Synthesis of Novel Tantalacalixarene Complexes: First Example of Intramolecular CH Activation of Monodepleted Aromatic Ring

The alkylation of (dichlorotantala)-p-tert-butylcalix[4]arenes 1 and 3 by two equivalents of NpMgCl (Np = neopentyl) affords the corresponding unreported (bisneopentyltantala)-p-tert-butylcalix[4]arenes 4 and 5. These complexes can undergo dealkylation by modest increase of the temperature to yield 6 and 7. The characterization and structural conformation of the complexes 6 and 7 were determined by 1H, 13C, NOESY, and COSY spectroscopic studies and single-crystal X-ray diffraction. 7 exhibits unprecedented use of an intramolecular C–H activation of an aromatic ring and simultaneously releases of one equivalent of neopentane. DFT calculation supports a mechanism in which the metal passes through a transition state involving a coplanar arrangement with its three ligands (CAr, HAr, CNp).