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 CH t Bu(OTf) (L 1 - = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe 2 ) 2 C 6 H 3 ) with LiCH 2 SiMe 3 resulted in formation of the alkylidene−alkyl species (L 1 )Ti CH t Bu(CH 2 SiMe 3 ) ( 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 (L 1 )Ti CH t Bu(OTf) with KCH 2 Ph 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 (L 2 )Ti CH t Bu(THF) ( 2; L 2 2 - = [Ar]NC(Me)CH(CH 2 )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 CH t Bu(OTf) (L 3 - = [Ar]NC( t Bu)CHC( t Bu)N[Ar], Ar = 2,6-(CHMe 2 ) 2 C 6 H 3 ) with LiCH 2 SiMe 3 or KCH 2 Ph resulted in clean formation of (L 3 )Ti CH t Bu(R) (R = CH 2 SiMe 3 ( 3 ), CH 2 Ph ( 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-(CMe 2 )(CHMe 2 )C 6 H 3 ]NC(Me)CHC(Me)N[2,6-(CMe 2 )(CHMe 2 )C 6 H 3 ](CH 2 Si(Me) 3 ) ( 5 ) and dimer [Ti NAr([Ar]NC(Me)CHC(μ-CH 2 ) CH t Bu)] 2 ( 6 ) were formed. Complex 5 was prepared in better yield through an independent synthesis involving Ti[2,6-(CMe 2 )(CHMe 2 )C 6 H 3 ]NC(Me)CHC(Me)N[2,6-(CMe 2 )(CHMe 2 )C 6 H 3 ](OTf) and LiCH 2 SiMe 3 . In THF complex 6 dissociated into the corresponding monomer ( t BuHC C(CH 2 )CHC(CH 3 )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 CH t Bu(OTf) and (L 3 )Ti CH t Bu(OTf) afforded the Ti(III) metallacycles ([Ar]NC(R)CHC(R)N[2,6-(CHMe 2 )(CH(CH 2 )(Me))C 6 H 3 ])TiCH 2 t Bu (L 4 2 -, R = Me ( 9 ); L 5 2 -, R = t Bu ( 10 )), both resulting from 1,2-addition of the proximal isopropyl CH 3 group across the Ti CH t Bu bond. One-electron oxidation of 10 with AgOTf promotes α-abstraction to generate back the alkylidene precursor (L 3 )Ti CH t Bu(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.