Formation of alkyne-bridged ferrocenophanes using ring-closing alkyne metathesis on 1,1'-diacetylenic ferrocenes.

Novel alkyne-bridged ferrocenophanes [fc{CO2(CH2) n C≡}2] (2a: n = 2; 2b: n = 3) were synthesized from the corresponding terminal diacetylenic ferrocenes [fc{CO2(CH2) n C≡CH}2] (1a: n = 2; 1b: n = 3) through ring-closing alkyne metathesis (RCAM) utilizing the highly effective molybdenum catalyst [MesC≡Mo{OC(CF3)2CH3}3] (MoF6; Mes = 2,4,6-trimethylphenyl). The metathesis reaction occurs in short time with high yields whilst giving full conversion of the terminal alkynes. Furthermore, the solvent-dependant reactivity of 2a towards Ag(SbF6) is investigated, leading to oxidation and formation of the ferrocenium hexafluoroantimonate 4 in dichloromethane, whereas the silver(I) coordination polymer 5 was isolated from THF solution.

Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on Activity Revealed by Kinetics, Solid-State NMR, and Chemical Shift Analysis.

Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH3)3-x(CF3)x}3] (MoF0, x = 0; MoF3, x = 1; MoF6, x = 2; MoF9, x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The 13C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF0 < MoF3 < MoF6 before sharply decreasing for MoF9, with a similar effect for the supported systems (MoF0 ≈ MoF9 < MoF6 < MoF3). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF9 derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF9 for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π*(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF6, prior to a sharp decrease in reactivity for MoF9 due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their 13C NMR chemical shift tensors.

Alkyne Metathesis with Silica-Supported and Molecular Catalysts at Parts-per-Million Loadings.

Improvement of the activity, stability, and chemoselectivity of alkyne-metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBuF6 )3 ] (1, Mes=2,4,6-trimethylphenyl, OtBuF6 =hexafluoro-tert-butoxy) onto partially dehydroxylated silica gave a well-defined silica-supported active alkyne-metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBuF6 )2 ] (1/SiO2-700 ). Both 1 and 1/SiO2-700 showed very high activity, selectivity, and stability in the self-metathesis of a variety of carefully purified alkynes, even at parts-per-million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO2-700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO2-700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.

Identification of a Grain Beetle Macrolide Pheromone and Its Synthesis by Ring-Closing Metathesis Using a Terminal Alkyne.

A major C18-macrolide was found during analysis of the frass of the storage beetle Oryzaephilus surinamensis to be (9Z,12Z,15R)-octadeca-9,12-dien-15-olide (10, cucujolide XI). The synthesis used ring-closing alkyne metathesis as a key step. The highly active 2,4,6-trimethylbenzylidyne molybdenum complex [MesCMo{OC(CF3)2Me}3] (12) allowed the use of a terminal alkyne and afforded the product in excellent yield. Bioassays proved the activity of the R-enantiomer 10 in the aggregation of the beetle. Cucujolide XI is the first macrolide pheromone oxidized at the ω-4 position.