Allenylidene Induced 1,2-Metalate Rearrangement of Indole-Boronates: Diastereoselective Access to Highly Substituted Indolines

Iridium-Catalyzed Asymmetric Difunctionalization of C-C σ-Bonds Enabled by Ring-Strained Boronate Complexes

Enantioenriched organoboron intermediates are important building blocks in organic synthesis and drug discovery. Recently, transition metal-catalyzed enantioselective 1,2-metalate rearrangements of alkenylboronates have emerged as an attractive protocol to access these valuable reagents by installing two different carbon fragments across C═C π-bonds. Herein, we report the development of an iridium-catalyzed asymmetric allylation-induced 1,2-metalate rearrangement of bicyclo[1.1.0]butyl (BCB) boronate complexes enabled by strain release, which allows asymmetric difunctionalization of C-C σ-bonds, including dicarbonation and carboboration. This protocol provides a variety of enantioenriched three-dimensional 1,1,3-trisubstituted cyclobutane products bearing a boronic ester that can be readily derivatized. Notably, the reaction gives trans diastereoisomers that result from an anti-addition across the C-C σ-bond, which is in contrast to the syn-additions observed for reactions promoted by Pd^II-aryl complexes and other electrophiles in our previous works. The diastereoselectivity has been rationalized based on a combination of experimental data and density functional theory calculations, which suggest that the BCB boronate complexes are highly nucleophilic and react via early transition states with low activation barriers.

Copper-Catalysed Rearrangement of Cyclic Ethynylethylene Carbonates: Synthetic Applications and Mechanistic Studies

Transition-metal-catalysed reactions of cyclic ethynylethylene carbonates have been intensively studied because of their robustness in new bond formation and diversified molecule construction. Known reaction modes usually involve a substitution step occurring at either the propargylic or terminal alkyne positions. Here, we report an unprecedented reaction pattern in which cyclic ethynylethylene carbonates first undergo a rearrangement to release allenal intermediates, which subsequently react with diverse nucleophiles to furnish synthetically useful allylic and propargylic allenols, phosphorus ylides, and cyclopropylidene ketones through an addition process rather than a substitution pathway. The products enable various further transformations, and mechanistic studies and theoretical calculations reveal that the reaction does not proceed via a semipinacol type [1,2]-hydride shift, but through base-mediated deprotonation as the key step to induce the rearrangement.

Palladium-Catalyzed Conjunctive Cross-Coupling with Electronically Asymmetric Ligands

Palladium-catalyzed conjunctive cross-coupling can be accomplished with the use of chiral phosphine-oxazoline based ligand structures. Of note, the reaction can be conducted on Grignard-based boron ate complexes and operates without the use of halide-scavenging additives, which are required for other catalyst systems.

1,2-Metallate Rearrangement Using Indole Boronate Species to Access 2,3-Diarylindoles and Indolines

A transition metal-free multicomponent reaction using lithiated indole, boronic ester, pyridine, and ethyl chloroformate was developed to access C2,C3 bis-arylated indoles, which are present in several marketed drugs and bioactive compounds. One-pot access to unsymmetrical C2,C3-diaryl indole from the parent indole remains a huge synthetic challenge. Our group was able to achieve this goal through a transition metal-free 1,2-metalate rearrangement of the indole boronate complex. The reaction of indole boronate species with activated pyridine allows 1,2-migration to access pyridyl-indoleboronate species, which will convert to the corresponding indole upon oxidation and indoline after deborylation. The reaction tolerates substituted pyridines, quinolone, isoquinoline, and more. Both aryl and alkyl boronic esters were accommodated under optimized reaction conditions. Apart from mechanistic studies using ¹¹B-NMR, this methodology has been applied to the gram-scale synthesis of several bioactive compounds.

Stereoselective gem-C,B-Glycosylation via 1,2-Boronate Migration

A novel protocol is established for the long-standing challenge of stereoselective geminal bisglycosylations of saccharides. The merger of PPh₃ as a traceless glycosidic leaving group and 1,2-boronate migration enables the simultaneous introduction of C-C and C-B bonds at the anomeric stereogenic center of furanoses and pyranoses. The power of this method is showcased by a set of site-selective modifications of glycosylation products for the construction of bioactive conjugates and skeletons. A scarce metal-free 1,1-difunctionalization process of alkenes is also concomitantly demonstrated.

Annulative Coupling of Vinylboronic Esters: Aryne-Triggered 1,2-Metallate Rearrangement

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed. An annulation reaction that proceeded through the formation of two C–C bonds and a C–B bond was realized by exploiting a 1,2-metallate rearrangement of boronate triggered by the addition of a vinyl group to the strained triple bond of an aryne. The generated aryl anion would then cyclize to a boron atom to complete the annulation cascade. The annulated borinic ester could be converted to boronic acids and their derivatives by oxidation, halogenation, and cross-coupling. Particularly, halogenation and Suzuki–Miyaura coupling proceeded in a site-selective fashion and produced highly substituted alkylboronic acid derivatives.

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed.

Chemo- and Stereoselective Dearomative Coupling of Indoles and Bielectrophilic β-Imino Boronic Esters via Imine-Induced 1,2-Boronate Migration

A new imine-induced 1,2-boronate migration has been developed for achieving chemo- and stereoselective dearomative coupling of C3-substituted indoles and bi-electrophilic β-imino boronic esters, providing rapid access to complex chiral indoline boronic esters with four stereocenters including an all-carbon quaternary stereocenter and a tertiary α-aminoboronic ester. In contrast, coupling of indoles without C3 substitution and β-imino boronic esters provided tetrahydro-1H-pyrido[4,3-b]indoles via imine-induced 1,2-boronate migration followed by deborylative rearomatization.

Nickel-Catalyzed Dearomative Arylboration of Indoles: Regioselective Synthesis of C2- and C3-Borylated Indolines

Indole dearomatization is an important strategy to access indolines: a motif present in a variety of natural products and biologically active molecules. Herein, a method for transition-metal catalyzed regioselective dearomative arylboration of indoles to generate diverse indolines is presented. The method accomplishes intermolecular dearomatization of simple indoles through a migratory insertion pathway on substrates that lack activating or directing groups on the C2- or C3-positions. Synthetically useful C2- and C3-borylated indolines can be accessed through a simple change in N-protecting group in high regio- and diastereoselectivities (up to >40:1 rr and >40:1 dr) from readily available starting materials. Additionally, the origin of regioselectivity was explored experimentally and computationally to uncover the remarkable interplay between carbonyl orientation of the N-protecting group on indole, electronics of the C2-C3 π-bond, and sterics. The method enabled the first enantioselective synthesis of (-)-azamedicarpin.