A computational study of the mechanism of the elimination hydrogen halides on alkenes to allenes: C-H activation mechanism

The study have been examined by density functional theory (DFT/B3LYP/6-31G*)calculations.All reactionsdid on hydrogen halides elimination reactions withalkenes toachieve allenesin the syn and anti-eliminations(X=F,Cl, Br).In this study, we report a newmechanism for the hydrogen halides elimination reaction in syn form.This reaction pathway isconcerted by C-H activation.Examination of the energy minimum and IRC diagram ondifferent transition state structures reveals that the elimination is done anti form forfluorineand syn for bromine and iodine.Also, in the case of syn elimination, we haveinvestigated the quickly remove of halogens. The results showed that quickly remove thehalogen elements follows asBr>Cl>F in both syn and anti-eliminations. According these resultswe found a good correlation between quickly remove of halogens and their atomic radius,atoms with larger radiuscould be quickly removed by Syn elimination, so fluorineatom due tothe small radius could not be removed quickly and elimination could just be done in antiformwithtwostage.Inanti-elimination,the first stage is removingthe hydrogen by baseandmaking the carbanionwould take place,then in second stage halogen go out with a backcarbanion (similar to the E1 mechanism), but syn-elimination is just done in one-step, in thecase of this mechanism, halogen insert to C-H next bond (C-H activation)and with formationof a three-membered ring, hydrogen halide removed. This reaction passes from a threememberedtransition state.

کلیدواژه ها:

Alkene ، Allene ، Transition state ، C-H activation ، Mechanism ، Anti elimination ، Syn elimination