Single-Step Synthesis of Secondary Phosphine Oxides

We report that in the presence of trifluoroacetic acid primary phosphines undergo efficient addition to aldehydes to form the corresponding secondary phosphine oxides in 47-97% yield. This transformation is compatible with aryl and alkyl phosphines, as well as a broad range of aldehydes, including formaldehyde. By using 1,5-dialdehydes as reaction partners, the addition provides a straightforward route to bis(phosphine oxides), which are difficult to prepare by alternative methods. In the presence of boron trifluoride diethyl etherate as reagent, benzophenone was shown to couple to phenylphosphine and cyclohexylphosphine in 92% and 72% yield, respectively. Twenty-three examples are presented.