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B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO2, Isocyanates, or Carbodiimides
Ist Teil von
Angewandte Chemie (International ed.), 2021-06, Vol.60 (24), p.13500-13506
Auflage
International ed. in English
Ort / Verlag
Weinheim: Wiley Subscription Services, Inc
Erscheinungsjahr
2021
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B2H5]− have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1H]− in cycloaddition reactions with the homologous series of heterocumulenes CO2, iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO2 selectively activates the B−B bond of [1H]−, while the μ‐H ligand is left untouched ([2H]−). The carbodiimide iPrNCNiPr, in contrast, neglects the B−B bond and rather adds the B‐bonded H− ion to its central C atom to generate a formamidinate bridge across the B2 pair ([3]−). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H]−) is related to [2H]−, the other ([5]−) is an analog of [3]−. We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.
Unprecedented cycloaddition reactions of two isomeric (hydrido)diborane(4) anions with a homologous heterocumulene series proceed via B–B or B–H bond activation. Our diborane starting materials exhibit sterically exposed BHB cores, allowing us to contribute experimental evidence to the body of theoretical work on this class of [B2H5]− anion analogs.