Reduction to aldehydes

Selective reduction to aldehydes can also be achieved using... 

Acid chlorides, reduction to aldehydes, 53, 55 Acid chlorides, aromatic, diazoketones from, 53, 37... 

Complete reduction of acyl chlorides to primary alcohols is not nearly as important as the reduction to aldehydes since alcohols are readily obtained by reduction of more accessible compounds such as aldehydes, free carboxylic acids or their esters [83,968]. Because aldehydes are the primary products of the reduction of acyl chlorides strong reducing agents convert acyl chlorides directly to alcohols. 

Reduction of amides to aldehydes was accomplished mainly by complex hydrides. Not every amide is suitable for reduction to aldehyde. Good yields were obtained only with some tertiary amides and lithium aluminum hydride, lithium triethoxyaluminohydride or sodium bis 2-methoxyethoxy)aluminum hydride. The nature of the substituents on nitrogen plays a key role. Amides derived from aromatic amines such as JV-methylaniline [1103] and especially pyrrole, indole and carbazole were found most suitable for the preparation of aldehydes. By adding 0.25 mol of lithium aluminum hydride in ether to 1 mol of the amide in ethereal solution cooled to —10° to —15°, 37-60% yields of benzaldehyde were obtained from the benzoyl derivatives of the above heterocycles [1104] and 68% yield from N-methylbenzanilide [1103]. Similarly 4,4,4-trifluorobutanol was prepared in 83% yield by reduction of N-(4,4,4-trifluorobutanoyl)carbazole in ether at —10° [1105]. 

Nitriles of carboxylic acids are an important source of primary amines which are produced by many reducing agents. However, a few reagents effect partial reduction to aldehydes 285. ... 

Selective reduction to aldehydes can also be achieved using /V-methoxy-iV-methyla-mides.49 Lithium aluminum hydride and diisobutylaluminum hydride have both been used as the hydride donor. The partial reduction is believed to be the result of the stability of the initial reduction product. The /V-mcthoxy substituent permits a chelated structure which is... 

The use of acyl chlorides in Friedel-Craft acylations of benzene rings, as well as their reactions with organometallics and reductions to aldehydes, has been discussed in Section 15.2. 

Reduction to Aldehydes Reduction of carboxylic acids to aldehydes is difficult because aldehydes are more reactive than carboxylic acids toward most reducing agents. Almost any reagent that reduces acids to aldehydes also reduces aldehydes to primary alcohols. In Section 18-10, we saw that lithium tri-ferf-butoxyaluminum hydride, LiAlH(0-f-Bu)3 is a weaker reducing agent than lithium aluminum hydride. It reduces acid chlorides to aldehydes because acid chlorides are strongly activated toward nucleophilic addition of a hydride ion. Under these conditions, the aldehyde reduces more slowly and can be isolated. Therefore, reduction of an acid to an aldehyde is a two-step process Convert the acid to the acid chloride, then reduce using lithium tri-ferf-butoxyaluminum hydride. 

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