871558-39-3Relevant articles and documents
Intermolecular C-H bond activation promoted by a titanium alkylidyne
Bailey, Brad C.,Fan, Hongjun,Baum, Erich W.,Huffman, John C.,Baik, Mu-Hyun,Mindiola, Daniel J.
, p. 16016 - 16017 (2005)
The transient titanium alkylidyne complex (PNP)Ti≡CtBu (PNP = N-[2-P(CHMe2)2-4-methylphenyl]2-), prepared from α-hydrogen abstraction of the corresponding alkylidene-alkyl species (PNP)Ti=CHtBu(CH2tBu), can readily undergo intermolecular 1,2-addition of C-H bonds of benzene and SiMe4. Synthesis and reactivity, isotopic labeling, kinetics, and theoretical studies strongly favor an alkylidyne pathway and the α-H abstraction step to be the rate-determining step. Copyright
Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti=C linkage
Bailey, Brad C.,Fan, Hongjun,Huffman, John C.,Baik, Mu-Hyun,Mindiola, Daniel J.
, p. 8781 - 8793 (2007)
The neopentylidene-neopentyl complex (PNP)Ti=CHtBu(CH 2tBu) (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 parametete 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 C 6H6 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(C 6H5), 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) x 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-C6H3D 3 was found to be 1.36(7). When 1 is alkylated with LiCH 2SiMe3 and KCH2Ph, the complexes (PNP)Ti=CHtBu(CH2SiMe3) (4) and (PNP)Ti=CH tBu(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 de-generate complex (PNP)Ti=CHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)Ti=CHPh(CH2SiMe 3) (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-F 2C6H4, and 1,4-F2C6H 4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)Ti=CH tBu(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me3C6H3, SiMe 4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)Ti=CHR(R′) (R = 3,5-Me 2C6H2, R′ = CH2-3,5-Me 2C6H2; R = SiMe3, R′ = CH 2SiMe3; R = SiMe2C=CSiMe3, R′ = CH2SiMe2C=CSiMe3; R = SiMe 2OSiMe3, R′ = CH2SiMe 2OSiMe3).
