. The computational investigation suggests the formation of a tetra-coordinated magnesium complex as the transition state for the Grignard reaction