N–H deprotonation of a diaminodialkoxido diborane(4) – a structural study on bifunctional Lewis acids/bases and their dimerisation to B(sp2)2B(sp3)2N2 six membered rings

The N–H deprotonation of the diaminodialkoxido diborane(4) pinB–Bdab (1) (pin: (OCMe2)2, dab: 1,2-(NH)2C6H4), is crucial for the electrophilic N-functionalisation towards unsymmetrical diborane(4) reagents. An N–H deprotonated diborane(4) comprises Lewis basic nitrogen atoms and at the same time Lewis acidic boron atoms. This bifunctionality governs its reactivity and structural chemistry. Whilst bases such as Na(hmds), tBuLi or Li(tmp) readily effect a single deprotonation of 1, the second deprotonation is less straightforward and cleanly only achieved with Li(tmp) as a strong but little nucleophilic base. The N–H deprotonated diborane(4) derivatives readily dimerise to give B(sp2)2B(sp3)2N2 six-membered ring Lewis base adducts. The structural chemistry of this class of compounds was studied in detail in the solid state by single crystal X-ray diffraction as well as in solution by NMR spectroscopy.