One-pot transformation of cyclic phosphine oxides to phosphine-boranes by dimethyl sulfide-borane

Different types of cyclic phosphine oxides, such as tetrahydrophosphole oxide 1, phosphabicyclo[3.1.0]hexane 3-oxide 8 and phosphabicyclo[2.2.1]heptene 7-oxides 10 and 12 were efficiently converted to phosphine-boranes 2, 9, 11 and 13, respectively, under relatively mild conditions by reaction with 4.4 equivalents of dimethyl sulfide-borane. The more strained hetero-ring the starting phosphine oxide (in general 16) has, the easier to accomplish the change in the P-function, that takes place through the corresponding phosphine intermediate (20). It is noteworthy that the imide carbonyl groups in starting materials 10 and 12 were fully reduced by the borane to give 11 and 13 respectively.