Four-coordinate titanium alkylidene complexes: Synthesis, reactivity, and kinetic studies involving the terminal neopentylidene functionality

A series of titanium complexes containing a terminal neopentylidene functionality have been prepared by a one electron oxidatively induced a-hydrogen abstraction from the corresponding bis-neopentyl precursor (Nacnac)Ti(CH ₂ ^tBu) ₂ (Nacnac ^- = [Ar]NC(CH ₃)CHC-(CH ₃)N[Ar], Ar = 2,6-(CHMe ₂) ₂C ₆H ₃), among them (Nacnac)Ti=CH ^tBu(OTf) and (Nacnac)-Ti=CH ^tBu(I). It was determined that bulky alkyl groups bound to titanium as well as a bulky coordinating anion from the oxidant are needed to promote a-hydrogen abstraction. Complex (Nacnac)Ti=CHtBu(OTf) serves as a template for other four-coordinate titanium neopentylidene complexes such as (Nacnac)Ti=CH ^tBu(X) (X ^- = Cl, Br, and BH ₄). Complexes (Nacnac)Ti=CH ^tBu(X) undergo cross-metathesis reactivity with the imine functionality of the Nacnac- ligand forming the imido complexes (H ^tBuC=C(Me)CHC(Me)N[Ar])Ti=NAr-(X) (X ^- = OTf, Cl, Br, I, BH ₄). In addition, C-H activation of two tertiary carbons also takes place to afford the titanacycles Ti[2,6-(CMe ₂)(CHMe ₂)C ₆H ₃]NC(Me)CHC(Me)N[2,6-(CMe ₂)-(CHMe ₂)C ₆H ₃](X) (X ^- = OTf, Cl, Br and η ²-BH ₄). Kinetic studies in C ₆D ₆ reveal the formation of (H ^tBuC=C(Me)CHC(Me) N[Ar])Ti=NAr(I) from (Nacnac)Ti=CH ^tBuCI) to be independent of solvent (C ₆D ₆, Et ₂O-d ₁₀, THF-ds) and the reaction to be first order in titanium (k = 8.06 × 10 ^-4 s ^-1 at 57°C, with activation parameters ΔH‡ = 21.3(2) kcal/mol, ΔS‡ = -8(3) cal/mol K). Compound (Nacnac)Ti=CH ^tBu(OTf) reacts with various substrates to afford products in which the alkylidene functionality has been significantly transformed. When the alkylidene derivatives (Nacnac _tBu)Ti=CH ^tBu(X) (X ^- = OTf, I; Nacnac _tBu ^- = [Ar]NC( ^tBu)CHC( ^tBu)N[Ar]) were prepared, the intramolecular cross-metathesis transformation observed with (Nacnac)Ti=CHtBu(X) was inhibited completely.