Wolff-Kishner type reductive deoxygenation

Synthetic strategy Multi-component one-pot protocol for the Wolff-Kishner type reductive deoxygenation of acyl phosphonates... 

Keywords Carboxylic acids, acyl chlorides, dichloromethane, room temperature, Arbuzov reaction, acyl phosphonates, Wolff-Kishner type reductive deoxygenation, alkyl phosphonates... 

Kedrowski and Dougherty developed a one-pot, four-step room temperature protocol for the synthesis of allylphosphonates (6) from acyl chlorides/carboxylic acids (5) via a Wolff-Kishner-type reductive deoxygenation (Scheme 3). This method offers a low-temperature alternative to the Arbuzov reaction that works well for a variety of aliphatic acids and shows a functional group tolerance similar to that of other hydrazone-forming reactions. The investigators also proposed a mechanism for this transformation (Scheme 4). 

Scheme 3 Synthesis of alkylphosphonates from acyl chlorides/carboxylic acids via a Wolff-Kishner-type reductive deoxygenation.

Since its introduction early in this century, the deoxygenation of aldehydes and ketones to methyl or methylene derivatives, respectively, via base treatment of hydrazone intermediates (equation 1) has proven to be one of the most convenient and synthetically useful processes available for this important type of transformation. The reaction is termed the Wolff-Kishner reduction in recognition of the two original independent discoverers.However, the initial recipes introduced proved tedious and unreliable with many structural, especially hindered, examples. This led to substantial efforts devoted over the years to developing more convenient and successful experimental procedures, resulting in a number of improved and more reliable modifications which are most often utilized at present. More recently, modified procedures have been provided which utilize hydride reductions of p-toluenesulfonylhydrazone (to-sylhydrazone) derivatives and subsequent decomposition to release the hydrocarbon products under much milder and less basic conditions than those normally required for Wolff-Kishner reductions (equation 2). 

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