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)TiCHtBu(CH2tBu) (2; PNP- = N[2-P(CHMe2)2-4-methylphenyl]2), prepared from the precursor (PNP)TiCHtBu(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)TiCHtBu(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)TiCHtBu(CH2SiMe3) (4) and (PNP)TiCHtBu(CH2Ph) (6)
are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3(CH2tBu) (5) and
(PNP)TiCHPh(CH2tBu) (7). By means of similar alkylations of (PNP)TiCHSiMe3(OTf) (8), the degenerate complex (PNP)TiCHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene−alkyl complex
(PNP)TiCHPh(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)TiCHR(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)TiCHtBu(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)TiCHR(R‘) (R = 3,5-Me2C6H2, R‘ = CH2-3,5-Me2C6H2; R = SiMe3, R‘ = CH2SiMe3; R =
SiMe2C⋮CSiMe3, R‘ = CH2SiMe2C⋮CSiMe3; R = SiMe2OSiMe3, R‘ = CH2SiMe2OSiMe3).