Abstract
The Cambridge Structural Database comprises 89 structures with M-Prosphos chelate rings. On this basis a compilation analysis of the conformation of the M-Prophos chelate ring has been carried out. Typical conformational features, such as the puckering of the chelate ring and the arrangement of the phenyl rings, important for the transmission of the chiral information in enantioselective ...
Abstract
The Cambridge Structural Database comprises 89 structures with M-Prosphos chelate rings. On this basis a compilation analysis of the conformation of the M-Prophos chelate ring has been carried out. Typical conformational features, such as the puckering of the chelate ring and the arrangement of the phenyl rings, important for the transmission of the chiral information in enantioselective catalysts, are attributed to CH/pi interactions within the M-Prophos system, although intramolecular interactions, in particular with Cp and Cp* ligands, also play a role. Distances below the van der Waals radii indicate weak bonding, provided the overlap angles are large enough. Normally, intermolecular interactions and packing effects are not structure-determining. Otherwise, recurrent motifs would not be as dominant, as established in the present analysis. The most important contribution comes from the interaction of the methyl group at the asymmetric center in the equatorial position of the five-membered chelate ring with the axial phenyl ring of the adjacent PPh2 group. The distances from the methyl group to the ipso and ortho carbon atoms of the axial phenyl ring are much shorter than those to C-i and C-o of the equatorial phenyl ring. In addition, the overlap angles are better. In an active process the methyl group attracts one of the two sides of the phenyl rings to establish an effective CH/pi interaction with one of the ortho carbon atoms. Within the PPh2 groups the interactions of the o-CH bond of one phenyl ring with the ipso and ortho carbon atoms of the other phenyl ring vary from strong to weak. Weak attractions are also observed between the hydrogen atoms of the CHMeCH2 backbone and the neighboring phenyl rings. The fact that the methyl group at the asymmetric center overwhelmingly occupies the equatorial position in the chelate ring is attributed to the missing phenyl-phenyl stabilization, when the methyl group switches into the exceptional axial position. Decreasing CH/pi intensity, based on distance and overlap angle, is visualized by broad, normal, thin, and dashed arrows.
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