Tandem Cyclization of Enynes Containing a Thioether or Ether Linkage via Ruthenium Allenylidene and Vinylidene 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.

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.

Reactions of Ruthenium Complexes Containing Pentatetraenylidene Ligand

Two ruthenium acetylide complexes [Ru]-C≡C-C≡C-C(OR)(C₃ H₅ )₂ (2, R=H and 2 a, R=CH₃ ; [Ru]=Cp(PPh₃ )₂ Ru) each with two cyclopropyl rings were synthesized from TMS-C≡C-C≡C-C(OH)(C₃ H₅ )₂ (1; TMS=trimethylsilyl). Treatments of 2 and 2 a with allyl halide in the presence of KPF₆ afforded the vinylidene complexes 3 and 3 a, respectively. When NH₄ PF₆ was used, instead of KPF₆ , additional ring-opening reaction took place on one of the three-membered ring. Treatment of [Ru]Cl with 1,3-butadiyne (6), bearing an epoxide ring, afforded acetylide complex 7 with a furyl ring. Treatment of 2 a with Ph₃ CPF₆ presumably afforded pentatetraenylidene complex {[Ru]=C=C=C=C=C(C₃ H₅ )₂ }[PF₆ ] (10), which was not isolated. Additions of various alcohols in a solution of 10 generated a number of disubstituted allenylidene complexes {[Ru]=C=C=C(OR)-C=C(C₃ H₅ )₂ }[PF₆ ] (11). Treatment of 11 with K₂ CO₃ afforded the acetylide complex 12 bearing a carbonyl group, characterized by single X-ray diffraction analysis. Addition of a primary amine to 10 caused cleavage of the farthermost C=C bond and several allenylidene complexes {[Ru]=C=C=C(Me)(NHR)}[PF₆ ] (18) were isolated.

Reactions of aromatic S- and O-enynes with two methyl substituents on the olefinic unit assisted by a ruthenium complex: tandem cyclization and product selectivity

Ruthenium-assisted cyclizations of two enynes, HC≡CCH(OH)(C6H4)X-CH2CH=CMe2 (X=S (1a), O (1b)), each of which contains two terminal methyl substituents on the olefinic parts, are explored. The reaction of 1a in CH2Cl2 gives the vinylidene complex 2a from the first cyclization and two side products, 3a and the carbene complex 4a with a benzothiophene ligand. The same reaction in the presence of HBF4 affords 4a exclusively. Air oxidation of 4a in the presence of Et3N readily gives an aldehyde product. In MeOH, tandem cyclizations of 1a generate a mixture of the benzothiochromene compound 10a and the carbene complex 7a also with a benzothiochromene ligand. First, cyclization of 1b likewise proceeds in CH2Cl2 to give 2b. Tandem cyclization of 1b in MeOH yields comparable products 10b and 7b with benzochromene moieties, yet with no other side product. The reaction of [Ru]Cl with HC≡CCH(OH)(C6H4)S-CH2CH=CH2 (1c), which contains no methyl substituent in the olefinic part, in MeOH gives the carbene complex 15c with an unsubstituted thiochromene by means of a C-S bond formation. Structures of 3a and 15c are confirmed by X-ray diffraction analysis. The presence of methyl groups of enynes 1a and 1b promotes sequential cyclization reactions that involve C-C bond formation through carbocationic species.