Abstract
We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti{PhC(NiPr)2}(NOtBu) (19), with heterocumulenes, aldehydes, ketones, organic nitriles, ArF5CCH, and B(ArF5)3 (ArF5 = C6F5). Compound 19 was synthesized via imide/alkoxyamine exchange from Cp*Ti{PhC(NiPr)2}(NtBu) and tBuONH2. Reaction of 19 with CS2 and Ar′NCO (Ar′ = 2,6-C6H3iPr2) gave the [2 + 2] cycloaddition products Cp*Ti{PhC(NiPr)2}{SC(S)N(OtBu)} and Cp*Ti{PhC(NiPr)2}{N(OtBu)C(NAr′)O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti{PhC(NiPr)2}{OC(NTol)N(Tol)C(NOtBu)O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NOtBu group transfer was observed with both tBuNCO and PhC(O)R, yielding the oxo-bridged dimer [Cp*Ti{PhC(NiPr)2}(μ-O)]2 and either the alkoxycarbodiimide tBuNCNOtBu or the oxime ethers PhC(NOtBu)R (R = H (25a), Me (25b), Ph (25c)). DFT studies showed that in the reaction with PhC(O)R (R = H, Me) the product distribution between the syn and anti isomers of PhC(NOtBu)R was under kinetic control. Reaction of 19 with ArCN gave the Ti═Nα insertion products Cp*Ti{PhC(NiPr)2}{NC(Ar)NOtBu} (Ar = Ph (28), 2,6-C6H3F2 (27), ArF5 (26)) containing tert-butoxybenzimidamide ligands. Reaction of 19 or 26 with an excess of ArF5CN gave Cp*Ti{PhC(NiPr)2}{NC(ArF5)NC(ArF5)N(C{ArF5}NOtBu)} (29) following net head-to-tail coupling of 2 equiv of ArF5CN across the Ti═Nα bond of 26. Reductive Nα–Oβ bond cleavage was observed with ArF5CCH, forming Cp*Ti(OtBu){NC(ArF5)C(H)N(iPr)C(Ph)N(iPr)} (30). Addition of 2 equiv of [Et3NH][BPh4] to 19 in THF-d8 resulted in protonolysis of the amidinate ligand, forming [PhC(NHiPr)2][BPh4] and the cationic alkoxyimido complex [Cp*Ti(NOtBu)(THF-d8)2]+. In contrast, reaction with B(ArF5)3 resulted in elimination of isobutene and formation of Cp*Ti{PhC(NiPr)2}{η2-ON(H)B(ArF5)3}.