Nucleophilic reducing

Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

Pyridines are reduced more easily than the corresponding benzenoid compounds. The greater the electron-withdrawing power of the substituents attached to the pyridine ring the easier is reduction by nucleophilic reducing agents. 

A second more subtle effect may also be operative in the metal ion control of nucleophilic reactions. When amines, thiolates or alkoxides are used as nucleophiles, they are expected to be highly reactive and hence relatively unselective. However, we saw in Chapter 2 that the proximity of the metal cation to the nucleophile reduces the charge density on the donor atom, and is thus expected to reduce the reactivity. We can use the reduced reactivity, and greater selectivity, of such co-ordinated nucleophiles to direct reaction towards the cyclic products. 

Super Hydride is one of the most powerful nucleophilic reducing agents available, capable of reducing many functional groups. It is also highly selective. The exocyclic double bond in aryl-substituted fulvenes has an increased polarity, due to the inductive effects of their respective aryl groups. This increased polarity allows for selective nucleophilic attack at this double bond and not at the diene component of the fulvenes. Other examples of the nucleophilic addition of hydrides to substituted fulvenes (albeit with alkyl or unsubstituted phenyl group functionality)... 

With other nucleophiles (Figures 5.42-5.45) aryldiazonium salts react according to other mechanisms to form substitution products. These substitutions are possible because certain nucleophiles reduce aryldiazonium salts to form radicals Ar—N=N-. These radicals lose molecular nitrogen. A highly reactive aryl radical remains, which then reacts directly or indirectly with the nucleophile. 

Nucleophilic reducing reagents such as LiAlH4 and NaBH4... 

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