M═Nα Cycloaddition and Nα−Nβ Insertion in the Reactions of Titanium Hydrazido Compounds with Alkynes: A Combined Experimental and Computational Study
Résumé
A combined experimental and DFT study of the reactions of diamide-amine supported titanium hydrazides with alkynes is presented. Reaction of Ti(N2Npy)(NNPh2)(py) (1, N2Npy = (2-NC5H4)CMe(CH2NSiMe3)2) with terminal and internal aryl alkynes ArCCR (Ar = Ph or substituted phenyl, R = Me or H) at room temperature gave the fully authenticated azatitanacyclobutenes Ti(N2Npy){N(NPh2)C(R)CAr} via ArCCR [2 + 2] cycloaddition to the Ti═Nα bond of the hydrazide ligand. In contrast, reaction of 1 with PhCCMe at 60 °C, or of Ti(N2NMe)(NNPh2)(py) (11, N2NMe = MeN(CH2CH2NSiMe3)2) with RCCMe (R = Me, Ph or substituted phenyl) at room temperature or below, gave vinyl imido compounds of the type Ti(N2NR′){NC(R)C(Me)NPh2}(py), in which RCCMe had undergone net insertion into the Nα−Nβ bond. These are the first examples of this type of reaction for any metal hydrazide. The reaction of 11 with PhCCMe had the activation parameters ΔH = 18.8(4) kcal mol−1, ΔS = 1(1) cal mol−1 K−1 and ΔG298 = 18.5(7) kcal mol−1. Mechanistic and DFT studies for 1 and 11 found that the Nα−Nβ insertion event is preceded by alkyne cycloaddition to Ti═Nα, and that Nα−Nβ bond “insertion” is really an intramolecular Nα atom migration process within the azatitanacyclobutenes following intramolecular chelation of NPh2 of the hydrazide ligand. Electron-withdrawing aryl groups on ArCCMe stabilize the azatitanacyclobutenes and also promote a specific regiochemistry (ArC carbon bound to Ti). This in turn defines the regiochemistry of the overall Nα−Nβ insertion reaction (ArC carbon bound to Nα). In contrast, electron-releasing aryl groups promote the final Nα migration stage of the mechanism, and a Hammett analysis of the rates of insertion of (4-C6H4X)CCMe into the Nα−Nβ bond of 11 found a reaction constant, ρ, of −0.74(5), consistent with NPA charge changes of ArC along the DFT reaction coordinate.