Department of Chemistry, University of California, Davis, California 95616, USA.
J Am Chem Soc. 2012 Sep 5;134(35):14595-603. doi: 10.1021/ja305853d. Epub 2012 Aug 23.
Formally, triple-bonded dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)] have been previously shown to activate aliphatic, allylic C-H bonds in cyclic olefins, cyclopentadiene (CpH), cyclopentene (c-C(5)H(8)) and 1,4-cyclohexadiene, with intriguing selectivity. In the case of the five-membered carbocycles, cyclopentadienyl species ArECp [E = Ge (3), Sn (4)] are formed. In this study, we examine the mechanisms for activation of CpH and c-C(5)H(8) using experimental methods and describe a new product found from the reaction between 1 and c-C(5)H(8), an asymmetrically substituted digermene ArGe(H)Ge(c-C(5)H(9))Ar (5), crystallized in 46% yield. This compound contains a hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with 3, explaining the fate of the liberated H atoms following triple C-H activation. We show that when these C-H activation reactions are carried out in the presence of tert-butyl ethylene (excess), compounds {ArE(CH(2)CH(2)tBu)}(2) [E = Ge(8), Sn(9)] are obtained in addition to ArECp; in the case of CpH, the neohexyl complexes replace the production of H(2) gas, and for c-C(5)H(8) they displace cyclopentyl product 5 and account for all the hydrogen removed in the dehydroaromatization reactions. To confirm the source of 8 and 9, it was demonstrated that these molecules are formed cleanly between the reaction of (ArEH)(2) [E = Ge(6), Sn(7)] and tert-butyl ethylene, new examples of noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type 6 and 7 can be surmised to be reactive intermediates in the production of 3 and 4, along with H(2), from 1 and 2 and CpH (respectively), or the formation of 3 and 5 from 1. The reaction of 6 or 7 with CpH gave 3 or 4, respectively, with concomitant H(2) evolution, demonstrating the basic nature of these low-valent group 14 element hydrides and their key role in the 'cascade' of C-H activation steps. Additionally, during the course of these studies a new polycyclic compound (ArGe)(2)(C(7)H(12)) (10) was obtained in 60% yield from the reaction of 1,6-heptadiene and 1 via double [2 + 2] cycloaddition and gives evidence for a nonradical mechanism for these types of reactions.
形式上，三键二价二烯基甲硅烷基 ArEEAr [E = Ge（1），Sn（2）; Ar = C（6）H（3）-2,6-（C（6）H（3）-2,6-（i）Pr（2））（2）] 先前已被证明可激活环状烯烃、环戊二烯（CpH）、环戊烯（c-C（5）H（8））和 1,4-环己二烯中的脂肪族、烯丙基 C-H 键，具有有趣的选择性。在五元碳环的情况下，形成了 ArECp [E = Ge（3），Sn（4）]环戊二烯基物种。在这项研究中，我们使用实验方法研究了 CpH 和 c-C（5）H（8）的活化机制，并描述了从 1 和 c-C（5）H（8）之间的反应中发现的一种新产物，不对称取代的二锗烯 ArGe（H）Ge（c-C（5）H（9））Ar（5），以 46%的产率结晶。该化合物含有氢化环戊基部分，被发现与 3 以 3:2 的比例生成，解释了三重 C-H 活化后释放的 H 原子的命运。我们表明，当这些 C-H 活化反应在叔丁基乙烯（过量）存在下进行时，除了 ArECp 之外，还获得了{ArE（CH（2）CH（2）tBu）}（2）[E = Ge（8），Sn（9）];对于 CpH，新己基配合物取代了 H（2）气体的产生，而对于 c-C（5）H（8），它们取代了环戊基产物 5，并解释了脱氢芳构化反应中除去的所有氢。为了确认 8 和 9 的来源，证明了这些分子是在（ArEH）（2）[E = Ge（6），Sn（7）]与叔丁基乙烯之间的反应中干净地形成的，这是新的非催化氢锗化和氢锡化的例子。因此，可以推测在 1 和 2 与 CpH（分别）反应或 1 与 3 和 5 的形成过程中，存在类型为 6 和 7 的瞬态氢化物是产生 3 和 4 以及 H（2）的反应性中间体，6 或 7 与 CpH 的反应分别得到 3 或 4，同时伴随着 H（2）的释放，证明了这些低价第 14 族元素氢化物的碱性及其在 C-H 活化步骤“级联”中的关键作用。此外，在这些研究过程中，从 1 和 1,6-庚二烯的反应中以 60%的产率获得了新的多环化合物（ArGe）（2）（C（7）H（12））（10），通过双[2 + 2]环加成得到，为这些类型的反应提供了非自由基机制的证据。
