Carbonyl compounds reductions, zinc-acetic acid

Other cobalt catalysts that have been used for enantioselective C—C double bond reduction of a,/ -unsaturated carbonyl compounds are vitamin B12 in combination with zinc/acetic acid (up to 33% ee)S7 and bis(dimcthylglyoximato)cobalt(II) (12) with chiral amines as cocatalysts88 and molecular hydrogen as reducing agent. The best results have been obtained with complex 12 and quinine or (2.S )-A -[(/ )-1 -phenethyl]-2-quinuclidinecarboxaniide (hydrogenation of 1,2-diphenyl-2-propen-l-one 49% ee A, /V -dimethyl-5-benzylidenehydantoin 79% ee). 

Low -molecular-weight ozonides are explosive and are theretore not isolated. Instead, the ozonide is immediately treated with a reducing agent such as zinc metal in acetic acid to convert it to carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the C=C bond is cleaved and oxygen becomes doubly bonded to each of the original alkene carbons. If an alkene with a letrasubstituted double bond is ozonized, two ketone fragments result if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result and so on. 

Purely aromatic ketones generally do not give satisfactory results pinacols and resinous products often predominate. The reduction of ketonic compounds of high molecular weight and very slight solubility is facilitated by the addition of a solvent, such as ethanol, acetic acid or dioxan, which is miscible with aqueous hydrochloric acid. With some carbonyl compounds, notably keto acids, poor yields are obtained even in the presence of ethanol, etc., and the difficulty has been ascribed to the formation of insoluble polymolecular reduction products, which coat the surface of the zinc. The adffition of a hydrocarbon solvent, such as toluene, is beneficial because it keeps most of the material out of contact with the zinc and the reduction occurs in the aqueous layer at such high dilution that polymolecular reactions are largdy inhibited (see Section IV,143). 

Fischli has used a related reagent, Cob(I)alamine,2 obtained by reduction of vitamin B,2 with zinc and acetic acid, as a catalyst for reduction with zinc and acetic acid of various unsaturated systems such as a,/ -unsaturated nitriles,3 esters,4 and carbonyl compounds,5 allylic alcohols and amines,6 and also isolated double bonds.6... 

Dibromoalcohols, which are readily prepared by addition of dibromomethyl-lithium to aldehydes or ketones, undergo reductive elimination on treatment with zinc and acetic acid in refluxing methylene chloride to give vinyl bromides as mixtures of E- and Z-isomers (Scheme 30). Conjugated 1-bromo-dienes or -trienes can be prepared from the appropriate unsaturated carbonyl compounds. 

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