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 α-hydrogen abstraction from the
corresponding bis-neopentyl precursor (Nacnac)Ti(CH2tBu)2 (Nacnac- = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-(CHMe2)2C6H3), among them (Nacnac)TiCHtBu(OTf) and (Nacnac)TiCHtBu(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 α-hydrogen abstraction.
Complex (Nacnac)TiCHtBu(OTf) serves as a template for other four-coordinate titanium
neopentylidene complexes such as (Nacnac)TiCHtBu(X) (X- = Cl, Br, and BH4). Complexes
(Nacnac)TiCHtBu(X) undergo cross-metathesis reactivity with the imine functionality of
the Nacnac- ligand forming the imido complexes (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(X) (X- = OTf, Cl, Br, I, BH4). In addition, C−H activation of two tertiary carbons also takes
place to afford the titanacycles Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](X) (X- = OTf, Cl, Br and η2-BH4). Kinetic studies in C6D6 reveal the formation
of (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(I) from (Nacnac)TiCHtBu(I) to be independent
of solvent (C6D6, Et2O−d10, THF-d8) 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CHtBu(OTf) reacts with various substrates to afford
products in which the alkylidene functionality has been significantly transformed. When
the alkylidene derivatives (NacnactBu)TiCHtBu(X) (X- = OTf, I; NacnactBu- = [Ar]NC(tBu)CHC(tBu)N[Ar]) were prepared, the intramolecular cross-metathesis transformation
observed with (Nacnac)TiCHtBu(X) was inhibited completely.