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((CH2Bu)-Bu-t)(2) (Nacnac- = [Ar]NC(CH3)CHC(CH8)N[Ar], Ar = 2,6-(CHMe2)(2)C6H3), among them (Nacnac)Ti=(CHBu)-Bu-t(OTf) and (Nacnac)Ti=(CHBu)-Bu-t(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 alpha-hydrogen abstraction. Complex (Nacnac)Ti=CHtBu(OTf) serves as a template for other four-coordinate titanium neopentylidene complexes such as (Nacnac)Ti=(CHBu)-Bu-t(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 ((HBuC)-Bu-t=C(Me)CHC(Me)N[Ar])Ti=NAr(X) (X- = OTf, Cl, Br, 1, 13114). 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 q2 -BH4). Kinetic studies in C6D6 reveal the formation of ((HBuC)-Bu-t=C(Me)CHC(Me)N[Ar])Ti=NAr(l) from (Nacnac)Ti=CH'Bu(I) to be independent of solvent (C6D6, Et2O-d(10), THF-d(8)) and the reaction to be first order in titanium (k = 8.06 x 10(-4) s(-1) at 57 degrees C, with activation parameters Delta H-double dagger = 21.3(2) kcal/mol, Delta S-double dagger = -8(3) cal/mol K). Compound (Nacnac)Ti=CH'Bu(OTf) reacts with various substrates to afford products in which the alkylidene functionality has been significantly transformed. When the alkylidene derivatives (Nacnac(tBu))Ti=(CHBu)-Bu-t(X) (X- = OTf, I; Nacnac(tBu)- = [Ar]NC(Bu-t)CHC(Bu-t)N[Ar]) were prepared, the intramolecular cross-metathesis transformation observed with (Nacnac)Ti=CHtBu(X) was inhibited completely.