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Ab initio study of reactions of 1‐halogen‐3‐methoxy‐1‐propynes with Grignard reagent

“…According to semiempirical calculations [15], the polarization of halogenOC ␣ -atom bond is X (Table I), although one should be cautious with the Mulliken charge distributions and should understand the possible inaccuracies. At the same time, the similar XOC ␣ bond polarization was recently obtained for the mono-substituted halogen acetylenes at the B3LYP/6-31ϩϩG** level, by natural population analysis [18].…”

“…The trans-position of the halogen atom and the MgCl group should be conducive to their elimination because the acetylene bent structure appearing by trans-decomposition passes into an equilibrium linear form much more easily than the structure formed by cis-decomposition [19]. According to MNDO calculations with counting the solvated Grignard reagent [15], the activation barrier in this reaction stage is a little lower than in the first stage. The performance of ab initio calculations for the decomposition of intermediate A have been hampered by the coordination of magnesium with the oxygen of the methoxy group, when both MgCl and the X atom are taken out of the plane of the ethylene fragment.…”

“…The semiempirical Modified Neglect of Diatomic Overlap (MNDO) calculations previously carried out for the reaction of 1-halogen-3-methoxy-1-propynes (I) with Grignard reagents (molecule CH 3 MgBr and its solvated form CH 3 MgBr ⅐ 2O(CH 3 ) 2 ) [15] show that the initial magnesium coordination at the -system of the triple bond of molecules I promotes the nucleophilic substitution of the halogen atom by an addition-elimination mechanism with the formation of the intermediate of vinylic structure (A): (4) PM3 calculations predict the complexes, with the coordination bonding of magnesium and halogen atoms being more stable [16].…”