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Introduction to Oxidation and Reduction

Reduction

In organic chemistry, very few methods are developed as well as reductions. A number of selective reagents are available for different applications. The most important reducing agents are:

  • Hydrogen + metal catalysts e.g. H2 /Ni, H2 /Pd, H2 /Pt
  • Metal hydrides e.g. LiAlH4 , NaBH4
  • Metals e.g. Li, Na, Mg, Zn
  • Low-valent nitrogen, phosphorus, and sulfur reagents e.g. HN=NH, PEt3 , Na2S2O4 , S8

Catalytic hydrogenations

Catalytic hydrogenations are frequently used for the following transformations:

Metal hydrides

Metal hydrides, such as lithium aluminun hydride (LiAlH4 ) and sodium borohydride (NaBH4 ), are suitable for the reduction of carbonyl compounds. NaBH4 is less reactive than LiAlH4 reducing aldehydes and ketones but not carboxylic acids.

Diborane

Diborane under hydroboration conditions reacts with alkenes and alkynes to give intermediates that can be transformed into various important compound classes:

Tab.3
Hydroboration
Alkenes to alkylboranes
Alkynes zu vinylboranes

Metals

Metals, such as lithium and sodium, reduce arenes and carbonyl compounds by electron transfer generating radical anions which undergo subsequent reactions.

Tab.4
Reductions with metals
Benzene derivatives to cycloalkadienes (Birch reduction)
Dimerization of esters to acyloins (acyloin condensation)

Low-valent nitrogen, phosphorus and sulfur reagents

Low-valent nitrogen, phosphorus and sulfur reagents are utilized for diverse purposes.

Tab.5
Reduction with low-valent reagents
Alkenes to alkanes with diimide
Peroxides to ethers with triethyl phosphite
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