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 tBuCCH 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 nBu₃SnCCtBu through Stille-type coupling reactions furnishes the alkynyl derivative [p-tBuCCC₆H₄B(μ-pz)₃Bp-C₆H₄CCtBu]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 14a–14d (Hpz^R = 3,5-bis(trifluoromethyl)pyrazole (14a), 4-(trimethylsilyl)pyrazole (14b), 3,5-dimethylpyrazole (14c), 3,5-di(tert-butyl)pyrazole (14d)).