Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

Abstract

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to as eries of 9-boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopyand X-raydiffraction of the Lewis adducts of these non-planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of at rivalent boron compounds with the Tf 2 N À anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using ac ationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity.I ntroduction of ap hosphorus and ab oron atom at each edge of at riptycene framework, allowed access to new bifunctional Lewis acid-base 9-phospha-10-boratriptycenes featuring promising reactivity for the activation of carbon-halogen bonds.

Non-planar Boron Lewis Acids Taking the Next Step: Development of Tunable Lewis Acids, Lewis Superacids and Bifunctional Catalysts

“…Considering that the boron pyramidalization was a key parameter, conferring to 9-boratriptycene a very high Lewis acidity, we then reasoned that switching the atom at the second bridged position of the triptycene should have a significant impact on the boron atom ground-state properties and thus deeply affect its Lewis acidic properties. 23 Accordingly, a series of 9-phosphonium-10-boratriptycene-atecomplexes was prepared by following the seminal method of Sawamura (Scheme 6). 24 A. Chardon et al…”

“…Remarkably, the halide anion attached to the boron atom in 44-48 originated from the reaction solvent, indicating that the intermediately generated phosphonium boratriptycenes 43 abstracted a halide ion from the reaction solvent CH 2 X 2 . 23 The Lewis acidity of 43 was quantified accurately by computing its fluoride and hydride ion affinities and comparing them with those of other cationic Lewis acids (Figure 8). 25 The fluoride ion affinity (FIA) of the catechol borenium ion 53 26 substantially surpassed that of 43 and 52, showing that the highly pyramidalized boron atom in addi-…”

Controlled Generation of 9‐Boratriptycene by Lewis Adduct Dissociation: Accessing a Non‐Planar Triarylborane

“…In an effort to access 9‐boratriptycene derivatives, we recently developed a method to generate the strongly pyramidalized 9‐bora‐10‐phosphatriptycene 8 as a transient Lewis acid with exceptionally high Lewis acidity (Scheme a) . Though the protodeboronation of 10 proceeded selectively at the exocyclic C−B bond (Scheme a), disappointingly, protodeboronation of the 9‐phenyl‐boratriptycene ate complex 13 lacking a phosphonium bridge occurred at an intracyclic C−B bond, thus preventing the formation of 9‐boratriptycene 9 (Scheme b) …”

Pincer‐Type Ligand‐Assisted Catalysis and Small‐Molecule Activation by non‐VSEPR Main‐Group Compounds

“…For example, constraining molecular planarity using tethers leads to a reduced Lewis acidity of boron (31 a-b; Scheme 36a), whereas incorporating boron into cage-like molecules gave enhanced Lewis acidity (31 e; Scheme 36a). [49] The increased Lewis acidity of boron in 31 e is because of the preorganized pyramidal geometry, which not only provides a vacant sp 3 orbital directed ideally to accept the fourth electron-donating ligand but also allow geometry reorganization with minimal energy barrier (Scheme 36b). Therefore, there have been several efforts to isolate non-VSEPR structures of group-13 compounds for exploring the structurereactivity relationships.…”