Homoannular Electronic Effects

Since all aromatic organometallic 7T-complex systems possess several possible sites for which substituent effects could be analyzed, it seems best that the various sites be studied independently of one another. One particular site, the substituted ring, has been shown to be amenable to such analysis. The electronic effects on the substituted ring in ferrocenes, i.e., homoannular electronic effects, have recently been the subject of an extensive review (75). For this reason discussion of ferrocene systems in this section will be abbreviated and attention will be focused upon recent advances in related metallocene and other aromatic organometallic systems. 

Two distinct approaches to the elucidation of homoannular electronic effects within the transition metal-complexed aromatic ring have been utilized (1) relative site reactivities and (2) NMR spectroscopy. Because of the relative availability of many ferrocene derivatives considerable attention has been devoted to this system. In several instances results for the ferrocene system have provided a basis for the analysis of electronic effects in related organometallic Tr-complexes. 

The study of relative site reactivities for aromatic electrophilic substitution reactions has provided useful information for assessing homoannular electronic effects within the ferrocene system. Ferrocenes of the type (I) 

Relative Site Reactivities of Monosubstituted and 1, T-Disi bstitl ted Ferrocenes Towards Electrophiles  

In each class, unequivocal assignments have been made viaspecific deuterium labeling techniques (65, 77, 78). 

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All four organometallic cyclopentadienyl systems discussed heretofore have had their homoannular electronic effects examined by PMR spectroscopy. Of the four, only the cobalticinium salts exhibited shielding for the... 

Electronic effects of substituents can affect organic molecules in diverse ways. Usually, loci near the substituent are most influenced, but often electron density is altered several bonds removed from the substituent. Aromatic organometallic tr-complexes provide fascinating systems for the study of substituent effects, for the three-dimensional nature of these complexes adds interesting complexities to the analysis of the distribution of electron density in such molecules. This chapter is intended to summarize homoannular, interannular, and metal-ring substituent effects in metallocene and similar systems with the idea of spurring additional research in the area. 

Most of the data concerning ferrocene electronic effects correlate rather well, f errocene emerges as a system which homoannularly allows electron withdrawal to take place principally from the 2-position and secondarily... 

Acylation experiments show that disubstitution occurs heteroannularly indicating that as expected —I substitutents relatively deactivate the substituted ring. In those cases where small yields of homoannularly di-acetylated products are isolated, the acyl substituents enter the 1,2-positions of the r-cyclopentadienyl ring rather than the 1,3-podtions. Thus in the monoacetylated ring the 3-position is deactivated relative to the 2-position. The transmission of electronic effects from one ring to the... 

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