Evidence of ketenimine formation during the multiple C-C coupling of isocyanides by stabilized Group 4 metallacyclobutanes

η2‐Iminoacyl and η2‐Acyl Monocyclopentadienyl Tantalum Complexes Bearing Oxo and Oxo‐Borane Ligands

“…The 13 C NMR spectrum of the new oxo iminoacyl compound 4a showed the η 2 -iminoacyl C sp 2 resonance at δ = 240.3 ppm. A comparison of this 13 C resonance and that assigned to the CH 2 -iminoacyl group in the 1 H NMR spectrum with the corresponding resonances in the parent compound 2a, showed a behaviour that is opposite to that observed for 2d and [TaCp*Cl{η 2 -C(Me)=NAr}(O)].…”

“…We previously reported [10] 3 , which afforded 2d in high yield. The formation of 2d was confirmed by 13 C and 19 F NMR spectroscopy. The 13 C NMR spectrum shows the resonance corresponding to the C sp 2 atom of the η 2 -iminoacyl ligand (δ = 240.1 ppm) to be slightly low-field shifted with respect to that of the starting compound [TaCp*Cl{η 2 -C(Me)=NAr}(O)] (δ = 236.8 ppm).…”

“…The formation of 2d was confirmed by 13 C and 19 F NMR spectroscopy. The 13 C NMR spectrum shows the resonance corresponding to the C sp 2 atom of the η 2 -iminoacyl ligand (δ = 240.1 ppm) to be slightly low-field shifted with respect to that of the starting compound [TaCp*Cl{η 2 -C(Me)=NAr}(O)] (δ = 236.8 ppm). An analogous behaviour was observed in the 1 H NMR spectrum for the methyl-iminoacyl group, which was shifted from δ = 2.65 ppm in [TaCp*Cl{η 2 -C(Me)=NAr}(O)] to δ = 2.94 ppm in 2d.…”

“…[1] For a given metal atom, the stability and further evolution of these compounds are determined by the nature of the ancillary ligands. Many reaction pathways may follow to give a broad variety of products: [1] (a) migratory insertion of a second alkyl or aryl group to give η 2 -imine or η 2 -ketene complexes; [2][3][4][5] (b) intra-or intermolecular coupling of iminoacyl or acyl units affording diaza-or dioxobutene complexes; [6,7] (c) transfer of the NR or O moieties to the metal centre; [8][9][10][11] (d) insertion of a second CNR [12][13][14][15][16] or CO [17] molecule into the new M-C bond formed after the first insertion process and (e) hydrogen migration. [18,19] We reported the results of our studies on the insertion reactions of CNR into the Ta-C(methyl) bond of monocyclopentadienyl complexes of the type [TaCp*Cl x Me 4-x ] [8][9][10] for which processes (a) and (c) were observed.…”

“…These pale yellow complexes are air and thermally stable in solution. The 13 C NMR resonance at ca. δ = 240 ppm is the most apparent spectroscopic feature and confirms the presence of the η 2 -iminoacyl ligand in complexes 2.…”