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(C2H4)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(C2H4)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.