Unlocking regioselectivity: steric effects and conformational constraints of Lewis bases in alkyllithium-initiated butadiene polymerization
Jian Tang, Yuan Fu, Jing Hua, Jiahao Zhang, Shuoli Peng, Zhibo Li
Abstract
NMR spectroscopy, isotope labeling studies, and density functional theory (DFT) calculations were employed to compare the impact of electronic and steric effects of various Lewis bases on regioselectivity. Our findings demonstrate that during 1,4-addition, the butadiene monomer is forced into close proximity with the Lewis base ligand, leading to significant steric interference and thus favoring 1,2-addition. Furthermore, we applied the concepts of "conformational restriction" to explain the enhanced 1,2-selectivity observed with ring-containing Lewis bases such as 1,2-dipiperidylethane. Building on this understanding, we have designed several highly efficient and cost-effective Lewis bases which achieves close to 100% 1,2-selectivity under mild conditions and significantly outperforms the best previously reported Lewis base, 1,2-dipiperidylethane, across a broad temperature range.