Synthesis and characterisation of the complete series of B–N analogues of triptycene

Abstract

The reaction between the bisborate Li2[o-C6H4(BH3)2] and 2 equivalents of an appropriate pyrazole derivative (Hpz(R)) in the presence of Me3SiCl yields o-phenylene-bridged pyrazaboles HB(μ-pz(R))2(μ-o-C6H4)BH (3a-3e; Hpz(R) = 4-iodopyrazole (3a), 4-(trimethylsilyl)pyrazole (3b), 3,5-dimethylpyrazole (3c), 3,5-di(tert-butyl)pyrazole (3d), 3,5-bis(trifluoromethyl)pyrazole (3e)). The synthesis approach thus provides access to uncharged B-N triptycenes bearing (i) functionalisable groups, (ii) electron-donating or -withdrawing substituents and (iii) pyrazole rings of varying steric demand. Treatment of p-R*C6H4BBr2 with the potassium tris(pyrazol-1-yl)borates K[HBpz3] or K[p-R*C6H4Bpz3] yields cationic pyrazolyl-bridged pyrazaboles [p-BrC6H4B(μ-pz)3BH]Br ([4a]Br) and [p-R*C6H4B(μ-pz)3Bp-C6H4R*]Br (R* = Br ([4b]Br), I ([4c]Br), SiMe3 ([4d]Br)), which can be regarded as full B-N analogues of triptycene. The B-H bonds of 3b and [4a]Br are unreactive towards tBuC[triple bond, length as m-dash]CH even at temperatures of 80 °C, thereby indicating an appreciable thermal stability of the corresponding B-N cage bonds. Most of the cage compounds are sufficiently inert towards water to allow quick aqueous workup. However, NMR spectroscopy in CD3OD solution reveals degradation of 3b or [4a]Br to the corresponding pyrazoles and o-C6H4(B(OCD3)2)2 or p-BrC6H4B(OCD3)2/B(OCD3)3. The diphenylated species [4b]Br is significantly more stable under the same measurement conditions; even after 76 d, most of the material degrades only to the stage of the syn/anti-pyrazaboles p-BrC6H4(CD3O)B(μ-pz)2B(OCD3)p-C6H4Br (11a/11b). A derivatisation of [4c]Br with nBu3SnC≡CtBu through Stille-type coupling reactions furnishes the alkynyl derivative [p-tBuC≡CC6H4B(μ-pz)3Bp-C6H4C≡CtBu]Br ([4e]Br). Larger B-N aggregates are also accessible: treatment of the tetrakisborate Li4[1,2,4,5-C6H2(BH3)4] with 4 equivalents of Hpz(R) in the presence of Me3SiCl leads to the corresponding B-N pentiptycenes (Hpz(R) = 3,5-bis(trifluoromethyl)pyrazole (14a), 4-(trimethylsilyl)pyrazole (14b), 3,5-dimethylpyrazole (14c), 3,5-di(tert-butyl)pyrazole (14d)).

How Boron Doping Shapes the Optoelectronic Properties of Canonical and Phenylene‐Containing Oligoacenes: A Combined Experimental and Theoretical Investigation

“…We therefore decided to tackle these issues by quantum chemical calculations. Moreover, we sought to confirm that DBI , a potentially ditopic Lewis acid, forms solely monoadducts with MeCN. Finally, reasons for the large difference in quantum yields of DBP and i DBP , as well as for the lack of emissivity of DBI were uncovered by theoretical means.…”

“…Yet, further trends in the electron affinities and Lewis acidities of DBA, DBP,a nd DBI cannots traightforwardly be explained by generally accepted qualitative concepts, such as the numbersa nd distributions of Clar's sextetso rt he relative sizes of the p-electron systems. We therefore decided to tackle these issues by quantum chemical calculations.M oreover,w es ought to confirm that DBI, ap otentially ditopic Lewis acid, [2,3] formss olely monoadducts with MeCN. Finally,r easons for the large differencei nq uantum yields of DBP and iDBP,a sw ell as for the lack of emissivity of DBI were uncovered by theoretical means.…”

“…Especially 9,10‐dihydro‐9,10‐diboraanthracenes ( 1 , Figure ), which provide two vicinal boron atoms embedded in a rigid, π‐conjugated cyclic scaffold, are currently receiving attention. As a result of their peculiar structure, compounds 1 act as ditopic Lewis acids that are able to bind ditopic Lewis bases in a chelating manner . Taking advantage of this reactivity, 1 ‐containing compounds have been employed to catalyze inverse electron‐demand Diels–Alder reactions or the dehydrogenation of ammonia‐borane .…”

“…[1] Especially 9,10-dihydro-9,10-diboraanthracenes (1, Figure 1), which provide two vicinal boron atomse mbedded in ar igid, p-conjugated cyclic scaffold, are currently receiving attention.A saresult of their peculiar structure, compounds 1 act as ditopic Lewis acids that are able to bind ditopicL ewis bases in ac helating manner. [2,3] Ta king advantage of this reactivity, 1-containing compounds have been employed to catalyze inverse electron-demand Diels-Alder reactions [4,5] or the dehydrogenation of ammonia-borane. [6] 9,10-Dihydro-9,10-diboraanthracenesa lso act as strong electron acceptors, and numerous derivatives undergo fully reversible, stepwise two-electron reduction, [7] which offersn ew opportunities fort he reductive activationo fH 2 [8] andt erminal alkynes.…”

C‐Halogenated 9,10‐Diboraanthracenes: How the Halogen Load and Distribution Influences Key Optoelectronic Properties

“…Despitet he recent progress in the field, comprehensive insight into key structure-property relationships of B-PAHs is still lacking, because the required access to entire series of system-atically modulatable B-PAH derivatives still imposes af ormidable challenge to the synthetic chemist. [4][5][6] Herein, we report an array of halogenated 9,10-dihydro-9,10-diboraanthracenes (DBAs; Figure 1), which differ in the nature (i.e., 1/5, 2/3/6, 4/7)o rt he numbersa nd positions of their halogen substituents( i.e., [5][6][7][8][9][10]. In all cases, compound I serves as the benchmark for comparison purposes.…”