The conversion of carboxylic acids to aldehydes is normally conducted in two steps by reduction of the acids or their derivatives to the corresponding alcohols followed by mild oxidation.
Mechanism of the Fukuyama Reduction
Compared to other direct reductions of carboxylic acids or carboxylic acid derivates such as using DIBAL-H or Rosenmund conditions, the Fukuyama Reduction is a mild alternative, offering outstanding functional group tolerance (see recent literature).
An initial oxidative addition of Pd(0) to the C(sp2)-S bond is followed by transmetallation of the resultant acylpalladium species with Et3SiH. Reductive elimination from the acylpalladium hydride leads to the desired aldehyde.
On the basis of this mechanism, it was surmised that substitution of Et3SiH by an appropriate organometallic reagent would provide access to ketones. Extensive screening of various transition metal catalysts and organometallic reagents have revealed suitable conditions, which are currently used in the Fukuyama-Coupling.
For a review see: T. Fukuyma, H. Tokuyama, Aldrichimica Acta 2004, 37, 87.