Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

• Published in: Angewandte Chemie International Edition Vol. 57; no. 40; pp. 13293 - 13297
• Main Authors: Fasano, Valerio, Cid, Jessica, Procter, Richard J., Ross, Emily, Ingleson, Michael J.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.10.2018; John Wiley and Sons Inc
• Edition: International ed. in English
• Subjects: 1,2-migration; Activation; Alkanes; Anions; Aromatic compounds; boranes; Boron; borylation; Communication; Communications; Diborane; Grignard reagents; Lewis acids; Reagents
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201808216

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B2Pin2)]− using “soft” BR3 electrophiles (BR3=BPh3 or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR3‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B2Pin2 and BR3 and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B2Pin2, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}− transfer to BPh3, thus enabling simple access to unsymmetrical sp2−sp3 diboranes. Selective π fishing: Selective activation of the π system in a vinyl sp2−sp3 diborane with “soft” borane Lewis acids led to intramolecular 1,2‐boryl‐anion migration from a boron atom to a carbon atom (see scheme) as opposed to oxygen coordination and B−O activation. This borane‐induced 1,2‐boryl‐anion migration enabled the triple borylation of a vinyl Grignard reagent to form differentially protected 1,1,2‐triborylated alkanes.