Resonance electron donating power

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. 

At this point we want to consider the relative reactivity of carboxylic acid derivatives and other carbonyl compounds in general terms. We return to the subject in more detail in Chapter 7. Let us first examine some of the salient structural features of the carbonyl compounds. The strong polarity of the C=0 bond is the origin of its reactivity toward nucleophiles. The bond dipole of the C-X bond would be expected increase carbonyl reactivity as the group X becomes more electronegative. There is another powerful effect exerted by the group X, which is resonance electron donation. 

The crystal structures of these aldol products reveal that a strong intramolecular hydrogen bond exists between hydroxyl and a-ketone groups. It is easy to envisage the existence of such a hydrogen bond if one considers the effect of the electron-donating power of CpMo(CO)2 that increases the basicity of the a-ketone group with a resonance represented by B. 

Like other compounds with carbon-carbon double bonds, enols are electron rich, so they react as nucleophiles. Enols are even more electron rich than alkenes, though, because the OH group has a powerful electron-donating resonance effect. A second resonance structure can be drawn for the enol that places a negative charge on one of the carbon atoms. As a result, this carbon atom is especially nucleophilic, and it can react with an electrophile to form a new bond to carbon. Loss of a proton then forms a neutral product. 

An acetate group is more powerfully electron donating (via resonance) than a methyl group (via hyperconjugation), so vinyl acetate is expected be the most reactive toward cationic polymerization. A nitro group is electron withdrawing, so nitroethylene is... 

The second and often more powerful influence is the resonance or delocalization effect. For any molecule for which more than one electron-pair bond structure can be written, the true structure will be intermediate between these structures and the molecule will be more stable than expected. Well-known examples include benzene (Figure 3.3a) and charged structures such as the allyl cation (Figure 3.3b). A carbocation can be stabilized either by an adjacent lone pair, which can be donated to form a double bond (Figure 3.3c) or by donation of electrons from an adjacent multiple bond (Figure 3.3b). Anions can be stabilized by donation of the lone pair to an adjacent multiply bonded atom (Figure 3.3d). 

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