Density Functional Theory Studies of Titanium-Catalyzed Hydroboration of Olefins

Hydroboration of vinylsilanes providing diversity-oriented hydrophobic building blocks for biofunctional molecules

Hydroboration of vinylsilanes with BH3 affords two silylethanol regioisomers. Herein, we investigated the regioisomeric ratio of hydroboration products from various vinylsilanes, focusing on the characteristic reaction profile. All investigated vinylsilanes afforded both regioisomers, and greater bulkiness increased the proportion of the Markovnikov products. The obtained silylethanols were used as hydrophobic building blocks for constructing nuclear progesterone receptor (PR) modulators. Notably, structural conversions from an α-isomer (silylethan-1-oxy derivative) to a β-isomer (2-silylethoxy derivative) caused complete activity-switching from a PR agonist to an antagonist. Our results indicate that silylethanols are useful for structural development, and vinylsilanes are a versatile source of hydrophobic building blocks for obtaining biofunctional molecules.

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.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

Titanium redox catalysis: insights and applications of an earth-abundant base metal

π-Acid ancillary ligands, reactants, or products can stabilize reactive low valent Ti intermediates through backbonding, and present opportunities for the development of vast new classes of Ti-catalyzed redox reactions with practical applications.

Dehydrogenative borylation: the dark horse in metal-catalyzed hydroborations and diborations?

Metal-catalyzed hydroborations and diborations frequently give “unwanted” side products arising from a competing dehydrogenative borylation reaction. While catalyst development initially focused on avoiding this “deleterious” pathway, fine tuning of a catalyst system could provide an efficient method to generate unsaturated alkenylboronate esters, synthetically valuable synthons for the Suzuki-Miyaura cross-coupling reaction. The aim of this review is to highlight the history and development of the dehydrogenative borylation reaction in the metal-catalyzed synthesis of alkenylboronate esters.

Theoretical Study of Trans-metalation Process in Palladium-Catalyzed Borylation of Iodobenzene with Diboron

Trans-metalation process in the palladium-catalyzed borylation of iodobenzene with diboron was theoretically investigated with the DFT method. Palladium(II) hydroxo phenyl complex, Pd(OH)(Ph)(PH(3))(2), and the fluoro analogue easily undergo the trans-metalation with diboron, B(2)(eg)(2) (eg = -OCH(2)CH(2)O-), to afford Pd(Ph)(Beg)(PH(3))(HO-Beg) and Pd(Ph)(Beg)(PH(3))(F-Beg), respectively, where B(2)(eg)(2) is adopted as a model of bis(pinacolato)diboron used experimentally. The electron re-distribution in the trans-metalation clearly indicates that the B-B bond scission occurs in a heterolytic manner. In the chloro analogue, PdCl(Ph)(PH(3))(2), however, the trans-metalation occurs in a homolytic manner with much difficulty, which is consistent with the experimental result. The significant differences between the chloro complex and the other hydroxo and fluoro complexes are easily interpreted in terms that hydroxo and fluoro ligands can form strongly bonding interaction with B(2)(eg)(2) but the chloro ligand cannot.