Easy Access to Phosphine‐Borane Building Blocks

In this paper, we highlight the synthesis of a variety of primary phosphine‐boranes (RPH2⋅BH3) from the corresponding dichlorophosphines, simply by using Li[BH4] as reductant and provider of the BH3 protecting group. The method offers facile access not only to alkyl‐ and arylphosphine‐boranes, but also to aminophosphine‐boranes (R2NPH2⋅BH3) that are convenient building blocks but without the protecting BH3 moiety thermally labile and notoriously difficult to handle. The borane‐protected primary phosphines can be doubly deprotonated using n‐butyllithium to provide soluble phosphanediides Li2[RP⋅BH3] of which the phenyl‐derivative Li2[PhP⋅BH3] was structurally characterized in the solid state. A range of differently substituted primary phosphine‐boranes (RPH2⋅BH3) are now accessible that as unprotected phosphines difficult to handle or even unstable. In particular, the synthesis of the aminophosphane‐boranes (R2NPH2⋅BH3) offers new opportunities to use as versatile building blocks in the syntheses of organophosphorus compounds.