Paracyclophanes as Chiral Ligands

The synthesis of enantiomerically pure compounds is one of the major challenges in organic synthesis. On the basic principle of asymmetric catalysis using metal 

The element of planar chirality plays a pivotal role in many modern ligand systems. The particularly huge success of ferrocenyl ligands has not been matched by any other chiral backbone to date. Metallocene and metal-arene-based ligand backbones exhibit the common feature that they become planar chiral only upon addition of (at least) two substituents on one ring fragment. [2.2]Paracyclophanes, however, need only one substituent (Fig. 2.1.3.1) to be chiral. 

Since the initial reports of Reich and Cram [1], the field of [2.2]paracyclophane chemistry has developed considerably, and the chemical behavior of [2.2]paracy-clophanes is well understood nowadays. Furthermore, based on our experience, these compounds are coined rocks that is, they are usually extremely stable. 

This implies, however, that they are sometimes very unreactive toward desired reactions. In general, they are able to offer reasonable resistance against synthetic efforts and are therefore an interesting challenge for dedicated chemists. 

When we first ventured into the field of [2.2]paracyclophane ligand synthesis, successful applications of such ligands were relatively rare [2]. The most prominent example was clearly the PHANEPHOS ligand developed by Rossen and Pye [3], who have found several successful applications in asymmetric hydrogenation reactions. A comprehensive survey of [2.2]paracyclophane-based ligands can be found in recent reviews [4, 5]. 

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