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 con-ditions and significantly outperforms the best previously reported Lewis base, 1,2-dipiperidylethane, across a broad temperature range.

中文翻译：

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解锁区域选择性：烷基锂引发的丁二烯聚合中路易斯碱的空间位阻和构象限制


在非极性溶剂中，烷基锂引发的 1,3-丁二烯聚合表现出高 1,4-选择性，在添加路易斯碱后转变为 1,2-选择性。在过去的 50 年里，流行的假设表明 Lewis 碱基主要通过电子效应影响区域选择性。然而，我们的研究表明，空间位阻也起着至关重要的作用。利用 X 射线单晶衍射，我们分析了活性物质的结构，并提出了一种链生长过渡态的新模型。采用原位 NMR 波谱、同位素标记研究和密度泛函理论 （DFT） 计算等技术来比较各种路易斯碱基的电子和空间效应对区域选择性的影响。我们的研究结果表明，在 1,4-加成过程中，丁二烯单体被迫与 Lewis 碱基配体靠近，导致显着的空间干扰，从而有利于 1,2-加成。此外，我们应用了“构象限制”的概念来解释用含环的 Lewis 碱（如 1,2-二哌啶基乙烷）观察到的增强的 1,2-选择性。基于这种理解，我们设计了几种高效且具有成本效益的 Lewis 碱，它们在温和条件下可实现接近 100% 的 1,2-选择性，并且在很宽的温度范围内明显优于以前报道的最佳 Lewis 碱 1,2-二哌啶基乙烷。