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%0 Thesis
%A Amit, Amit
%T Organogermanes : synthesis and their application as orthogonal cross-coupling partner
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2023-08897
%P 1 Online-Ressource : Illustrationen, Diagramme
%D 2022
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2023
%Z Dissertation, RWTH Aachen University, 2022
%X Transition metal-catalyzed cross-coupling technology has become widely adopted and played an important role in transforming modern synthetic organic chemistry. Certainly, the continuous development of synthetic methodologies is required for rapid, modular, and selective access to drugs, agrochemicals, and functional materials. While research has produced new bespoke catalysts and reaction conditions have evolved, the same traditional coupling partners are still employed. In this context, this thesis demonstrates organogermanes as robust, convenient, and nontoxic reagents in C-C and C-X bond formation in an orthogonal fashion - namely tolerating traditional cross-coupling functionalities (e.g., silanes, boronic esters, halogens). A number of synthetic strategies where the Ge-functionality was more reactive than any of the established coupling-partners are demonstrated in this thesis. The first chapter of this thesis describes a biaryl coupling using aryl germanes with subsequent direct C-H activation of electron-rich arenes via gold-catalysis. The investigation started by exploring the reactivity of trialkyl aryl germane (ArGeEt3) with an electrophilic gold complex. We found that aryl germanes are reactive with both Au(I) and Au(III) species whereas corresponding silanes can only be activated by Au(III). Our computational studies show a relatively low distortion energy for the C-Ge bond which is the main reason for the higher reactivity of aryl germanes. However, the oxidative gold-catalyzed C-H bond functionalization requires electron-rich arenes as coupling partners owing to the C-H bond activation proceeding by a SEAr-type mechanism. To remedy this electronic requirement, we employed an Umpolung strategy by in situ transferring polyfluoarenes to the gold-complex with the assistance of silver, enabling a chemoselective coupling of polyfluoarenes with aryl germanes. By using this strategy, both electron-poor and -rich aryl germanes were successfully coupled with polyfluoroarenes, widening the coupling space for oxidative gold-catalysis to electron poor examples as well while accessing electron-poor/electron-poor biaryls. Beyond C-H activation (by direct SEAr or Ag assitance), we have also developed a fully selective procedure with diazonium salts. The coupling of ArGeEt3 with aryldiazonium salts proceeds in the presence of valuable functionalities such as C-Bpin, C-SiMe3, C-I, C-Br, C-Cl under light activated gold-catalysis. As opposed to silanes, aryl germanes are also reactive with electron-rich aryldiazonium salts by using a photoredox approach. Encouraged by these findings, we next showcased the reactivity of alkynylgermanes for challenging C(sp)-C(sp2) bond formations via gold-catalysis. Notably, the reaction is highly chemoselective as the selective coupling of the alkynylgermane is obtained in the presence of aryl halides, silane (SiMe3), boronic ester (Bpin) and even aryl germane (GeEt3), showcasing its utility as an orthogonal functional handle in late-stage synthesis. Another part of this thesis is directed towards C-X (C-N, C-O, </td><td width="150">
%X  C-C) bond formation from aryl germanes. In terms of C-N bond formation, we developed a mild and selective photocatalytic condition to access various nitroarenes via ipso-nitration of aryl germanes. Mechanistic studies strongly support the addition of the nitro radical to aryl germanes –an unprecedented mechanistic activation of aryl germanes. Moreover, we have explored the reactivity of aryl germanes with a more electrophilic palladium species i.e. in Pd(II)/Pd(IV) -catalytic cycle. In this regard, a highly efficient and reliable method for the oxygenation of aryl germanes using Pd(OAc)2 and PhI(TFA)2 as an oxidant have been developed. The reaction proceeds at room temperature and is fully selective for the Ge-site, while tolerating enabling functionalities such as the C-I bond which would not be possible in a traditional Pd-catalyzed cross-coupling reaction. Furthermore, using a similar set of conditions, we have also established Heck coupling of aryl germanes. Although normally Heck coupling requires elevated temperatures, our reaction proceeds at room temperature and is fully selective again for Ar-GeEt3 in presence of typical Heck coupling functionalities (for e.g., C-Br </td><td width="150">
%X  C-I).The last chapter of this thesis discusses the synthesis of organogermanes where we developed a base-mediated direct C-H germylation of arenes and heteroarenes. This method utilizes lithium tetramethylpiperidide (LiTMP) as a base and triethylgermanium chloride (Et3GeCl) as the germane source. The reaction is operationally simple, rapid, and performed at room temperature in a one-pot procedure. Moreover, we established a B(C6F5)3-catalyzed direct C(sp)-H germylation of terminal alkynes by utilizing 2,6-lutidine as an organic base. This provides a straightforward route towards accessing a wide range of alkynylgermanes.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2023-08897
%U https://publications.rwth-aachen.de/record/968938