Reductive functionalization of a rhodium(iii)–methyl bond by electronic modification of the supporting ligand
作者:Matthew E. O'Reilly、Dale R. Pahls、Joanna R. Webb、Nicholas C. Boaz、Subhojit Majumdar、Carl D. Hoff、John T. Groves、Thomas R. Cundari、T. Brent Gunnoe
DOI:10.1039/c4dt00234b
日期:——
Net reductive elimination (RE) of MeX (X = halide or pseudo-halide: Clâ, CF3CO2â, HSO4â, OHâ) is an important step during Pt-catalyzed hydrocarbon functionalization. Developing Rh(I/III)-based catalysts for alkane functionalization is an attractive alternative to Pt-based systems, but very few examples of RE of alkyl halides and/or pseudo-halides from RhIII complexes have been reported. Here, we compare the influence of the ligand donor strength on the thermodynamic potentials for oxidative addition and reductive functionalization using [tBu3terpy]RhCl (1) tBu3terpy = 4,4â²,4â²â²-tri-tert-butylpyridine} and [(NO2)3terpy]RhCl (2) (NO2)3terpy = 4,4â²,4â²â²-trinitroterpyridine}. Complex 1 oxidatively adds MeX X = Iâ, Clâ, CF3CO2â (TFAâ)} to afford [tBu3terpy]RhMe(Cl)(X) X = Iâ (3), Clâ (4), TFAâ (5)}. By having three electron-withdrawing NO2 groups, complex 2 does not react with MeCl or MeTFA, but reacts with MeI to yield [(NO2)3terpy]RhMe(Cl)(I) (6). Heating 6 expels MeCl along with a small quantity of MeI. Repeating this experiment but with excess [Bu4N]Cl exclusively yields MeCl, while adding [Bu4N]TFA yields a mixture of MeTFA and MeCl. In contrast, 3 does not reductively eliminate MeX under similar conditions. DFT calculations successfully predict the reaction outcome by complexes 1 and 2. Calorimetric measurements of [tBu3terpy]RhI (7) and [tBu3terpy]RhMe(I)2 (8) were used to corroborate computational models. Finally, the mechanism of MeCl RE from 6 was investigated via DFT calculations, which supports a nucleophilic attack by either Iâ or Clâ on the RhâCH3 bond of a five-coordinate Rh complex.
MeX(X = 卤素或伪卤素:Cl⁻、CF3CO2⁻、HSO4⁻、OH⁻)的净还原消除(RE)是铂催化的碳氢化合物官能化过程中的一个重要步骤。开发基于Rh(I/III)的催化剂用于烷烃官能化是铂基系统的一个有吸引力的替代方案,但报道的RhIII配合物中烷基卤化物和/或伪卤化物的RE的例子非常少。在这里,我们比较了配体施电子能力对氧化加成和还原官能化的热力学势的影响,使用[tBu3terpy]RhCl (1) tBu3terpy = 4,4′,4′′-三叔丁基吡啶}和[(NO2)3terpy]RhCl (2) (NO2)3terpy = 4,4′,4′′-三硝基萜吡啶}。配合物1能氧化加成MeX X = I⁻、Cl⁻、CF3CO2⁻ (TFA⁻)},生成[tBu3terpy]RhMe(Cl)(X) X = I⁻ (3)、Cl⁻ (4)、TFA⁻ (5)}。由于具有三个吸电子的NO2基团,配合物2与MeCl或MeTFA无反应,但与MeI反应生成[(NO2)3terpy]RhMe(Cl)(I) (6)。加热6可以同时排出MeCl和少量MeI。重复这个实验但使用过量的[Bu4N]Cl只产生MeCl,而加入[Bu4N]TFA则生成MeTFA和MeCl的混合物。相比之下,3在类似条件下无法还原消除MeX。DFT计算成功预测了配合物1和2的反应结果。对[tBu3terpy]RhI (7)和[tBu3terpy]RhMe(I)2 (8)的热量计测量被用来验证计算模型。最后,通过DFT计算研究了从6中MeCl的RE机制,支持I⁻或Cl⁻对五配位Rh配合物的Rh–CH3键的亲核攻击。