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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)).
Highlights
Triptycenes 1 (Fig. 1) are a class of organic compounds possessing a three-dimensional paddle-wheel structure
The synthesis protocol tolerates various degrees of steric bulk at the 3,5-positions of the pyrazolyl rings: for example, derivatives carrying hydrogen atoms (3a, 3b; Fig. 2), methyl groups (3c) or tert-butyl groups (3d) are all formed in good yields
We found that the o-phenylene ring may be replaced by otherorganic fragments, because the synthesis sequence can be applied to the 1,1′-ferrocenylene-bridged bisborate Li2[1,1′-fc(BH3)2]24,25 to give compounds like 3f (Fig. 2). 3f can be viewed as an Fe-containing B–N triptycene, and as an ansa-ferrocene with pyrazabole26 bridge
Summary
Triptycenes 1 (Fig. 1) are a class of organic compounds possessing a three-dimensional paddle-wheel structure. Due to their rigid bicyclo[2.2.2]octatriene core, angles close to 120° are maintained between the three o-phenylene ring panels, which leads to the formation of three sized compartments lined with π-electron clouds. This unique geometry often frustrates space-efficient packing and thereby. Further developments in these fields are critically dependent on the availability of broadly functionalised triptycene derivatives. The Diels–Alder reactions between anthracenes and p-benzoquinones or benzynes have offered the most important methods for the assembly of triptycene scaffolds (Scheme 1). General reaction conditions: (i) Et2O, rt, between 12 h and 15 h. (ii) CH3CN-toluene or toluene, between rt and 120 °C, between 8 h and 24 h
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