Reducing agents, nucleophilic

Asymmetric induction is the preferential formation of one enantiomer or diastereomer over another due to the influence of a chiral center (or chiral element) either in the substrate, reagent, catalyst, or solvent of the reaction. 

In an enantioselective reaction, an achiral substrate is converted selectively to one of two enantiomers by a chiral reagent or catalyst. This process is an example of asymmetric induction. 

Enantiomeric excess or % ee (or % optical purity) = [observed specific rotation] divided by [specific rotation of pure enantiomer] X 100. Note that there are examples where the linear relationship between enantiomeric excess and optical rotation fails. A percent enantiomeric excess (% ee) of less than 100% indicates that the compound is contaminated with the other enantiomer. The ratio of enantiomers in a sample of known (measured) optical purity may be calculated as follows fraction of the major isomer = [(% ee) + 0.5 (100 - % ee)]. Thus, 

86% ee represents a sample consisting of a 93 7 ratio of enantiomers [major enantiomer = 86 + 0.5 (14)]. 

Diastereomeric excess or % de = [% of major diastereomer - % of minor diastereomer]. 

---

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. 

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)... 

The indole ring system is not reduced by nucleophilic reducing agents, such as lithium aluminium hydride or sodium borohydride lithium/liquid ammonia does, however, reduce the benzene ring 4,7-dihydroindole is the main product. ... 

HO-/H2O Hi, cat. or HBr H + Zn, MeOH Weak bases, electrophiles Strong bases, nucleophiles, reducing agents... 

The alternative scheme would be to reduce the ketone first and the alkene second. This order must be wrong though, because simple alkenes are nucleophihc and are not reduced by NaBtb. NaBH4 is a nucleophilic reducing agent and attacks alkenes only if they are conjugated with an... 

Asoln.of cycloheptyl bromide in tetrahydrofuran injected at 25° into a vigorously stirred soln. of lithium hydridotriethylborate in the same solvent, and stirring continued 3 hrs. cycloheptane. Y 99%. - Lithium hydridotriethylborate is a nucleophilic reducing agent far superior to LiAlH4. Compared to LiBH4, its nucleophilicity is enhanced by a factor of 10,000. F. e. s. H. C. Brown and S. Krishnamurthy, Am. Soc. 95, 1669 (1973) replacement with NaAlH iOCH.,-in the presence of palladium chloride complexes cf. I. Simunek and M. Kraus, Coll. 38, 1786 (1973). 

Lithium trialkylboron hydrides are very attractive new reducing agents. Thus lithium triethylboron hydride is a nucleophilic reducing agent for alkyl halides far superior to LiAlH. Compared with LiBH, its nucleophilicity is enhanced by a factor of 10,000 . 

Epoxides can be reduced to alcohols. Lithium aluminum hydride is a nucleophilic reducing agent and the hydride is therefore added at the less substituted carbon atom. Lithium triethylborohydride is more reactive than LiAlH4 and is superior for epoxides that are resistant to reduction.Reduction by metals, particularly lithium in ethylenediamine, " also gives good yields. 

---