Divergent Synthesis of Enantioenriched Silicon-Stereogenic Benzyl-, Vinyl- and Borylsilanes via Asymmetric Aryl to Alkyl 1,5-Palladium Migration

Functionalization of Si-bound methyl group provides an efficient access to diverse organosilanes. However, the asymmetric construction of silicon-stereogenic architectures by functionalization of Si-bound methyl group has not yet been described despite recent significant progress in producing chiral silicon. Herein, we disclosed the enantioselective silylmethyl functionalization involving the aryl to alkyl 1,5-palladium migration to access diverse naphthalenes possessing an enantioenriched stereogenic silicon center, which are inaccessible before. It is worthy to note that the realization of asymmetric induction at the step of metal migration itself remains challenging. Our study constitutes the first enantioselective aryl to alkyl 1,5-palladium migration reaction. The key to the success is the discovery and fine-tuning of the different substituents of α,α,α,α-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL)-based phosphoramidites, which ensure the enantioselectivity and desired reactivity.

Solvent-Switchable Remote C-H Activation via 1,4-Palladium Migration Enables Site-Selective C-P Bond Formation: A Tool for the Synthesis of P-Chiral Phosphinyl Imidazoles

Solvent-switchable and site-selective phosphorylation of imidazoles at the C2 or C5 position of the imidazole ring was achieved via 1,4-palladium migration. P-Chiral tert-butyl(aryl)phosphine oxides were cross-coupled to 1-(2-bromophenyl)-1H-imidazoles with high enantiospecificity, thereby leading to a novel class of chiral imidazole-based phosphine oxides. As proof of concept, reduction of an analogue yielded the corresponding P-chiral 2-phosphinyl imidazole ligand, which was shown to induce high enantioselectivity in the formation of axially chiral molecules synthesized via Pd-catalyzed Suzuki-Miyaura cross-coupling.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Copper-Catalyzed Regio- and Stereoselective Formal Hydro(borylmethylsilyl)ation of Internal Alkynes via Alkenyl-to-Alkyl 1,4-Copper Migration

Catalytic reactions involving 1,n-metal migration from carbon to carbon enable a nonclassical way of constructing organic molecular skeletons, rapidly providing complex molecules from relatively simple precursors. By utilization of this attractive feature, a new and efficient synthesis of alkenylsilylmethylboronates has been developed by formal hydro(borylmethylsilyl)ation of unsymmetric internal alkynes with silylboronates under copper catalysis. The reaction proceeds regioselectively and involves an unprecedented alkenyl-to-alkyl 1,4-copper migration. The reaction mechanism has been investigated by a series of kinetic, NMR, and deuterium-labeling experiments.

Rapid Access to Fused Tetracyclic N-Heterocycles via Amino-to-Alkyl 1,5-Palladium Migration Coupled with Intramolecular C(sp3)-C(sp2) Coupling

An unprecedented route for the preparation of fused tetracyclic N-heterocycles is presented through the palladium-catalyzed cyclization of isocyanides with alkyne-tethered aryl iodides. In this transformation, a novel amino-to-alkyl 1,5-palladium migration/intramolecular C(sp3)-C(sp2) coupling sequence was observed first. More importantly, isocyanide exhibited three roles, serving simultaneously as a C1 synthon, a C1N1 synthon, and the donor of C(sp3) for C(sp3)-C(sp2) coupling, and the reaction was the sole successful example that achieved C(sp3)-H activation of isocyanide.

Accessing indole-isoindole derivatives via palladium-catalyzed [3+2] cyclization of isocyanides with alkynyl imines

A facile protocol for the preparation of indole-isoindole derivatives was developed and proceeds via a palladium-catalyzed [3+2] cyclization of isocyanides with alkynyl imines. In this transformation, the palladium catalyst has a triple role, serving simultaneously as a π acid, a transition-metal catalyst and a hydride ion donor, thus enabling the dual function of isocyanide both as a C1 synthon for cyanation and a C1N1 synthon for imidoylation. Significantly, the reaction is the sole successful example for accessing indole-isoindole derivatives, and will open up new avenues to assemble unique N-heterocycle frameworks. Furthermore, the synthetic value of this protocol is demonstrated in the late-stage modification of physiologically active molecules and in the construction of aggregation-induced emission compounds.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Tunable Construction of Multisubstituted 1,3-Dienes and Allenes via a 1,4-Palladium Migration/Carbene Insertion Cascade

Efficient palladium-catalyzed vinylic C-H alkenylation and allenylation of gem-disubstituted ethylenes with N-tosylhydrazones of aryl alkyl and diaryl ketones were achieved to access trisubstituted 1,3-dienes and tetrasubstituted allenes, respectively. An aryl to vinyl 1,4-palladium migration/carbene insertion/β-hydride elimination sequence proceeded to switch the chemo- and regioselectivities to give structurally diverse products. Use of 2-FC₆H₄OH additive enables enhancement of the reaction efficiency through accelerating the key 1,4-palladium migration process.

Rhodium-Catalyzed Three-Component Reaction of Alkynes, Arylzinc Chlorides, and Iodomethanes Producing Trisubstituted/Tetrasubstituted Alkenes with/without 1,4-Migration

A three-component reaction of alkynes, arylzinc chlorides, and iodomethanes was found to proceed in the presence of a rhodium catalyst to give high yields of trisubstituted/tetrasubstituted alkenes. The usual arylzinc chlorides only gave trisubstituted alkenes, generated through a migratory carbozincation-cross-coupling sequence, where 1,4-Rh migration from an alkenyl carbon to an aryl carbon occurred. In contrast, 5-membered heteroarylzinc chlorides only gave the tetrasubstituted alkenes via a carborhodation-cross-coupling pathway without 1,4-migration.

Phosphorylation of C(sp3)-H Bonds via 1,4-Palladium Migration

1,4-Palladium migration has emerged as a reliable method for directed C-H functionalization. In contrast to coupling with carbon nucleophiles, limited examples with heteroatom nucleophiles have been reported. Herein we report a palladium-catalyzed intermolecular C(sp³)-H phosphorylation reaction via 1,4-palladium migration, which is often difficult because of the strong coordination of phosphorus reagents to palladium catalysts. Phosphorylation of C(sp³)-H bonds is accomplished in good reaction yields with excellent regioselectivity. The judicious selection of the phosphine ligand proved to be the key to the success of this cascade process.

Highly Diastereoselective Synthesis of Octahydro-1H-cyclpenta[c]pyridine Skeleton via Pd/Au-Relay Catalyzed Cascade Reaction of (Z)-1-Iodo-1,6-diene and Alkyne

The Octahydro-1H-cyclopenta[c]pyridine Skeletons exist in a broad spectrum of bioactive natural products, and the development of efficient and convenient protocols to construct this skeleton remains a challenging task. Herein, we...

Intermolecular C-H silylation through cascade carbopalladation and vinylic to aryl 1,4-palladium migration

A palladium-catalyzed remote C-H silylation reaction has been developed through vinylic to aryl 1,4-palladium migration. By using alkyne-tethered aryl iodides as the starting materials and hexamethyldisilane as the silylating reagent, the reaction involves cascade intramolecular carbopalladation, 1,4-palladium migration, and silylation with hexamethyldisilane, and leads to the formation of exocyclic alkene-containing 5-silylisoquinolines as the final products.

Quadruple Role of Pd Catalyst in Domino Reaction Involving Aryl to Alkyl 1,5-Pd Migration to Access 1,9-Bridged Triptycenes

A Pd-catalyzed domino reaction of 1,8,13-tribromo-9-methoxytriptycenes is reported. Under conventional Suzuki coupling conditions, the triptycenes underwent multiple transformations to give 1,9-bridged triptycenes. Based on mechanistic investigations, a single Pd catalyst functions as Pd⁰ , Pd^II and Pd^IV species to catalyze four distinct processes: (1) aryl to alkyl 1,5-Pd migration, (2) intramolecular arylation, (3) homocoupling of phenylboronic acid and (4) Suzuki coupling. DFT calculations revealed that 1,5-Pd migration likely proceeds via both concerted Pd^II and stepwise Pd^IV routes. Asymmetric synthesis of the chiral triptycenes, as well as optical resolution, and further transformation are also reported.