Iridium-catalyzed dehydrocoupling of primary amine-borane adducts: A route to high molecular weight polyaminoboranes, boron-nitrogen analogues of polyolefins
Iridium-catalyzed dehydrocoupling of primary amine-borane adducts: A route to high molecular weight polyaminoboranes, boron-nitrogen analogues of polyolefins
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DOI：
10.1002/anie.200801197
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Manners, Ian
中科院分区：
文献类型：
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作者：
Staubitz, Anne;Soto, Alejandro Presa;Manners, Ian
尽管硼氮化合物与碳化合物之间存在历史相似性[1]，并且人们对基于主族元素的聚合物材料的兴趣日益浓厚，[2]基于硼氮键的大分子化学仍然极其有限。[3]特别是，聚烯烃的高分子量硼氮类似物 ([R2N-BR’2] n) 从未得到令人信服的表征，[4] 尽管此类结构在计算研究方面引起了人们的关注。 [5]在此，我们报告了这些材料的第一个可溶性大分子实例的催化路线。我们之前报道过通过多种后过渡金属（例如 Ru、Rh、Ir、Pd、Pt）预催化剂介导的脱氢偶联/脱氢作用，从膦-硼烷加合物催化形成磷-硼键。 [6]发现使用仲膦-硼烷加合物 R2PH·BH3 作为底物可得到线性或环状低聚物。此外，Rh 催化的空间阻碍较小的伯膦-硼烷加合物 RPH2·BH3 的脱氢偶联导致消除一当量的 H2，并得到高分子量的聚膦硼烷 [RPH-BH2] n。我们还发现仲胺-硼烷加合物 R2NH·BH3 产生环状低聚物 (R2NÀBH2) x (x= 2 或 3)。 [7]然而，我们最初尝试将这种催化脱氢偶联化学扩展到使用Rh预催化剂从伯胺-硼烷加合物RNH2·BH3和氨-硼烷NH3·BH3形成具有硼-氮骨架的聚氨基硼烷，但没有成功。相反，我们观察到消除了多达大约两当量的 H2，产生了环硼嗪 [RNBH] 3 和性质不明确（可能是支链或环状）不溶性低聚物材料的混合物 [Eq.(1)]。 [7]最近人们对胺-硼烷作为储氢材料的浓厚兴趣[8]催生了一系列新型的、在某些情况下改进的基于各种早过渡金属和晚过渡金属的胺-硼烷加合物脱氢偶联催化剂。 [9]特别是，Goldberg、Heinekey 及其同事 [9a] 的报告称，Brookhart s Ir 催化剂 1 [10] 成功快速地将稀溶液中的 NH3·BH3 脱氢偶合，并消除一当量的 H2，生成被认为是五聚体 [NH2BH2] 5 的产物，促使我们重新探索聚氨基硼烷的合成。当 N-甲胺硼烷 (MeNH2·BH3) 的 THF 浓溶液在 08C 下用 0.3 mol% 的 1 处理时，我们观察到剧烈的鼓泡。随着溶液变得越来越粘稠，将反应混合物在208℃搅拌约20分钟[方程(2)]。然后将所得产物通过沉淀到作为非溶剂的正烷烃中进行分离和纯化，得到白色固体（图1a），随后通过NMR光谱、元素分析（EA）、红外光谱以及通过凝胶渗透色谱（GPC）和动态光散射（DLS）测量的分子量将其表征为聚丙烯类似物2a。
Despite the historic analogy between boron–nitrogen compounds and those of carbon [1] and the expanding interest in polymeric materials based on main-group elements,[2] macromolecular chemistry based on boron–nitrogen linkages is extremely limited.[3] In particular, high molecular weight boron–nitrogen analogues of polyolefins ([R2N-BR’2] n) have never been convincingly characterized,[4] although such structures have attracted attention with respect to computational studies.[5] Herein we report a catalytic route to the first, soluble macromolecular examples of these materials. We have previously reported the catalytic formation of phosphorus–boron bonds from phosphine–borane adducts through dehydrocoupling/dehydrogenation mediated by a variety of late-transition-metal (eg Ru, Rh, Ir, Pd, Pt) precatalysts.[6] The use of secondary phosphine–borane adducts R2PH· BH3 as substrates was found to give linear or cyclic oligomers. In addition, the Rh-catalyzed dehydrocoupling of less sterically encumbered primary phosphine–borane adducts RPH2· BH3 led to the elimination of a single equivalent of H2 and afforded high molecular weight polyphosphinoboranes [RPH-BH2] n. We also found that secondary amine–borane adducts R2NH· BH3 yield cyclic oligomers (R2NÀBH2) x (x= 2 or 3).[7] However, our initial attempts to extend this catalytic dehydrocoupling chemistry to the formation of polyaminoboranes with a boron–nitrogen backbone from primary amine–borane adducts RNH2· BH3 and ammonia–borane NH3· BH3 with Rh precatalysts were unsuccessful. Instead, we observed the elimination of up to approximately two equivalents of H2 to yield mixtures of borazines [RNBH] 3 and poorly defined (and presumably branched or cyclic) insoluble oligomeric material [Eq.(1)].[7] The recent intense interest in amine–boranes as hydrogen-storage materials [8] has led to a series of new and in several cases improved dehydrocoupling catalysts for amine–borane adducts based on a variety of early and late transition metals.[9] In particular, the report by Goldberg, Heinekey, and co-workers [9a] that Brookhart s Ir catalyst 1 [10] successfully and rapidly dehydrocouples NH3· BH3 in dilute solution with the elimination of a single equivalent of H2 to yield a product that is believed to be the pentamer [NH2BH2] 5 prompted us to reexplore the synthesis of polyaminoboranes. When a concentrated solution of N-methylamine–borane (MeNH2· BH3) in THF was treated with 0.3 mol% of 1 at 08C, we observed vigorous bubbling. As the solution became increasingly viscous, the reaction mixture was allowed to stir at 208C for about 20 minutes [Eq.(2)]. The resulting product was then isolated and purified by precipitation into n-alkane as a nonsolvent to give a white solid (Figure 1a), which was subsequently characterized as the polypropylene analogue 2a by NMR spectroscopy, elemental analysis (EA), IR spectroscopy, and in terms of molecular weight by gel permeation chromatography (GPC) and dynamic light scattering (DLS) measurements.