Stepwise versus Concerted Reductive Elimination Mechanisms in the Carbon–Iodide Bond Formation of (DPEphos)RhMeI2 Complex,Organometallics

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Stepwise versus Concerted Reductive Elimination Mechanisms in the Carbon–Iodide Bond Formation of (DPEphos)RhMeI2 Complex
Organometallics ( IF 2.5 ) Pub Date : 2018-12-05 , DOI: 10.1021/acs.organomet.8b00723
Jing-Lu Yu ₁ , Shuo-Qing Zhang ₁ , Xin Hong ₁

Affiliation

Reductive elimination is the key bond formation process of organometallic reactions. Goldberg and co-workers recently revealed an unprecedented competition of parallel stepwise reductive elimination pathways for the carbon–iodide bond formation of (DPEphos)RhMeI₂ complex. To understand the controlling factors that differentiate the concerted and stepwise pathways, we performed density functional theory (DFT) calculations to elucidate the mechanistic details. The competing stepwise pathways were identified as the anionic and zwitterionic stepwise pathways. The anionic pathway involves the direct S_N2 attack of the external iodide anion to the methyl group, leading to the observed carbon–iodide bond formation. Alternatively, heterolytic Rh–I bond cleavage generates the cationic (DPEphos)RhMeI⁺ intermediate, and the subsequent S_N2 attack of the iodide anion to the methyl group occurs via the zwitterionic transition state. In comparison with the stepwise reductive elimination pathways, the classic concerted pathways require significantly higher barriers. This is due to the energy penalty associated with the orientation change of the methyl group during the classic three-centered reductive elimination. The energy required for this orientation change is highly related to the hybrization of carbon; thus, the selectivity for the stepwise reductive elimination pathways can be switched if the C(sp²) or C(sp) group participates in the carbon–iodide bond formation.

中文翻译：

（DPEphos）RhMeI _2配合物碳碘键形成中的逐步与协同还原消除机理

还原消除是有机金属反应的关键键形成过程。戈德堡及其同事最近发现，对于（DPEphos）RhMeI ₂络合物的碳碘键形成而言，平行逐步还原消除途径的竞争前所未有。为了了解区分协调的和逐步的途径的控制因素，我们进行了密度泛函理论（DFT）计算以阐明机理细节。竞争性逐步途径被鉴定为阴离子和两性离子逐步途径。阴离子途径涉及直接的S _N2外部碘化物阴离子攻击甲基，导致观察到碳-碘键形成。另外，Rh–I杂合键裂解可产生阳离子（DPEphos）RhMeI ⁺中间体，以及随后的S _N通过两性离子过渡态发生碘化物阴离子对甲基的2攻击。与逐步还原消除途径相比，经典的协同途径需要更高的屏障。这是由于在经典的三中心还原消除过程中，与甲基的方向变化有关的能量损失。这种方向变化所需的能量与碳的高度相关。因此，如果C（sp ²）或C（sp）基团参与碳碘键的形成，则可以切换逐步还原消除途径的选择性。

更新日期：2018-12-05

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