13C NMR Spectra of the dianions Li2-n resulting from double deprotonation of unsaturated carboxylicacids 1 to 6 by lithium dialkylamides, are examined. Higher field resonance for C2 than for C4 atoms are observed, which reveal higher elctron density at C2 than at C4, in agreement with the kinetic α-regioselectivity exhibited by these dianions on reaction with electrophiles.
考察了由不饱和羧酸1至6被二烷基酰胺锂双重去质子化后的二价阴离子Li 2 -n的13 C NMR光谱。观察到C 2的原子共振高于C 4原子,这表明C 2的电子密度高于C 4的电子,这与这些二价阴离子与亲电子试剂反应所表现出的动力学α-区域选择性一致。
Unsaturated carboxylic acid dienolates. Addition to substituted cyclohexanones. Inverted kinetic and thermodynamic stereoselectivities.
Addition of the lithium dienolate derived from crotonic acid to monosubstituted cyclohexanones occurs through the α and γ carbon atoms of the dienolate in the cold and on heating, respectively. For any regioselectivity, equatorial approach is found under kinetic conditions, but equilibration favours products from axial attack and selectivity may be inverted.