Intruduction of 4-iodopyrazole
The acknowledgment amid the bisborate Li2[o-C6H4(BH3)2] and 2 equivalents of an adapted pyrazole acquired (Hpz(R)) in the attendance 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 amalgam admission appropriately provides admission to uncharged B-N triptycenes address (i) functionalisable groups, (ii) electron-donating or -withdrawing substituents and (iii) pyrazole rings of capricious steric demand. Analysis 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 admired as abounding B-N analogues of triptycene.
The B-H bonds of 3b and [4a]Br are apathetic appear tBuC[triple bond, breadth as m-dash]CH even at temperatures of 80 °C, thereby advertence an apparent thermal adherence of the agnate B-N cage bonds. A lot of of the cage compounds are abundantly apathetic appear baptize to acquiesce quick aqueous workup. However, NMR spectroscopy in CD3OD band-aid reveals abasement of 3b or [4a]Br to the agnate pyrazoles and o-C6H4(B(OCD3)2)2 or p-BrC6H4B(OCD3)2/B(OCD3)3.
The diphenylated breed [4b]Br is decidedly added abiding beneath the aforementioned altitude conditions; even afterwards 76 d, a lot of of the actual degrades alone to the date 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 acquired [p-tBuC≡CC6H4B(μ-pz)3Bp-C6H4C≡CtBu]Br ([4e]Br). Larger B-N aggregates are aswell accessible: analysis of the tetrakisborate Li4[1,2,4,5-C6H2(BH3)4] with 4 equivalents of Hpz(R) in the attendance of Me3SiCl leads to the agnate 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)).