M=N_α Cycloaddition and N_α-N_β Insertion in the Reactions of Titanium Hydrazido Compounds with Alkynes: A Combined Experimental and Computational Study

摘要

A combined experimental and DFT study of the reactions of diamide-amine supported titanium hydrazides with alkynes is presented. Reaction of Ti(N_2N~(py))(NNPh_2)(py) (1, N_2N~(py) = (2-NC_5H_4)CMe(CH_2NSiMe_3)_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(N_2N~(py)){N(NPh_2)C(R)CAr} via ArCCR [2 + 2] cycloaddition to the Ti=N_α bond of the hydrazide ligand. In contrast, reaction of 1 with PhCCMe at 60 ℃, or of Ti(N_2N~(Me))(NNPh_2)(py) (11, N_2N~(Me) = MeN(CH_2CH_2NSiMe_3)_2) with RCCMe (R = Me, Ph or substituted phenyl) at room temperature or below, gave vinyl imido compounds of the type Ti(N_2N~R'){NC(R)C(Me)NPh_2}(py), in which RCCMe had undergone net insertion into the N_α-N_β 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 ΔG_(298)~≠ = 18.5(7) kcal mol~(-1). Mechanistic and DFT studies for 1 and 11 found that the N_α-N_β insertion event is preceded by alkyne cycloaddition to Ti=N_α, and that N_α-N_β bond "insertion" is really an intramolecular N_α atom migration process within the azatitanacyclobutenes following intramolecular chelation of NPh_2 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 N_α-N_β insertion reaction (ArC carbon bound to N_α). In contrast, electron-releasing aryl groups promote the final N_α migration stage of the mechanism, and a Hammett analysis of the rates of insertion of (4-C_6H_4X)CCMe into the N_α-N_β bond of 11 found a reaction constant, p, of -0.74(5), consistent with NPA charge changes of ArC along the DFT reaction coordinate.