Selective Cross-Metathesis Versus Ring-Closing Metathesis of Terpenes, Taking the Path Less Travelled.

A diiodo ruthenium olefin metathesis pre-catalyst was employed to achieve remarkably selective cross-metathesis reactions of prenylated 1,6-dienes, effectively overcoming the entropically favored intramolecular ring-closing metathesis. This reaction was investigated using Density Functional Theory (DFT) computations and fine-tuned through the application of a Design of Experiments (DoE) approach. The potential of this innovative process was demonstrated through the unprecedented functionalization of various terpene natural products via cross-metathesis, resulting in the synthesis of new derivatives in a single step.

Reactivity and Selectivity in Ruthenium Sulfur-Chelated Diiodo Catalysts.

A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF3 -I, was synthesized and characterized. This latent precatalyst provides a distinct activity and selectivity profiles for olefin metathesis reactions depending on the substrate. For example, 1,3-divinyl-hexahydropentalene derivatives were efficiently obtained by ring-opening metathesis (ROM) of dicyclopentadiene (DCPD). Ru-S-CF3 -I also presented a much more effective photoisomerization process from the inactive cis-diiodo to the active trans-diiodo configuration after exposure to 510 nm (green light), allowing for a wide scope of photoinduced olefin metathesis reactions. DFT calculations suggest a faster formation and enhanced stability of the active trans-diiodo species of Ru-S-CF3 -I compared with Ru-S-Ph-I, explaining its higher reactivity. In addition, the photochemical release of chloride anions by irradiation of Cl-BODIPY in the presence of DCPD derivatives with diiodo Ru benzylidenes, led to in situ generation of chloride complexes, which quickly produced the corresponding cross-linked polymers. Thus, novel selective pathways that use visible light to guide olefin metathesis based synthetic sequences is presented.

Optimization of hydrogen-evolving photochemical molecular devices.

A molecular photocatalyst consisting of a Ru(II) photocenter, a tetrapyridophenazine bridging ligand, and a PtX2 (X=Cl or I) moiety as the catalytic center functions as a stable system for light-driven hydrogen production. The catalytic activity of this photochemical molecular device (PMD) is significantly enhanced by exchanging the terminal chlorides at the Pt center for iodide ligands. Ultrafast transient absorption spectroscopy shows that the intramolecular photophysics are not affected by this change. Additionally, the general catalytic behavior, that is, instant hydrogen formation, a constant turnover frequency, and stability are maintained. Unlike as observed for the Pd analogue, the presence of excess halide does not affect the hydrogen generation capacity of the PMD. The highly improved catalytic efficiency is explained by an increased electron density at the Pt catalytic center, this is confirmed by DFT studies.