Abstract
Stereodynamic ligands offer intriguing possibilities in enantioselective catalysis. “NU-BIPHEPs” are a class of stereodynamic diphosphine ligands which are easily accessible via rhodium-catalyzed double [2 + 2 + 2] cycloadditions. This study explores the preparation of differently functionalized “NU-BIPHEP(O)” compounds, the characterization of non-covalent adduct formation and the quantification of enantiomerization barriers. In order to explore the possibilities of functionalization, we studied modifications of the ligand backbone, e.g., with 3,5-dichlorobenzoyl chloride. Diastereomeric adducts with Okamoto-type cellulose derivatives and on-column deracemization were realized on the basis of non-covalent interactions. Enantioselective dynamic HPLC (DHPLC) allowed for the determination of rotational barriers of ΔG‡298K = 92.2 ± 0.3 kJ mol−1 and 99.5 ± 0.1 kJ mol−1 underlining the stereodynamic properties of “NU-BIPHEPs” and “NU-BIPHEP(O)s”, respectively. These results make the preparation of tailor-made functionalized stereodynamic ligands possible and give an outline for possible applications in enantioselective catalysis.
Highlights
Chiral biaryl compounds such as BINAP (2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) represent widely used and highly efficient ligands that can be applied in a variety of enantioselective catalytic transformations
In contrast to 1a and 1b, three coexisting isomeric species were observed with NMR spectroscopy in CDCl3 for tetrahydrobiisoindole “NU-BIPHEP(O)” 1c
We report a strategy to use unprotected tetrahydrobiisoindole “NU-BIPHEP(O)” for functionalization with substituents at the secondary amine position, in this study namely by formation of tertiary amide binding sites with 3,5-dichlorobenzoyl chloride
Summary
Chiral biaryl compounds such as BINAP (2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) represent widely used and highly efficient ligands that can be applied in a variety of enantioselective catalytic transformations. Unlike BINAP, the related stereodynamic BIPHEP (2,2’-bis(diphenylphosphino)1,1’-biphenyl) ligands have a significantly lower barrier of rotation around the central C–C bond regarding the conversion of the enantiomers into one another. This enables fast enantiomerization at room temperature. The rotational barrier around the central C–C bond of BIPHEP ligands is a key property of stereodynamic ligands that determines the temperature required for ligand enantiomerization as well as the half-life of isolated enantiomers The latter are of particular importance if chiral co-ligands are cleaved off prior to catalysis and if the remaining stereochemically aligned. The attachment of a 3,5dichlorobenzoyl binding site is reported and non-covalent interactions as well as rotational barriers are studied in solution by (D)HPLC techniques
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