Ruthenium‐Catalyzed Transfer‐Hydrogenative Cyclization of 1,6‐Diynes with Hantzsch 1,4‐Dihydropyridine as a H2 Surrogate

The transfer‐hydrogenative cyclization of 1,6‐diynes with Hantzsch 1,4‐dihydropyridine as a H₂ surrogate was performed in the presence of a cationic ruthenium catalyst of the type [Cp′Ru(MeCN)₃PF₆]. Exocyclic 1,3‐dienes or their 1,4‐hydrogenation products, cycloalkenes, were selectively obtained, depending on the substrate structure and the reaction conditions.     

Stereospecific and Stereoselective Rhodium(I)‐Catalyzed Intramolecular [2+2+2] Cycloaddition of Allene‐Ene‐Ynes: Construction of Bicyclo[4.1.0]heptenes

Treatment of the allene‐ene‐yne substrates with [{RhCl(CO)₂}₂] effected the intramolecular [2+2+2]‐type ring‐closing reaction to produce various of tri‐ and tetracyclic derivatives containing a cyclopropane ring. The reaction is highly stereoselective as well as stereospecific with good to excellent yields.     

Hydroaminomethylation Beyond Carbonylation: Allene–Imine Reductive Coupling by Ruthenium‐Catalyzed Transfer Hydrogenation

Ruthenium(II)‐catalyzed hydrogen transfer from 2‐propanol mediates reductive coupling of 1,1‐disubstituted allenes with formaldimines with complete branch‐regioselectivity, thus representing a new method for hydroaminomethylation beyond classical hydroformylation/reductive amination.     

Ruthenium‐Catalyzed C−C Bond Cleavage of 2H‐Azirines: A Formal [3+2+2] Cycloaddition to Fused Azepine Skeletons

2H‐azirines can serve as three‐atom synthons by C?C bond cleavage, however, it involves a high energy barrier under thermal conditions (>50.0 kcal mol^?1). Reported is a ruthenium‐catalyzed [3+2+2] cycloaddition reaction of 2H‐azirines with diynes, thus leading to the formation of fused azepine skeletons. This approach features an unprecedented metal‐catalyzed C?C bond cleavage of 2H‐azirines at room temperature, and the challenging construction of aza‐seven‐membered rings from diynes. The results of this study provide a new reaction pattern for constructing nitrogen‐containing seven‐membered rings and may find applications in the synthesis of other complex heterocycles.     

Selective Ruthenium‐Catalyzed Hydrochlorination of Alkynes: One‐Step Synthesis of Vinylchlorides

An unprecedented ruthenium‐catalyzed direct and selective alkyne hydrochlorination is reported and leads to vinylchlorides in excellent yields with atom economy. The reaction proceeds at room temperature from terminal alkynes and provides a variety of chloroalkenes. Only the regioisomer resulting from the formal Markovnikov addition is selectively formed. Mechanistic studies show the stereoselective syn addition of HCl to alkynes at room temperature and suggest a chloro hydrido Ru^IV species as a key intermediate of the reaction.     

Ruthenium‐Catalyzed [2+2+2] Cycloaddition of 1,6‐Enynes and Unactivated Alkynes: Access to Ring‐Fused Cyclohexadienes

The [2+2+2] intermolecular carbocyclization reactions between 1,6‐enynes and alkynes catalyzed by [RuCl(cod)(Cp*)] (cod=1,5‐cyclooctadiene, Cp*=pentamethylcyclopentadienyl) are reported to provide bicyclohexa‐1,3‐dienes. The presented reaction conditions are compatible with internal and terminal alkynes and the chemo‐ and regioselectivity issues are controlled by the presence of substituents at the propargyl carbon center of the alkyne(s) partner(s).     

Stereoselective Rhodium‐Catalysed [2+2+2] Cycloaddition of Linear Allene–Ene/Yne–Allene Substrates: Reactivity and Theoretical Mechanistic Studies

Allene–ene–allene ( 2 and 5 ) and allene–yne–allene ( 3 and 7 ) N‐tosyl and O‐linked substrates were satisfactorily synthesised. The [2+2+2] cycloaddition reaction catalysed by the Wilkinson catalyst [RhCl(PPh₃)₃] was evaluated. Substrates 2 and 5 , which bear a double bond in the central position, gave a tricyclic structure in a reaction in which four contiguous stereogenic centres were formed as a single diastereomer. The reaction of substrates 3 and 7 , which bear a triple bond in the central position, gave a tricyclic structure with a cyclohexenic ring core, again in a diastereoselective manner. All cycloadducts were formed by a regioselective reaction of the inner allene double bond and, therefore, feature an exocyclic diene motif. A Diels–Alder reaction on N‐tosyl linked cycloadducts 8 and 10 allowed pentacyclic scaffolds to be diastereoselectively constructed. The reactivity of the allenes on [2+2+2] cycloaddition reactions was studied for the first time by density functional theory calculations. This mechanistic study rationalizes the order in which the unsaturations take part in the catalytic cycle, the reactivity of the two double bonds of the allene towards the [2+2+2] cycloaddition reaction, and the diastereoselectivity of the reaction.     

Stereoselective Nickel‐Catalyzed [2+2] Cycloadditions of Ene‐Allenes

A stereoselective nickel‐catalyzed [2+2] cycloaddition of ene‐allenes is reported. This transformation encompasses a broad range of ene‐allene substrates, thus providing efficient access to fused cyclobutanes from easily accessed π‐components. A simple and inexpensive first‐row catalytic system comprised of [Ni(cod)₂] and dppf was used in this process, thus constituting an attractive approach to synthetically challenging cyclobutane frameworks under mild reaction conditions.     

Preparation,structure, and unique thermal [2+2], [4+2], and [3+2] cycloaddition reactions of 4-vinylideneoxazolidin-2-one

The terminal allene C(alpha)=C(beta) bonds of 4vinylidene-2-oxazolidinone (2) readily undergo [2+2] cycloaddition with a wide variety of terminal alkynes, alkenes, and 1,3-dienes irrespective of their electronic nature under strictly thermal activation conditions (70-100 degrees C) and provide 3substituted (Z)-methylenecyclobutenes 6, 3substituted methylenecyclobutanes 7 and 8, and 3vinylmethylenecyclobutanes 9, respectively, in good to excellent yields. Alkenes react with 2 with complete retention of configuration. The [2+2] cycloaddition is concluded to proceed via a concerted [(pi(2s)+pi(2s))(allene) + pi(2s)] Hückel transition state on the basis of experimental evidences and quantum mechanical methods. Some highly polarized enones and nitrile oxide, on the other hand, react with 2 selectively at the internal C(4)=C(alpha) double bonds and give spiro compounds 10 and 11, respectively. The bent allene bonds (173-176 degrees) and the unique reactivity associated with 2 are attributed to a low-lying LUMO (C(alpha)=C(beta)) that is substantiated by a through-space sigma*(N-SO(2))-pi*(C(alpha)=C(beta)) orbital interaction.     

Chemo‐ and Regioselective Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo‐ and regioselective partially intramolecular rhodium(I)‐catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.     

Ruthenium‐Catalyzed Oxidation of Alkenes,Alkynes, and Alcohols to Organic Acids with Aqueous Hydrogen Peroxide

A protocol that adopts aqueous hydrogen peroxide as a terminal oxidant and [(Me₃tacn)(CF₃CO₂)₂Ru^III(OH₂)]CF₃CO₂ ( 1 ; Me₃tacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane) as a catalyst for oxidation of alkenes, alkynes, and alcohols to organic acids in over 80 % yield is presented. For the oxidation of cyclohexene to adipic acid, the loading of 1 can be lowered to 0.1 mol %. On the one‐mole scale, the oxidation of cyclohexene, cyclooctene, and 1‐octanol with 1 mol % of 1 produced adipic acid (124 g, 85 % yield), suberic acid (158 g, 91 % yield), and 1‐octanoic acid (129 g, 90 % yield), respectively. The oxidative C?C bond‐cleavage reaction proceeded through the formation of cis‐ and trans‐diol intermediates, which were further oxidized to carboxylic acids via C? C bond cleavage.     

Efficient Access to Bicyclo[4.3.0]nonanes: Copper‐Catalyzed Asymmetric Silylative Cyclization of Cyclohexadienone‐Tethered Allenes

The creation of three consecutive chiral carbon centers in one step is achieved using Cu‐catalyzed asymmetric silylative cyclization of cyclohexadienone‐tethered allenes. Through regioselective β‐silylation of the allene and subsequent enantioselective 1,4‐addition to cyclohexadienone, this tandem reaction could afford cis‐hydrobenzofuran, cis‐hydroindole, and cis‐hydroindene frameworks with excellent yields (80–98 %) and enantioselectivities (94–98 % ee) bearing vinylsilane and enone substructures. Meanwhile, this mild transformation is generally compatible with a wide range of functional groups, which allows further conversion of the bicyclic products to bridged and tricyclic ring structures.     

Ruthenium‐Catalyzed Allylation–Cyclization Reactions of Cyclic 1,3‐Dicarbonyl Compounds with 1‐Vinyl Propargyl Alcohols

Ruthenium‐catalyzed allylation–cyclization reactions of cyclic 1,3‐dicarbonyl compounds with 1‐vinyl propargyl alcohols that lead to diverse carbo‐ or heterocyclic products in a one‐pot cascade process are reported. These mechanistically distinct reactions are catalyzed by a single ruthenium(0) complex that contains a redox‐coupled dienone ligand. The reaction pathway strongly depends on the substrate substitution pattern, which determines the mode of activation of the 1‐vinyl propargyl alcohol. The environmentally benign process, which generates water as the only waste product, is of wide scope and allows the atom‐economic synthesis of highly functionalized furans, pyrans, and spirocarbocycles.