Organogermanes as Orthogonal Coupling Partners in Synthesis and Catalysis

Copper-Catalyzed Three-Component Germyl Peroxidation of Alkenes

The concurrent incorporation of a germyl fragment and another functional group (beyond the hydrogen atom) across the C═C double bond is a highly appealing yet challenging task. Herein we demonstrate the efficient germyl peroxidation of alkenes with germanium hydrides and tert-butyl hydroperoxide via a copper-catalyzed three-component radical relay strategy. This protocol exhibits excellent functional group tolerance and exquisite chemo- and regioselectivity under mild conditions and represents a rare example of constructing synthetically challenging metal-embedded organic peroxides.

Nickel-Catalyzed Reductive Csp3-Ge Coupling of Alkyl Bromides with Chlorogermanes

Reductive cross-coupling provides facile access to organogermanes, but it remains largely unexplored. Herein we report a nickel-catalyzed reductive Csp³-Ge coupling of alkyl bromides with chlorogermanes. This work has established a new method for producing alkylgermanes. The reaction proceeds under very mild conditions and tolerates various functionalities including ether, alcohol, alkene, nitrile, amine, ester, phosphonates, amides, ketone, and aldehyde. The application of this method to the modification of bioactive molecules is demonstrated.

Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)-carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)-metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM-E and TM-H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid-hydrogen and metalloid-metalloid bonds to unsaturated compounds; activation and functionalization of C-H bonds and C-X bonds with hydrometalloids and bismetalloids; activation and functionalization of C-H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

Hydrogermylation initiated by trialkylborohydrides: a living anionic mechanism

A trialkylborohydide-initiated hydrogermylation of aromatic alkenes is reported along with a DFT-supported mechanism following the course of a ‘living’ process.

Alkyl-GeMe3: Neutral Metalloid Radical Precursors upon Visible-Light Photocatalysis

Single-electron transfer (SET) oxidation of ionic hypervalent complexes, representatively alkyltrifluoroborates (Alkyl-BF3-) and alkylbis(catecholato)silicates (Alkyl-Si(cat)2-), have contributed substantially to alkyl radical generation compared to alkali or alkaline earth organometallics because of their excellent activity-stability balance. Herein, we report another proposal using neutral metalloid compounds, Alkyl-GeMe3, as radical precursors. Compared to Alkyl-BF3- and Alkyl-Si(cat)2-, Alkyl-GeMe3 show comparable activity in radical addition reactions. Moreover, Alkyl-GeMe3 gives the first success of group 14 tetraalkyl nucleophiles in nickel catalyzed cross-coupling. Meanwhile, the neutral nature of these organogermanes supplemented the limination of ionic precursors in purification and derivatization. A preliminary mechanism study corresponds to the procedure that alkyl radical generates from tetraalkylgermane radical cation with the assistance of a nucleophile, which may also enlighten the development of more non-ionic alkyl radical precursors with metalloid center.

Allene C(sp2)-H Activation and Alkenylation Catalyzed by Palladium

The selective transition-metal-mediated activation of C(sp²)-H bonds of allenes is a formidable challenge because of the competitive, intrinsic reactivity of cumulated double bonds. Herein, we report a Pd-catalyzed C-H alkenylation of electronically unbiased allenes, affording penta-1,2,4-triene products in up to 94% yield. A picolinamide directing group enables the formation of putative allenyl-palladacycles, which subsequently participate in a turnover-limiting Heck-type reaction with electron-deficient alkene coupling partners. This mechanistic proposal is consistent with experimental and computational investigations. Additionally, we report for the first time the use of picolinamide N,O-acetals as readily removable auxiliaries for C-H activation reactions, allowing the efficient alkenylation of allenyl carbinol derivatives. Successful removal of the directing groups without affecting the reactive penta-1,2,4-triene substructure of the products is demonstrated.

Hydrogermylation of Alkenes via Organophotoredox-Initiated HAT Catalysis

This Letter discloses the straightforward hydrogermylation of olefins under visible-light organophotoredox-initiated HAT catalysis conditions to yield primary and secondary alkyl germanes at room temperature. The protocol is operationally simple, metal-free, and tolerant of various functional groups. The synthesized alkyl germanes proved to be highly robust toward acidic, basic, or oxidizing conditions and chemical transformations of C_sp²-GeEt₃ or C_sp²-BPin functionalities in their presence.

Nickel-Catalyzed Reductive C-Ge Coupling of Aryl/Alkenyl Electrophiles with Chlorogermanes

Cross-electrophile coupling has emerged as a promising tool for molecular synthesis; however, current studies have focused mainly on forging C-C bonds. We report a cross-electrophile C-Ge coupling reaction and thereby demonstrate the possibility of constructing organogermanes from carbon electrophiles and chlorogermanes. The reaction proceeds under mild conditions and offers access to both aryl and alkenyl germanes. Electron-rich, electron-poor, and ortho-/meta-/para-substituted (hetero)aryl electrophiles, as well as cyclic and acyclic alkenyl electrophiles, were coupled. Gram-scale reaction, incorporation of the -GeR₃ moiety into complex biologically active molecules, and derivatization of formed organogermanes are demonstrated.

Germatranes and carbagermatranes: (hetero)aryl and alkyl coupling partners in Pd-catalyzed cross-coupling reactions

In the past few decades, palladium-catalyzed cross-coupling reactions have taken root in the construction of a complex synthetic community. The development of organometallics has been an important objective in this field. Our group has focused on exploiting new germanium-based reagents and the corresponding catalytic processes. In the past three years, we have established new methods for the synthesis of structure-modified (hetero)aryl germatranes and alkyl carbagermatranes. Particularly for alkyl carbagermatranes, the stability to be compatible with various derivatization reactions and the high activity for transmetallation (e.g. base/additive-free for primary alkyl carbagermatranes) distinguish them from many reported nucleophiles. In this article, we would introduce (1) the development process of organogermanium reagents in palladium-catalyzed cross-couplings; (2) the history of germatrane-type systems and the breakthrough we have made in the field; (3) the outlook for (carba)germatranes and alkyl-GeMe₃.

Iron-catalysed radical cyclization to synthesize germanium-substituted indolo[2,1-a]isoquinolin-6(5H)-ones and indolin-2-ones

A simple and efficient strategy for iron-catalysed cascade radical cyclization was developed, by which an array of germanium-substituted indolo[2,1-a]isoquinolin-6(5H)-ones and indolin-2-ones were obtained in one pot with germanium hydrides as radical precursors. A rapid intramolecular radical trapping mode enabled the selective arylgermylation of alkenes over the prevalent hydrogermylation reaction.

Synthesis of N-CF3 Alkynamides and Derivatives Enabled by Ni-Catalyzed Alkynylation of N-CF3 Carbamoyl Fluorides

The expansion of chemical space associated with ubiquitous motifs is key to unleash new properties and functions. In this context, alkynamides, prevalent in numerous drugs and materials, represent an untapped resource. We herein report the first synthetic access to N-trifluoromethyl alkynamides. Our strategy relies on a mild and operationally simple Ni-catalyzed coupling of N-CF₃ carbamoyl fluorides with alkynyl silanes. The synthesized N-CF₃ alkynamides proved to be highly robust and readily functioned as a platform to unlock access to valuable derivatives, such as N-CF₃ decorated alkenyl amides, oxindoles, or quinolones, all of which were inaccessible to date.

Rh(II)-Catalyzed Alkynylcyclopropanation of Alkenes by Decarbenation of Alkynylcycloheptatrienes

Alkynylcyclopropanes have found promising applications in both organic synthesis and medicinal chemistry but remain rather underexplored due to the challenges associated with their preparation. We describe a convenient two-step methodology for the alkynylcyclopropanation of alkenes, based on the rhodium(II)-catalyzed decarbenation of 7-alkynyl cycloheptatrienes. The catalytic system employed circumvents a fundamental problem associated with these substrates, which usually evolve via 6-endo-dig cyclization or ring-contraction pathways under metal catalysis. This unique performance unlocks a rapid access to a diverse library of alkynylcyclopropanes (including derivatives of complex drug-like molecules), versatile intermediates that previously required much lengthier synthetic approaches. Combining experiments and DFT calculations, the complete mechanistic picture for the divergent reactivity of alkynylcycloheptatrienes under metal catalysis has been unveiled, rationalizing the unique selectivity displayed by rhodium(II) complexes.

The dioxasilepanyl group as a versatile organometallic unit: studies on stability, reactivity, and utility

Organic synthesis is performed based on precise choices of functional groups and reactions employed. In a multistep synthesis, an ideal functional group should be compatible with various reaction conditions and unaltered until it is subjected to a selective conversion. The current study was set out to search for a silicon functionality that meets these criteria. Here we have established a new silicon-based synthetic methodology centred on a bulky 7-membered dialkoxysilyl group (2,4,4,7,7-pentamethyl-1,3,2-dioxasilepan-2-yl) that uniquely has both stability and on-demand reactivity. The exceptional stability of this functional group was corroborated by both experimental and computational studies which demonstrated that key factors for its stability were a 7-membered structure and steric hindrance. In turn, the dioxasilepanyl group was found to become reactive and to be easily transformed in the presence of appropriate activators. Combined with the development of easy and robust methods to introduce the dioxasilepanyl group onto aryl rings, these findings have allowed a shorter and more efficient synthesis of a bioactive molecule, thus demonstrating the potential utility of the easily accessible dioxasilepanyl group in organic synthesis.

Base-Mediated Direct C-H Germylation of Heteroarenes and Arenes

The direct C-H germylation of heteroarenes, arenes, and benzylic C-H bonds promoted by lithium tetramethylpiperidide (LiTMP) is reported. The method is rapid, selective, and operationally simple, consisting of direct addition of all reagents at room temperature (one-pot procedure). The synthetic utility of these newly accessed aryl germanes as viable coupling partners in Pd catalysis is also showcased.

Synthesis of Dialkyl-Substituted Monofluoroalkenes via Palladium-Catalyzed Cross-Coupling of Alkyl Carbagermatranes

An unprecedented cross-coupling reaction of alkyl carbagermatranes with bromofluoroolefins to deliver dialkyl-substituted monofluoroalkenes was achieved. This cross-coupling reaction was performed under base/additive-free conditions with excellent functional group tolerance, therefore offering an opportunity for challenging dialkyl-substituted monofluoroalkenes. The preparation of bioactive agent analogues including an antitubercular agent mimic and a COX-2 inhibitor analogue and the late-stage fluoroalkenylation of drug-molecule derivatives proved the utility of this strategy.

Nickel-Catalyzed Ipso-Borylation of Silyloxyarenes via C-O Bond Activation

The conversion of silyloxyarenes to boronic acid pinacol esters via nickel catalysis is described. In contrast to other borylation protocols of inert C-O bonds, the method is competent in activating the carbon-oxygen bond of silyloxyarenes in isolated aromatic systems lacking a directing group. The catalytic functionalization of benzyl silyl ethers was also achieved under these conditions. Sequential cross-coupling reactions were achieved by leveraging the orthogonal reactivity of silyloxyarenes, which could then be functionalized subsequently.

Cobalt-Catalyzed (E)-β-Selective Hydrogermylation of Terminal Alkynes

A cobalt-catalyzed method for the hydrogermylation of alkynes is reported, providing a selective and accessible route to (E)-β-vinyl(trialkyl)germanes from terminal alkynes and HGeBu₃. As shown in multiple examples, the developed method demonstrates a broad functional group tolerance an practical utility for late-stage hydrogermylation of natural products. The method is compatible with alkynes bearing both aryl and alkyl substituents, providing unrivaled selectivity for previously challenging 1° alkyl-substituted alkynes. Moreover, the catalyst used in this method, Co₂(CO)₈, is a cheap and commercially available reagent. Conducted mechanistic studies supported the syn-addition of Bu₃GeH to an alkyne π-complex.