Role of Regioisomeric Bicyclo[3.3.0]octa-2,5-diene Ligands in Rh Catalysis: Synthesis, Structural Analysis, Theoretical Study, and Application in Asymmetric 1,2- and 1,4-Additions.

In order to study the impact of regioisomeric diene ligands on the formation and catalytic activity of Rh complexes, a series of C2 - and CS -symmetric 2,5-disubstituted bicyclo[3.3.0]octa-2,5-dienes C2 -L and CS -L, respectively, were synthesized from Weiss diketone by simultaneous deprotonation/electrophilic trapping of both oxo functions, and the catalytic behavior was studied in the presence of [RhCl(C2 H4 )2 ]2 . Complexes [RhCl(C2 -L)]2 bearing C2 -symmetric ligands catalyzed effectively the asymmetric arylation of N-tosylaldimines to (S)-diarylamines with yields and ee values up to 99%. In Hayashi-Miyaura reactions, however, the complexes showed poor catalytic activity. When complexes [RhCl(CS -L)]2 with CS -symmetric ligand or mixtures of [RhCl(C2 -L)]2 and [RhCl(CS -L)]2 were employed in 1,2-additions, racemic addition products were observed, suggesting a C═C isomerization of the diene ligands. X-ray crystal structure analysis of both Rh complexes formed from the [RhCl(C2 H4 )2 ]2 precursor and ligands C2 -L and CS -L revealed that only the C2 -symmetric ligand C2 -L coordinated to the Rh, whereas CS -L underwent a Rh-catalyzed C═C isomerization to rac-C2 -L, which then gave the racemic [RhCl(rac-C2 -L)]2 complex. DFT calculations of the relative stabilities of the Rh complexes and the proposed intermediates provided a mechanistic rationale via Rh-mediated hydride transfer.