Controlled alkene and alkyne insertion reactivity of a cationic zirconium complex stabilized by an open diamide ligand

The chemistry of electrophilic zirconium complexes stabilized by a sterically open diamide ligand has been studied. Treatment of Me2Si(NLiCMe3)(2) with ZrCl4(THF)(2) afforded {Me2Si(NCMe3)(2)}ZrCl2(THF)(2) (1), which, in solution, was in equilibrium with a dimeric zirconium dichloride species [{Me2Si(NCMe3)(2)}ZrCl2](2)(THF). Complex 1 was converted to dialkyl complexes {Me2Si(NCMe3)(2)}ZrR2 (R = CH2Ph, 4; CH2CMe3, 5) using MgBz(2)(dioxane)(0.5) and LiCH2CMe3, respectively, but dimethylation was unsuccessful. Alkyl abstraction from 4 using B(C6F5)(3) cleanly afforded {Me2Si(NCMe3)(2)}Zr(CH2Ph){eta(6)-PhCH2B(C6F5)(3)} (6), in which the anion strongly coordinates to the benzylzirconium cation ria the aromatic ring. Protonolysis of 4 using [PhMe2NH][B(C6F5)(4)] afforded the Lewis-base adduct [{Me2Si(NCMe3)(2)}Zr{CH2Ph)(NMe2Ph)](+) (7), whereas [Ph3C][B(C6F5)(4)] gave 1 equiv of Ph3CCH2Ph and a mixture of two cationic species, 9a/b, proposed to be monomeric and dimeric benzylzirconium cations, Reaction of 4 with 0.5 equiv of the trityl reagent afforded the dizirconium complex [{Me2Si(NCMe3)(2)}(2)Zr-2(CH2Ph)(3)](+). Cations 6, 7, and 9a/b cleanly and rapidly reacted with 2-butyne to afford the single insertion product, [{Me2Si(NCMe3)(2)}Zr{eta(1),eta(6)-C(Me)=C(Me)CH2Ph](+), stabilized by a chelating pi-coordination of the benzene ring of the hydrocarbyl ligand. Structurally similar insertion products were obtained from the reaction of 9a with a range of alkenes, [{Me2Si(NCMe3)(2)}Zr{eta(1),eta(6)-CH2CH(R)CH2Ph](+) (R = H, Me, n-Bu, CH2Ph). Benzylborate adduct 6 also underwent single alkene insertion giving {Me2Si(NCMe3)(2)}Zr{eta(1)-CH2CH(R)CH2Ph}{eta(6)-PhCH2B(C6F5)(3)} (R = H, Me), stabilized by anion coordination to zirconium. The dramatic effects of anion, Lewis base, solvent, and substrate variation on the rate of insertion have been rationalized in terms of the facility of anion or base dissociation from the benzylzirconium cation.