| Sumario: | The neopentylidene-neopentyl complex (PNP)TiCHBu(CHBu) (2; PNP = N[2-P(CHMe)-4-methylphenyl]), prepared from the precursor (PNP)TiCHBu(OTf) (1) and LiCHBu, extrudes neopentane in neat benzene under mild conditions (25 °C) to generate the transient titanium alkylidyne, (PNP)TiCBu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the TiC linkage to generate (PNP)TiCHBu(CH) (3). Kinetic, mechanistic, and theoretical studies suggest the CH activation process to obey pseudo-first-order in titanium, the α-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d conversion to 3/3-d = 3.9(5) at 40 °C) with activation parameters ΔH‡ = 24(7) kcal/mol and ΔS‡ = -2(3) cal/mol·K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 °C for the intermolecular C-H activation reaction in CH vs CD). A KIE of 1.33(3) at 25 °C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-CHD. For the activation of aromatic C-H bonds, however, the formation of the σ-complex becomes rate-determining via a hypothetical intermediate (PNP)TiCBu(CH), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in CD at 95 °C over 48 h generates 3-d, thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) × 10 s, and with activation parameters ΔH‡ = 31(16) kcal/mol and ΔS‡ = 3(9) cal/mol·K. At 95 °C for one week, the EIE for the 2 → 3 reaction in 1,3,5-CHD was found to be 1.36(7). When 1 is alkylated with LiCHSiMe and KCHPh, the complexes (PNP)TiCHBu(CHSiMe) (4) and (PNP)TiCHBu(CHPh) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe(CHBu) (5) and (PNP)TiCHPh(CHBu) (7). By means of similar alkylations of (PNP)TiCIHSiMe(OTf) (8), the degenerate complex (PNP)TiCHSiMe(CHSiMe) (9) or the non-degenerate alkylidene-alkyl complex (PNP)TiCHPh(CHSiMe) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)TiCHR(CH) (R = SiMe (10), Ph (12)). Substrates such as FCH, 1,2-FCH, and 1,4-FCH react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)TiCHBu(aryl).…
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