Cobalt‐Catalyzed Asymmetric Sequential Hydroboration/Isomerization/Hydroboration of 2‐Aryl Vinylcyclopropanes

Access to Cyclic Borates by Cu-Catalyzed Borylation of Unactivated Vinylcyclopropanes

We report the copper-catalyzed borylation of unactivated vinylcyclopropanes to form six-membered cyclic borate salts. A copper complex bearing an N-heterocyclic ligand in combination with bis(pinacolato)diboron and LiOtBu catalyzes the ring-opening of the substrate under mild reaction conditions. The protocol can be applied to aryl- and heteroaryl-substituted vinylcyclopropanes and can be conducted on a gram scale. The synthetic utility of the lithium salts of the cyclic borate has been demonstrated through regioselective ring-opening functionalizations.

Ligand-Controlled Cobalt-Catalyzed Regiodivergent and Stereoselective Ring-Opening Isomerization of Vinyl Cyclopropanes

A ligand-controlled regiodivergent and stereoselective ring-opening isomerization of vinylcyclopropane was developed with cobalt catalysis. Employing the commercially available Xantphos ligand, the reactions afforded exclusively linear-type 1,3-dienes as the products. Interestingly, when switching the ligand to an amido-diphosphine ligand (PNP), branched-type 1,3-dienes were obtained with high regioselectivity and stereoselectivity. Preliminary mechanistic investigations suggested that a π-allyl metal and a metal-hydride species are involved as key intermediates in the two transformations, respectively.

Rhodium-Catalyzed Highly Enantioselective Hydroboration of Acyclic Tetrasubstituted Alkenes Directed by an Amide

Although progress has been made in enantioselective hydroboration of di- and trisubstituted alkenes over the past decades, enantioselective hydroboration of tetrasubstituted alkenes with high diastereo- and enantioselectivities continues as an unmet challenge since the 1950s due to its extremely low reactivity and the difficulties to simultaneously control the regio- and stereoselectivity of a tetrasubstituted alkene. Here, we report highly regio-, diastereo-, and enantioselective catalytic hydroboration of diverse acyclic tetrasubstituted alkenes. The delicate interplay of an electron-rich rhodium complex and coordination-assistance forms a highly adaptive catalyst that effectively overcomes the low reactivity and controls the stereoselectivity. The generality of the catalyst system is exemplified by its efficacy across various tetrasubstituted alkenes with diverse steric and electronic properties.

Enantioselective Copper-Catalyzed Ring-Opening Diboration of Arylidenecyclopropanes to Access Chiral Skipped 1,4- and 1,3-Diboronates

Two enantioselective approaches to synthesize chiral skipped diboronate compounds have been developed, relying on copper-catalyzed one-pot asymmetric ring-opening diboration of arylidenecyclopropanes. A wide range of arylidenecyclopropanes react smoothly with HBpin in the presence of CuOAc and (R)-DTBM-Segphos, affording chiral 1,4-diboronates with high enantioselectivity (up to 99% ee). Meanwhile, a variety of arylidenecyclopropanes react selectively with HBpin and B2pin2 in the presence of CuOAc and (S,S)-Ph-BPE with the sequential addition of MeOH, providing chiral 1,3-diboronates with high enantioselectivity (up to 98% ee). These enantioenriched 1,3- and 1,4-diboronates can undergo various enantiospecific transformations with minimal loss of their enantiopurity. Mechanistic studies reveal that these two diboration processes start with CuH-catalyzed ring-opening hydroboration of arylidenecyclopropanes to form a mixture of Z/E-homoallyl boronate intermediates, which subsequently undergo enantioselective CuH-catalyzed second hydroboration or Cu-Bpin-catalyzed protoboration to produce chiral 1,4-diboronates or 1,3-diboronates, respectively.

Pd-Catalyzed Regioselective Cyclopropanation of 2-Substituted 1,3-Dienes

A Pd-catalyzed 3,4-regioselective cyclopropanation of 2-substituted 1,3-dienes by decomposition of diazo esters is reported. The vinylcyclopropanes generated are isolated in practical chemical yields with high levels of regioselectivity but low diastereoselectivity. The system operates under mild reaction conditions, is scalable, and tolerates various sensitive functional groups. A series of original postcatalytic derivatizations is presented to highlight the synthetic potential of the catalytic method.

Selective Electroreductive Hydroboration of Olefins with B2pin2

Organoboron showed great potential in the synthesis of various high-value chemical compounds. Direct hydroboration of olefins has been witnessed over time as a mainstream method for the synthesis of organoboron compounds. In this work, an electroreductive anti-Markovnikov hydroboration approach of olefins with readily available B₂pin₂ to synthesize valuable organoboron compounds with high chemo- and regioselectivities under metal catalyst-free conditions was reported. This protocol exhibited broad substrate scope and good functional-group tolerance on styrenes and heteroaromatic olefins, providing synthetically useful alkylborons with high efficiency and even various deuterium borylation products with good D-incorporation when CD₃CN was employed as solvent. Furthermore, gram-scale reactions and extensive functional derivatization further highlighted the potential of this method.

Applications of Vanadium, Niobium, and Tantalum Complexes in Organic and Inorganic Synthesis

Organometallic catalysis is a powerful strategy in chemical synthesis, especially with the cheap and low toxic metals based on green chemistry principle. Thus, the selection of the metal is particularly important to plan relevant and applicable processes. The group VB metals have been the subject of exciting and significant advances in both organic and inorganic synthesis. In this Review, we have summarized some reports from recent decades, which are about the development of group VB metals utilized in various types of reactions, such as oxidation, reduction, alkylation, dealkylation, polymerization, aromatization, protein synthesis, and practical water splitting.