Controlling Regiochemistry in the Syntheses of Boraindanes from Diborane(4) Starting Materials

Abstract

The reaction of tert-butylisonitrile (tBuNC) with 1,2-dihalo-1,2-diduryldiborane leads initially to the formation of the mono-base adduct of the symmetrical diborane(4), which then undergoes an intramolecular cyclization resulting in the formation of a 1-boraindane. This result is in contrast to a previously reported cyclization of a mono-isonitrile adduct of an unsymmetrical 1,1-pinacol-2,2-diaryldiborane(4), which results in the formation of a 1-boraindane. This latter result is herein confirmed by the reaction of 1,1-difluoro-2,2-dimesityldiborane(4) with tBuNC, which yielded the 2-boraindane compound. The mechanism of the former reaction has been computationally elucidated, and the differences between this route and the pathway to 1-boraindanes is discussed. These reactions further the understanding of the chemistry of the increasingly popular mono-base adducts of diborane(4), demonstrate the versatility of isonitriles in comparison to standard two-electron donors, and elucidate selective routes to boron-containing polycyclics, such as those being proposed as analogues for conventional organic pharmaceuticals.

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

“…The B−X b bonds are elongated by approximately 13 % in [B 2 Cl 3 (IMes) 2 ]BCl 4 and 7 % in [B 2 I 3 (CAAC Me ) 2 ]I compared with the B−X t bonds (Table ). Unlike the previously reported unsymmetrically and neutral inverted halide‐bridged diborane mono‐adducts, the presence of the two Lewis bases in the ionic [B 2 X 3 L 2 ]A species generates a pseudo ‐ C 2 v symmetry around the axis passing through the bridging halide and the center of the B−B bond, leading to two relatively similar B−X b bond lengths (B−Cl b 2.033(2), 2.078(3) Å; B−I b 2.400(5), 2.422(5) Å) and B‐B‐X b angles (B‐B‐Cl b 64.66(12), 67.51(12)°; B‐B‐I b 69.0(3), 70.3(3)°) in each case, suggesting that the electrons are equally shared between the two B−X b bonds. It is noteworthy that these ionic species are isoelectronic and isostructural to cyclic organic halonium species, which act as intermediates in the halogenation of olefins .…”

“…Although hydrodiboranes are well‐known to form μ 2 ‐hydride‐bridged structures displaying three‐center‐two‐electron bonding, examples of μ 2 ‐halogen‐bridged diboranes, in which a lone pair of the bridging halide acts as a donor to the second boron atom in a three‐center‐four‐electron bonding mode, are much rarer. To date only a handful of such structures, albeit of neutral, mono(base)‐stabilized dihalodiboranes, have been reported: a series of μ 2 ‐halide‐bridged mono‐phosphine adducts of B 2 X 2 Mes 2 (X=Cl, Br, I; Mes=2,4,6‐Me 3 C 6 H 2 ), a series of μ 2 ‐chloro‐bridged mono‐pyridine adducts of B 2 Cl 2 Ar 2 (Ar=Mes, Dur=2,3,5,6‐Me 4 C 6 H) and a μ 2 ‐iodo‐bridged 1,2,3‐azadiborolidine . Similarly to these species, the two boron, terminal halogen, and carbene carbon atoms in [B 2 Cl 3 (IMes) 2 ]BCl 4 and [B 2 I 3 (CAAC Me ) 2 ]I (Figure ) are quasi‐coplanar, with a maximum deviation from the mean B 2 (X t ) 2 C 2 plane (X t =terminal halide) of 0.15 and 0.24 Å, respectively.…”

Synthesis of Complex Boron–Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions

“…Herein we present the reactivity of four diaryl­(dihalo)­diboranes(4) with phenyl and/or trimethylsilyl azide. These diboranes(4) were chosen as substrates as our previous work has shown them to be quite reactive and often provide structurally interesting products, most likely due to the combination of labile, accessible B–X bonds and steric stabilization from the adjacent aryl substituents. − We demonstrate that these reactions yield unprecedented boron-substituted triazenes, found as exotic five- and six-membered rings and resulting from unexpected 1,4-aryl-migrations, and tetrazaboroles, resulting from cycloaddition reactions on novel boryliminoboranes.…”

Effects of the Substituents of Boron Atoms on Conjugated Polymers Containing B←N Units

“…The boron–nitrogen coordination bond (B←N) is one typical kind of Lewis acid/base pair . The intermolecular B←N Lewis pair has been employed to modulate molecular orbitals and thereby to tune absorption and fluorescence spectra of conjugated molecules and polymers . Our group has recently discovered that intramolecular B←N Lewis pair formation can significantly downshift LUMO/HOMO energy levels ( E LUMO / E HOMO ) of conjugated polymers .…”

“…N Lewis pair has been employed to modulate molecular orbitals and thereby to tune absorption and fluorescences pectra of conjugated molecules and polymers. [28][29][30][31][32][33][34][35][36][37] Our group has recently discovered that intramolecular B ! NL ewis pair formation can significantly downshift LUMO/HOMO energy levels (E LUMO /E HOMO )o fc onjugated polymers.…”