Eck Martin, Würtemberger-Pietsch Sabrina, Eichhorn Antonius, Berthel Johannes H J, Bertermann Rüdiger, Paul Ursula S D, Schneider Heidi, Friedrich Alexandra, Kleeberg Christian, Radius Udo, Marder Todd B
Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
Institut für Anorganische und Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106 Braunschweig, Germany.
Dalton Trans. 2017 Mar 14;46(11):3661-3680. doi: 10.1039/c7dt00334j.
In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (Bpin) and bis(neopentyl glycolato)diboron (Bneop), as well as bis(ethylene glycolato)diboron (Beg) and tetrakis(dimethylamino)diboron (B(NMe)), and their reactivity, along with that of bis(catecholato)diboron (Bcat) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for Beg, at room temperature for Bneop and at higher temperature for Bcat. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of Beg with sterically less demanding NHCs such as MeIm and iPrIm in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [LB(OR)] and borenium [LB(OR)] cations have been observed from the reaction of the bis-borate Beg with the NHCs iPrIm and MeIm, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.
在这项详细研究中，我们报告了广泛使用的二硼（4）化合物双（频哪醇合）二硼（Bpin）、双（新戊二醇合）二硼（Bneop）、双（乙二醇合）二硼（Beg）和四（二甲基氨基）二硼（B(NMe)）的结构及其反应活性，以及双（儿茶酚合）二硼（Bcat）与具有不同空间需求的主链饱和和主链不饱和氮杂环卡宾（NHC）的反应活性。根据二硼（4）化合物和所用NHC的性质，观察到了源于B-B键和C-N键活化的路易斯酸/路易斯碱加合物或NHC扩环产物。通过固态和溶液核磁共振光谱以及X射线衍射对相应的NHC加合物和NHC扩环产物进行了分离和表征。一般来说，我们观察到Beg在低温下、Bneop在室温下以及Bcat在较高温度下发生B-B键和C-N键活化。反应活性强烈取决于NHC和二硼（4）化合物的空间效应，以及相应的路易斯碱性和路易斯酸性。我们的结果为这些二硼（4）试剂与如今无处不在的NHC的化学性质、相应NHC加合物的稳定性以及一般使用路易斯碱进行B-B键活化提供了深刻见解。我们证明，即使在低至-40°C至-30°C的温度下，无需任何金属物种参与，也可能引发B-B键活化。例如，Beg与空间需求较小的NHC（如MeIm和iPrIm）在溶液中的反应在低温下生成了相应的扩环产物。此外，从双硼酸酯Beg与NHC iPrIm和MeIm的反应中观察到了硼鎓[LB(OR)]和硼正离子[LB(OR)]阳离子，这得出结论：用NHC（或一般的路易斯碱）活化双硼酸酯可能是获得此类物种的一种简便途径。
