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 (L₁)TiCH^tBu(OTf) (L₁^- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe₂)₂C₆H₃) with LiCH₂SiMe₃ resulted in formation of the alkylidene−alkyl species (L₁)TiCH^tBu(CH₂SiMe₃) (1) in 82% yield. Compound 1 was fully characterized, and the molecular structure disclosed a four-coordinate titanium complex having significant α-hydrogen agostic interaction and possessing terminal alkylidene and alkyl functionalities. Attempts to alkylate (L₁)TiCH^tBu(OTf) with KCH₂Ph in THF resulted in clean deprotonation of the methyl group attached to the β-carbon of the diketiminate ligand to form the four-coordinate titanium(IV) neopentylidene−tetrahydrofuran complex (L₂)TiCH^tBu(THF) (2; L₂^2- = [Ar]NC(Me)CH(CH₂)N[Ar], 64% isolated yield). Complex 2 was fully characterized and revealed a low-coordinate titanium(IV) in a C₁ environment, which is supported by a chelating bis-anilide ligand. Alkylation of the alkylidene derivative (L₃)TiCH^tBu(OTf) (L₃^- = [Ar]NC(^tBu)CHC(^tBu)N[Ar], Ar = 2,6-(CHMe₂)₂C₆H₃) with LiCH₂SiMe₃ or KCH₂Ph resulted in clean formation of (L₃)TiCH^tBu(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(μ-CH₂)CH^tBu)]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 (^tBuHC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 (L₁)TiCH^tBu(OTf) and (L₃)TiCH^tBu(OTf) afforded the Ti(III) metallacycles ([Ar]NC(R)CHC(R)N[2,6-(CHMe₂)(CH(CH₂)(Me))C₆H₃])TiCH₂^tBu (L₄^2-, R = Me (9); L₅^2-, R = ^tBu (10)), both resulting from 1,2-addition of the proximal isopropyl CH₃ group across the TiCH^tBu bond. One-electron oxidation of 10 with AgOTf promotes α-abstraction to generate back the alkylidene precursor (L₃)TiCH^tBu(OTf). The redox-controlled 1,2-addition and α-abstraction reactions are specific only to the isopropyl methyl attached to the aryl group of the β-diketiminate ligand.