Platinum-catalyzed bis-germylation of alkynes with organodigermanes and cyclic oligogermanes

Hexamethyldigermane, Me3CeGeMe3, reacted with various alkynes in the presen ce of platinum complexes at 120 degreesC to afford Z-1,2-bis(germyl)ethenes in moderate to good yields. Terminal alkynes exhibit higher reactivities t han internal ones. [Pt(acac)(2)] and [Pt(dba)(2)] serve as efficient cataly sts, while [Pt(PPh3)(4)], [PtCl2(PPh3)(2)], and [Pt(dba)(2)]-phosphite were found to be inactive. Four- and six-membered cyclic oligogermanes, such as dodecamethylcyclohexagermane. (Me2Ge)(6), reacted with alkynes in the pres ence of platinum catalysts to yield 1,4-digermacyclohexa-2,5-dienes in ca. 30% yield. The reactions of phenylacetylene with 1,2-digermacyclohexa-3,5-d ienes afforded the corresponding 1,4-digermacyclohexa-2,5,7-trienes in 93% yield. Bis(germyl)platinum complexes having various tertiary phosphine liga nds have been prepared as models of a key intermediate in the above mention ed catalytic bis-germylation of alkynes, and their structures have been est ablished by spectroscopic methods and X-ray crystallography. Bis(germyl)pla tinum complexes reacted with phenylacetylene to give the corresponding inse rtion products, germyl(germylvinyl)platinum species, whose structures have been determined by spectroscopic and X-ray analysis. Germyl(germylvinyl)pla tinum complexes were found to liberate a bis-germylation product of the alk yne upon heating. The result supports a mechanism involving the oxidative a ddition of a digermane to a Pt(0) complex, the insertion of an alkyne into one of the two Pt-Ge bonds to give a germyl(germylvinyl)platinum species, a nd the reductive elimination of the bis-germylation product of the alkyne. Evidence suggesting the extrusion of a germylene unit from the bis-germylpl atinum species has been obtained, accounting for the generation courses of other by-products.