Rhodium-catalysed enantioselective hydroboration of olefins is a valuable synthetic transformation, typically employing a chiral catalyst and an achiral borane source. The pertinent chemo-, regio-and enantioselectivity issues of this reaction are discussed. However, the main emphasis of this review is on the evolution of catalytic asymmetric hydroboration. This has primarily relied upon the development and application of chiral bidentate P,P and P,N ligands which have exhibited varying degrees of success in this transformation.

The preparation and resolution of a series of axially chiral quinazoline-containing ligands is described in which the key steps are the metal-catalyzed naphthyl-phosphorus bond formation, the naphthalene-quinazoline Suzuki coupling, and the preparation of the Suzuki electrophilic components from the corresponding imidate and anthranilic acid. Diastereomeric palladacycles derived from the racemic phosphinamines and (+)-di-mu-chlorobis[(R)-dimethyl(1-(1-naphthyl)ethyl)aminato-C2,N]dipalladium(II) were separated by fractional crystallization. The configuration of the resulting diastereomers was determined by X-ray crystallographic analysis. Displacement of the resolving agent by reaction with 1,2-bis(diphenylphosphino)ethane afforded enantiopure ligand in each case. Their rhodium complexes were prepared and applied in the enantioselective hydroboration of a range of vinylarenes. The quinazolinap catalysts were found to be extremely active, giving excellent conversions, good to complete regioselectivities, and the highest enantioselectivities obtained to date for several members of the vinylarene class, including cis-beta-methylstyrene (97%), cis-stilbene (99%), and indene (99.5%).

Nature is a well-recognized source of compounds of interest, but access is often an issue. One pertinent example is the cinchona alkaloids from the bark of Cinchona calisaya. In this experiment, students at the third-year undergraduate level undertake the selective isolation and characterization of two of the four main alkaloids present in the bark. Beginning with powdered bark, acid−base extraction, followed by selective crystallization, serves to yield cinchonine, nature's chiral catalyst, from the complex mixture. Slow crystallization provides suitable quality crystals for X-ray analysis. Students readily appreciate the three-dimensional nature of this chiral catalyst, which aids subsequent NMR spectroscopic analysis. Manipulation of the mother liquor by thin-layer and flash column chromatographic techniques proves a simple but elegant method to furnish quinine, nature's anti-malarial agent. Both alkaloids are treated to an intensive structure elucidation workshop comprising 1-and 2dimensional NMR, infrared, and mass spectrometry. The method is economical, polished, and robust, bringing the student on a journey from crude plant material to medicinally important natural products in three, 2-h laboratory sessions. Moreover, questions in the student handout and model answers in instructor's notes, respectively, require that students engage further in topics associated with the context of this practical.

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