Lowering the Reduction Potential of a Boron Compound by Means of the Substituent Effect of the Boryl Group: One‐Electron Reduction of an Unsymmetrical Diborane(4)

Abstract

We have clarified and observed the high electron affinity of pinB-BMes2 (1; Mes = mesityl, pin = pinacolato). By using electrochemistry, it was shown that 1 has a higher electron affinity than those of B2pin2 and Mes3B. One-electron reduction of 1 gave the corresponding radical anion. The ESR spectroscopy and DFT calculation revealed the unsymmetrical distribution of electron density over the B-B bond. UV/Vis spectroscopy showed that the SOMO-related absorption supports the deep purple color of the radical anion. DFT studies on the torsion angle dependency of the LUMO levels and relative energies revealed the reason why 1 has high electron affinity as a result of the substituent effect of the Bpin group.

Reaction of B2(o‐tol)4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C−H Borylations

“…[7] We have recently reported am odification of B 2 pin 2 with two 2,4,6-Me 3 C 6 H 2 (Mes) groups to form the unsymmetric diborane(4), pinB À BMes 2 ,w hich exhibits ap inacolato and two mesityl groups. [12] Even though it can be expected that diboranes (4) without the stabilization from heteroatoms should exhibit higher reactivity than heteroatom-stabilized diboranes (4), examples for the reactivity of such diboranes(4) remain limited. [12] Even though it can be expected that diboranes (4) without the stabilization from heteroatoms should exhibit higher reactivity than heteroatom-stabilized diboranes (4), examples for the reactivity of such diboranes(4) remain limited.…”

“…Thec yclization of diboranyl ketone 5 results in the formation of af our-membered ring in 6.H owever,D FT calculations suggested ah igher activation energy for the formation of 5 in comparison with that for 10 and avery high activation energy (43.0 kcal mol À1 )for the subsequent migration of the o-tolyl group in 6 to furnish 7,which would be able to dissociate into boraalkene 8 and oxoborane 9. [22] Selected bond lengths []and angles [8 8]: B1-C1 1.558(2), B1-C8 1.613(2), B1-C21 1.582(2), C21-C16 1.507(2), C16-C15 1.395(2), C15-C8 1.524(2);C 1-B1-C8 123.65 (13), C1-B1-C21 129.39 (13), C8-B1-C21 106.88 (12), B1-C8-C15 102.92 (11),C 8-C15-C161 12.82 (13),C 15-C16-C21 112.67 (13), B1-C21-C16 104.51 (12). It should be noted that 11 can be considered as ac arbene that is stabilized by a p-donating oxygen atom and a p-accepting boron atom.…”

“…[8] Thea bsence of heteroatoms at one of the two boron atoms renders this molecule highly electrophilic.Infact, pinBÀBMes 2 reacts with tert-butylisocyanide leading to cleavage of the NCt riple bond, [8] with CO to provide aC O-coordinated boraalkene, [8] with alkynes to form diborylalkenes, [9] with 2,6-dimethylphenylisocyanide to furnish a1 ,2-oxaboretane ring, [10] with pyridine to give C À Hf unctionalized pyridines, [11] and with electrons to afford the corresponding radical anion. [12] Even though it can be expected that diboranes (4) without the stabilization from heteroatoms should exhibit higher reactivity than heteroatom-stabilized diboranes (4), examples for the reactivity of such diboranes(4) remain limited. [13] We have recently reported one such example,t hat is,t etra(otolyl)diborane (4) (1), which exhibits high electron affinity and reactivity toward H 2 (Scheme 1).…”

N‐Heterocyclic Carbene Stabilized Boryl Radicals

“…[2] To the best of our knowledge,h owever,t he tris(2,4,6-trimethylphenyl)boron radical anion is the only example that was structurally characterized in the solid state by X-ray diffraction analysis of its lithium salt [Li(12-crown-4) 2 ][BMes 3 ], [3] while significantly more molecular structures could be established for related diboron radical species. [4][5][6][7][8][9][10] Recently,n umerous neutral analogues of the type LBR 2 C have been reported, which can be formally regarded as five-electron boryl species BR 2 C stabilized by asuitable Lewis base (L). [11] N-Heterocyclic carbenes (NHC) proved particularly useful for stabilization of the boryl moiety, [12] and the acridinyl radical 1 can be regarded as the first example of as eries of structurally characterized (NHC)BR 2 C adducts ( Figure 1).…”

Konstruktion linearer und verzweigter Tetraborane durch 1,1‐ und 1,2‐Diborierung von Diborenen

“…Das Borylborylen 4 reagierte ebenfalls rasch mit B 2 cat 2 bei À30 8 8Cu nd lieferte ein ähnliches 11 [14] Dies kann dem Mangel an p-donierenden Substituenten an B1 zugeschrieben werden, da dieses Atom jetzt an drei trigonalen Boratomen gebunden ist. [23] Die B1-B2-Bindung von 10 ist mit 1.685(4) wesentlich kürzer als die von 7 aufgrund der sp 2 -Hybridisierung des B2-Atoms,w ährend der B1-P1-Abstand (…”