Why You Really Should Consider Using Palladium-Catalyzed Cross-Coupling of Silanols and Silanolates

Understanding Site Selectivity in the Palladium-Catalyzed Cross-Coupling of Allenylsilanolates

Allenylsilanolates can undergo cross-coupling at the α- or γ-terminus, and site selectivity appears to be determined by the intrinsic transmetalation mechanism. Fine-tuning of concentration, nucleophilicity, and steric bulk of the silanolate moiety allows for the selective formation of one isomer over the other. Whereas the α-isomer can be obtained in synthetically useful yield, the γ-isomer is favored only when employing reaction conditions that are inevitably associated with diminished reactivity.

Highly Enantioselective Cobalt-Catalyzed Hydrosilylation of Alkenes

A cobalt-catalyzed highly Markovnikov-type and enantioselective hydrosilylation of alkenes is developed for the efficient synthesis of valuable chiral dihydrosilanes. This protocol is operationally simple and atom-economy, and using relatively simple and readily available starting materials. The reaction is suitable for both aryl and aliphatic alkenes with excellent functional group tolerability. The reaction could be easily carried out in a gram-scale. The TOF and TON is up to 1800 and 860, respectively.

Highly chemoselective ruthenium(ii)-catalyzed direct arylation of cyclic and N,N-dialkyl benzamides with aryl silanes

The ruthenium(ii)-catalyzed oxidative cross-coupling of C(sp2)-H bonds with organosilanes has been accomplished for the first time. This novel protocol enlists challenging cyclic and N,N-dialkyl benzamides as weakly-coordinating substrates to achieve highly regioselective C(sp2)-H arylation as a proof-of-concept, taking advantage of the attractive features of organosilanes as coupling partners. This innovative method is characterized by very high chemoselectivity, installing halide functional groups (I, Br, Cl) that are incompatible with Ru(ii)-carboxylate systems employing halides as cross-coupling partners, while obviating the need for sensitive organometallic reagents and cryogenic temperatures typical to the classic directed-ortho-metallation (DoM) techniques, employing benzamides to afford bioactive structural motifs.

Asymmetric Copper Hydride-Catalyzed Markovnikov Hydrosilylation of Vinylarenes and Vinyl Heterocycles

We report a highly enantioselective CuH-catalyzed Markovnikov hydrosilylation of vinylarenes and vinyl heterocycles. This method has a broad scope and enables both the synthesis of isolable silanes and the conversion of crude products to chiral alcohols. Density functional theory calculations support a mechanism proceeding by hydrocupration followed by σ-bond metathesis with a hydrosilane.

Pincer cobalt complex-catalyzed Z-selective hydrosilylation of terminal alkynes

Co-Catalyzed Z-selective hydrosilylation of terminal alkynes produces (Z)-β-vinylsilanes, which are further converted to (Z)-disubstituted alkenes by Pd-catalyzed cross couplings.

Ruthenium(ii)-catalyzed ortho-C–H arylation of diverse N-heterocycles with aryl silanes by exploiting solvent-controlled N-coordination

We report the first method for the direct, regioselective Ru(ii)-catalyzed oxidative arylation of C–H bonds in diverse N-heterocycles with aryl silanes by exploiting solvent-controlled N-coordination.

Cooperative redox activation for carbon dioxide conversion

A longstanding challenge in production chemistry is the development of catalytic methods for the transformation of carbon dioxide into useful chemicals. Silane and borane promoted reductions can be fined-tuned to provide a number of C1-building blocks under mild conditions, but these approaches are limited because of the production of stoichiometric waste compounds. Here we report on the conversion of CO₂ with diaryldisilanes, which through cooperative redox activation generate carbon monoxide and a diaryldisiloxane that actively participate in a palladium-catalysed carbonylative Hiyama-Denmark coupling for the synthesis of an array of pharmaceutically relevant diarylketones. Thus the disilane reagent not only serves as the oxygen abstracting agent from CO₂, but the silicon-containing ‘waste', produced through oxygen insertion into the Si–Si bond, participates as a reagent for the transmetalation step in the carbonylative coupling. Hence this concept of cooperative redox activation opens up for new avenues in the conversion of CO₂.

While CO₂ can be converted to CO for further reactivity, typically this involves the generation of large amounts of waste. Here the authors report the conversion of CO₂ to CO, with the siloxane by-product being further used in a carbonylative Hiyama-Denmark coupling to form diarylketones.

Pd-Catalyzed Suzuki-Miyaura and Hiyama-Denmark Couplings of Aryl Sulfamates

Using a recently discovered precatalyst, the first Pd-catalyzed Suzuki-Miyaura reactions using aryl sulfamates that occur at room temperature are reported. In complementary work, it is demonstrated that a related precatalyst can facilitate the coupling of aryl silanolates, which are less toxic and reactive nucleophiles than boronic acids with aryl chlorides. By combining our results using modern electrophiles and nucleophiles, the first Hiyama-Denmark reactions using aryl sulfamates are reported.

Copper-Mediated C-N Coupling of Arylsilanes with Nitrogen Nucleophiles

A method for the oxidative coupling of arylsilanes with nitrogen nucleophiles is reported. This method occurs with a broad range of heptamethyltrisiloxylarenes and nitrogen nucleophiles, proceeds with the arylsilane as limiting reagent, and does not require a fluoride activator with electron-poor arylsilanes. The combination of this method with C-H silylation generates arylamines from unactivated arenes with site selectivity controlled by steric effects. This combination of steps gives direct access to many compounds that cannot be accessed via alternative C-H functionalization methods, including direct C-H amination or the combination of C-H borylation and amination.

Palladium-Catalyzed Defluorinative Coupling of 1-Aryl-2,2-Difluoroalkenes and Boronic Acids: Stereoselective Synthesis of Monofluorostilbenes

The palladium-catalyzed defluorinative coupling of 1-aryl-2,2-difluoroalkenes with boronic acids is described. Broad functional-group tolerance arises from a redox-neutral process by a palladium(II) active species which is proposed to undergo a β-fluoride elimination to afford the products. The monofluorostilbene products were formed with excellent diastereoselectivity (≥50:1) in all cases, and it is critical, as traditional chromatographic techniques often fail to separate monofluoroalkene isomers. As a demonstration of this method's unique combination of reactivity and functional-group tolerance, a Gleevec® analogue, using a monofluorostilbene as an amide isostere, was synthesized.

Trifluoromethylation of Arylsilanes with [(phen)CuCF3 ]

A method for the trifluoromethylation of arylsilanes is reported. The reaction proceeds with [(phen)CuCF3 ] as the CF3  source under mild, oxidative conditions with high functional-group compatibility. This transformation complements prior trifluoromethylation of arenes in several ways. Most important, this method converts arylsilanes formed by the silylation of aryl C-H bonds to trifluoromethylarenes, thereby allowing the conversion of arenes to trifluoromethylarenes. The unique capabilities of the reported method are demonstrated by the conversion of a C-H bond into a C-CF3 bond in active pharmaceutical ingredients which do not undergo this overall transformation by alternative functionalization processes, including a combination of borylation and trifluoromethylation.

In Situ Generation of ArCu from CuF2 Makes Coupling of Bulky Aryl Silanes Feasible and Highly Efficient

A bimetallic system of Pd/CuF2, catalytic in Pd and stoichiometric in Cu, is very efficient and selective for the coupling of fairly hindered aryl silanes with aryl, anisyl, phenylaldehyde, p-cyanophenyl, p-nitrophenyl, or pyridyl iodides of conventional size. The reaction involves the activation of the silane by Cu(II), followed by disproportionation and transmetalation from the Cu(I)(aryl) to Pd(II), upon which coupling takes place. Cu(III) formed during disproportionation is reduced to Cu(I)(aryl) by excess aryl silane, so that the CuF2 system is fully converted into Cu(I)(aryl) and used in the coupling. Moreover, no extra source of fluoride is needed. Interesting size selectivity towards coupling is found in competitive reactions of hindered aryl silanes. Easily accessible [PdCl2 (IDM)(AsPh3)] (IDM = 1,3-dimethylimidazol-2-ylidene) is by far the best catalyst, and the isolated products are essentially free from As or Pd (<1 ppm). The mechanistic aspects of the process have been experimentally examined and discussed.