Allenylidene/Alkenylcarbyne Synthesis and Reactivity in Ruthenium Complexes with Monodentate Phosphine Ligands

Constraint of a ruthenium-carbon triple bond to a five-membered ring

The incorporation of a metal-carbon triple bond into a ring system is challenging because of the linear nature of triple bonds. To date, the synthesis of these complexes has been limited to those containing third-row transition metal centers, namely, osmium and rhenium. We report the synthesis and full characterization of the first cyclic metal carbyne complex with a second-row transition metal center, ruthenapentalyne. It shows a bond angle of 130.2(3)° around the sp-hybridized carbyne carbon, which represents the recorded smallest angle of second-row transition metal carbyne complexes, as it deviates nearly 50° from the original angle (180°). Density functional theory calculations suggest that the inherent aromatic nature of these metallacycles with bent Ru≡C-C moieties enhances their stability. Reactivity studies showed striking observations, such as ambiphilic reactivity, a metal-carbon triple bond shift, and a [2 + 2] cycloaddition reaction with alkyne and cascade cyclization reactions with ambident nucleophiles.

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.