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Intramolecular C−H Activation Reactions Derived from a Terminal Titanium Neopentylidene Functionality. Redox-Controlled 1,2-Addition and α-Hydrogen Abstraction Reactions

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posted on 20.06.2005, 00:00 authored by Falguni Basuli, Brad C. Bailey, John C. Huffman, Daniel J. Mindiola
Alkylation of the terminal neopentylidene titanium(IV) complex (L1)TiCHtBu(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CHtBu(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CHtBu(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CHtBu(THF) (2; L22- = [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CHtBu(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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(μ-CH2)CHtBu)]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CHtBu(OTf) and (L3)TiCHtBu(OTf) afforded the Ti(III) metallacycles ([Ar]NC(R)CHC(R)N[2,6-(CHMe2)(CH(CH2)(Me))C6H3])TiCH2tBu (L42-, R = Me (9); L52-, 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 α-abstraction to generate back the alkylidene precursor (L3)TiCHtBu(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.