Carboxylic Acids and Their Derivatives

Reducing Carboxylic Acids to Aldehydes

We have seen that carboxylic acids can be reduced with LiAlH4 and boranes to primary alcohols. We also know that esters, nitriles, and acid chlorides can be reduced to aldehydes using DIBAL or at lowered temperatures (typically -78 oC):

 

 

Now, as there are no textbook methods for the direct reduction of carboxylic acids to aldehydes, you will need to use a workaround for these types of questions. You can reduce the acid to an alcohol and oxidize it with a mild oxidizing agent such as PCC, PDC, or DMP. Alternatively, you can convert the acid to an ester or an acid chloride and reduce those to an aldehyde:

 

 

Direct Reduction of Carboxylic Acids to Aldehydes

These reactions are generally not covered in undergraduate organic chemistry courses, so ask your instructor before using them in exam questions.

One is the one-pot reduction of carboxylic acids to aldehydes using DIBAL and TMSCl (Tetrahedron Letters 39, 1998, 909-910).

 

 

TMSCl is trimethylsilyl chloride, and it is a common reagent for preparing TMS ether for the protection of alcohols. DCM is dichloromethane, and it is the solvent of the reaction. The mechanism of the reduction is likely similar to what we have seen in the reduction of regular esters with DIBAL.

It is not exactly a direct conversion of the acid to an aldehyde, as it is first converted into a silyl ester. However, the ester is formed in situ, and the substrate is the carboxylic acid itself.

There are also Nickel-catalyzed reactions for the direct conversion of carboxylic acids to aldehydes. Here are the references of a couple of those that you can check out for further reading:

Org. Lett. 2019, 21, 19, 7804–7808,  ACS Catal. 2022, 12, 11, 6313–6324

 

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