We have quantum chemically explored the competition between the S_N2 and S_N2′ pathways for X^– + H₂C═CHCH₂Y (X, Y = F, Cl, Br, I) using a combined relativistic density functional theory and coupled-cluster theory approach. Bimolecular nucleophilic substitution reactions at allylic systems, i.e., C^γ═C^β–C^α–Y, bearing a leaving-group at the α-position, proceed either via a direct attack at the α-carbon (S_N2) or via an attack at the γ-carbon, involving a concerted allylic rearrangement (S_N2′), in both cases leading to the expulsion of the leaving-group. Herein, we provide a physically sound model to rationalize under which circumstances a nucleophile will follow either the aliphatic S_N2 or allylic S_N2′ pathway. Our activation strain analyses expose the underlying physical factors that steer the S_N2/S_N2′ competition and, again, demonstrate that the concepts of a reaction’s “characteristic distortivity” and “transition state acidity” provide explanations and design tools for understanding and predicting reactivity trends in organic synthesis.

中文翻译：

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SN2 与 SN2' 竞争

我们使用_组合的_{相对论密度泛函}理论和^_ _ _{_}耦合聚类理论方法。烯丙基体系中的双分子亲核取代反应，即 C ^γ = C ^β -C ^α -Y，在 α 位带有离去基团，通过直接攻击 α-碳 (S _N 2) 或通过对 γ-碳的攻击，涉及一致的烯丙基重排（S _N2'），在这两种情况下都会导致离去基团的驱逐。在这里，我们提供了一个物理上合理的模型来合理化在哪些情况下亲核试剂将遵循脂肪族 S _N 2 或烯丙基 S _N 2' 途径。我们的活化应变分析揭示了引导 S _N 2/S _N 2' 竞争的潜在物理因素，并再次证明反应的“特征畸变”和“过渡态酸度”的概念为理解和设计工具提供了解释和设计工具。预测有机合成中的反应趋势。