The Electronic Influence of Ferrocenyl and Related Groups as Substituents

THE ELECTRONIC INFLUENCE OF FERROCENYL AND RELATED GROUPS AS SUBSTITUENTS 

In the previous sections the various inffuences of substituents on metallocenes and related molecules have been examined. To complete a study of the electronic nature of these molecules, it remains that their behavior as substituents themselves be discussed. 

The very strong electron-donating power of the ferrocene nucleus is clearly demonstrated in the extraordinary stability of the a-ferrocenyl carbonium ion (X). Questions have been raised concerning the mode of stabilization of such ions, i.e., (1) whether the77-orbitals of the ring overlap the empty p-orbital (resonance), (2) whether the f-orbitals on iron participate or overlap the empty p orbital, and (3) whether there is ring movement. 

These ions have received considerable attention and have been extensively reviewed 13). The chemistry of such ions is thus outside the scope of this review additional information may be found in the latest papers of Nugent and Richards (55). However, the more quantitative aspects of the electron donor properties of the ferrocenyl and related groups, such as those provided by determination of their Hammett j-constants, are most appropriate for inclusion here. 

Relatively extensive studies have been performed on the solvolysis of several arenechromium tricarbonyl tosylates (6, 7). Comparison of the solvolysis data of the complexed and noncomplexed optically active threo-and ervfAro-3-phenyl-2-butyl tosylates led the authors to conclude that complexation of these tosylates inhibited phenyl and hydrogen migration, prevented racemization in the threo case, and reduced direct displacement 

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