Gunay Ahmet, Jones William D
Department of Chemistry, University of Rochester, Rochester, New York 14627, USA.
J Am Chem Soc. 2007 Jul 18;129(28):8729-35. doi: 10.1021/ja071698k. Epub 2007 Jun 20.
Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt0-acetylene complexes with eta2-coordination of the alkyne, along with the corresponding PtII C-C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C-C bonds or formation of PtII complexes was not observed. However, the reverse reaction, C-C reductive coupling (PtII --> Pt0), did occur under thermal conditions, allowing the determination of the energy barriers for C-C bond formation from the different PtII complexes. For the reaction (dtbpe)Pt(-Ph)(-CCPh) (2) --> (dtbpe)Pt(eta2-PhCCPh) (1), DeltaG was 32.03(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C6F5)(CCC6F5) (6) --> (dtbpe)Pt(eta2-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C bond in (dtbpe)Pt(eta2-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term "reductive coupling" is defined as the formation of (dtbpe)Pt(eta2-acetylene) complex from the PtII complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.
通过光解实现了二苯乙炔及其几种取代衍生物的碳 - 碳键活化并进行了研究。合成并表征了具有炔烃η2配位的Pt0 - 乙炔配合物以及相应的PtII C - C活化光解产物，包括X射线晶体结构分析。虽然C - C裂解反应在光化学条件下容易发生，但未观察到C - C键的热活化或PtII配合物的形成。然而，逆反应，即C - C还原偶联（PtII→Pt0），在热条件下确实发生了，这使得能够从不同的PtII配合物确定C - C键形成的能垒。对于反应（dtbpe）Pt（-Ph）（-CCPh）（2）→（dtbpe）Pt（η2-PhCCPh）（1），ΔG为32.03(3) kcal/mol。相比之下，在缺电子体系中C - C键形成的能垒，即（dtbpe）Pt（C6F5）（CCC6F5）（6）→（dtbpe）Pt（η2 - 双（五氟苯基）乙炔）（5），为47.30 kcal/mol。通过用吸电子或供电子基团对底物进行电子修饰，可以调节C - C键形成的能垒。在（dtbpe）Pt（η2 - （对氟苯基 - 对甲苯基乙炔）（9）中的C - C键裂解后，得到了（dtbpe）Pt（对氟苯基）（对甲苯基乙炔基）（10）和（dtbpe）Pt（对甲苯基）（对氟苯基乙炔基）（11）。逆反应的动力学研究证实，在假设两条途径的过渡态能量相同的情况下，10比11对还原偶联[术语“还原偶联”定义为从PtII配合物形成（dtbpe）Pt（η2 - 乙炔）配合物]更稳定1.22 kcal/mol。10和11的产物比例为55:45，表明缺电子的C - C键仅略微优先裂解。
