Reduction of a dihydroboryl cation to a boryl anion and its air-stable, neutral hydroboryl radical through hydrogen shuttling

Abstract

A doubly cyclic (alkyl)(amino)carbene-stabilised dihydroboronium cation undergoes fully reversible 2e− reduction to a stable hydroboryl anion via B-to-C hydrogen migration. Subsequent 1e− oxidation yields a bench-stable neutral hydroboryl radical.

Probing the Boundaries between Lewis‐Basic and Redox Behavior of a Parent Borylene

“…Given that the 11 B NMR shift of Lewis base adducts of boranes is dependent on the overall electron‐donor strength of the Lewis base, a comparison with the 11 B NMR shifts of literature‐known donor complexes of BCl 3 (Figure 4) [16] shows that borylene 1 is a comparatively weak Lewis base, similar to dimethyl ether and dimethyl sulfide. The 31 P NMR shift of 1‐BCl 3 at −10.9 ppm is significantly downfield‐shifted from that of 1 (δ 11B =−25.4 ppm), and comparable to that of the [(CAAC)(PMe 3 )BH 2 ] + cation, [1‐H] + (δ 31P =−10.6 ppm) [17] . In the 1 H{ 11 B} NMR spectrum the B H resonance appears at 1.80 ppm as a broad doublet coupling to the neighboring phosphorus nucleus ( 2 J 1H‐31P =12.3 Hz), while the CAAC ligand resonances are all split due to the presence of the chiral borylene center and the hindered rotation around the B−C CAAC bond.…”

“…While the 1 : 1 reactions with the heavier group 13 trichlorides, GaCl 3 and InCl 3 , also resulted in the formation of the corresponding borylene‐gallane and ‐indane adducts ( 1‐GaCl 3 : δ 11B =−25.2 ppm; 1‐InCl 3 : δ 11B =−24.6 ppm), these proved less selective (Scheme 2b) [19] . In both cases the NMR spectra of the reaction mixture showed the formation of varying amounts of the known [1‐H] + cation [17,20] . For the InCl 3 ‐based reaction single‐crystal X‐ray diffraction analysis confirmed the formation of the by‐product [1‐H][In 2 Cl 6 ] 0.5 (see Figure S41 in the Supporting Information) [21] .…”

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

“…Thankfully, recently radical chemistry of boron has seen tremendous developments and several stable boron-based cAAC radicals and radical cations have been stabilized (Scheme 65 and 66). [15,17,118,121,136,[282][283][284] cAACs have been increasingly used as neutral donor atoms in the stabilization of the main group, transition metal, and organic radical species. However, neutral boron-based triplet diradical species were unknown until Braunschweig et al isolated such species featuring two cAAC stabilized boron centers (209,210,211).…”

Can a Wanzlick-like equilibrium exist between dicoordinate borylenes and diborenes?

“…However, despite the fact that dicoordinate borylenes [LRB:] and their diborene 14 cousins [LRB BRL] have now been both isolated and inferred from reactivity, no evidence for a bora-Wanzlick equilibrium has been presented. It is also important to note that the reduction of base-stabilised dihaloorganyl boranes [LBRX 2 ] most often leads to the diborene [LRB BRL] when the L unit is a poor π-acceptor, but in contrast, boryl radicals, 15 borylenes, or intramolecular C–C/C–H activation products 16 are formed when the L unit is a strong π-acceptor. The elucidation of the underlying reasons is one of the topics of the present work.…”