Planar from pinenes

First examples of chiral boratabenzene complexes, which are of interest owing to potential applications as enantioselective catalysts, have recently been reported. The planar chiral chrominm tricarbonyl complex (137) was obtained by addnct formation between borabenzene and an (S)-valinol-derived oxazoline ligand. The Cr(CO)3 fragment stereoselectively complexes with this borabenzene addnct. A different approach has been chosen by Herberich, who developed the chiral boratabenzene ligand (138), which is derived from a-pinene. ... 

Planar chiral compounds should also be accessible from the chiral pool. An example (with limited stereoselectivity) of such an approach is the formation of a ferrocene derivative from a -pinene-derived cyclopentadiene (see Sect. 4.3.1.3 [81]). A Cj-symmetric binuclear compound (although not strictly from the chiral pool, but obtained by resolution) has also been mentioned [86]. Another possibility should be to use the central chiral tertiary amines derived from menthone or pinene (see Sect. 4.3.1.3 [75, 76]) as starting materials for the lithiation reaction. In these compounds, the methyl group at the chiral carbon of iV,iV-dimethyl-l-ferrocenyl-ethylamine is replaced by bulky terpene moieties, e.g., the menthane system (Fig. 4-2 le). It was expected that the increase in steric bulk would also increase the enantioselectivity over the 96 4 ratio, as indicated by the results with the isopropyl substituent [118]. However, the opposite was observed almost all selectivity was lost, and lithiation also occurred in the position 3 and in the other ring [134]. Obviously, there exists a limit in bulkiness, where blocking of the 2-position prevents the chelate stabilization of the lithium by the lone pair of the nitrogen. 

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