Cyclizations of Aryl Enynes Containing Propargyl Alcohol and Diallylamine Groups to Yield Indolecarbaldehydes Induced by Ruthenium Complexes

Conventional and unconventional alkyne activations by Ru and Os for unprecedented dimetalated quinolizinium complexes

Two types of unexpected quinolizinium complexes were obtained from the reactions between pyridine-functionalized propargylic alcohol HC[triple bond, length as m-dash]CC(OH)(Ph)(CH₂(2-py)) (L1) and cis-[M(L^L)₂Cl₂] (M = Ru, Os; L^L = 1,1-bis(diphenylphosphino)methane (dppm), 2,2'-bipyridine (bpy)). Their molecular structures revealed that L1 can be activated by Ru and Os via the conventional "vinylidene-involving" or unconventional "non-vinylidene-involving" pathways.

Ruthenium-Induced Cyclization of Heteroatom-Functionalized Alkynes: Progress, Challenges and Perspectives

Metal-induced cyclization of functionalized alkynes represents one of the most general approaches to prepare organic heterocycles. Although Ru^II centers are well-established to promote alkyne to vinylidene rearrangements and many Ru^II -mediated alkyne cyclizations have been rationalized to be the results of post-vinylidene transformations, recent discoveries indicate that Ru^II centers can serve as electrophiles and induce alkyne cyclizations without vinylidene intermediacy. In this Minireview, an overview of the Ru^II -induced cyclization of heteroatom-functionalized alkynes in the last decade is provided, with an emphasis on the discoveries and validations of the unconventional "non-vinylidene-involving" pathways.

Selective oxymetalation of terminal alkynes via 6-endo cyclization: mechanistic investigation and application to the efficient synthesis of 4-substituted isocoumarins

The cyclization of heteroatom-containing alkynes with π acidic metal salts is an attractive method to prepare heterocycles because the starting materials are readily available and the organometallic compounds are useful synthetic intermediates. A new organometallic species in the heterocyclization provides an opportunity to synthesize heterocycles that are difficult to obtain. Herein, we describe a novel cyclic oxymetalation of 2-alkynylbenzoate with indium or gallium salts that proceeds with an unusual regioselectivity to give isocoumarins bearing a carbon-metal bond at the 4-position. This new type of metalated isocoumarin provided 3-unsubstituted isocoumarins that have seldom been investigated despite their important pharmacological properties. Indium and gallium salts showed high performance in the selective 6-endo cyclization of terminal alkynes while boron or other metals such as Al, Au, and Ag caused 5-exo cyclization or decomposition of terminal alkynes, respectively. The metalated isocoumarin and its reaction intermediate were unambiguously identified by X-ray crystallographic analysis. The theoretical calculation of potential energy profiles showed that oxyindation could proceed via 6-endo cyclization under thermodynamic control while previously reported oxyboration would give a 5-membered ring under kinetic control. The investigation of electrostatic potential maps suggested that the differences in the atomic characters of indium, boron and their ligands would contribute to such a regioselective switch. The metalated isocoumarins were applied to organic synthetic reactions. The halogenation of metalated isocoumarins proceeded to afford 4-halogenated isocoumarins bearing various functional groups. The palladium-catalyzed cross coupling of organometallic species with organic halides gave various 4-substituted isocoumarins. A formal total synthesis of oosponol, which exhibits strong antifungal activity, was accomplished.

Cyclization Reactions of Aryl Propargyl Acetates with Tethered Epoxide Induced by Ruthenium Complex

Reactions of the ruthenium complex [Ru]Cl ([Ru]=Cp(PPh₃ )₂ Ru; Cp=η⁵ -C₅ H₅ ) with several aryl propargyl acetates, each with an ortho-substituted chain of various length containing an epoxide on the aromatic ring and with or without methyl substitutents on the epoxide ring, bring about novel cyclizations. The cyclization reactions of HC≡CCH(OAc)(C₆ H₄ )CH₂ (RC₂ H₂ O) (R=H, 6 a; R=CH₃ , 6 b, where RC₂ H₂ O is an epoxide ring) in MeOH give the vinylidene complexes 5 a-b, respectively, each with the Cβ integrated into a tetrahydro-5H-benzo[7]annulen-6-ol ring. A C-C bond formation takes place between the propargyl acetate and the less substituted carbon of the epoxide ring. Further cyclizations of 5 a-b induced by HBF₄ give the corresponding vinylidene complexes 8 a-b each with a new 8-oxabicyclo-[3.2.1]octane ring by removal of a methanol molecule in high yield. For similar aryl propargyl acetates with a shorter epoxide chain, the cyclization gives a mixture of a vinylidene complex with a tetrahydronaphthalen-1-ol ring and a carbene complex with a tricyclic indeno-furan ring. For the cyclization of 18, with a longer epoxide chain, opening of the epoxide is required to afford the vicinal bromohydrin 22, then tandem cyclization occurs in one pot. Products are characterized by spectroscopic methods as well as by XRD analysis.