Department of Chemistry, Duke University, Durham, North Carolina 27708, USA.
Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
Nat Chem. 2015 Apr;7(4):323-7. doi: 10.1038/nchem.2185. Epub 2015 Feb 23.
Forbidden reactions, such as those that violate orbital symmetry effects as captured in the Woodward-Hoffmann rules, remain an ongoing challenge for experimental characterization, because when the competing allowed pathway is available the reactions are intrinsically difficult to trigger. Recent developments in covalent mechanochemistry have opened the door to activating otherwise inaccessible reactions. Here we report single-molecule force spectroscopy studies of three mechanically induced reactions along both their symmetry-allowed and symmetry-forbidden pathways, which enables us to quantify just how 'forbidden' each reaction is. To induce reactions on the ~0.1 s timescale of the experiments, the forbidden ring-opening reactions of benzocyclobutene, gem-difluorocyclopropane and gem-dichlorocyclopropane require approximately 130 pN less, 560 pN more and 1,000 pN more force, respectively, than their corresponding allowed analogues. The results provide the first experimental benchmarks for mechanically induced forbidden reactions, and in some cases suggest revisions to prior computational predictions.
禁反反应，如那些违反轨道对称效应的反应，如伍德沃德-霍夫曼规则所捕获的，仍然是实验特征描述的一个持续挑战，因为当竞争允许的途径可用时，这些反应本质上很难触发。共价机械化学的最新进展为激活原本无法进入的反应打开了大门。在这里，我们报告了三种机械诱导反应的单分子力谱研究，沿着它们的对称允许和对称禁阻途径进行，这使我们能够量化每个反应的禁阻程度。为了在实验的~0.1 秒时间尺度上诱导反应，苯并环丁烯、偕二氟环丙烷和偕二氯环丙烷的禁阻环开反应分别需要大约 130 pN、560 pN 和 1000 pN 的力，才能与相应的允许类似物相比。结果为机械诱导禁反反应提供了第一个实验基准，在某些情况下，还对先前的计算预测提出了修正。
