Reversible Photothermal Isomerization of Carborane‐Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene‐Stabilized Carborane‐Fused Borirane

Abstract

Af ully reversible photothermal isomerization between carborane-fused trigonal-planar azaborole (darkpurple) and tetrahedral borirane (pale-yellow) has been observed, leading to the isolation and structural characterization of the first example of carborane-fused borirane.DFT calculations indicate that the azaborole is thermodynamically more stable than the borirane by 11.2 kcal mol À1 ,a nd the energy barrier for the thermal conversion from azaborole to borirane is 35.5 kcal mol À1 .The reactivity studies show that the BÀC(cage) bond in borirane can be broken in the reaction with CuCl, HCl, or elemental sulfur. Scheme 1. Photothermal transformations of four-coordinate organoboranes.Scheme 2. Photothermal isomerization between 1 and 2.

Access to cationic polyhedral carboranes via dynamic cage surgery with N-heterocyclic carbenes

“…Taking these considerations into account, one could expect that the addition of a strong σ-donor such as NHCs would increase the electron density on those traditionally taken electrondeficient skeletons and stabilise them like in the many cases of low-valent or cationic main-group elements, described specifically for boron compounds by Braunschweig [11][12][13] . Surprisingly enough, the number of studies dealing with the reactivity of polyhedral boranes and heteroboranes with NHCs is limited to 12-vertex closo-heteroboranes (o-carborane -1,2-dicarbadodecaborane(12) and 1-thiadodecaborane( 12)) [33][34][35] and bridged/ strained 13-vertex heteroboranes [36][37][38][39][40] . The reactions of o-carborane and Xie's 13-vertex dicarbaboranes mostly lead to deprotonated or deboronated species with one less vertex; in the reactions of thiaborane, however, also cage distortion and the rearrangement of one hydrogen atom from terminal to bridging position have been observed.…”