Alkyl-Substituted Allylic Lithium Compounds: Structure and Dynamic Behavior

Abstract

Several methyl-substituted allylic lithium compounds have been prepared by CH3Li cleavage of their corresponding bis(methyl)bis(allylic)stannanes. Low-temperature 13C and proton NMR studies of 1:1 complexes of these allylic lithium compounds with TMEDA establish their structures. NMR line shape changes with temperature provide barriers to rotation. Results are listed in order as follows (allyl substituents, compound number, barrier to rotation in kcal·mol-1, and bonds undergoing rotation): 1,1-dimethyl, 26, 18, 2−3; endo-1-methyl, 27, 19, 2−3; endo-1-exo-3-dimethyl, 28, 21, 1−2 and 2−3. These observations together with the allylic 13C NMR chemical shifts indicate that in the case of unsymmetrical alkyl substitution at the termini the allyl C−C bond to the more substituted terminus is of higher bond order than that to the less substituted terminus. Unsymmetrical substitution is proposed to reduce the degree of delocalization compared to the symmetrically substituted allylic lithium compounds. A mechanism ...