Synthesis of Vinylgermanes via the Au/TiO₂-Catalyzedcis-1,2-Digermylation of Alkynes and the Regioselective Hydrogermylation of Allenes

Abstract

In the presence of Au/TiO2 (1 mol %), terminal alkynes react quantitatively with stoichiometric amounts of the unactivated digermane Me3Ge-GeMe3, forming exclusively cis-1,2-digermylated alkenes. We also establish the Au/TiO2-catalyzed hydrogermylation of terminal allenes with Et3GeH, which exhibits a highly regioselective mode of addition on the more substituted double bond forming vinylgermanes. Additionally, we provide preliminary results regarding the Pd nanoparticle-catalyzed C–C coupling of 1,2-digermyl alkenes with aryl iodides.

Deoxygenation of Epoxides with Hexamethyldigermane Catalyzed by Au Nanoparticles on TiO2

“…[27] The studies on this transformation were triggered in our group following the recent observation of the smooth activation of the GeÀ Ge linkage of hexamethyldigermane on Au/TiO 2 , and its subsequent cis-1,2-addition to alkynes. [28] We envisioned the chemisorbed on Au nanoparticle digermane serving as a formal oxygen atom acceptor from epoxides, as indeed occurs experimentally. In our preliminary studies we examined parent styrene oxide (1 a) as a model compound.…”

“…[30] If, however, Me 3 GeGeMe 3 (2) is present under the same conditions, the isomerization pathway is completely suppressed and deoxygenation into styrene 3 a occurs quantitatively (Scheme 2) within 30 min. Since digermane is sensitive to moisture in the presence of Au/TiO 2 forming digermoxane, [28] the solvent must be as dry as possible, otherwise suitable excess of it must be used to compensate its destruction. Typically, however, 1.5 equiv.…”

“…In Scheme 6, we propose the mechanism of the Au nanoparticle‐catalyzed deoxygenation of epoxides with hexamethyldigermane. After insertion of the Ge−Ge linkage on the less coordinated atoms [20e] of Au nanoparticle (Au n ), forming Me 3 Ge−Au n −GeMe 3 species, [28] and coordination of the epoxide on them follows initiation of the deoxygenation via the postulated polar transition state TS I . Similar coordination and polarity profile had been proposed in the Au/TiO 2 ‐catalyzed silaboration of epoxides and oxetanes [36] .…”

“…In this manuscript we report that Au nanoparticles supported on titania (Au/TiO 2 ) can catalyze the deoxygenation of epoxides using the commercially available hexamethyldigermane (Me 3 GeGeMe 3 ) as the oxygen atom acceptor, yielding an alkene and the volatile hexamethyldigermoxane (Me 3 GeOGeMe 3 ), [26] which appears in equilibrium with germanol Me 3 GeOH if moisture is present [27] . The studies on this transformation were triggered in our group following the recent observation of the smooth activation of the Ge−Ge linkage of hexamethyldigermane on Au/TiO 2 , and its subsequent cis ‐1,2‐addition to alkynes [28] . We envisioned the chemisorbed on Au nanoparticle digermane serving as a formal oxygen atom acceptor from epoxides, as indeed occurs experimentally.…”

Radical Germylzincation of Aryl- and Alkyl-Substituted Internal Alkynes

“…For this, carbon–germanium bond-forming addition reactions to alkynes using hydrogermanes as a germanium source are most popular. The regio- and stereoselective hydrogermylation of alkynes was disclosed under several conditions, including thermal, sonochemical, or microwave activation, transition-metal catalysis, and main-group Lewis-acid catalysis as well as in the presence of radical initiators. ,, However, these methods share the common intrinsic limitation that they cannot give access to tetrasubstituted or 1,2,2-substituted vinylgermanes, which have two substituents distal to the germyl group . Alkyne germylmetalation is attractive in this context, as the newly formed C­(sp 2 )–metal bond can serve as a linchpin for further functionalization, but it remains virtually unexplored: Reports on alkyne germylcupration are scarce , and are limited to terminal alkynes .…”