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.