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 the titanium alkylidene-fluoride (PNP)TiCHtBu(F) (14) by a β-fluoride elimination, concurrent with formation of o-benzyne. β-Fluoride elimination to yield 14 occurs from 2 under elevated temperatures with kaverage = 4.96(16) × 10−5 s−1 and with activation parameters ΔH⧧ = 29(1) kcal/mol and ΔS⧧ = −3(4) cal/mol·K. It was found that β-fluoride elimination is accelerated when electron-rich groups are adjacent to the fluoride group, thus implying that a positive charge buildup at the arylfluoride ring occurs in the activated complex of 2. The alkylidene derivative (PNP)TiCHSiMe3(CH2SiMe3) (15) also undergoes α-hydrogen abstraction to form the putative (PNP)Ti’CSiMe3 (B) at higher temperatures (>70 °C) and dehydrofluorinates the same series of HArF when the reaction mixture is thermolyzed at >100 °C over 72 h to produce o-benzyne products and the fluoride analogue (PNP)TiCHSiMe3(F) (26). Only in the case of the substrate 1,2-F2C6H4 can the kinetic C−H activation product (PNP)TiCHSiMe3(o,m-F2C6H3) be isolated and crystallographically characterized. 1-Fluorohexane and fluorocyclohexane can also be dehydrofluorinated by intermediates A and B. No intermediates are observed, but in the case of 1-fluorohexane, the terminal olefin is spectroscopically identified. The dehydrofluorination of HArF and hydrofluoroalkanes (HAlF) can be made cyclic via the quantitative conversion of the alkylidene-fluorides to 1 and 15, by means of transmetalation with LiCH2XMe3 (X = C and Si), and the reactivity of 1 with halobenzenes is also presented and discussed