C-F and C-H bond activation of fluorobenzenes and fluoropyridines at transition metal centers: how fluorine tips the scales

Double role of the hydroxy group of phosphoryl in palladium(II)-catalyzed ortho-olefination: a combined experimental and theoretical investigation

Density functional theory calculations have been carried out on Pd-catalyzed phosphoryl-directed ortho-olefination to probe the origin of the significant reactivity difference between methyl hydrogen benzylphosphonates and dimethyl benzylphosphonates. The overall catalytic cycle is found to include four basic steps: C-H bond activation, transmetalation, reductive elimination, and recycling of catalyst, each of which is constituted from different steps. Our calculations reveal that the hydroxy group of phosphoryl plays a crucial role almost in all steps, which can not only stabilize the intermediates and transition states by intramolecular hydrogen bonds but also act as a proton donor so that the η(1)-CH3COO(-) ligand could be protonated to form a neutral acetic acid for easy removal. These findings explain why only the methyl hydrogen benzylphosphonates and methyl hydrogen phenylphosphates were found to be suitable reaction partners. Our mechanistic findings are further supported by theoretical prediction of Pd-catalyzed ortho-olefination using methyl hydrogen phenylphosphonate, which is verified by experimental observations that the desired product was formed in a moderate yield.

C(sp3)-F bond activation of CF3-substituted anilines with catalytically generated silicon cations: spectroscopic evidence for a hydride-bridged Ru-S dimer in the catalytic cycle

Heterolytic splitting of the Si-H bond mediated by a Ru-S bond forms a sulfur-stabilized silicon cation that is sufficiently electrophilic to abstract fluoride from CF(3) groups attached to selected anilines. The ability of the Ru-H complex, generated in the cooperative activation step, to intramolecularly transfer its hydride to the intermediate carbenium ion (stabilized in the form of a cationic thioether complex) is markedly dependent on the electronic nature of its phosphine ligand. An electron-deficient phosphine thwarts the reduction step but, based on the Ru-S catalyst, half of an equivalent of an added alkoxide not only facilitates but also accelerates the catalysis. The intriguing effect is rationalized by the formation of a hydride-bridged Ru-S dimer that was detected by (1)H NMR spectroscopy. A refined catalytic cycle is proposed.