Synthesis of novel stereodefined vinylgermanes bearing an allyl group or an allenyl group: (E)‐2‐aryl‐1‐germylalka‐1,4‐dienes and (E)‐4‐aryl‐5‐germylpenta‐1,2,4‐trienes

Abstract

Stereodefined synthesis of an unprecedented family of vinylgermanes bearing an allyl group, (E)-2-aryl-1-germylalka-1,4-dienes, or an allenyl group, (E)-4-aryl-5-germylpenta-1,2,4-trienes, via a cross-coupling reaction of (Z)-germyl(stannyl)ethenes with the respective allyl halide (Br, Cl) and propargyl bromide is described. In the reaction with allyl halides, either a Pd(dba) 2 -CuI combination (dba: dibenzylideneacetone) or CuI alone readily catalyzes or mediates the coupling reaction of (Z)-germyl(stannyl)ethenes, producing the novel vinylgermanes bearing an allyl group. The thienyl group or hydroxy group of the (Z)-germyl(stannyl)ethene survives the reaction. Copper(I) iodide alone readily mediates the reaction with allyl chloride or methallyl chloride upon addition of sodium bromide to produce the respective cross-coupled product in good yield. In contrast, crotyl halides (Br, Cl) or prenyl chloride couple with/without allylic transposition in the bromide or the chloride. In the reaction with propargyl bromide, a Pd(dba) 2 and CuI combination efficiently drives the coupling reaction of (Z)-germyl(stannyl)ethenes in NMP (N-methylpyrrolidone), providing the stereochemically defined allenyl vinylgermanes, (E)-4-aryl-5-germylpenta-1,2,4-trienes, in good yields.

Radical Germylzincation of α-Heteroatom-Substituted Alkynes

“…[24][25][26] Nonetheless, following this approach, subsequent elaboration of the C(sp 2 )-Sn bond required an additional synthetic step. 27…”

“…In that case, however, the addition was found to be reversible and in favor of the starting materials; hence vinylgermane formation only proceeded in the presence of a proton donor and the C­(sp 2 )–Cu bond formed was not available for subsequent elaboration . It is also worthy of mention that protocols for the related alkyne germylstannation were developed using the oxidative addition chemistry of Ge–Sn compounds. − Nonetheless, following this approach, subsequent elaboration of the C­(sp 2 )–Sn bond required an additional synthetic step …”

“…24−26 Nonetheless, following this approach, subsequent elaboration of the C(sp 2 )−Sn bond required an additional synthetic step. 27 Here we focus on α-heteroatom-substituted alkynes, a substrate class that has not been considered previously for the above-described methods and that would deliver valuable heteroatom-substituted alkenes. 28 We describe the regio-and stereoselective germylzincation of both terminal and internal nitrogen-, oxygen-, sulfur-, and phosphorus-substituted alkynes by reaction with a combination of a hydrogermane and Et 2 Zn (Scheme 1, bottom).…”

Radical Germylzincation of Aryl- and Alkyl-Substituted Internal Alkynes

“…9 Moreover, subsequent C(sp 2 )−Cu bond functionalization is poorly effective or impeded by the reversible character of the addition step. Alkyne germylstannylation was also considered for the same purpose 10 but, again, only on terminal alkynes, and C(sp 2 )−Sn bond functionalization required an additional synthetic step.…”