(Z)-Selective Hydroboration of Terminal Alkynes Catalyzed by a PSP-Pincer Rhodium Complex

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

LiOtBu-Promoted trans-Stereoselective and β-Regioselective Hydroboration of Propargyl Alcohols

A convenient and efficient trans-stereoselective and β-regioselective hydroboration of propargyl alcohols was achieved simply with LiOtBu as the base and (Bpin)2 as the boron reagent in dimethyl sulfoxide at room temperature. Both terminal and internal propargyl alcohols with diverse structures and functional groups underwent the transformation smoothly to produce β-Bpin-substituted (E)-allylic alcohols, of which the synthetic potentials were demonstrated by the downstream conversions of boronate, alkenyl, and hydroxyl groups.

Palladium-Catalyzed Multicomponent Assembly of (Z)-Alkenylborons via Carbopalladation/Boronation/Retro-Diels-Alder Cascade Reaction

A palladium-catalyzed multicomponent cascade reaction of aryl iodides, oxanorbornadiene, and diborns to access (Z)-alkenylborons is reported. This transformation proceeds through the sequential carbopalladation/boronation/retro-Diels-Alder domino reaction. The oxanorbornadiene used in this reaction serves as an acetylene surrogate, which is generated via a retro-Diels-Alder reaction. Such a stereoselective and scalable approach has a wide range of functional group tolerance and good substrate universality.

An ionic Fe-based metal-organic-framework with 4'-pyridyl-2,2':6',2''-terpyridine for catalytic hydroboration of alkynes

An ionic metal-organic-framework (MOF) containing nanoscale channels was readily assembled from ditopic 4'-pyridyl-2,2':6',2''-terpyridine (pytpy) and a simple iron(ii) salt. X-ray structural analysis revealed a two-dimensional grid-like framework assembled by classic octahedral (pytpy)2FeII cations as linkers (with pytpy as a new ditopic pyridyl ligand) and octa-coordinate FeCl2 centers as nodes. The layer-by-layer assembly of the 2-D framework resulted in the formation of 3-D porous materials consisting of nano-scale channels. The charges of the cationic framework were balanced with anionic Cl3FeOFeCl3 in its void channels. The new Fe-based MOF material was employed as a precatalyst for syn-selective hydroboration of alkynes under mild, solvent-free conditions in the presence of an activator, leading to the synthesis of a range of trans-alkenylboronates in good yields. The larger scale applicability and recyclability of the new MOF catalyst was further explored. This represents a rare example of an ionic MOF material that can be utilized in hydroboration catalysis.

Effect of Internal Ligand Strain on Coordination Behavior of PSP Pincer Ligands

Chelating ligands and most specifically pincer ligands, with their characteristic co-planar binding, usually undergo deformations upon coordination, resulting in a significant ligand strain. Such an effect on the properties of the so formed complex has rarely been explored. This study is an attempt to analyze this strain and its contribution to the overall binding energy and coordination behavior of PSP pincer ligands. Hence, we designed a rigid thioxanthone-based PSP pincer ligand (I) and studied the difference in the coordination properties with the more flexible thioxanthene and thioether-based PSP pincer ligands (II and III). Although with one equivalent of Pd(II) precursor, the three ligands exhibited a similar coordination behavior leading to similar κ³-P,S,P pincer complexes, an in-depth computational analysis pointed out the different contributions of the internal strain energy in lowering the binding energy of these complexes. This effect was clearly reflected when we calculated the enthalpy change of these ligand-exchange reactions. As these exchange reactions are enthalpy-driven, these results could also be confirmed experimentally. With two equivalents of Pd(II), the three ligands diverged in their coordination behavior. Specifically, ligands I and III gave each a binuclear complex, with different coordination modes, whereas the pincer complex of ligand II remained unaffected by excess of Pd(II). Our calculations suggest that the driving force for the formation of binuclear Pd(II) complexes is the relief of the internal ligand strain. With Pt(II), only the mononuclear κ³-P,S,P pincer complexes were obtained irrespectively of the amount of the Pt(II) precursor. In these cases, we assume that kinetic inertness of the formed mononuclear pincer Pt(II) complexes prevents binding of an additional Pt(II) nucleus. This study points out the important role of the internal ligand strain in PSP pincer ligand coordination behavior. We believe that our findings can be extended to other pincer ligands systems as well.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.