Anorganisch-Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8037 Zürich, Switzerland.
J Am Chem Soc. 2010 Dec 29;132(51):18233-47. doi: 10.1021/ja107187r. Epub 2010 Dec 8.
Dibromonitrosyl(dihydrogen)rhenium(I) complexes [ReBr(2)(NO)(PR(3))(2)(η(2)-H(2))] (1; R = iPr, a; Cy, b) and Me(2)NH·BH(3) (DMAB) catalyze at either 90 °C or ambient temperature under 10 bar of H(2) the hydrogenation of various terminal and cyclic alkenes (1-hexene, 1-octene, cyclooctene, styrene, 1,5-cyclooctadiene, 1,7-octadiene, α-methylstyrene). Maximum turnover frequency (TOF) values of 3.6 × 10(4) h(-1) at 90 °C and 1.7 × 10(4) h(-1) at 23 °C were achieved in the hydrogenation of 1-hexene. The extraordinary catalytic performance of the 1/DMAB system is attributed to the formation of five-coordinate rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (2; R = iPr, a; Cy, b) and the action of the Lewis acid BH(3) originating from DMAB. The related 2/BH(3)·THF catalytic system also exhibits under the same conditions high activity in the hydrogenation of various alkenes with a maximum turnover number (TON) of 1.2 × 10(4) and a maximum TOF of 4.0 × 10(4) h(-1). For the hydrogenations of 1-hexene with 2a and 2b, the effect of the strength of the boron Lewis acid was studied, the acidity being in the following order: BCl(3) > BH(3) > BEt(3) ≈ BF(3) > B(C(6)F(5))(3) > BPh(3) ≫ B(OMe)(3). The order in catalytic activity was found to be B(C(6)F(5))(3) > BEt(3) ≈ BH(3)·THF > BPh(3) ≫ BF(3)·OEt(2) > B(OMe)(3) ≫ BCl(3). The stability of the catalytic systems was checked via TON vs time plots, which revealed the boron Lewis acids to cause an approximate inverse order with the Lewis acid strength: BPh(3) > BEt(3) ≈ BH(3)·THF > B(C(6)F(5))(3). For the 2a/BPh(3) system a maximum TON of 3.1 × 10(4) and for the 2a/B(C(6)F(5))(3) system a maximum TOF of 5.6 × 10(4) h(-1) were obtained in the hydrogenation of 1-hexene. On the basis of kinetic isotope effect determinations, H(2)/D(2) scrambling, halide exchange experiments, Lewis acid variations, and isomerization of terminal alkenes, an Osborn-type catalytic cycle is proposed with olefin before H(2) addition. The active rhenium(I) monohydride species is assumed to be formed via reversible bromide abstraction with the "cocatalytic" Lewis acid. Homogeneity of the hydrogenations was tested with filtration and mercury poisoning experiments. These "rhenium(I) hydride/boron Lewis acid" systems demonstrate catalytic activities comparable to those of Wilkinson- or Schrock-Osborn-type hydrogenations accomplished with precious metal catalysts.
二溴亚硝酰基（二氢）铼（I）配合物 [ReBr2(NO)(PR3)2(η2-H2)]（1；R=iPr，a；Cy，b）和 Me2NH·BH3（DMAB）在 90°C 或 10 巴 H2 下，在环境温度下催化各种末端和环状烯烃（1-己烯、1-辛烯、环辛烯、苯乙烯、1,5-环辛二烯、1,7-辛二烯、α-甲基苯乙烯）的加氢反应。在 1-己烯的加氢反应中，在 90°C 时达到的最大转化频率（TOF）值为 3.6×10(4)h(-1)，在 23°C 时为 1.7×10(4)h(-1)。1/DMAB 体系的非凡催化性能归因于五配位铼（I）氢化物配合物[Re(Br)(H)(NO)(PR3)2]（2；R=iPr，a；Cy，b）的形成和路易斯酸 BH3（来自 DMAB）的作用。在相同条件下，相关的 2/BH3·THF 催化体系在各种烯烃的加氢反应中也表现出高活性，最大转化率（TON）为 1.2×10(4)，最大 TOF 为 4.0×10(4)h(-1)。对于 1-己烯与 2a 和 2b 的加氢反应，研究了硼路易斯酸强度的影响，酸度顺序如下：BCl3>BH3>BEt3≈BF3>B(C6F5)3>BPh3≫B(OMe)3。发现催化活性的顺序为 B(C6F5)3>BEt3≈BH3·THF>BPh3≫BF3·OEt2>B(OMe)3≫BCl3。通过 TON 与时间的关系图检查了催化体系的稳定性，结果表明路易斯酸的稳定性与路易斯酸的强度呈近似反比关系：BPh3>BEt3≈BH3·THF>B(C6F5)3。对于 2a/BPh3 体系，在 1-己烯的加氢反应中获得了 3.1×10(4)的最大 TON，对于 2a/B(C6F5)3 体系，在 1-己烯的加氢反应中获得了 5.6×10(4)h(-1)的最大 TOF。基于动力学同位素效应测定、H2/D2 混合、卤化物交换实验、路易斯酸变化和末端烯烃的异构化，提出了一种 Osborn 型催化循环，其中烯烃在 H2 加成之前。假定活性铼（I）单氢化物物种是通过与“共催化”路易斯酸可逆地夺取溴化物形成的。通过过滤和汞中毒实验测试了氢化的均相性。这些“铼（I）氢/硼路易斯酸”体系表现出与使用贵金属催化剂完成的 Wilkinson 或 Schrock-Osborn 型氢化相当的催化活性。
