Catalytic Synthesis of Silanols by Hydroxylation of Hydrosilanes: From Chemoselectivity to Enantioselectivity

Cationic and neutral ruthenium(II) complexes bearing silyl-phosphines: synthesis, structures, and catalytic studies in selective hydroboration of carbonyl compounds

The reactivity of ortho-silylated phosphines featuring SiMe3 and SiR2H (R = Me, Ph) units has been studied towards ruthenium(II) metal precursors. The reaction of phosphino-trimethylsilane (1a) with [Ru(p-cymene)Cl2]2 in the presence of KPF6 afforded the cationic complex [Ru(η6-p-cymene)(CH3CN)(1a)]PF6 (3). Phosphine ligands bearing hydrosilane groups afforded P-silanol ligated ruthenium complexes [Ru(η6-cymene)(Ph2PSi(CH3)2OH)Cl]Cl (5) and [Ru(η6-cymene)(Ph2PSi(Ph)2OH)Cl]Cl (6) via intramolecular hydroxylation of 1b and 1c. This process involves the formation of a reactive intermediate metal complex [(1b)Ru(η6-cymene)Cl2] (A) which was structurally characterized. Complex 5 was subsequently converted to its neutral analog [Ru(η6-p-cymene)(Ph2PSi(CH3)2O)Cl] (5'). Unlike alkoxy-phosphines, which are known to undergo tautomerism, silyl phosphine derivatives afford bench stable P,O-ligated ruthenium(II) complexes, indicating a unique reactivity profile of phosphine-silanols. Ruthenium complexes 3 and 5 were employed as molecular catalysts in the hydroboration of a series of aliphatic and aromatic aldehydes and ketones which yielded boronate esters. Mechanistic investigations revealed the formation of a silylated phosphine-ruthenium hydride species, which acts as the active catalyst. Furthermore, complex 3 displayed excellent chemoselective hydroboration of aldehydes over ketones at very low catalyst loadings (0.05 mol%) under solvent-free conditions.

Catalytic Asymmetric Dehydrogenative Si-H/X-H Coupling toward Si-Stereogenic Silanes

ConspectusChiral organosilicon compounds bearing a Si-stereogenic center have attracted increasing attention in various scientific communities and appear to be a topic of high current relevance in modern organic chemistry, given their versatile utility as chiral building blocks, chiral reagents, chiral auxiliaries, and chiral catalysts. Historically, access to these non-natural Si-stereogenic silanes mainly relies on resolution, whereas their asymmetric synthetic methods dramatically lagged compared to their carbon counterparts. Over the past two decades, transition-metal-catalyzed desymmetrization of prochiral organosilanes has emerged as an effective tool for the synthesis of enantioenriched Si-stereogenic silanes. Despite the progress, these catalytic reactions usually suffer from limited substrate scope, poor functional-group tolerance, and low enantioselectivity. The growing demand for Si-stereogenic silanes with structural diversity has continued to drive the development of new practical methods for the assembly of these chiral molecules.Five years ago, our research group embarked on a project aimed at developing a general catalytic approach that can unlock access to various functionalized Si-stereogenic organosilanes with high efficiency. This Account describes our laboratory's endeavor in the exploration and development of catalytic asymmetric dehydrogenative Si-H/X-H coupling toward Si-stereogenic silanes. This approach features (1) readily accessible dihydrosilane starting materials; (2) diverse X-H (X═C, N, O, etc.) coupling partners; (3) platform transformable Si-stereogenic monohydrosilane products; and (4) high efficiency and atomic economy.At the initial stage of the research, a biaryl dihydrosilane was selected as the model substrate to conduct an enantioselective intramolecular C-H/Si-H dehydrogenative coupling reaction. Rh/Josiphos catalytic system was found to be effective at the early stage of this process, while the final enantiocontrol was elusive. Mechanistic studies indicated that a rhodium silyl dihydride complex is the resting state in the catalytic cycle, which may undergo racemization of the Si-stereogenic center. Enlightened by the mechanistic investigations, two strategies, the tandem alkene hydrosilylation strategy and bulky alkene-assisted dehydrogenative strategy, were adopted to avoid racemization, delivering the corresponding Si-stereogenic 9-silafluorenes with excellent yields and enantioselectivities. Further enantioselective intramolecular C(sp2)-H or C(sp3)-H silylation gave access to a series of five-, six- and seven-membered Si-stereogenic heterocycles with high efficiency. Next, we extended the reaction to an intermolecular version, realizing asymmetric Si-H/C-H, Si-H/O-H, and Si-H/N-H dehydrogenative coupling reactions toward a variety of acyclic Si-stereogenic monohydrosilanes, silyl ethers, siloxanes, silanols, and silazanes. We also presented our endeavors to apply the resulting Si-stereogenic compounds, including further derivatization, polymerization, and chiroptical property investigations, which successfully introduced Si-stereocenters into bioactive molecules, polymers, and chiroptical materials. Lastly, based on the understanding of silyl metal species, we developed a new type of chiral silyl ligand that can be applied to enable an atroposelective intermolecular C-H/Si-H dehydrogenative coupling reaction. We anticipate that our research, including synthetic methodology, mechanistic insights, and property studies, will not only inspire the further development of chiral organosilicon chemistry but also contribute to the creation of novel chiral molecules to be applied in synthetic chemistry, medicinal chemistry, and materials science.

Electroreductive Cross-Coupling between Aromatic Aldehydes and Chlorosilanes Enabling the Synthesis of α-Silyl Alcohols

α-Silyl alcohols are powerful structural motifs for pharmaceutical chemistry, materials chemistry, and organic synthesis. The limitations of current synthetic techniques encompass a requirement for difficult-to-obtain silyl precursors, noble-metal catalysts, and narrow substrate scopes. Here, we developed a general synthetic method for α-silyl alcohols through electroreductive cross-coupling of aldehydes and chlorosilane. This method features easily available reagents, mild conditions, and a wide substrate scope. The establishment of this protocol will provide an alternative for access to α-silyl alcohols.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.