1239367-85-1Relevant articles and documents
M=Nα Cycloaddition and Nα-N β Insertion in the reactions of titanium hydrazido compounds with alkynes: A combined experimental and computational study
Schofield, A. Daniel,Nova, Ainara,Selby, Jonathan D.,Manley, Catherine D.,Schwarz, Andrew D.,Clot, Eric,Mountford, Philip
, p. 10484 - 10497 (2010/08/22)
A combined experimental and DFT study of the reactions of diamide-amine supported titanium hydrazides with alkynes is presented. Reaction of Ti(N 2Npy)(NNPh2)(py) (1, N2N py = (2-NC5H4)CMe(CH2NSiMe 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(N2Npy){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 °C, or of Ti(N2NMe)(NNPh 2)(py) (11, N2NMe = MeN(CH2CH 2NSiMe3)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-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 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 N α-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.
