In an earlier blog the catalytic Wittig was introduced as an interesting way to remove the presence of a large amount of triphenylphosphine oxide (TPPO) waste-product from a regular Wittig reaction. In a novel approach to the same problem (increasing atom efficiency) Jun-Jie Cao et al. (DOI) use TPPO as a catalyst for a secondary reaction. The second part in this tandem reaction is a conjugate reduction with reagent trichlorosilane.
The investigators agree that as TPPO is present in an stoichiometric amount , the label catalyst is dubious but on the other hand the reduction will not take place at all without the oxide. They are less clear about the possibility of reduction of TPPO back to the phosphine by the silane which is a well known reaction. And the atom efficiency does not really improve: the value for the Wittig reaction alone is about 45% and due to the introduction of two equivalents of silane the tandem total (despite the free catalyst) drops to 35%.