ABSTRACT
The role of liquid confinement on the asymmetric Rh catalysis was studied using the 1,2-addition of phenylboroxine (2) to N-tosylimine 1 in the presence of [RhCl(C2 H4 )2 ]2 and chiral diene ligands as benchmark reaction. To get access to Rh complexes of different polarity, enantiomerically pure C2 -symmetric p-substituted 3,6-diphenylbicyclo[3.3.0]octadienes 4 and diastereomerically enriched unsymmetric norbornadienes 5 and 6 carrying either the Evans or the SuperQuat auxiliary were synthesized. A microemulsion containing the equal amounts of H2 O/KOH and toluene/reactants was formulated using the hydrophilic sugar surfactant n-octyl ß-d-glucopyranoside (C8 G1 ) to mediate the miscibility between the nonpolar reactants and KOH, needed to activate the Rh-diene complex. Prominent features of this organized reaction medium are its temperature insensitivity as well as the presence of water and toluene-rich compartments with a domain size of 55â Å confirmed by small-angle X-ray scattering (SAXS). Although bicyclooctadiene ligands 4 a,b,e performed equally well under homogeneous and microemulsion conditions, ligands 4 c,d gave a different chemoselectivity. For norbornadienes 5, 6, however, microemulsions markedly improved conversion and enantioselectivity as well as reaction rate, as was confirmed by kinetic studies using ligand 5 b.
ABSTRACT
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 CâC 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 CâC 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.
