Organogermanes have recently emerged as prospective coupling partners to circumvent the limitations of traditional reagents used in Pd-catalyzed C-C cross-coupling due to their low toxicity, resistance to protonolysis, and benchtop stability. Consequently, new catalytic methods for producing vinylgermanes, useful synthetic blocks, with high regio- and stereoselectivities are highly desirable, and the most atom-economical approach would be a direct hydrogermylation of alkynes. A variety of catalysts has been studied for such reactions since the middle of the 20th century, including Lewis acids, precious metals (such as Pd, Ru, Rh, Pt), and even some earth-abundant metals (Mn, Fe); however, control over regio- and stereoselectivity still remains a significant challenge. Given the limitations of reported catalysts, there exists an opportunity for other earth-abundant metals to serve as catalysts in those transformations. Hence, we developed a new cobalt-catalyzed protocol, which demonstrated a broad functional group tolerance in hydrogermylation of terminal alkynes with both aryl and alkyl substituents. Regioselectivity of the developed method for previously challenging 1° alkyl-substituted alkynes by far surpasses all previously reported reactions. Our protocol is also suitable for the late-stage hydrogermylation of natural compounds and pharmaceuticals, as demonstrated on the regiospecific hydrogermylation of Mestranol, an estrogen used in birth control pills and menopausal hormone therapy. The catalyst employed in this reaction, Co2(CO)8, is a cheap and widely available reagent. In conclusion, for the first time Co was used as a viable catalyst for hydrogermylation of alkynes, paving the way to a selective and accessible method for the synthesis of (E)-β-vinyl(trialkyl)germanes. The key mechanism of syn-addition of Bu3GeH via π-alkyne complex is supported by experimental studies, which also ruled out radical pathways, and the crucial role of HCo(CO)4 in the catalytic cycle was elucidated to rationalize the (E)-β-vinyl regioselectivity.

Regioselective access to (E)-β-vinylgermanes by cobalt-catalyzed hydrogermylation of terminal alkynes

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ACS Fall 2022

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