Palladium(ii)-catalyzed vinylic geminal double C-H activation and alkyne annulation reaction: synthesis of pentafulvenes

Synthesis of Multisubstituted Cyclopentadiene Derivatives from 3,3-Disubstituted Cyclopropenes and Internal Alkynes Catalyzed by Low-Valent Niobium Complexes

A low-valent niobium species generated from NbCl5 and 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (Si-Me-CHD) in combination with PPh3 catalyzed a [2+2+1]-cycloaddition reaction of 3,3-disubstituted cyclopropenes and 2 equiv of diaryl/dialkylalkynes, leading to isomeric mixtures of multisubstituted cyclopentadienes 3-5. The initial catalyst activation process was a one-electron reduction of NbCl5 with Si-Me-CHD to provide [NbCl3(μ-Cl) (L)]2 (L = PMe2Ph (6), L = PPh3 (7)) in the presence of phosphine ligands. An NMR spectroscopic time course experiment using complex 7 as the catalyst revealed an induction period for the product formation, corresponding to an additional one-electron reduction of 7 by the substrates to give catalytically active η2-alkyne complexes of NbCl3. A combined computational and experimental study clarified the mechanism of this unprecedented [2+2+1]-cyclopentadiene synthesis; a rate-determining 1,2-insertion of cyclopropene into η2-alkyne niobium species to form cyclopropane-fused metallacyclopentene followed by ring-opening β-C elimination provides a dienylalkylidene intermediate prior to incorporation of the second alkyne through carbene/alkyne metathesis. We also demonstrated the synthetic utility of the multisubstituted cyclopentadienes as the cyclopentadienyl ligands by derivatizing to the corresponding lithium cyclopentadienide, which is applicable for the synthesis of ferrocene 10.

Rh(III)-catalyzed (5 + 2)-cycloaddition reactions of ortho-hydroxyethyl phenols with internal alkynes: efficient synthesis of benzoxepines

An unprecedented (5 + 2)-cycloaddition reaction of ortho-hydroxyethyl phenol and internal alkyne was developed. This Rh(III)-catalyzed reaction provided benzoxepine derivatives which have very high biological significance. A wide range of ortho-hydroxyethyl phenols and internal alkynes were studied to provide the benzoxepines in high yields.

Construction of polysubstituted pentafulvenes via palladium-catalyzed deacetylation of enones

Herein, we report an efficient synthetic method for polysubstituted pentafulvenes via palladium-catalyzed deacetylative [2+2+1] annulation of enones with alkynes. Aryl-, alkenyl-, and alkyl-substituted α,β-enones were suitable substrates, affording the pentafulvene products in moderate to good yields. This protocol shows excellent compatibility with sensitive halides, free hydroxyl groups, and heterocycles. One-pot gram-scale synthesis and further applications in the late-stage modification of natural products demonstrate the synthetic utility of this method.

Diverse reactivity of alkynes in C-H activation reactions

Alkynes occupy a prominent role as a coupling partner in the transition metal-catalysed directed C-H activation reactions. Due to low steric requirements and linear geometry, alkynes can effectively coordinate with metal d-orbitals. This makes alkynes one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the pattern of reactivity of alkynes also changes. Due to the varied reactivity of alkynes, such as alkenylation, annulation, alkylation, and alkynylation, they have been extensively used for the synthesis of valuable organic molecules. Despite enormous explorations with alkynes, there are still a lot more possible ways by which they can be made to react with M-C bonds generated through C-H activation. Practically there is no limit for the creative use of this approach. In particular with the development of new high and low valent first-row metal catalysts, there is plenty of scope for this chemistry to evolve as one of the most explored areas of research in the coming years. Therefore, a highlight article about alkynes is both timely and useful for synthetic chemists working in this area. Herein, we have highlighted the diverse reactivity of alkynes with various transition metals (Ir, Rh, Ru, Pd, Mn, Fe, Co, Ni, Cu) and their applications, along with some of our thoughts on future prospects.

Rhodium-Catalyzed Coordination-Assisted Regioselective and Migratory Three-Point Double Annulation of o-Alkenyl Phenols with Tertiary Propargyl Alcohols

We disclose herein a Rh(III)-catalyzed migratory three-point double annulation of o-alkenyl phenols with propargyl alcohols for de novo construction of naphtho furan derivatives in a regio- and chemoselective manner. The protocol orchestrates two new rings with four new bonds in one operation without the need for any additive. Necessary labeled and control experiments are conducted to elucidate the reaction mechanism. A tertiary hydroxyl group is found to be crucial both for controlling the regioselective insertion of alkyne through chelation with rhodium to form a key spiro cyclic intermediate and for forcing ring expansion via unusual and selective olefin reshuffling, apart from forming an extra (furan) ring. The protocol is scalable and shows tolerance for late stage functionalization of natural products.

Unexpected formation of poly-functionalized fulvenes by the reaction of a tetraaryl[5]cumulene with iodine

Longer cumulenes have come to draw considerable attention due to their unique properties and reactivities, leading to various hydrocarbons. In this manuscript, we describe the reaction of tetrakis(p-methoxyphenyl)[5]cumulene with iodine to afford poly-functionalized fulvenes via unexpected migration of a terminal aryl ring under ambient conditions. The obtained iodinated fulvenes were utilized in Suzuki-Miyaura cross-coupling reactions affording penta- and fully-arylated fulvenes successfully.

Ru(ii)-Catalyzed cascade decarbonylative annulation and dehydrogenative alkenylation reactions: synthesis of phthalides

A novel Ru(ii)-catalyzed decarbonylative annulation and direct C-H functionalization reaction of phthalic anhydrides with acrylates is reported. This reaction provides an efficient route for the synthesis of 3,7-disubstituted phthalides.

Ruthenium(ii)-catalyzed decarbonylative and decarboxylative coupling of isatoic anhydrides with salicylaldehydes: access to aryl 2-aminobenzoates

A ruthenium(ii)-catalyzed coupling reaction of isatoic anhydrides and salicylaldehydes has been developed for the synthesis of 2-aminobenzoates. This reaction proceeds through metal-catalyzed decarbonylation and decarboxylation to afford good yields of aryl 2-aminobenzoates.

Palladium(II)-Catalyzed Oxidative Decarboxylative [2 + 2 + 1] Annulation of Cinnamic Acids with Alkynes: Access to Polysubstituted Pentafulvenes

An unprecedented palladium(II)-catalyzed oxidative decarboxylative [2 + 2 + 1] annulation of cinnamic acids with alkynes has been developed for the synthesis of polysubstituted pentafulvenes. Ag₂CO₃ and DMSO are essential for the reaction. This protocol features readily available starting materials, a wide substrate scope, and moderate to excellent yields. Moreover, various significant frameworks can be easily obtained from the late-stage transformations of pentafulvenes via oxidation, reduction, and Scholl-type reaction.