Estudo teórico-experimental do estado de transição da redução de cicloexanona s-2-monossubstituídas, com hidretos metálicos (LiAIH4 e NaBH4)

Abstract

Transition state (TS) structures for the reduction of the 2-X cyclohexanones series with LiAlH4 (X = SMe, OMe, SeMe, Me and iso-propyl) and NaBH4 (X = F, Cl and Br), were optimized by density functional theory (B3LYP/6-31G(d,p). Four transition state structures corresponding to the axial and equatorial reductor attacks on the carbonyl π-plane and also the way that it is complexes with the carbonyl oxigen were located for each ketone conformer. Electronic potential maps (EPM) and natural bond orbitals (NBO) analysis were used to investigate the electronic effect of the substituent group on the stabilization of transition states (TSs). Furthermore, it was analyzed the uneven carbonyl orbital distribution in LUMO (π*). The observed results to the 2-X-monosubstituted cyclohexanones series, for X = SMe, OMe e SeMe, are in disagreement between energetic preference and the TSs tension, showing the Li---X interaction as the main factor responsible for the energetic preference. The ketone conformational equilibrium showed to be determinant in the TSs formation and the products ratio. For X = methyl and iso-propyl series, the conformational equilibrium as well torsional tension present in the TS, were responsible for the stereoselectivity, once the Li--X interaction was not observed. The obtained results from halogenated compounds (X = F, Cl e Br) were similar between F and Cl, mainly in torsional tension and energetic balance between some TSs, which are responsible for the stereoselectivity. The brominated ketone reduction was different from F and Cl derivatives, presenting in some ETs the Li---X interaction. This effect together with torsional tension in the TSs and ketone conformational equilibrium are responsible for the stereoselectivity. In general, reduction stereoselectivity of the 2-X-monosubstituted ketones series showed to be dependent on both ketone conformational ratio, substituent nature and reaction transition state.