Hydrocarbons from carbonyl compound reduction

The anion BH, formed in Eq. (3), is thermodynamically a stronger base than B and will react with another molecule of substrate, as in Eq. (4). Each PB will therefore consume a total of two protons (and two electrons). An exception to the fast removal of BH by further reduction is sometimes found for radical anion oxygen bases derived from carbonyl compounds, as discussed in Sec. III.B.2. Radical anion EGBs are usually derived from aromatic systems such as aromatic hydrocarbons, A-heteroaromatic systems or azo-arenes. An example was given in Scheme 3 [3]. Radical anion EGBs are normally pro-... 

The reduction of carbonyl compounds to hydrocarbons may be achieved under acidic conditions e.g. the Clemmensen reduction with zinc and concentrated hydrochloric acid), basic conditions (e.g. the Wolff-Kishner reduction of a hydrazone with alkali) or neutral conditions (e.g. the catalytic reduction of thioketals with Raney nickel). The carbonyl group may represent the residue from an earlier step in the synthesis of a compound. 

Carbonyl compounds are reduced to symmetrical ethers, probably by way of reduction of some of the starting material to a silyl ether (9), reaction to form the mixed ketal (10) and then reductive replacement of the silyloxy group. Some hydrocarbon may be obtained as a by-product by reduction of (9 Scheme 4). Among the acid partners that have been used are trifluoroacetic acid, trityl perchlorate (with aldehydes) and electrogenerated protons. With Nafion resin symmetrical ethers are obtained from aldehydes, but silyl ethers are obtained from ketones. ... 

Electrolysis of carbonyl compounds provides pinacols, alcohols or hydrocarbons, depending on the conditions, such as pH, the nature of the electrode, and its potential. Fundamental studies have been carried out on the mechanisms of hydrocarbon formation using acetone as a substrate. Although several electrodes, such as Cd, Pt, Pb or Zn, are recommended, carbonyl compounds, including aryl and alkyl derivatives, require strong aqueous acidic media for reduction to the hydrocarbons. The mechanism of the electrolytic reduction is probably similar to that of Clemmensen reduction, which starts from anion radical formation by one-electron transfer, as indicated in Scheme 3. The difference is that electrolytic reduction takes place in an electric double layer, rather than on the surface of the zinc metal. 

The better defined participation of carbonyl radical anions is evident [56] in the electrocatalyzed reaction between aromatic carbonyl compounds (or other easily reduced carbonyl compounds) and dialkyl phosphonates (Scheme 18). In similar vein, and also in Scheme 18, radical anions generated from aromatic aldehydes may abstract a proton from an added acidic hydrocarbon such as fluorene (pXa22.6) or indene (pXa20.1), and the resulting carbanion adds to unreduced aldehyde. The chain reaction is propagated by protonation of the addition product by another molecule of hydrocarbon [57]. Reaction is by controlled potential coelectrolysis in THE, at the aldehyde reduction potential, and substantial yields are only obtained with 2,6-dichlorobenzaldehyde. 

In situ reduction of tosylhydrazones by NaBHjCN provides an efficient method for the deoxygenation of carbonyl compounds to furnish the corresponding hydrocarbons (see also Section 3.4). In the case of tosylhydrazones derived from a,P-unsaturated carbonyl compounds, the reduction leads to a stereoselective migration of the double bond to give the corresponding tran -alkene. 

Deoxygenation of carbonyl compounds. Hydrous Sn02 is prepared from SnCU by precipitation with aqueous ammonia then drying and calcination at 300°C for 5 h. The reduction of aliphatic carboxylic acids gives the corresponding alcohols, whereas aromatic acids are further reduced to the hydrocarbons. Aromatic ketones also give hydrocarbons. 

Complete removal of the oxygen from a carbonyl compound is possible via Clemmensen reduction or by Wolff-Kishner reduction. The functional group exchange for this process shows the hydrocarbon with a ketone or aldehyde precursor. 

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