Modular Synthesis of a New Class of Bis(amino-oxazoline) Using Palladium-Catalyzed Buchwald−Hartwig Amination Methodology

Abstract

The palladium-catalyzed Buchwald−Hartwig aryl amination between 2,2‘-diaminobiphenyl and 2 equiv of 2-(2‘-bromophenyl)oxazoline has been used to prepare a new class of tetradentate bis(amino-oxazoline). The reaction between 2,2‘-diaminobiphenyl and 1 equiv of 2-(2‘-bromophenyl)oxazoline is highly selective for monoarylation, and the resulting products have been reacted in a second arylation to afford unsymmetrically substituted bis(amino-oxazolines). These new diamido/donors belong to the tropos class of ligand and exist as an equilibrium mixture of S,aS,S- and S,aR,S-diastereoisomers, which interconvert by rotation about the biaryl axis. Variable-temperature 1H NMR studies and line-shape analysis of simulated spectra gave ΔH⧧ and ΔS⧧ values of 51.5−57 kJ mol-1 and −25.9 to −57.0 J mol-1 K-1, respectively, for diastereointerconversion. Interestingly, reaction of one of the symmetrical bis(amino-oxazoline) with [Cu(MeCN)4][PF6] results in dynamic resolution to afford diastereopure (S,aS,S)-[Cu(L)][PF6], which has a highly unusual sawhorse geometry at copper with the two oxazolines occupying trans coordination sites and two weak interactions to the nitrogen atoms of the secondary amino groups. 1H and 13C NMR spectra of this copper complex are entirely consistent with a single C2-symmetric diastereoisomer in solution that shows no sign of diastereointerconversion even after standing for one week.