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 [beta]-Fluoride Elimination

Journal Article · Wed Jan 07 04:00:00 EST 2009 · Organometallics

DOI:https://doi.org/10.1021/om800910q· OSTI ID:1005445

Fout, A R; Scott, J; Miller, D L; Bailey, B C; Pink, M; Mindiola, D J

The neopentylidene-neopentyl complex (PNP)Ti=CH{sup t}Bu(CH{sub 2}{sup t}Bu) (1); (PNP{sup -} = N[2-P(CHMe{sub 2}){sub 2}-4-methylphenyl]{sub 2}) extrudes neopentane in neat fluorobenzene under mild conditions (25 C) to generate the transient titanium alkylidyne (PNP)Ti-C{sup t}Bu (A), which subsequently undergoes regioselective 1,2-CH bond addition of a fluorobenzene across the Ti-C linkage to generate (PNP)Ti=CH{sup t}Bu(o-FC{sub 6}H{sub 4}) (2). Kinetic and mechanistic studies suggest that the C-H activation process is pseudo-first-order in titanium, with the {alpha}-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 (HAr{sub F}), to form a family of alkylidene-arylfluoride derivatives of the type (PNP)Ti=CH{sup t}Bu(Ar{sub F}). Thermolysis of the latter compounds generates the titanium alkylidene-fluoride (PNP)Ti=CH{sup t}Bu(F) (14) by a {beta}-fluoride elimination, concurrent with formation of o-benzyne. {beta}-Fluoride elimination to yield 14 occurs from 2 under elevated temperatures with k{sub average} = 4.96(16) x 10{sup -5} s{sup -1} and with activation parameters {Delta}H{sub {-+}} = 29(1) kcal/mol and {Delta}S{sub {-+}} = -3(4) cal/mol {center_dot}K. It was found that {beta}-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=CHSiMe{sub 3}(CH{sub 2}SiMe{sub 3}) (15) also undergoes {alpha}-hydrogen abstraction to form the putative (PNP)Ti'-CSiMe{sub 3} (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=CHSiMe{sub 3}(F) (26). Only in the case of the substrate 1,2-F{sub 2}C{sub 6}H{sub 4} can the kinetic C-H activation product (PNP)Ti=CHSiMe{sub 3}(o,m-F{sub 2}C{sub 6}H{sub 3}) 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 LiCH{sub 2}XMe{sub 3} (X = C and Si), and the reactivity of 1 with halobenzenes is also presented and discussed.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: USDOE
• OSTI ID: 1005445
• Journal Information: Organometallics, Journal Name: Organometallics Journal Issue: (1) ; 01, 2009 Vol. 28; ISSN 0276-7333; ISSN ORGND7
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

2-2-DIMETHYLPROPANE
36 MATERIALS SCIENCE
ALKENES
BENZENE
BUILDUP
FLUORIDES
KINETICS
MIXTURES
SUBSTRATES
TITANIUM
TRANSIENTS