Unlocking regioselectivity: steric effects and conformational constraints of Lewis bases in alkyllithium-initiated...

In nonpolar solvents, alkyllithium-initiated 1,3-butadiene polymerization exhibits high 1,4-selectivity, which shifts towards 1,2-selectivity upon the addition of Lewis bases. For the past 50 years, the prevailing hypothesis has suggested that Lewis bases primarily influence regioselectivity through electronic effects. However, our study reveals that steric hindrance also plays a crucial role. Using X-ray single-crystal diffraction, we analyzed the structure of the active species and proposed a new model for the chain-growth transition state. Techniques such as in situ 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.