Taming the Lewis Superacidity of Non‐Planar Boranes: C−H Bond Activation and Non‐Classical Binding Modes at Boron

Abstract

The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It enabled the formation of non-classical electron deficient BÀHÀB type of bonding, which was supported by spectroscopic and structural parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations, such as Csp 2 ÀH and Csp 3 ÀH activation under a frustrated Lewis pair regime and the cleavage of Csp 3 ÀSi bonds. The demonstration of such rich chemical behaviour and flexibility on a single molecular compound makes it a unique mediator of chemical transformations generally restricted to transition metals.

A Selenenium-Bridged 10-Boratriptycene Lewis Acid

“…such peculiar species in several challenging chemical transformations (Figure 1). 7 With a Lewis acidity further exceeding the previously reported 9-boratriptycene derivatives, the sulfonium derivative showed a propensity to activate several strong and weakly reactive bonds such as Csp 2 -H, Csp 3 -H, Csp 3 -Csp 3 , and Csp 3 -Si bonds. 7 In the context of the development of new pyramidal organoboranes and of the investigation of the structural features affecting the Lewis acidity of such pyramidal boron Lewis acids, we have been stimulated to synthesize a selenium derivative of 9-boratriptycene.…”

“…7 With a Lewis acidity further exceeding the previously reported 9-boratriptycene derivatives, the sulfonium derivative showed a propensity to activate several strong and weakly reactive bonds such as Csp 2 -H, Csp 3 -H, Csp 3 -Csp 3 , and Csp 3 -Si bonds. 7 In the context of the development of new pyramidal organoboranes and of the investigation of the structural features affecting the Lewis acidity of such pyramidal boron Lewis acids, we have been stimulated to synthesize a selenium derivative of 9-boratriptycene. With similar electronegativity as sulfur but with a 35% longer atomic radius leading to longer C-Se than C-S bonds, the replacement of a sulfur bridgehead triptycene with a selenium one will lead to decreased pyramidalization at the boron atom.…”

“…In the solid state, a B-O bond of 1.541(3) Å is calculated, slightly longer than the sulfonium derivative (1.537( 9) Å) although shorter than the phosphonium derivative (1.548(3) Å). 6b,7 Scheme 3 Formation of the Lewis adducts 11, 12, and 13 via protodeborylation of 9 and molecular structures of 11 and 12, counteranion and hydrogen atoms have been omitted for clarity IR spectroscopy showed that the CN stretching vibration of 12 (2373 cm -1 ) is slightly higher to that of 3 (2345 cm -1 ), 7 whereas the C=O of 11 (1592 cm -1 ) is similar to that of 2-EtOAc (1580 cm -1 ), 6b suggesting similar Lewis acidic properties. A closer look at the carboxyl moiety of the coordinat-ed ethyl acetate moiety revealed that the C=O bond is elongated (1.263(3) Å) compared to free ethyl acetate (1.203 Å) [10][11][12] while the carbonyl C-O bond (1.288(3) Å) is considerably shortened (1.345 Å).…”

“…13 C NMR (126 MHz, CDCl 3 ):  = 143.7 (Cq), 137.2 (Cq), 136. 7,133.7 (Cq),129.6,128.3,127.6,124.1,27.1 (CH 3 ), 21.1 (CH 3 ). Under glovebox conditions, triflimidic acid (14 mg, 0.050 mmol, 1.1 equiv) was added to a suspension of 10-mesityl-9-selenenium-10-boratriptycene-ate complex 9 (20 mg, 0.046 mmol, 1.0 equiv) in toluene (2.0 mL) at r.t.…”

“…The synthesis (Scheme 1) started with the reaction of 2-bromophenyl Grignard reagent with selenium oxychloride followed by reduction of the formed selenoxide 6 with PCl 3 to produce 7. The triarylborane 8 was prepared from bis(2-bromophenyl) selenide (7) with reduced yield compared to the sulfur derivative. The borane then undergoes a [4+2] cycloaddition with in situ generated benzyne to form the triptycene scaffold, providing the mesityl-boron-ate complex 9 in good yield.…”