B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO2, Isocyanates, or Carbodiimides. Issue 24 (7th May 2021)

• Record Type: Journal Article
• Authors: Trageser, Timo; Bebej, Dariusz; Bolte, Michael; Lerner, Hans‐Wolfram; Wagner, Matthias
• Is Part Of: Angewandte Chemie international edition. Volume 60:Issue 24(2021)
• Page Start: 13500
• Page End: 13506
• Publication Date: 2021-05-07
• Subjects: B−B bonds -- B−H bonds -- bond activation -- cycloaddition -- diboranes; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.202103427
• Languages: English
• ISSNs: 1433-7851

Abstract

Abstract: The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B2 H5 ] − have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1 H] − in cycloaddition reactions with the homologous series of heterocumulenes CO2, i PrNCO, and i PrNCN i Pr. We show that a C=O bond of CO2 selectively activates the B−B bond of [1 H] −, while the μ‐H ligand is left untouched ([2 H] − ). The carbodiimide i PrNCN i Pr, 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 i PrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4 H] − ) is related to [2 H] −, 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. Abstract : 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 [B2 H5 ] − anion analogs.