Enantiospecific Synthesis of ortho-Substituted 1,1-Diarylalkanes by a 1,2-Metalate Rearrangement/anti-SN 2' Elimination/Rearomatizing Allylic Suzuki-Miyaura Reaction Sequence

Photoredox-Catalyzed Decarboxylative Cross-Coupling Reaction to Synthesis Unsymmetrical Diarylmethanes

The photoredox-catalyzed decarboxylative cross-coupling reaction of aryl acetic acids and aryl nitriles has been achieved under an argon atmosphere in high yields. This method provides a fast way to obtain prevalent aryl acetic acids from an abundant natural source. A tentative radical mechanism has been proposed.

Orthogonal sp3-Ge/B Bimetallic Modules: Enantioselective Construction and Enantiospecific Cross-Coupling

In this study, a series of enantioenriched sp3-Ge/B bimetallic modules were successfully synthesized via an enantioselective copper-catalyzed hydroboration of carbagermatrane (Ge)-containing alkenes. Orthogonal cross-coupling selectivity under different Pd-catalyzed conditions was achieved in an enantiospecific manner. Notably, the chiral secondary Ge exhibited a remarkable transmetallation ability prior to primary or secondary Bpin. The effectiveness of this Ge/B bimetallic strategy was further demonstrated through the development of new functional small molecules with Aggregation-Induced Emission (AIE) and Circularly Polarized Luminescence (CPL) performance. This represents the first successful example of synthesis of enantioenriched alkylgermanium reagents that permit enantiospecific cross-coupling reactions.

Enantioselective catalytic 1,2-boronate rearrangements

[Figure: see text].

The Stereochemical Course of Pd-Catalyzed Suzuki Reactions Using Primary Alkyltrifluoroborate Nucleophiles

Using deuterium-labeled stereochemical probes, we show that primary alkyltrifluoroborate nucleophiles undergo transmetalation to palladium exclusively via a stereoretentive pathway and that the resulting stereospecificity is broadly independent of electronic and steric effects. This stands in stark contrast to the stereochemical course of transmetalation for secondary alkyltrifluoroborates, which varies between net stereoretention and net stereoinversion depending upon the electronic properties of the supporting phosphine ligand, the electronic properties of the aryl electrophile, and the steric properties of the alkylboron nucleophile. In this study, we additionally show that the stereochemical course of transmetalation for secondary alkylboron reagents can be under reagent steric control, while no such steric control exists for analogous primary alkylboron nucleophiles. The combined study reveals fundamental mechanistic differences between transmetalations of primary and secondary alkylboron reagents in Pd-catalyzed Suzuki reactions.

Stereoselectivity in Pd-catalysed cross-coupling reactions of enantioenriched nucleophiles

Advances in Pd-catalysed cross-coupling reactions have facilitated the development of stereospecific variants enabling the use of configurationally stable, enantioenriched, main-group organometallic nucleophiles to form C(sp ³)-C(sp ²) bonds. Such stereospecific cross-coupling reactions constitute a powerful synthetic approach to attaining precise 3D control of molecular structure, allowing new stereogenic centres to be readily introduced into molecular architectures. Examples of stereospecific, Pd-catalysed cross-coupling reactions have been reported for isolable enantioenriched alkylboron, alkyltin, alkylgermanium and alkylsilicon nucleophiles. In these reactions, a single, dominant stereospecific pathway of transmetallation to palladium is required to effect efficient chirality transfer to the cross-coupled product. However, the potential for competing stereoretentive and stereoinvertive pathways of transmetallation complicates the stereochemical transfer in these processes and impedes the rational development of new stereospecific cross-coupling variants. In this Review, we describe the use of enantioenriched organometallic nucleophiles in stereospecific, Pd-catalysed cross-coupling reactions. We focus on systems involving well-defined, isolable, enantioenriched nucleophiles in which a clear stereochemical course of transmetallation is followed. Specific modes of electronic activation that influence the reactivity of alkylmetal nucleophiles are described and presented in the context of their impact on the stereochemical course of cross-coupling reactions. We expect that this Review will serve as a valuable resource to assist in deconvoluting the many considerations that potentially impact the stereochemical outcome of Pd-catalysed cross-coupling reactions.

Stereospecific 1,2-Migrations of Boronate Complexes Induced by Electrophiles

The stereospecific 1,2-migration of boronate complexes is one of the most representative reactions in boron chemistry. This process has been used extensively to develop powerful methods for asymmetric synthesis, with applications spanning from pharmaceuticals to natural products. Typically, 1,2-migration of boronate complexes is driven by displacement of an α-leaving group, oxidation of an α-boryl radical, or electrophilic activation of an alkenyl boronate complex. The aim of this article is to summarize the recent advances in the rapidly expanding field of electrophile-induced stereospecific 1,2-migration of groups from boron to sp2 and sp3 carbon centers. It will be shown that three different conceptual approaches can be utilized to enable the 1,2-migration of boronate complexes: stereospecific Zweifel-type reactions, catalytic conjunctive coupling reactions, and transition metal-free sp2 -sp3 couplings. A discussion of the reaction scope, mechanistic insights, and synthetic applications of the work described is also presented.

Enantioselective benzylic C-H arylation via photoredox and nickel dual catalysis

The asymmetric cross-coupling reaction is developed as a straightforward strategy toward 1,1-diaryl alkanes, which are a key skeleton in a series of natural products and bioactive molecules in recent years. Here we report an enantioselective benzylic C(sp³)−H bond arylation via photoredox/nickel dual catalysis. Sterically hindered chiral biimidazoline ligands are designed for this asymmetric cross-coupling reaction. Readily available alkyl benzenes and aryl bromides with various functional groups tolerance can be easily and directly transferred to useful chiral 1,1-diaryl alkanes including pharmaceutical intermediates and bioactive molecules. This reaction proceeds smoothly under mild conditions without the use of external redox reagents.

Chiral 1,1-diaryl alkanes are important targets in pharmaceutical industry. Here, the authors report report a redox-neutral enantioselective benzylic C−H bond arylation via photoredox/nickel dual catalysis accessing chiral 1,1-diarylalkane compounds under mild conditions.

An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition: Construction of gem-Bis(boryl)alkanes

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B₂ Pin₂ (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem-bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical-polar crossover, specifically, a 1,2-boryl-anion shift from boron to the α-carbon sp² center. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical-chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Computational assessments of diastereoselective [4+2] cycloaddition and 1,3-borotopic shift of a dearomatized tertiary boronic ester intermediate: reactivities explained through transition-state distortion energies

Interception of a dearomatized tertiary boronic ester, formed through a kinetically and thermodynamically favorable 1,2-metalate rearrangement/anti-S_N2′ elimination of an activated ortho-lithiated benzyl amine, in a [4+2] cycloaddition or 1,3-borotopic shift has been investigated by density functional theory (DFT). Although superacitvated “naked” Li⁺ was found to greatly promote 1,3-borotopic shift, the diastereoselective [4+2] cycloaddition was favored. It was revealed that the factor that controls the diastereoselectivity was the steric bulk provided by the diene, which is in agreement with experimental diastereoselectivity. A comparison of unreactive dienophiles such as maleic anhydride, diethyl maleate, and others with 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione (PTAD) was found to be in an excellent agreement with the experiments; where their lack of reactivity is attributed to the high deformation energies of the interacting components to achieve the transition state structure which was pronounced with the high energy of LUMO orbitals.

Interception of dearomatized tertiary boronic ester in a diastereoselective [4+2] cycloaddition or 1,3-borotopic shift in the presence or absence of “naked” Li⁺, understanding reactivities by activation/strain model, were evaluated by DFT calculations.

Enantiospecific Synthesis of ortho-Substituted 1,1-Diarylalkanes by a 1,2-Metalate Rearrangement/anti-SN 2' Elimination/Rearomatizing Allylic Suzuki-Miyaura Reaction Sequence

The one‐pot sequential coupling of benzylamines, boronic esters, and aryl iodides has been investigated. In the presence of an N‐activator, the boronate complex formed from an ortho‐lithiated benzylamine and a boronic ester undergoes stereospecific 1,2‐metalate rearrangement/anti‐S_N2′ elimination to form a dearomatized tertiary boronic ester. Treatment with an aryl iodide under palladium catalysis leads to rearomatizing γ‐selective allylic Suzuki–Miyaura cross‐coupling to generate 1,1‐diarylalkanes. When enantioenriched α‐substituted benzylamines are employed, the corresponding 1,1‐diarylalkanes are formed with high stereospecificity.