Tormena Claudio F, Dias Luiz C, Rittner Roberto
Departamento de Química, Faculdade de Filosofia Ciências e Letras de Ribeirão Preto, Universidade de São Paulo (USP), Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto SP, Brazil.
J Phys Chem A. 2005 Jul 14;109(27):6077-82. doi: 10.1021/jp052048a.
A theoretical study is employed to describe the orbital interactions involved in the conformers' stability, the energies for the stereoelectronic interactions, and the corresponding effects of these interactions on the molecular structure (bond lengths) for cis- and trans-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes. For cis-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes, two LPO --> sigmaC(2)-Me(8) interactions are extremely important and the energies involved in these interactions are in the range 6.81-7.58 kcal mol(-1) for the LP(O)(1) --> sigmaC(2)-Me(8) and 7.58-7.71 kcal mol(-1) for the LP(O)(3) --> sigmaC(2)-Me(8) interaction. These two LP(O) --> sigmaC(2)-Me(8) interactions cause an upfield shift, indicating an increased shielding (increased electron density) of the ketal carbon C(2) as well as the axial Me(8) group in the chair conformation. These LP(O) --> sigma*C(2)-Me(8) hyperconjugative anomeric type interactions can explain the 13C NMR chemical shifts at 19 ppm for the axial methyl group "Me(8)" and 98.5 ppm for the ketal carbon "C(2)". The observed results for the trans derivatives showed that for compounds 2a-c (R = -CN, -C[triple bond]CH, and -CHO, respectively) the chair conformation is predominant, whereas for 2d,f-h [-CH3, -Ph, -C6H4(p-NO2), -C6H4(p-OCH3), respectively] the twist-boat is the most stable compound and for 2e [-C(CH3)3] is the only form.
采用理论研究来描述顺式和反式4,6-二取代-2,2-二甲基-1,3-二氧六环构象异构体稳定性中涉及的轨道相互作用、立体电子相互作用的能量以及这些相互作用对分子结构（键长）的相应影响。对于顺式4,6-二取代-2,2-二甲基-1,3-二氧六环，两个LPO→σC(2)-Me(8)相互作用极其重要，对于LP(O)(1)→σC(2)-Me(8)相互作用，这些相互作用涉及的能量范围为6.81 - 7.58 kcal mol(-1)，对于LP(O)(3)→σC(2)-Me(8)相互作用为7.58 - 7.71 kcal mol(-1)。这两个LP(O)→σC(2)-Me(8)相互作用导致了高场位移，表明在椅式构象中缩酮碳C(2)以及轴向Me(8)基团的屏蔽增加（电子密度增加）。这些LP(O)→σ*C(2)-Me(8)超共轭异头型相互作用可以解释轴向甲基“Me(8)”在19 ppm处和缩酮碳“C(2)”在98.5 ppm处的13C NMR化学位移。反式衍生物的观察结果表明，对于化合物2a - c（R分别为 - CN、-C≡CH和 - CHO），椅式构象占主导，而对于2d,f - h [-CH3、-Ph、-C6H4(p - NO2)、-C6H4(p - OCH3)，分别]，扭船式是最稳定的化合物，对于2e [-C(CH3)3]是唯一的形式。
