An unusual synthesis of 2,3-dihydrophosphindole 1-oxides

A very simple route to dimeric phosphole oxides is reported as is the pyrolytic decomposition of these phosphorus-bridged dimers. Like similar dimeric phosphole sulfides, these phosphole oxide dimers readily eliminate the bridging phosphorus grouping upon strong heating but, unlike the analogous sulfides, hydrogen transfer to give a 2,3-dihydrophosphindole 1-oxide rather than hydrogen elimination to give the fully unsaturated phosphindole oxide occurs. This hydrogen transfer is shown to be non-stereospecific and some mechanistic implications of this are briefly discussed.