Palladium-catalysed cross-coupling of organosilicon reagents

Pd-Catalyzed Vinylation of Aryl Halides with Inexpensive Organosilicon Reagents Under Mild Conditions

Pd-catalyzed Hiyama vinylation reaction of non-activated aryl chlorides and bromides under mild conditions was developed. The use of efficient vinyl donors and electron-rich sterically hindered phosphine ligands was critical for the success of the reaction. The products of this transformation can be used for Am/Cm separation, an important challenge in nuclear fuel reprocessing. The substituent effect on Am/Cm separating selectivity was also achieved, which could contribute to the development of new chromatographic materials for the separation of Am and Cm.

The influence of NHCs on C-Si and C-C reductive elimination: a computational study of the selectivity of Ni-catalyzed C-H activation of arenes with vinylsilanes

Density functional theory calculations were performed to investigate the mechanism and origins of the NHC-controlled selectivity of Ni-catalyzed C-H activation of arenes with vinylsilanes. The key to the selectivity is the different impacts of NHCs on the C-Si/C-C reductive elimination of the square-planar/T-shaped intermediate.

Palladium-Catalyzed Direct Arylation of Polyfluoroarenes for Accessing Tetra- ortho-Substituted Biaryls: Buchwald-type Ligand Having Complementary -PPh2 Moiety Exhibits Better Efficiency

The first general examples of direct C-H arylation of electron-deficient polyfluoroarenes with challenging di- ortho-substituted aryl(heteroaryl) chlorides for tetra- ortho-substituted biaryl synthesis are reported. Key to success is the use of Buchwald-type biaryl phosphine ligand, notably with inexpensive -PPh₂ moiety (instead of -PCy₂ group). Pd(OAc)₂ associated with ligand L9 exhibits even higher efficiency than the corresponding SPhos toward this reaction. A wide range of sterically hindered di- ortho-substituted chloroarenes bearing electron-donating or -withdrawing groups are found applicable. Excellent product yields are obtained under mild reaction conditions, and the catalyst loading down to 0.25 mol % of Pd can also be achieved.

Fluoride-mediated rearrangement of phenylfluorosilanes

Combining Ph3SiF and fluoride ion under conditions used for the Hiyama coupling causes rapid formation of the expected [Ph3SiF2]−; however, real-time electrospray mass spectrometric analysis reveals that phenyl-fluoride exchange occurs concomitantly, also producing substantial quantities of [PhnSiF5–n]− (n = 0–2). The exchange process is verified using 19F NMR spectroscopy. This observation may have implications for Hiyama reaction protocols, which use transmetallation from triaryldifluorosilicates as a key step in cross-coupling.

Si-Directed regiocontrol in asymmetric Pd-catalyzed allylic alkylations using C1-ammonium enolate nucleophiles

Cooperative catalysis enables the direct enantioselective α-allylation of linear prochiral esters using Si-substituted allyl electrophiles. The Si-substituent directs the regioselectivity of enantioselective bond formation and provides products containing synthetically versatile pentafluorophenyl ester and vinylsilane moieties. Critical to the efficacy of this process was the recognition that the ancillary ligand on palladium could be altered to prevent formation of a deleterious ether by-product, whilst retaining enantioselectivity through the Lewis base catalyst. Flexibility such as this is unique to cooperative catalysis events and provides efficient access to an array of enantioenriched products that are orthogonally functionalized and easily modified.

An access to 1,3-azasiline-fused quinolinonesviaoxidative heteroannulation involving silyl C(sp3)–H functionalization

A Mn-promoted intermolecular oxidative heteroannulation ofN-(2-cyanoaryl)-acrylamides with tertiary silanes toward 1,3-azasiline-fused quinolinones is presented.

Visible Light-Induced Room-Temperature Heck Reaction of Functionalized Alkyl Halides with Vinyl Arenes/Heteroarenes

The first visible light-induced Pd-catalyzed Heck reaction of α-heteroatom substituted alkyl iodides and -bromides with vinyl arenes/heteroarenes has been developed. This transformation efficiently proceeds at room temperature and enables synthesis of valuable functionalized allylic systems, such as allylic silanes, boronates, germanes, stannanes, pivalates, phosphonates, phthalimides, and tosylates from the corresponding α-substituted methyl iodides. Notably, synthesis of the latter substrates failed under existing thermally induced Pd-catalyzed conditions, which highlights the importance of visible light for this transformation.

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.

Synthesis of substituted Z-styrenes by Hiyama-type coupling of oxasilacycloalkenes: application to the synthesis of a 1-benzoxocane

Several Hiyama cross-coupling reactions of oxasilacycloalkenes and aryl iodides are described that produce trisubstituted Z-styrenes in moderate to excellent yields. Both electron-rich and electron-poor aryl iodides are tolerated in the cross-coupling reaction. The oxasilacycloalkene coupling partners were prepared by ruthenium-catalyzed intramolecular anti-hydrosilylation of alkynols. One of the cross-coupling products was converted to a 1-benzoxocane, albeit in low yield, using an intramolecular Buchwald–Hartwig etherification. The cyclic ether produced contains the carbon skeleton of heliannuol A.

Stereoselective Synthesis of Z-Vinylsilanes via Palladium-Catalyzed Direct Intermolecular Silylation of C(sp2)-H Bonds

An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp²)-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.

Heck-like Reactions Involving Heteroatomic Electrophiles

Since the discovery of the Heck reaction in the early seventies, this reaction has become a powerful tool in synthetic organic chemistry. By employing heteroatomic instead of traditional carbon electrophiles, the Heck reaction shows an intriguing flexibility. These "hetereoatomic-Heck reactions" reinvigorate the area, offering new routes to highly useful synthetic precursors and structural motifs present in biologically active compounds. This microreview focuses on early developments leading to the heteroatomic-Heck reactions (silyl-Heck, boryl-Heck and intramolecular aza-Heck), current state of the emerging area, as well as that of a few related processes.

B(C6F5)3-Catalyzed C-Si/Si-H Cross-Metathesis of Hydrosilanes

The substituent redistribution of hydrosilanes on silicon through C-Si and Si-H bond cleavage and reformation is of great interest and importance, but this transformation is usually difficult to achieve in a selective fashion. By using electron-rich aromatic hydrosilanes, we have achieved for the first time the selective C-Si/Si-H bond homo- and cross-metathesis of a series of hydrosilanes in the presence of a boron catalyst B(C₆F₅)₃. This protocol features simple reaction conditions, high chemoselectivity, wide substrate scope, and high functionality tolerance, offering a new pathway for the synthesis of multisubstituted functional silanes.

Protein modification via alkyne hydrosilylation using a substoichiometric amount of ruthenium(ii) catalyst

Transition metal catalysis has emerged as a powerful strategy to expand synthetic flexibility of protein modification. Herein, we report a cationic Ru(ii) system that enables the first example of alkyne hydrosilylation between dimethylarylsilanes and O-propargyl-functionalized proteins using a substoichiometric amount or low-loading of Ru(ii) catalyst to achieve the first C-Si bond formation on full-length substrates. The reaction proceeds under physiological conditions at a rate comparable to other widely used bioorthogonal reactions. Moreover, the resultant gem-disubstituted vinylsilane linkage can be further elaborated through thiol-ene coupling or fluoride-induced protodesilylation, demonstrating its utility in further rounds of targeted modifications.

Palladium-Mediated Labeling of Nucleic Acids

New applications of Pd-catalyzed coupling reactions (Suzuki-Miyaura, Sonogashira, and Stille-Migita coupling) for post-conjugation of nucleic acids have been developed recently. Breakthroughs in this area might now pave the way for the development of sophisticated DNA probes, which might be of great interest in chemical biology, nanotechnology, and bioanalysis, as well as in diagnostic domains.

Rhodium(iii)-catalyzed directed C–H benzylation and allylation of indoles with organosilicon reagents

Rhodium(iii)-catalyzed C2-benzylation and allylation of indoles with organosilicon reagents via chelation-assisted C–H activation has been developed.

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.

Rhodium-catalyzed carbonylative synthesis of silyl-substituted indenones

A novel and efficient rhodium-catalyzed procedure for the preparation of silyl-substituted indenones has been developed. With silanes and internal alkynes as the substrates in the presence of CO, good to excellent yields of the desired indenones can be isolated.

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.

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.