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

Abstract

A highly bent triarylborane, 9‐boratriptycene, was generated in solution by selective protodeboronation of the corresponding tetra‐aryl boron ate complex with the strong Brønsted acid HNTf2. The iptycene core confers enhanced Lewis acidity to 9‐boratriptycene, making it unique in terms of structure and reactivity. We studied the stereoelectronic properties of 9‐boratriptycene by quantifying its association with small N‐ and O‐centered Lewis bases, as well as with sterically hindered phosphines. The resultant Lewis adducts exhibited unique structural, spectroscopic, and photophysical properties. Beyond the high pyramidalization of the 9‐boratriptycene scaffold and its low reorganization energy upon Lewis base coordination, quantum chemical calculations revealed that the absence of π donation from the triptycene aryl rings to the boron vacant pz orbital is one of the main reasons for its high Lewis acidity.

Boron-Doped Polycyclic π-Electron Systems with an Antiaromatic Borole Substructure That Forms Photoresponsive B–P Lewis Adducts

“…Indeed, the B−P bond length in 7c• PT is comparable to that in a 9-boratriptycene•PPh 3 adduct [1.976(4) Å]. 38 A UV−vis titration experiment provided further insight into the associative behavior of 7 and PT (Figure S10). Upon addition of PT to a toluene solution of 7a, the absorption band associated with 7a at 506 nm gradually diminished, and a new absorption band at 415 nm emerged with isosbestic points at 364 and 430 nm.…”

A Selenenium-Bridged 10-Boratriptycene Lewis Acid

“…3 5 It has been reported that, by preventing p-π conjugation while lowering the reorganization energy with a pre-pyramidalized boron compound, the Lewis acidity of perfluorinated triarylboranes can be reached without the introduction of electron-withdrawing substituents. 6a Using the 9-boratriptycene 1 as reference, we demonstrated that switching from carbon to a cationic phosphorous bridgehead triptycene scaffold 2 leads to a dramatic increase in Lewis acidity, approaching and even surpassing triarylsilylium species. 6b Very recently, we reported the generation of the transient sulfonium-bridged analogue 3 of the 9-boratriptycene 1 and, for the first time, the high reactivity of such peculiar species in several challenging chemical transformations (Figure 1 ).…”

Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6‐Collidine with the 10‐Sulfonium‐9‐boratriptycene

“…Whereas the boron atom in the precursor boron ate‐complex 8 resonates at −8.7 ppm in the 11 B NMR spectrum (CDCl 3 ), the neutral boron atom in the collidine Lewis adduct 11 a gives a signal at −1.5 ppm, and that in the pyridine Lewis adduct 11b gives a signal at −1.3 ppm, in a typical range for other N−B Lewis adducts of 9‐boratriptycene ( e. g . in pyridine‐9‐boratriptycene Lewis adduct 7 with a chemical shift of δ=−1.6 ppm in 11 B NMR in CD 2 Cl 2 ) [11] . The 11 B NMR spectrum of 11 a also featured a typical sharp signal at −17.6 ppm for the tetravalent boron atom in the anion [B(C 6 F 5 ) 4 ] ‐ .…”