Coupling of bulky, electron-deficient partners in aryl amination in the preparation of tridentate bis(oxazoline) ligands for asymmetric catalysis.

A new class of tridentate bis(oxazoline) ligands 7, in which an N-phenylaniline unit links the two oxazoline rings, has been prepared. The key step in their synthesis is a Hartwig-Buchwald type Pd-catalyzed aryl amination between the two bulky o-substituted coupling partners, 2-(2'-bromophenyl)oxazolines 8 and 2-(o-aminophenyl)oxazolines 9. By varying the substituent on the coupling partners, a range of 10 ligands has been prepared in good yield. During the synthesis of 2-(o-aminophenyl)oxazolines 9a-d, a number of products of unexpected side reactions were isolated in two of the three steps. Alternatively, the required 2-(o-aminophenyl)oxazolines 9 were obtained by a DAST-promoted cyclodehydration of hydroxyamides 12a-d without formation of any byproducts.