Bühringer Martina U, Padberg Kevin, Phleps Martin D, Maid Harald, Placht Christian, Neiss Christian, Ferguson Michael J, Görling Andreas, Tykwinski Rik R
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander University Erlangen-Nürnberg (FAU), Nikolaus-Fiebiger Str. 10, 91058, Erlangen, Germany.
Lehrstuhl für Theoretische Chemie & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstr. 3, 91058, Erlangen, Germany.
Angew Chem Int Ed Engl. 2018 Jul 2;57(27):8321-8325. doi: 10.1002/anie.201802137. Epub 2018 Jun 1.
Bonding is a fundamental aspect of organic chemistry, yet the magnitude of C=C bonding in [n]cumulenes as a function of increasing chain length has yet to be experimentally verified for derivatives longer than n=5. The synthesis of a series of apolar and unsymmetrically substituted tetraaryl[n]cumulenes (n=3, 5, 7, 9) was developed and rotational barriers for Z/E isomerization were measured using dynamic VTNMR spectroscopy. Both experiment and theory confirm a dramatic reduction in the rotational barrier (through estimation of ΔG for the isomerization) across the series, from >24 to 19 to 15 to 11 kcal in [n]cumulenes with n=3, 5, 7, 9, respectively. The reduction in cumulenic bonding in longer cumulenes thus affords bond rotational barriers that are more characteristic of a sterically hindered single bond than that of a double bond.
键合是有机化学的一个基本方面，然而对于链长大于n = 5的衍生物，[n]轮烯中C = C键合强度随链长增加的变化情况尚未得到实验验证。我们合成了一系列非极性且不对称取代的四芳基[n]轮烯（n = 3、5、7、9），并使用动态变温核磁共振光谱法测量了Z/E异构化的旋转势垒。实验和理论均证实，在该系列中，异构化的旋转势垒（通过估算异构化的ΔG）显著降低，对于n = 3、5、7、9的[n]轮烯，分别从>24 kcal降至19 kcal、15 kcal和11 kcal。因此，较长轮烯中轮烯键合的减弱使得键的旋转势垒更具空间位阻单键的特征，而非双键的特征。
