Synthesis and functionalization of organogermanes in the three-dimensional chemical space

Due to the ever-increasing demand for novel synthetic transformations of organic compounds, operationally simple and bench stable alternatives to literature reported examples are especially sought after. The introduction of a non-toxic and robust functional group, like the alkylgermanium handle, would open new possibilities to navigate the chemical landscape. In the two-dimensional space, transition metal catalyzed approaches have enabled a multitude of decoration possibilities in the last decades; however, the three-dimensional area lacks diverse and efficient transformation possibilities. In the first chapter, the C–GeEt3 handle is introduced via a rare base catalyzed remote hydrogermylation reaction of alkenes. The straightforward reaction conditions, combined with commercially available starting materials, underscore the operational simplicity of the novel method. Under optimized conditions, it was shown that catalytic amounts of LiOtBu and CsF isomerize alkene carbon chains of up to eight carbons and subsequently hydrogermylate the internal double bond to the corresponding product in a one-pot reaction. This methodology proved effective for a range of substrates, including aromatic systems bearing electron-donating and electron-withdrawing substituents, as well as heteroaryl and non-aromatic compounds such as allyl phosphine, silane, and Bpin derivatives. Interestingly, different germanium sources are viable for the herein reported remote hydrofunctionalzation, whereas analogous hydroborylation or hydrosilylation reagents failed to achieve productive results, highlighting the unique reactivity of the germanium group. Further mechanistic investigations have revealed that upon salt metathesis, the strong base CsOtBu might be formed and responsible for the unexpected reactivity. DFT calculations of the relevant pKa values are in line with the proposed anionic reaction mechanism, which was also supported by deuterium labeling studies. Additionally, ICP-MS analysis and control reactions with various transition metals were conducted to exclude the observed reactivity origins from metal contamination. The second chapter focuses on the full functionalization of the established building block containing three different handles on one carbon, namely the C–Cl, C–Bpin and C–GeEt3 group. In this context, several fully decorated compounds were synthesized to demonstrate rapid access to complex structures. To achieve this full functionalization various reported transformations of the C–Cl and C–Bpin units were applied. During these transformations the alkylgermanium group repeatedly proved its robustness under harsh conditions, e.g. organometallic reagents and peroxides, making it an ideal handle within the building block. In these cases, the C–Bpin group was successfully oxidized, (hetero)arylated and vinylated following the reported procedures. Beyond known transformations, such as Giese addition and dual photoredox catalysis with nickel, additional investigations were carried out to functionalize the C–GeEt₃ moiety. The photochemically generated alkyl radical was efficiently trapped by sulfonyl reagents, incorporating nitrile, vinyl and alkynyl groups. Furthermore, vinyl sulfonium salts were utilized to unlock a novel Giese type addition reaction, where formally an electron rich double bond ends up in the final product. Notably, when an alcohol in the α-position to the alkylgermanium group was present under dual photoredox conditions, the cross-coupled product featured selective oxidation of the alcohol to an α-aryl ketone. In addition to photochemical transformations, an electrochemical approach was applied to further expand the scope of possible functionalizations, particularly for C–heteroatom bond formations. This method provided rapid access to three additional complex structures.