Addition of boranes to iminophosphines: Synthesis and reactivity of a new bulky hydroboration reagent

Two sterically diverse iminophosphines have been prepared from the addition of 2-Ph2PC(O)H and 4-H2NC6H4OMe (L1) and 2,6-H2NC6H4(i-Pr)2 (L2), and the addition of boranes to these substrates has been examined in an effort to reduce the imine CN bond. Reactions using borane-dimethylsulfide with the smaller imine L1 afforded adducts arising from initial coordination to the phosphorus atom and then the imine nitrogen. Addition of excess borane-dimethylsulfide eventually led to reduction of the imine and generated an active H2BNRR′ unit, which went on to make several aminophosphine products. The sterically hindered imine L2 was used to retard the reactivity in the resulting aminoborane. Hydroboration studies with a new borane (7) showed good selectivity in reactions with 1-octene and vinyl arenes. Reactions of iminophosphines with alkyl and dialkylborane also gave adducts, making reduction of the imine increasingly difficult. Addition of the dialkoxyboranes, catecholborane and pinacolborane, were promising for the less hindered imine, but harsh reaction conditions (metal catalysts, microwave reactor and elevated temperatures and pressures) were required in reductions of the sterically-hindered imine, giving rise to complicated product distributions.

Graphical abstractThe addition of boranes to iminophosphines have been examined as a gentle method of reducing these species to the corresponding amines. A novel hydroboration reagent has been prepared and examined for its ability to add to unsaturated organics.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Addition of borane, alkyl- and dialkylboranes to iminophosphines gave initial adduct formation and then reduction. ► Reactions of dialkoxyboranes with sterically-accessible imine gave corresponding addition products. ► Addition of dialkoxyboranes with the bulky imine required harsh reaction conditions. ► A novel hydroborating reagent was eventually generated from a bulky imine.