The Role of Group 14 Element Hydrides in the Activation of C–H Bonds in Cyclic Olefins

Formally, triple-bonded dimetallynes ArEEAr [E = Ge (<b>1</b>), Sn (<b>2</b>); Ar = C₆H₃-2,6-(C₆H₃-2,6-<i>ⁱ</i>Pr₂)₂] have been previously shown to activate aliphatic, allylic C–H bonds in cyclic olefins, cyclopentadiene (CpH), cyclopentene (c-C₅H₈) and 1,4-cyclohexadiene, with intriguing selectivity. In the case of the five-membered carbocycles, cyclopentadienyl species ArECp [E = Ge (<b>3</b>), Sn (<b>4</b>)] are formed. In this study, we examine the mechanisms for activation of CpH and c-C₅H₈ using experimental methods and describe a new product found from the reaction between <b>1</b> and c-C₅H₈, an asymmetrically substituted digermene ArGe­(H)­Ge­(c-C₅H₉)Ar (<b>5</b>), crystallized in 46% yield. This compound contains a hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with <b>3</b>, 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 <i>tert</i>-butyl ethylene (excess), compounds {ArE­(CH₂CH₂tBu)}₂ [E = Ge­(<b>8</b>), Sn­(<b>9</b>)] are obtained in addition to ArECp; in the case of CpH, the neohexyl complexes replace the production of H₂ gas, and for c-C₅H₈ they displace cyclopentyl product <b>5</b> and account for all the hydrogen removed in the dehydroaromatization reactions. To confirm the source of <b>8</b> and <b>9</b>, it was demonstrated that these molecules are formed cleanly between the reaction of (ArEH)₂ [E = Ge­(<b>6</b>), Sn­(<b>7</b>)] and <i>tert</i>-butyl ethylene, new examples of noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type <b>6</b> and <b>7</b> can be surmised to be reactive intermediates in the production of <b>3</b> and <b>4</b>, along with H₂, from <b>1</b> and <b>2</b> and CpH (respectively), or the formation of <b>3</b> and <b>5</b> from <b>1</b>. The reaction of <b>6</b> or <b>7</b> with CpH gave <b>3</b> or <b>4</b>, respectively, with concomitant H₂ 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(C₇H₁₂) (<b>10</b>) was obtained in 60% yield from the reaction of 1,6-heptadiene and <b>1</b> via double [2 + 2] cycloaddition and gives evidence for a nonradical mechanism for these types of reactions