The Synthesis of B2(SIDip)2 and its Reactivity Between the Diboracumulenic and Diborynic Extremes

• Published in: Angewandte Chemie International Edition 2015-11, Vol.54 (46), p.13801-13805
• Main Authors: Böhnke, Julian, Braunschweig, Holger, Dellermann, Theresa, Ewing, William C., Hammond, Kai, Jimenez-Halla, J. Oscar C., Kramer, Thomas, Mies, Jan
• Format: Article
• Language: English
• Subjects: boron; carbenes; Ligands; main-group chemistry; multiple bonds; thermochemistry
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201506368

A new compound with the formula L‐B2‐L wherein the stabilizing ligand (L) is 1,3‐bis[diisopropylphenyl]‐4,5‐dihydroimidazol‐2‐ylidene (SIDip) has been synthesized, isolated, and characterized. The π‐acidity of the SIDip ligand, intermediate between the relatively non‐acidic IDip (1,3‐bis[diisopropylphenyl]imidazol‐2‐ylidene) ligand and the much more highly acidic CAAC (1‐[2,6‐diisopropylphenyl]‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) ligand, gives rise to a compound with spectroscopic, electrochemical, and structural properties between those of L‐B2‐L compounds stabilized by CAAC and IDip. Reactions of all three L‐B2‐L compounds with CO demonstrate the differences caused by their respective ligands, as the π‐acidities of the CAAC and SIDip carbenes enabled the isolation of bis(boraketene) compounds (L(OC)B‐B(CO)L), which could not be isolated from reactions with B2(IDip)2. However, only B2(IDip)2 and B2(SIDip)2 could be converted into bicyclic bis(boralactone) compounds. A happy middle ground: The synthesis and characterization of B2(SIDip)2 shows the capacity of carbene ligands to control the chemistry in which they are involved. The SIDip carbene, having a π‐acidity lying between those of the commonly used CAAC and IDip carbenes, allows the syntheses of compounds with structures and reactivities intermediate between those found in compounds constructed of CAAC or IDip.