Transition-Metal Free C-C Bond Cross-Coupling of Aryl Ethers with Diarylmethanes.

We describe a general and efficient transition-metal free C-C bond cross-coupling of (hetero)aryl ethers and diarylmethanes via C(sp2)-O bond cleavage. The coupling reactions mediated by KHMDS proceeded well with high efficiency, broad substrate scope, and good functional group tolerance. The robustness and practicability of this protocol also have been demonstrated by easy gram-scale preparation and diversified product derivatization.

Catalyst-free C(sp3)-H functionalization of methyl azaarenes with heteroaromatic trifluoromethyl ketone hydrates: "all-water" synthesis of α-trifluoromethyl tertiary alcohols.

An efficient catalyst-free C(sp3)-H bond functionalization of methyl azaarenes with heteroaromatic trifluoromethyl ketone hydrates in neat water has been developed for the synthesis of α-trifluoromethyl tertiary alcohols bearing N-heteroaromatics. This method not only features excellent efficiency, broad substrate scope, catalyst-free conditions, and easy gram-scale preparation but also represents a new and rare example of "all-water" synthesis of trifluoromethylated molecules.

Rhodium-Catalyzed Remote Borylation of Alkynes and Vinylboronates.

Remote functionalization involving a fascinating chain-walking process has emerged as a powerful strategy for the rapid access to value-added functional molecules from readily available feedstocks. However, the scope of current methods is predominantly limited to mono- and di-substituted alkenes. The remote functionalization of multi- and heteroatom-substituted alkenes is challenging, and the use of alkynes in the chain walking is unexplored. We herein report a rhodium catalyzed remote borylation of internal alkynes, offering an unprecedented reaction mode of alkynes for the preparation of synthetically valuable 1,n-diboronates. The regioselective distal migratory hydroboration of sterically hindered tri- and tetra-substituted vinylboronates is also demonstrated to furnish various multi-boronic esters. Synthetic utilities are highlighted through the selective manipulation of the two boryl groups in products such as the regioselective cross coupling, oxidation, and amination.

Cobalt-Catalyzed Asymmetric Remote Borylation of Alkyl Halides.

Enantioselective functionalization of racemic alkyl halides is an efficient strategy to assemble complex chiral molecules, but remains one of the biggest challenges in organic chemistry. The distant and selective activation of unreactive C-H bonds in alkyl halides has received growing interest as it enables rapid generation of molecular complexity from simple building blocks. Here, we reported a cobalt-catalyzed remote borylation of alkyl (pseudo)halides (alkyl-X, X=I, Br, Cl, OTs) with pinacolborane (HBpin) and presented a robust approach for the generation of valuable chiral secondary organoboronates from racemic alkyl halides. This migration borylation reaction is compatible with primary, secondary, and tertiary bromides, offering direct access to a broad range of alkylboronates. The extension of this catalytic system to the borylation of aryl halides was also demonstrated. Preliminary mechanistic studies revealed that this remote borylation involved a radical reaction pathway.

Mixed-Valence BN-Doped Corannulene Trimer Radical Cations.

Mixed-valence (MV) dimers have been extensively investigated, however, the structure and properties of purely organic MV trimers based on open-shell polycyclic aromatic hydrocarbons remain elusive. Herein, unprecedented MV BN-doped corannulene radical cations [BN-Cor1]3 ⋅⋅2+ ⋅ 2[BArylF 4 ]- and [BN-Cor2]3 ⋅⋅2+ ⋅ 2[BArylF 4 ]- were synthesized via chemical oxidation, and their structures were unambiguously confirmed by single-crystal X-ray diffraction. These uncommon radical cations consist of three corannulene cores and two [BArylF 4 ]- anions, and three corannulene motifs [BN-Cor1]3 ⋅⋅2+ and [BN-Cor2]3 ⋅⋅2+ in the unit cell exhibit a trimer structure with a slipped π-stacking configuration. Detailed structural analyses further revealed that the corannulene cores exhibit an infinite layered self-assembly configuration, allowing their potential applications as building blocks for molecular conductors. The detection of a forbidden transition (Δms =±2) by electron paramagnetic resonance (EPR) spectroscopy further confirmed the existence of two unpaired electrons in the π-trimers and the MV characteristic of these two species. Variable-temperature EPR and conductivity measurements suggested that the BN-doped π-trimers exhibited antiferromagnetic coupling and conductivity properties.

Rh-Catalyzed Coupling of Aldehydes with Allylboronates Enables Facile Access to Ketones.

We present herein a novel strategy for the preparation of ketones from aldehydes and allylic boronic esters. This reaction involves the allylation of aldehydes with allylic boronic esters and the Rh-catalyzed chain-walking of homoallylic alcohols. The key to this successful development is the protodeboronation of alkenyl borylether intermediate via a tetravalent borate anion species in the presence of KHF2 and MeOH. This approach features mild reaction conditions, broad substrate scope, and excellent functional group tolerance. Mechanistic studies also supported that the tandem allylation and chain-walking process were involved.

Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling of Oxygen-Substituted Allylboronates with Aryl/Vinyl (Pseudo)Halides.

An efficient palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of oxygen atom-substituted allylboronates with aryl/vinyl bromides, iodides, and triflates has been developed. The present coupling reactions proceeded smoothly to provide a variety of allylic siloxanes with high efficiency and excellent regioselectivity. This protocol features broad substrate scope, excellent functional group tolerance, and easy gram-scale preparation, and offers an alternative approach for the synthesis of allylic alcohols and their derivatives.

Enantioselective Cobalt-Catalyzed Hydroboration of Ketone-Derived Silyl Enol Ethers.

Catalytic asymmetric hydroboration of alkenes is a powerful tool for the synthesis of natural products, agrochemicals, and pharmaceuticals via the versatile transformations of chiral alkyl boronic esters. However, the scope of available alkenes is limited to styrenes, activated alkenes, and compounds with directing groups. The catalytic enantioselective hydroboration of heteroatom-substituted alkenes is rarely explored and those catalyzed by earth-abundant metals are yet to be reported. Herein, we report a cobalt-catalyzed asymmetric hydroboration of ketone-derived silyl enol ethers and provide a convenient approach to access valuable enantiopure ß-hydroxy boronic esters. This protocol features mild reaction conditions, a broad substrate scope, and excellent enantioselectivities (up to 99 % ee). This approach was applied in the successful synthesis of salmeterol and albuterol, demonstrating its potential to streamline complex molecule synthesis.

Asymmetric Oxidative Lactonization of Enynyl Boronates.

Oxidation of C-B bonds is extensively used in organic synthesis, materials science, and chemically biology. However, these oxidations are usually limited to the oxidation of C(sp3 )-B and C(sp2 )-B bonds. The C(sp)-B bond oxidation is rarely developed. Herein we present a novel strategy for the preparation of γ-lactones via the oxidation of enynyl boronates. This process successively involves the C(sp)-B bond oxidation, the epoxidation of C-C double bond and the lactonization. This protocol provided various γ-lactones and unsaturated butenolides efficiently that are prevalent in numerous nature products and bioactive molecules. Most importantly, asymmetric oxidative lactonization of enynyl boronates was also achieved, providing chiral γ-lactones in high enantioselectivities and diastereoselectivities. The versatile transformations and ubiquity of γ-lactones shed light on the importance of this strategy in the construction and late-stage functionalization of complex molecules.

Enantioselective Rh-Catalyzed Hydroboration of Silyl Enol Ethers.

The asymmetric hydroboration of alkenes has proven to be among the most powerful methods for the synthesis of chiral boron compounds. This protocol is well suitable for activated alkenes such as vinylarenes and alkenes bearing directing groups. However, the catalytic enantioselective hydroboration of O-substituted alkenes has remained unprecedented. Here we report a Rh-catalyzed enantioselective hydroboration of silyl enol ethers (SEEs) that utilizes two new chiral phosphine ligands we developed. This approach features mild reaction conditions and a broad substrate scope as well as excellent functional group tolerance, and enables highly efficient preparation of synthetically valuable chiral borylethers.

A mild and practical method for deprotection of aryl methyl/benzyl/allyl ethers with HPPh2 and tBuOK.

A general method for the demethylation, debenzylation, and deallylation of aryl ethers using HPPh2 and tBuOK is reported. The reaction features mild and metal-free reaction conditions, broad substrate scope, good functional group compatibility, and high chemical selectivity towards aryl ethers over aliphatic structures. Notably, this approach is competent to selectively deprotect the allyl or benzyl group, making it a general and practical method in organic synthesis.

Rhodium-Catalyzed Deoxygenation and Borylation of Ketones: A Combined Experimental and Theoretical Investigation.

The rhodium-catalyzed deoxygenation and borylation of ketones with B2pin2 have been developed, leading to efficient formation of alkenes, vinylboronates, and vinyldiboronates. These reactions feature mild reaction conditions, a broad substrate scope, and excellent functional-group compatibility. Mechanistic studies support that the ketones initially undergo a Rh-catalyzed deoxygenation to give alkenes via boron enolate intermediates, and the subsequent Rh-catalyzed dehydrogenative borylation of alkenes leads to the formation of vinylboronates and diboration products, which is also supported by density functional theory calculations.

Lewis Acid Mediated Electrophilic Cyanation of 2,2'-Biphenols.

A Lewis acid mediated electrophilic cyanation of 2,2'-biphenols with a trifluoromethanesulfonyl (Tf) protecting group is reported. The cyanation reactions with less toxic, commercially available MeSCN as a cyanating reagent afforded a range of 3-cyan-2,2'-biphenols in moderate to high yields. The use of trifluoromethanesulfonyl (Tf) as a protecting group is crucial to the success of this transformation. Moreover, the cyanated products were readily transformed into various synthetically useful molecules. This protocol features high efficiency, excellent regioselectivity, and good functional group compatibility and may provide a practical tool for the synthesis and modification of biologically active compounds, catalysts, and ligands.

Lewis acid-promoted site-selective cyanation of phenols.

An efficient Lewis acid-promoted site-selective electrophilic cyanation of 3-substituted and 3,4-disubstituted phenols has been developed. The cyanation reactions using MeSCN as the cyanating reagent proceeded efficiently to afford a wide range of 2-hydroxybenzonitriles with high efficiency and excellent regioselectivity. This protocol could provide a practical method for the synthesis and modification of biologically active molecules.

Triflimide (HNTf2) in Organic Synthesis.

Triflimide (HNTf2) is a commercially available and highly versatile super Brønsted acid. Owing to its strong acidity as well as good compatibility with organic solvents, it has been widely employed as an exceptional catalyst, promoter, or additive in a wide range of organic reactions. On many occasions, triflimide has been demonstrated to outperform triflic acid (TfOH). The uniquely outstanding performance of triflimide also benefits from the low nucleophilicity and noncoordinating property of its conjugate base (Tf2N-). Therefore, it has been employed as a precursor toward a variety of cationic metal complexes or organic intermediates with enhanced reactivity or catalytic activity. In this Review, we describe these features and applications of triflimide in organic synthesis, including its synthesis, physical properties, and role as catalyst or promoter in organic reactions. At the end of this Review, another closely related reagent, triflidic acid (HCTf3), is also briefly introduced.

Rhodium-Catalyzed Remote C(sp3 )-H Borylation of Silyl Enol Ethers.

A rhodium-catalyzed remote C(sp3 )-H borylation of silyl enol ethers (SEEs, E/Z mixtures) by alkene isomerization and hydroboration is reported. The reaction exhibits mild reaction conditions and excellent functional-group tolerance. This method is compatible with an array of SEEs, including linear and branched SEEs derived from aldehydes and ketones, and provides direct access to a broad range of structurally diverse 1,n-borylethers in excellent regioselectivities and good yields. These compounds are precursors to various valuable chemicals, such as 1,n-diols and aminoalcohols.

Divergent and Stereoselective Synthesis of Tetraarylethylenes from Vinylboronates.

The synthesis of a new tetraborylethylene (TBE) is reported, and its application in the preparation of [4+0]-tetraarylethenes (TAEs) is elucidated. TAEs have widespread applications in material science and supramolecular chemistry due to their aggregation-induced emission (AIE) properties. The divergent and stereoselective synthesis of [3+1]-, [2+2]-, and [2+1+1]-TAEs via multiple couplings of vinylboronates with aryl bromides is demonstrated. These couplings feature a broad substrate scope and excellent functional group compatibility due to mild reaction conditions. Facile access to various tetraarylethenes is provided. This strategy represents an important complement to the conventional methods employed for the synthesis of TAEs, and would be a valuable tool for synthesizing TAE-based molecules useful in functional materials, biological imaging and chemical sensing.

Oxidation of Alkynyl Boronates to Carboxylic Acids, Esters, and Amides.

A general efficient protocol was developed for the synthesis of carboxylic acids, esters, and amides through oxidation of alkynyl boronates, generated directly from terminal alkynes. This protocol represents the first example of C(sp)-B bond oxidation. This approach displays a broad substrate scope, including aryl and alkyl alkynes, and exhibits excellent functional group tolerance. Water, primary and secondary alcohols, and amines are suitable nucleophiles for this transformation. Notably, amino acids and peptides can be used as nucleophiles, providing an efficient method for the synthesis and modification of peptides. The practicability of this methodology was further highlighted by the preparation of pharmaceutical molecules.

Formation of a Supramolecular Polymeric Adhesive via Water-Participant Hydrogen Bond Formation.

A supramolecular polymeric adhesive was prepared from non-viscous, non-polymeric materials by water-participant hydrogen bonds. Pt-pyridine coordination and water-crown ether hydrogen bonding combine to effect the supramolecular polymerization. The supramolecular polymeric adhesive displays strong, reversible adhesion to hydrophilic surfaces, a property that forecasts the application of hydrogen bonding in advanced supramolecular materials.

Cascade Copper-Catalyzed 1,2,3-Trifunctionalization of Terminal Allenes.

A cascade cyanation/diborylation of terminal allenes proceeds efficiently with copper catalysis using bis(pinacolato)diboron (B2Pin2) and N-cyano-N-phenyl-p-methylbenzenesulfonamide (NCTS) as reagents. Mechanistic studies suggest that the process proceeds through cyanoborylation of the substituted π-system of the allene followed by hydroboration of the remaining π-component. A wide array of product derivatives may be accessed through site-selective cross-couplings and N-bromosuccinimide-promoted heteroarylations as well as standard oxidative and reductive conversions of the initially obtained adducts.