Kinetic stabilization allows structural analysis of a benzoborirene

Abstract

The successful synthesis and first structural characterization by single crystal X-ray crystallography of a highly strained benzoborirene motif, as well as its reactivity towards selected electrophiles and nucleophiles are reported.

Boosting Ring Strain and Lewis Acidity of Borirane: Synthesis, Reactivity and Density Functional Theory Studies of an Uncoordinated Arylborirane Fused to o‐Carborane

“…This unexpected result can be conveniently related to the fact that π delocalization to the boron center is 12), C4À C3À B1À C2 4.38 (12), C6À C5À C4À C3 84.75 (16). 19), P1À C2 1.8683 (15), O1À P1 2.0532 (10), B1À O1 1.3096(18), B1À C3 1.594(2), C1À B1À C3 122.09 (12), C1À B1À O1 111.80 (12), C3À B1À O1 126.04 (13).…”

“…Indeed, this unique reactivity pattern represents a striking example of dearomative 1,4-insertion of ArÀ (C=O)À into a CÀ B bond, especially when considering that CÀ B has a higher average σ-bond enthalpy than CÀ C bond (D 0 (kJ/mol) CÀ B 365 vs. CÀ C 358). The relevance of the boosted Lewis acidity and ring strain is further reflected by the fact that no reaction was observed between the benzoborirene [C 6 H 4 {μ-BTrip}] [10] (Trip = 2,4,6-i Pr 3 C 6 H 2 ), a 2D analogous compound of 1 that is less Lewis acidic and less strained, and benzophenone even at 60 °C for hours.…”

“…Inspired by the recent success in applying bulky terphenyl substituent for the isolation and structural characterization of a benzoborirene, [10] we set out to synthesize a three-dimensional (3D) analogue following the 2D/3D relationship [11] between benzene and carborane. As demonstrated in this work, this 3D analogue represents, in comparison to the conventional boriranes, a rare example with boosted Lewis acidity and boosted ring strain in one.…”

“…Thermal ellipsoids are drawn at the 50 % probability level. Selected bond lengths (Å) and angles (deg): B1À C1 1.616(2), C1À C2 1.6664(19), P1À C2 1.8683(15), O1À P1 2.0532(10), B1À O1 1.3096(18), B1À C3 1.594(2), C1À B1À C3 122.09(12), C1À B1À O1 111.80(12), C3À B1À O1 126.04(13).…”

Accessing unusual heterocycles: ring expansion of benzoborirenes by formal cycloaddition reactions

“…26,27 We recently described the synthesis of the kinetically stabilized benzoborirene 1 and provided the rst single crystal X-ray structure. 28 The sterically demanding Trip 2 C 6 H 3 (Trip = 2,4,6-iso-Pr 3 C 6 H 2 ) substituent provides considerable stabilization to the boron centre in 1 as evidenced by high thermal stability as well as inertness towards a number of weak nucleophiles (acetonitrile, 2,6-lutidine, and 2-picoline) and weak electrophiles (trimethylchlorosilane and benzophenone). 28 So far, only methanolysis and formation of a Lewis acid-base complex with pyridine have been observed for 1, 28 while the other two known benzoborirenes react with the stronger Lewis base I i Pr Me2 by opening the three-membered ring (Scheme 1a).…”