Cu2O/SiC高效催化卤代芳烃与酚类的Ullmann偶联反应

卤代苯与酚类化合物反应制取二芳基醚是现代有机合成中的一个重要反应.传统的二苯醚合成方法是铜催化卤代苯与酚类化合物的Ullmann型C-O偶联反应,但是这种方法需要苛刻的反应条件.后来,人们发现了Pd(0)和Cu(Ⅰ)基催化剂,但是前者成本较高,且需要使用昂贵的配体,因此其应用受到了限制,而铜作为一种成本较低的催化剂受到了越来越多的关注.铜催化剂可以分为均相和非均相两大类.均相铜催化剂使用的是铜盐,并且需要加入配体,成本较高,且不易分离和循环利用.非均相铜催化剂研究较多的是CuO,Cu2O及Cu纳米颗粒,其中Cu2O纳米颗粒催化剂对Ullmann型C-O偶联反应具有很高的催化活性,但是它在潮湿的空气中容易被氧化,因此需要寻找一种合适的载体防止Cu2O纳米颗粒被氧化.SiC具有优良的化学稳定性及导电导热性能,并且作为载体己经成功应用到很多热催化及光催化反应中.本文以高比表面积的SiC为载体,以二乙二醇作为溶剂和还原剂,采用传统的两步液相还原法制备了Cu2O/SiC催化剂,并通过X射线衍射、X射线光电子能谱、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和H2程序升温还原等方法对Cu2O/SiC催化剂进行了表征.SEM和TEM结果表明,Cu2O纳米颗粒均匀分散在SiC表面,同时上述表征结果都表明Cu在SiC上主要以Cu2O的形式存在.将制备的Cu2O/SiC催化剂用于催化卤代芳烃与酚类的Ullmann C-O偶联反应中.以碘苯和苯酚的Ullmann C-O偶联反应为模型实验,考察了反应温度、反应时间、溶剂、碱的种类及用量和催化剂用量等条件的影响,得到了碘苯与苯酚UllmannC-O偶联反应的最优反应条件为:卤代芳烃14 mmol,酚类14 mmol,1.0当量的Cs2CO3,Cu2O/SiC(5 wt％) 10 mg,四氢呋喃10mL,在Ar气氛下150℃反应3h.在该条件下,二苯醚收率达到97％,转化频率(TOF)高达1136 h-1.Cu2O/SiC催化剂对Ullmann C-O偶联反应具有很好的普适性,并且对Ullmann C-S偶联反应也表现出很高的活性,TOF高达1186h-1.以碘苯和苯酚的Ullmann C-O偶联反应为基准实验,对催化剂的循环稳定性进行了考察.Cu2O/SiC催化剂五次循环后二苯醚的收率从97％降低至64％,这主要是由于活性组分Cu2O的流失所致.

Cu2O/SiC as efficient catalyst for Ullmann coupling of phenols with aryl halides

A Cu2O/SiC heterogeneous catalyst was prepared via a two-step liquid-phase method using diethylene glycol as both the solvent and the reducing agent.The catalyst was characterized using X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy (SEM),transmission electron microscopy (TEM),and H2 temperature-programmed reduction.All the results indicate that Cu is present on the SiC support primarily as Cu2O.The SEM and TEM results show that cubic Cu2O nanoparticles are uniformly dispersed on the β-SiC surface.The reaction conditions,namely the temperature,reaction time,and amounts of base and catalyst used,for the Ullmann-type C-Ocross-coupling reaction were optimized.A model reaction was performed using iodobenzene (14.0 mmol) and phenol (14.0 mmol) with Cu2O/SiC (5 wt％ Cu) as the catalyst,Cs2CO3 (1.0 equiv.) as the base,and tetrahydrofuran as the solvent at 150 ℃ for 3 h;a yield of 97％ was obtained and the turnover frequency (TOF) was 1136 h-1.The Cu2O/SiC catalyst has a broad substrate scope and can be used in Ullmann-type C-O cross-coupling reactions of aryl halides and phenols bearing a variety of different substituents.The catalyst also showed high activity in the Ullmann-type C-S cross-coupling of thiophenol with iodobenzene and substituted iodobenzenes;a TOF of 1186 h-1 was achieved.The recyclability of the Cu2O/SiC catalyst in the O-arylation of phenol with iodobenzene was investigated under the optimum conditions.The yield decreased from 97％ to 64％ after five cycles.The main reason for the decrease in the catalyst activity is loss of the active component,i.e.,Cu2O.