Intermolecular C−H Bond Activation Reactions Promoted by Transient Titanium Alkylidynes. Synthesis, Reactivity, Kinetic, and Theoretical Studies of the Ti⋮C Linkage
The neopentylidene−neopentyl complex (PNP)TiCHtBu(CH2tBu) (2; PNP- = N[2-P(CHMe2)2-4-methylphenyl]2), prepared from the precursor (PNP)TiCHtBu(OTf) (1) and LiCH2tBu, extrudes neopentane in neat benzene under mild conditions (25 °C) to generate the transient titanium alkylidyne, (PNP)Ti⋮CtBu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti⋮C linkage to generate (PNP)TiCHtBu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C−H activation process to obey pseudo-first-order in titanium, the α-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d3 conversion to 3/3-d3 = 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 C6H6 vs C6D6). A KIE of 1.33(3) at 25 °C arose when the intramolecular C−H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C−H bonds, however, the formation of the σ-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti⋮CtBu(C6H5), and C−H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 °C over 48 h generates 3-d6, thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) × 10-5 s-1, 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-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)TiCHtBu(CH2SiMe3) (4) and (PNP)TiCHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3(CH2tBu) (5) and (PNP)TiCHPh(CH2tBu) (7). By means of similar alkylations of (PNP)TiCHSiMe3(OTf) (8), the degenerate complex (PNP)TiCHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene−alkyl complex (PNP)TiCHPh(CH2SiMe3) (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(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C−H bond with intermediate A, in some cases regioselectively, to form the neopentylidene−aryl derivatives (PNP)TiCHtBu(aryl). Intermediate A can also perform stepwise alkylidene−alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene−alkyls (PNP)TiCHR(R‘) (R = 3,5-Me2C6H2, R‘ = CH2-3,5-Me2C6H2; R = SiMe3, R‘ = CH2SiMe3; R = SiMe2C⋮CSiMe3, R‘ = CH2SiMe2C⋮CSiMe3; R = SiMe2OSiMe3, R‘ = CH2SiMe2OSiMe3).
