Seven Omer, Popp Sebastian, Bolte Michael, Lerner Hans-Wolfram, Wagner Matthias
Institut für Anorganische Chemie, J.W. Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany.
Dalton Trans. 2014 Jun 14;43(22):8241-53. doi: 10.1039/c4dt00442f.
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-RC6H4BBr2 with the potassium tris(pyrazol-1-yl)borates K[HBpz3] or K[p-RC6H4Bpz3] yields cationic pyrazolyl-bridged pyrazaboles [p-BrC6H4B(μ-pz)3BH]Br ([4a]Br) and [p-RC6H4B(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)).
双硼酸锂Li2[o-C6H4(BH3)2]与2当量的合适吡唑衍生物（Hpz(R)）在Me3SiCl存在下反应，生成邻亚苯基桥连的吡唑硼烷HB(μ-pz(R))2(μ-o-C6H4)BH（3a - 3e；Hpz(R) = 4-碘吡唑（3a）、4-(三甲基硅基)吡唑（3b）、3,5-二甲基吡唑（3c）、3,5-二叔丁基吡唑（3d）、3,5-双(三氟甲基)吡唑（3e））。因此，该合成方法能够得到不带电荷的B-N三联苯，其具有（i）可官能化的基团、（ii）供电子或吸电子取代基以及（iii）不同空间需求的吡唑环。用三(吡唑-1-基)硼酸钾K[HBpz3]或K[p-RC6H4Bpz3]处理p-RC6H4BBr2，得到阳离子型吡唑基桥连的吡唑硼烷[p-BrC6H4B(μ-pz)3BH]Br（[4a]Br）和[p-RC6H4B(μ-pz)3Bp-C6H4R]Br（R* = Br（[4b]Br）、I（[4c]Br）、SiMe3（[4d]Br）），它们可被视为三联苯的全B-N类似物。即使在80℃的温度下，3b和[4a]Br的B-H键对tBuC≡CH也没有反应活性，这表明相应的B-N笼状键具有相当高的热稳定性。大多数笼状化合物对水足够惰性，允许进行快速的水相后处理。然而，在CD3OD溶液中的NMR光谱显示3b或[4a]Br降解为相应的吡唑和o-C6H4(B(OCD3)2)2或p-BrC6H4B(OCD3)2/B(OCD3)3。在相同的测量条件下，二苯基化的物种[4b]Br明显更稳定；即使在76天后，大多数物质也仅降解到顺式/反式吡唑硼烷p-BrC6H4(CD3O)B(μ-pz)2B(OCD3)p-C6H4Br（11a/11b）的阶段。通过Stille型偶联反应，用nBu3SnC≡CtBu对[4c]Br进行衍生化，得到炔基衍生物[p-tBuC≡CC6H4B(μ-pz)3Bp-C6H4C≡CtBu]Br（[4e]Br）。更大的B-N聚集体也可得到：在Me3SiCl存在下，用4当量的Hpz(R)处理四硼酸锂Li4[1,2,4,5-C6H2(BH3)4]，得到相应的B-N五联苯（Hpz(R) = 3,5-双(三氟甲基)吡唑（14a）、4-(三甲基硅基)吡唑（14b）、3,5-二甲基吡唑（14c）、3,5-二叔丁基吡唑（14d））。
