Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti C linkage

Abstract

The neopentylidene-neopentyl complex (PNP)(TiCHBu)-Bu-t((CH2Bu)-Bu-t) (2;PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)), prepared from the precursor (PNP)(TiCHBu)-Bu-t(OTf) (1) and (LiCH2Bu)-Bu-t, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)(TiCBu)-Bu-t (A), which subsequently undergoes 1,2-CH bond addition of benzene across the TiC linkage to generate (PNP)(TiCHBu)-Bu-t(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters Delta H = 24(7) kcal/mol and Delta S = -2(3) cal/mol center dot K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees 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 sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)(TiCBu)-Bu-t(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 degrees C over 48 h generates 3-d(6), 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 Delta H = 31(16) kcal/mol and Delta S = 3(9) cal/mol.K. At 95 degrees 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)(TiCHBu)-Bu-t(CH2SiMe3) (4) and (PNP)(TiCHBu)-Bu-t(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3((CH2Bu)-Bu-t) (5) and (PNP)TiCHPh((CH2Bu)-Bu-t) (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)(TiCHBu)-Bu-t(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me<INF>3</INF>C<INF>6</INF>H<INF>3</INF>, SiMe<INF>4</INF>, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)TiCHR(R') (R = 3,5-Me<INF>2</INF>C<INF>6</INF>H<INF>2</INF>, R' = CH<INF>2</INF>-3,5-Me<INF>2</INF>C<INF>6</INF>H<INF>2</INF>; R = SiMe<INF></INF>, R' = CH<INF>2</INF>SiMe<INF>3</INF>; R = SiMe<INF>2</INF>CCSiMe<INF>3</INF>, R' = CH<INF>2</INF>SiMe<INF>2</INF>CCSiMe<INF>3</INF>; R = SiMe<INF>2</INF>OSiMe<INF>3</INF>, R' = CH<INF>2</INF>SiMe<INF>2</INF>OSiMe<INF>3</INF>).