Kantchev Eric Assen B, Pangestu Surya R, Zhou Feng, Sullivan Michael B, Su Hai-Bin
School of Chemistry and Chemical Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, Anhui (P.R. of China); School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 (Singapore).
Chemistry. 2014 Nov 17;20(47):15625-34. doi: 10.1002/chem.201402988. Epub 2014 Sep 26.
The C-H activation in the tandem, "merry-go-round", [(dppp)Rh]-catalyzed (dppp=1,3-bis(diphenylphosphino)propane), four-fold addition of norborene to PhB(OH)2 has been postulated to occur by a C(alkyl)H oxidative addition to square-pyramidal Rh(III) -H species, which in turn undergoes a C(aryl)-H reductive elimination. Our DFT calculations confirm the Rh(I) /Rh(III) mechanism. At the IEFPCM(toluene, 373.15 K)/PBE0/DGDZVP level of theory, the oxidative addition barrier was calculated to be 12.9 kcal mol(-1) , and that of reductive elimination was 5.0 kcal mol(-1) . The observed selectivity of the reaction correlates well with the relative energy barriers of the cycle steps. The higher barrier (20.9 kcal mol(-1) ) for norbornyl-Rh protonation ensures that the reaction is steered towards the 1,4-shift (total barrier of 16.3 kcal mol(-1) ), acting as an equilibration shuttle. The carborhodation (13.2 kcal mol(-1) ) proceeds through a lower barrier than the protonation (16.7 kcal mol(-1) ) of the rearranged aryl-Rh species in the absence of o- or m-substituents, ensuring multiple carborhodations take place. However, for 2,5-dimethylphenyl, which was used as a model substrate, the barrier for carborhodation is increased to 19.4 kcal mol(-1) , explaining the observed termination of the reaction at 1,2,3,4-tetra(exo-norborn-2-yl)benzene. Finally, calculations with (Z)-2-butene gave a carborhodation barrier of 20.2 kcal mol(-1) , suggesting that carborhodation of non-strained, open-chain substrates would be disfavored relative to protonation.
在串联的“旋转木马”反应中，[(dppp)Rh]（dppp = 1,3 - 双（二苯基膦基）丙烷）催化降冰片烯向PhB(OH)₂进行四次加成，据推测，该反应通过C(烷基)H向四方锥型Rh(III) - H物种的氧化加成发生，而后者又经历C(芳基)-H还原消除。我们的密度泛函理论（DFT）计算证实了Rh(I)/Rh(III)机理。在IEFPCM(甲苯, 373.15 K)/PBE0/DGDZVP理论水平下，计算得到氧化加成能垒为12.9 kcal mol⁻¹，还原消除能垒为5.0 kcal mol⁻¹。观察到的反应选择性与循环步骤的相对能垒密切相关。降冰片基 - Rh质子化的较高能垒（20.9 kcal mol⁻¹）确保反应朝着1,4 - 迁移（总能量为16.3 kcal mol⁻¹）进行，起到平衡穿梭的作用。在没有邻位或间位取代基的情况下，碳硼氢化反应（13.2 kcal mol⁻¹）的能垒低于重排芳基 - Rh物种的质子化反应（16.7 kcal mol⁻¹），这确保了会发生多次碳硼氢化反应。然而，对于用作模型底物的2,5 - 二甲基苯基，碳硼氢化反应的能垒增加到19.4 kcal mol⁻¹，这解释了在1,2,3,4 - 四（外向 - 降冰片 - 2 - 基）苯处观察到的反应终止现象。最后，用(Z)-2 - 丁烯进行的计算得出碳硼氢化反应能垒为20.2 kcal mol⁻¹，这表明相对于质子化反应，非张力开链底物的碳硼氢化反应是不利的。
