Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes

Metal-catalyzed highly Markovnikov-type selective hydrofunctionalization of terminal alkynes provides a straightforward and atom-economical route to access 1,1-disubstituted alkenes, which have a wide range of applications in organic synthesis. However, the highly Markovnikov-type selective transformations are challenging due to the electronic and steric effects during the addition process. With the development of metal-catalyzed organic synthesis, different metal catalysts have been developed to solve this challenge, especially for platinum group metal catalysts. In this perspective, we review homogeneous metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes according to the classified element types as well as reaction mechanisms. Future avenues for investigation are also presented to help expand this exciting field.

Mix and (Mis)match: further studies of the electronic structure and mixed-valence characteristics of 1,4-diethynylbenzene-bridged bimetallic complexes

The 1,4-diethynylbenzene motif is commonly employed as a bridging ligand in bimetallic molecular systems intended to show pronounced intramolecular electronic interactions, delocalized electronic structures and 'wire-like' properties between the metal fragments at the ligand termini. In contrast to these expectations, the donor-acceptor compounds [{Cp'(CO)xM'}(μ-C[triple bond, length as m-dash]CC6H4C[triple bond, length as m-dash]C){M(PP)Cp'}]n+ [n = 0, 1; M'(CO)xCp' = Fe(CO)2Cp, W(CO)3Cp*; M(PP)Cp' = Fe(dppe)Cp, Fe(dppe)Cp*, Ru(PPh3)2Cp, Ru(dppe)Cp, Ru(dppe)Cp*] display remarkably little bridge-mediated electronic interaction between the electron-rich {M(PP)Cp'} and electron-poor {M'(CO)xCp'} fragments in the ground state. However, a relatively high-energy (26 000-30 000 cm-1) M-to-M' charge transfer can be identified. One-electron oxidation is largely localized on the {M(C[triple bond, length as m-dash]CR)(PP)Cp'} fragment and gives rise to a new charge transfer band with bridging-ligand-to-{M(PP)Cp'}+ (M'(CO)xCp' = Fe(CO)2Cp) or M'-to-M(+) (M(CO)xCp' = W(CO)3Cp*) character. The localized electronic ground state of these complexes is better revealed through analysis of the IR spectra, taking advantage of the well-resolved ν(C[triple bond, length as m-dash]C) and ν(CO) bands and IR spectroelectrochemical methods, than through the more classical analysis based on the concepts of Marcus-Hush theory and analysis of the putative IVCT electronic transition. The conclusions are supported by DFT calculations using the BLYP35 functional.

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.