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 C- and C-symmetric 2,5-disubstituted bicyclo[3.3.0]octa-2,5-dienes C-L and C-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(CH)]. Complexes [RhCl(C-L)] bearing C-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(C-L)] with C-symmetric ligand or mixtures of [RhCl(C-L)] and [RhCl(C-L)] 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(CH)] precursor and ligands C-L and C-L revealed that only the C-symmetric ligand C-L coordinated to the Rh, whereas C-L underwent a Rh-catalyzed C═C isomerization to rac-C-L, which then gave the racemic [RhCl(rac-C-L)] complex. DFT calculations of the relative stabilities of the Rh complexes and the proposed intermediates provided a mechanistic rationale via Rh-mediated hydride transfer.