Direct Observation of Reductive Elimination of MeX (X = Cl, Br, I) from Rh^III Complexes: Mechanistic Insight and the Importance of Sterics

Rare cases of directly observed reductive elimination (RE) of methyl halides from RhIII complexes are described. Treatment of the coordinatively unsaturated complexes [(tBuPNP)­Rh­(CH3)­X]­[BF4] (1–3, X = I, Br, and Cl; tBuPNP = 2,6-bis-(di-tert-butylphosphinomethyl)­pyridine) with coordinating and noncoordinating compounds results in the formation of the corresponding free methyl halides and RhI complexes. The rate increase of CH3I and CH3Br RE in the presence of polar aprotic solvents argues in favor of an SN2 RE mechanism. However, the RE of CH3Cl is faster in polar protic solvents, which argues in favor of a concerted C–Cl RE. The RE of methyl halides from complexes 1–3 is induced by steric factors, as treatment of the less bulky complexes [(iPrPNP)­Rh­(CH3)­X]­[BF4] (19–21; X = I, Br, Cl, respectively) with coordinating compounds leads to the formation of the adducts complexes rather than RE of the methyl halides. The accumulated evidence suggests that the RE process is nonassociative.本文报道了可直接观测到的、从三价铑（RhIII）配合物中还原消除（reductive elimination，RE）生成甲基卤化物的罕见案例。将配位不饱和配合物[(tBuPNP)Rh(CH3)X][BF4]（1~3，X分别为I、Br、Cl；其中tBuPNP为2,6-双(二叔丁基膦甲基)吡啶）与配位性及非配位性化合物反应，可得到相应的游离甲基卤化物与一价铑（RhI）配合物。在极性非质子溶剂存在下，CH3I与CH3Br的还原消除反应速率显著提升，这支持了双分子亲核取代（SN2）型还原消除机理；但CH3Cl的还原消除在极性质子溶剂中速率更快，这支持了协同型C-Cl还原消除机理。配合物1~3的甲基卤化物还原消除反应受位阻因素调控：当将位阻更小的配合物[(iPrPNP)Rh(CH3)X][BF4]（19~21，X分别为I、Br、Cl，其中iPrPNP为2,6-双(二异丙基膦甲基)吡啶）与配位性化合物反应时，仅生成加合配合物，而非发生甲基卤化物的还原消除。综上积累的实验证据表明，该还原消除过程为非缔合机理。