Rhodium(III)-Catalyzed Controllable C−H Bond Functionalization of Benzamides and Vinylidenecyclopropanes: A Directing Group Determined Reaction Pathway

InI3-catalyzed polyene cyclization of allenes and its application in the total synthesis of seven abietane-type diterpenoids

A novel polyene cyclization using the allene group as the initiator has been successfully developed. This methodology provides an efficient strategy for the construction of an abietane-type tricyclic skeleton with a functionalizable C2-C3 double bond and features a wide substrate scope and excellent stereoselectivities. Potential utility of this approach has been well demonstrated by the collective total synthesis of seven abietane-type diterpenoids. Specifically, (±)-2,3-dihydroxyferruginol and (±)-2,3-dihydroxy-15,16-dinor-ent-pimar-8,11,13-triene were synthesized for the first time.

1,4,2-Dioxazol-5-ones as Isocyanate Equivalents: Chemoselective Non-Metal-Catalyzed Carboxamidation of Indoles

1,4,2-Dioxazol-5-ones are known to undergo decarboxylation under thermal conditions followed by Lossen’s rearrangement to give isocyanates. Described herein is the in situ trapping of the isocyanates by indoles to give indole-3-carboxamides in good to excellent yields.

Facile synthesis of O-acylhydroxamates via reaction of oxime chlorides with carboxylic acids

A simple and efficient method for the synthesis of O-acylhydroxamate derivatives from oxime chlorides and carboxylic acids was developed. The reaction affords clean and facile access to diverse O-acylhydroxamates in high yields (up to 85%). The chemical structure of a typical product was confirmed using single-crystal X-ray structure analysis.

Cu-catalyzed coupling of vinylidene cyclopropanes with allyl and allenyl boronates

The development of Cu-catalyzed coupling of vinylidene cyclopropanes with allyl or allenyl boronates is reported. The reaction forms a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes with concurrent opening of the cyclopropane ring. In addition, the resulting Cu-enolate intermediate can be intercepted by external electrophiles.

Redox-Neutral Rhodium(III)-Catalyzed Chemospecific and Regiospecific [4+1] Annulation between Indoles and Alkenes for the Synthesis of Functionalized Imidazo[1,5-a]indoles

Exploiting internal alkenes embedded with an oxidizing function/leaving group as a rare and unconventional one-carbon unit, a redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4+1] annulation between indoles and alkenes for the synthesis of functionalized imidazo[1,5-a]indoles has been achieved. Internal alkenes employed here can fulfill an unusual [4+1] annulation rather than normal [4+2] annulation/C-H alkenylation. This method is characterized by excellent chemo- and regioselectivity, broad substrate scope, good functional group tolerance, good to high yields, and redox-neutral conditions.

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H' elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Dual Role of the Rhodium(III) Catalyst in C-H Activation: [4 + 3] Annulation of Amide with Allylic Alcohols to 7-Membered Lactams

[4 + 3] annulation of primary and secondary benzamide and cinnamamide derivatives using allyl alcohol as a coupling partner catalyzed by Rh(III) is reported, where Rh(III) is playing a dual role of an oxidant and a catalyst for C-H activation. The Rh-catalyst oxidizes allyl alcohol to its carbonyl derivative, and the in situ-generated carbonyl compound reacts with benzamide in the presence of the Rh-catalyst, forming the corresponding alkylated products. Mechanistic studies show that AgSbF₆ is also playing a dual role. Apart from being a halide scavenger, AgSbF₆ catalyzes the cyclization of the alkylated product, forming the desired lactam. The current method has good synthetic application and is useful for synthesizing a few biologically active compounds that can act as the dopamine D₃ receptor ligand, including berberine-like analogues. The deuteration study and control experiments helped us to propose the mechanism.

Cu-catalyzed C-C bond formation of vinylidene cyclopropanes with carbon nucleophiles

The development of Cu-catalyzed addition of carbon nucleophiles to vinylidene cyclopropanes was reported. The reactions with 1,1-bisborylmethane provided homopropargylic boronate products by forming a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes. Alkynyl boronates are also suitable nucleophile precursors in reactions with vinylidene cyclopropanes, and skipped diynes were obtained in high yields. In addition, the Cu-enolate generated from the initial addition of nucleophilic copper species to vinylidene cyclopropanes can be intercepted by an external electrophile. As such, vinylidene cyclopropane serves as a linchpin to connect a nucleophile and an electrophile by forming two carbon-carbon bonds sequentially.

A rhodium(iii)-catalyzed tunable coupling reaction of indole derivatives with alkylidenecyclopropanes via C-H activation

We herein report a rhodium(iii)-catalyzed cross coupling of indole derivatives with alkylidenecyclopropanes (ACPs) in the presence of KCl, affording the alkene products exclusively with E-selectivity via C-H bond activation. The beta-H elimination to afford the conjugated diene derivatives has been suppressed by the addition of KCl. A plausible reaction mechanism has been proposed along with derivatization of the obtained product to demonstrate the practical usefulness of this synthetic protocol.

Trisannulation of benzamides and cyclohexadienone-tethered 1,1-disubstituted allenes initiated by Cp*Rh(iii)-catalyzed C–H activation

A trisannulation of benzamides and cyclohexadienone-tethered 1,1-disubstituted allenes was achieved via the initiation of Cp*Rh(iii)-catalyzed C–H bond activation.

Recent Advances in Transition-Metal-Catalyzed/Mediated Transformations of Vinylidenecyclopropanes

Vinylidenecyclopropanes (VDCPs), having an allene moiety connected to a highly strained cyclopropyl group, have attracted a substantial amount of attention since they are fascinating building blocks for organic synthesis. During recent years, the reactions of VDCPs in the presence of a Lewis acid or a Brønsted acid and those induced by heat or light have experienced significant advancements due to the unique structural and electronic properties of VDCPs. Transition-metal-catalyzed reactions of VDCPs were not intensely investigated until the last 5 years. Recently, significant progress has been made in transition-metal-catalyzed transformations of VDCPs, and they have emerged as a new direction for the chemistry of strained small rings, especially when new types of functionalized vinylidenecyclopropanes (FVDCPs) are used as substrates. To date, many interesting transformations have been explored using these novel VDCPs under the catalysis of transition metals, such as gold, palladium, or rhodium, and various novel and useful heterocyclic or polycyclic compounds have been generated. These new findings have enriched the chemistry of strained small carbocycles. This Account will describe the transition-metal-catalyzed transformations of VDCPs recently developed in our laboratory and by other groups. The chemistry of Au-catalyzed VDCPs has been enriched and extensively developed by our group. In this respect, a new process for generating gold carbenes from VDCPs has been disclosed. The reactivity of these new gold carbenoid species was fully investigated, and many novel reaction modes based on these new gold carbenoid species were explored, including oxidation reactions, intramolecular cyclopropanations, C(sp³)-H bond functionalizations, and C-O bond cleavage reactions. Rh-catalyzed reactions of VDCPs are another key field of transition-metal-catalyzed reactions of VDCPs. In particular, rhodium-catalyzed cycloadditions, Pauson-Khand reactions, and C-H bond activations of FVDCPs have been explored in detail by our group. A new trimethylenemethane rhodium (TMM-Rh) complex generated from VDCPs was discovered and utilized as an electrophilic Rh-π-allyl precursor. Moreover, some unprecedented highly regio- and enantioselective asymmetric allylic substitutions via this novel TMM-Rh complex were developed with different kinds of nucleophiles. This Account will also summarize the recent advances in palladium-, copper-, and iron-catalyzed cycloisomerization reactions of VDCPs reported by our group and others. These reactions always afford the desired products with excellent chemo-, regio-, diastereo-, and enantioselectivities, which will make them highly valuable for the synthesis of key scaffolds in natural products and pharmaceutical molecules in the future.