A General Strategy for the Asymmetric Preparation of α-Stereogenic Allyl Silanes, Germanes, and Boronate Esters via Dual Copper Hydride- and Palladium-Catalysis

Recent Developments with Organogermanes: their Preparation and Application in Synthesis and Catalysis

Within the sphere of traditional Pd0 /PdII cross coupling reactions, organogermanes have been historically outperformed both in terms of scope and reactivity by more conventional transmetalating reagents. Subsequently, this class of compounds has been largely underutilized as a coupling partner in bond-forming strategies. Most recent studies, however, have shown that alternative modes of activation of these notoriously robust building blocks transform organogermanes into the most reactive site of the molecule-capable of outcompeting other functional groups (such as boronic acids, esters and silanes) for both C-C and C-heteroatom bond formation. As a result, over the past few years, the literature has increasingly featured methodologies that explore the potential of organogermanes as chemoselective and orthogonal coupling partners. Herein we highlight some of these recent advances in the field of organogermane chemistry both with respect to their synthesis and applications in synthetic and catalytic transformations.

Ligand-Controlled On-Off Switch of a Silicon-Tethered Directing Group Enabling the Regiodivergent Hydroalkylation of Vinylsilanes under Ni-H Catalysis

A regiodivergent Ni-H-catalyzed hydroalkylation of vinylsilanes is described. Depending on the ancillary ligand at the nickel catalyst, the regioselectivity can be steered by a directing group attached to the silicon atom. The mild protocols allow for the selective synthesis of either branched or linear alkylsilanes. An example of a vinylgermane is also reported. The method features a broad scope with high functional-group tolerance and follows a radical mechanism.

Ligand-modulated nickel-catalyzed regioselective silylalkylation of alkenes

Organosilicon compounds have shown tremendous potential in drug discovery and their synthesis stimulates wide interest. Multicomponent cross-coupling of alkenes with silicon reagents is used to yield complex silicon-containing compounds from readily accessible feedstock chemicals but the reaction with simple alkenes remains challenging. Here, we report a regioselective silylalkylation of simple alkenes, which is enabled by using a stable Ni(II) salt and an inexpensive trans-1,2-diaminocyclohexane ligand as a catalyst. Remarkably, this reaction can tolerate a broad range of olefins bearing various functional groups, including alcohol, ester, amides and ethers, thus it allows for the efficient and selective assembly of a diverse range of bifunctional organosilicon building blocks from terminal alkenes, alkyl halides and the Suginome reagent. Moreover, an expedient synthetic route toward alpha-Lipoic acid has been developed by this methodology.

Regioselective Hydroalkylation of Vinyl- and Allylsilanes as Well as Vinylgermanes under Ni-H Catalysis

A Ni-H-catalyzed hydroalkylation of vinylsilanes and -germanes as well as allylsilanes with unactivated alkyl iodides is reported. Unlike related reactions of styrene or vinyl boronate esters, the addition across the C-C double bond proceeds with anti-Markovnikov selectivity to deliver the linear regioisomer. Mechanistic control experiments support a radical mechanism, and a competition experiment reveals that the chemoselectivity is in favor of the vinyl over the allyl group.

Enantio- and Regioconvergent Synthesis of γ-Stereogenic Vinyl Germanes and Their Use as Masked Vinyl Halides

A nickel-catalyzed enantio- and regioconvergent alkylation of regioisomeric mixtures of racemic germylated allylic electrophiles with alkyl nucleophiles is reported. Key to success is a newly developed hept-4-yl-substituted Pybox ligand that enables accessing various chiral γ-germyl α-alkyl allylic building blocks in excellent yields and enantioselectivities. The reason for the regioconvergence is the steering effect of the bulky germyl group. The resulting vinyl germanes can be easily halodegermylated without racemization of the allylic stereocenter to afford synthetically valuable γ-stereogenic vinyl halides.

A General Strategy for the Asymmetric Preparation of α-Stereogenic Allyl Silanes, Germanes, and Boronate Esters via Dual Copper Hydride- and Palladium-Catalysis

α-Stereogenic allyl metalloids are versatile synthetic intermediates which can undergo various stereocontrolled transformations. Most existing methods to prepare α-stereogenic allyl metalloids involve multi-step sequences that curtail the number of compatible substrates and are limited to the synthesis of boronates. Here, we report a general method for the enantioselective preparation of α-stereogenic allyl metalloids utilizing dual CuH- and Pd-catalysis. This approach leverages a stereoretentive Cu-to-Pd transmetalation of an in situ generated alkyl copper species to allow access to enantioenriched allyl silanes, germanes, and boronate esters with broad functional group compatibility.