Soon after its discovery, it was found, that ferrocene behaves in many respects like an aromatic electron-rich organic compound, which is activated towards electrophilic reactions almost like phenol. As a consequence, a rich organic chemistry of ferrocene was developed, in which the organometallic moiety is treated like a simple phenyl group. Thus, Friedel -Crafts acylation leads to ferrocenyl ketones, which can be reduced to secondary ferrocenyl alcohols containing an asymmetric carbon atom. Compounds of this type can be resolved into the enantiomers by standard resolution techniques [l] and do not provide any problems with respect to chemistry and nomenclature. However, ferrocene is different from phenyl with respect to stereochemistry : a derivative with at least two different substituents in the same ring cannot be superimposed with its mirror image, i.e., it is chiral. The first compound of this type to be prepared was 3,1'-dimethyl-ferrocenecarboxylic acid, in which both enantiomers were obtained by resolution of the racemate with cinchonidine and quinidine ( Fig. 4-1) [2]. Crystals of one enantiomer suitable for X-ray structure analysis could be grown; this result confirmed unequivocally the correlation with the true configuration of the molecule [3].Schlogl has investigated the problem systematically and suggested this isomerism to be an example of planar chirality fl, 41. A simple rule was developed to assign chirality descriptors to homoannular di-or polysubstituted ferrocenes: The observer regards the molecule from the side of the ring to be assigned (called the "upper" ring). The substituents are then analyzed for priority according to the Cahn -Ingold -Prelog (CIP) rules. If the shortest path from the substituent with highest priority to that following in hierarchy is clockwise, the chirality descriptor is (R), otherwise it is (S) [5]. If both rings bear at least two different substituents, the process is started with the ring containing the substituent of highest priority, the molecule is then turned upside down and the second ring subsequently assigned. In order to show clearly that the chirality descriptor belongs to a planar element of chirality, it is often written as (,R) or ($), respectively. Another convention exists, which states that in ferrocene derivatives with both central and planar chirality, the descriptor of the central chirality is written first. Thus, (R, S) should mean that the compound contains an element of central chirality with ( R ) configuration and an element of planar chirality with (S) configuration. This order is used in this book. However, the opposite order of the descriptors was suggested in a recent monograph 174
