1259294-27-3Relevant academic research and scientific papers
Methylation of ylidene-triazenes: Insight and guidance for 1,3-dipolar cycloaddition reactions
Tennyson, Andrew G.,Moorhead, Eric J.,Madison, Brian L.,Er, Joyce A. V.,Lynch, Vincent M.,Bielawski, Christopher W.
experimental part, p. 6277 - 6282 (2011/01/05)
Reaction of 1,3-dimesitylimidazolylidene (1) with p-functionalized phenyl azides 2 (a: H, b: OCH3, c: NO2) afforded the respective imidazolylidene-triazenes 3 in good yields (65-99%). Subsequent treatment with methyl iodide produced the corresponding methylated products 4 in near-quantitative yields (99%). Analysis by NOESY 1D NMR spectroscopy and single-crystal X-ray diffraction revealed that the methylation reaction was regioselective and occurred at the nitrogen atom most distal from the heterocycle. Consistent with the formation of ionic salts, the 1H NMR signals for the imidazole protons in 4 were shifted > 0.5 ppm downfield compared to 1, indicating the accumulation of positive charge. Similarly, the λmax values recorded for 4 exhibited a much narrower range than those for 3 (16 vs. 77 nm, respectively), suggesting that the frontier orbital energies within the former were dominated by Coulombic effects (i.e., the acquisition of positive charge) as opposed to electronic effects from the electron-donating or withdrawing groups of the aryl azide. Whereas computational analyses revealed that the observed regioselectivity of the methylation reaction may be explained by thermodynamics, kinetic factors (e.g., sterics) may also be important contributors and render one reaction pathway significantly more accessible due to a lower activation energy. Coupling of 1,3-dimesitylimidazolylidene (1) with a phenyl azide (2) afforded an ylidene-triazene 3, which was methylated with CH3I to yield the corresponding salt 4. Crystallographic, 1H and NOESY 1D NMR, as well as UV/Vis analyses provided insight into the structural and electronic features of 4. Subsequent computational studies revealed that 3 should engage in dipolar cycloadditions, and the absence thereof may arise from kinetic factors.
