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

Abstract

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.

Access to Base Adducts of Low‐Valent Organotin‐Hydride Compounds by Controlled, Stepwise Hydrogen Abstraction from a Tetravalent Organotin Trihydride

“…[7d, 9b] Their critical involvement in the CÀHa ctivation of cyclopentadiene and their reactivity towards C=Cd ouble bonds have been studied by Powera nd co-workers. [12] Recently we becamei nterested in probing the accessibility of low-valent organotin hydride derivatives by selectively removingh ydrogen from tetravalent organotin trihydrides. In this context we werea blet os how that nitrogen bases such as DMAP (4-(N,N-dimethylamino)pyridine) can induce dehydrogenationa nd finally the base adducts of terphenyltin(II) hy-drides were observed.…”

Reactivity of Monomeric N→Ge Coordinated Germanium(II) Hydrides

“…proceeded without a catalyst . Furthermore, C H activation was also observed by Power et al, who reported an isolation of the {(2,6‐Dipp 2 C 6 H 3 )Ge(η3‐C 5 H 9 )} by the treatment of the digermene {(2,6‐Dipp 2 C 6 H 3 )(H)Ge} 2 with CpH. Jones et al reported on the hydrogermylation reactions of alkenes and alkynes with two‐coordinated germanium(II) hydride {N(Ar)(Si i Pr 3 )}GeH (Figure B) that even reacted reversibly with cycloalkenes .…”

Activation of Small Molecules by Compounds that Contain Triple Bonds Between Heavier Group‐14 Elements

“…Instead, the reaction selectively activates the C(sp 3 )À Hb ond in CpH to give 23 [32] or 24 [33] (Scheme 14). The expected Diels-Alder [4+ +2] cycloaddition was not observed.…”