Dehydrofluorination of Hydrofluorocarbons by Titanium Alkylidynes via Sequential C−H/C−F Bond Activation Reactions. A Synthetic, Structural, and Mechanistic Study of 1,2-CH Bond Addition and β-Fluoride Elimination

Abstract

The neopentylidene-neopentyl complex (PNP)Ti═CHtBu(CH2tBu) (1; (PNP− = N[2-P(CHMe2)2-4-methylphenyl]2) extrudes neopentane in neat fluorobenzene under mild conditions (25 °C) to generate the transient titanium alkylidyne (PNP)Ti≡CtBu (A), which subsequently undergoes regioselective 1,2-CH bond addition of a fluorobenzene across the Ti≡C linkage to generate (PNP)Ti═CHtBu(o-FC6H4) (2). Kinetic and mechanistic studies suggest that the C−H activation process is pseudo-first-order in titanium, with the α-hydrogen abstraction being the rate-determining step and the post-rate-determining step being the C−H bond activation of fluorobenzene. At 100 °C complex 2 does not equilibrate back to A and the preference for C−H activation in benzene versus fluorobenzene is 2:3, respectively. Compound 1 also reacts readily, and in most cases cleanly, with a series of hydrofluoroarenes (HArF), to form a family of alkylidene-arylfluoride derivatives of the type (PNP)Ti═CHtBu(ArF). Thermolysis of the latter compounds generates ...