Intramolecular C-H activation reactions derived from a terminal titanium neopentylidene functionality.: Redox-controlled 1,2-addition and α-hydrogen abstraction reactions

Alkylation of the terminal neopentylidene titanium(IV) complex (L-1)Ti=(CHBu)-Bu-t(OTf) (L-1(-) = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe2)(2)C6H3) with LiCH2SiMe3 resulted in formation of the alkylidene-alkyl species (L-1)Ti=(CHBu)-Bu-t(CH2SiMe3) (1) in 82% yield. Compound 1 was fully characterized, and the molecular structure disclosed a four-coordinate titanium complex having significant a-hydrogen agostic interaction and possessing terminal alkylidene and alkyl functionalities. Attempts to alkylate (L-1)Ti=(CHBu)-Bu-t(OTf) with KCH2Ph in THF resulted in clean deprotonation of the methyl group attached to the beta-carbon of the diketiminate ligand to form the four-coordinate titanium(IV) neopentylidene-tetrahydrofuran complex (L-2)Ti=(CHBu)-Bu-t(THF) (2; L-2(2-) = [Ar]NC(Me)CH(CH2)N[Ar], 64% isolated yield). Complex 2 was fully characterized and revealed a low-coordinate titanium(IV) in a C-1 environment, which is supported by a chelating bis-anilide ligand. Alkylation of the alkylidene derivative (L-3)Ti=CHtBu(OTf) (L-3(-) = [Ar]NC(Bu-t)CHC(Bu-t)N[Ar1, Ar = 2,6-(CHMe2)(2)C6H3) with LiCH2SiMe3 or KCH2Ph resulted in clean formation of (L3)Ti=CHtBu(R) (R = CH2SiMe3 (3), CH2Ph (4)). Complexes 3 and 4 were fully characterized, and the structure of 4 was determined by single-crystal X-ray diffraction studies. Complex 1 was found to decompose rapidly to several products, of which the titanacycle Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](CH2Si(Me)(3)) (5) and dimer [Ti= NAr([Ar]NC(Me)CHC(mu-CH2)=(CHBu)-Bu-t)](2) (6) were formed. Complex 5 was prepared in better yield through an independent synthesis involving Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](OTf) and LiCH2SiMe3. In THF complex 6 dissociated into the corresponding monomer ((BuHC)-Bu-t=C(CH2)CHC(CH3)N[Ar])Ti=NAr(THF) (8), quantitatively. Unlike complex 1, complexes 3 and 4 are kinetically more stable to intramolecular Wittig-like and C-H abstraction reactions. It was also found that one-electron reduction of the four-coordinate titanium alkylidene complexes (L-1)Ti=(CHBu)-Bu-t(OTf) and (L-3)Ti=(CHBu)-Bu-t(OTf) afforded the Ti(III) metallacycles ([Ar]NC(R)CHC(R)N[2,6-(CHMe2)(CH(CH2)(Me))C6H3])(TiCH2Bu)-Bu-t (L-4(2-), R = Me (9); L-5(2-), R = tBu (10)), both resulting from 1,2-addition of the proximal isopropyl CH3 group across the Ti=CHtBu bond. One-electron oxidation of 10 with AgOTf promotes a-abstraction to generate back the alkylidene precursor (L3)Ti=CHtBu(OTf). The redox-controlled 1,2-addition and alpha-abstraction reactions are specific only to the isopropyl methyl attached to the aryl group of the beta-diketiminate ligand.