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

Formally, triple-bonded dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C6H3-2,6-(C6H3-2,6-iPr2)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 (3), Sn (4)] 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 1 and c-C5H8, an asymmetrically substituted
digermene ArGe­(H)­Ge­(c-C5H9)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(CH2CH2tBu)}2 [E = Ge­(8), Sn­(9)] 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 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 H2, 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 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) (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.