A Stable but Highly Reactive Phosphine‐Coordinated Borenium: Metal‐free Dihydrogen Activation and Alkyne 1,2‐Carboboration

Abstract

Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H2 in the presence of PtBu3 (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H2 in the absence of PtBu3 has been investigated using deuterium-labeling experiments and DFT calculations. Both experiments and calculations imply the side-on coordination of H2 to the B center, followed by heterolytic splitting and B-C bond cleavage. An uncommon syn 1,2-carboboration has also been observed upon reaction of the borenium ion with 3-hexyne.

Mono- and Polynuclear Boron Dications

“…The structure of [36] 3+• determined by X-ray crystallography contains a B 4 core in rhombic arrangement reminiscent of that observed in [34] 2+ . Nevertheless, structural disparities between [36] 3+ , two opposite sides of the B 4 rhombus are bridged by two hpp ligands. In contrast, each adjacent pair of boron atoms of the B 4 core is linked by a hpp ligand in [36] 3+• .…”

“…68 Accordingly, [36] 3+• features a relatively intense EPR signal at g = 2.006. The structure of [36] 3+• determined by X-ray crystallography contains a B 4 core in rhombic arrangement reminiscent of that observed in [34] 2+ . Nevertheless, structural disparities between [36] 3+ , two opposite sides of the B 4 rhombus are bridged by two hpp ligands.…”

“…Bourissou et al reported the synthesis and H 2 activation of the naphthyl spacer-supported borenium cations ( [11] + ) that are stabilized by the intramolecular R 3 P¼B interaction. 36 Recently, the same group investigated the protonolysis reaction of [11a][NTf 2 ] with Tf 2 NH to afford the triflimide-coordinated neutral species (Scheme 2). The subsequent replacement of the two coordinating triflimide anions with Et 3 P=O results in the isolation of boron dication [12][NTf 2 ] 2 .…”

“…Nevertheless, structural disparities between [36] 3+ , two opposite sides of the B 4 rhombus are bridged by two hpp ligands. In contrast, each adjacent pair of boron atoms of the B 4 core is linked by a hpp ligand in [36] 3+• . Moreover, all measured BB distances in [36] 3+• , particularly that between the transannular position of 1.7987(3) ¡, are significantly longer than those in [34] 2+ (1.703(4) ¡).…”

“…In contrast, each adjacent pair of boron atoms of the B 4 core is linked by a hpp ligand in [36] 3+• . Moreover, all measured BB distances in [36] 3+• , particularly that between the transannular position of 1.7987(3) ¡, are significantly longer than those in [34] 2+ (1.703(4) ¡). These structural variations are presumably due to the difference in the number of the skeletal electrons shared by the central four boron atoms.…”