Charge-Separated Anion Pairs: The Rationale, the Application in Selective Activation of Trimethoxysilane and Controlled Conversion of Carboxylic Acids to Aldehydes

Direct conversion of both aromatic and aliphatic carboxylic acids to the corresponding aldehydes is realized. Using carboxylate bearing bulky ammonium counter cations such as tetrabutylammonium (TBA) acetate as the activating reagent to activate trimethoxysilane, aromatic carboxylic acids can be converted to the corresponding aldehydes in good yield and excellent chemoselectivity. Using tetrafluoroborate bearing bulky ammonium counter cations such as TBABF4 as the activating reagent and pyridine derivative such as 1-(pyridine-4-yl)piperidine (PPDP) as an additive, conversion of aliphatic carboxylic acids to the corresponding aldehydes also proceeds with good yields and excellent chemoselectivity. The nucleophilicity of the carboxylate or tetrafluoroborate anions can be regulated by changing the size of the counter cation, and the interaction of such anions with hydrosilane leads to the controlled activation of the latter. In such a way, the progress of the reactions can be controlled to ensure the chemoselective conversion of carboxylic acids to the corresponding aldehydes. In addition to excellent functional group tolerance, the method is easy to operate; requires only easily available reagents; and is suitable for aromatic, α,β-unsaturated, and aliphatic carboxylic acids. Preliminary NMR experiments suggest that reaction of aromatic acids with the activated hydrosilane gives the corresponding silyl esters that can be reduced to the desired aldehydes by the activated hydrosilane.