金属有机铼配合物催化环氧化合物理论研究

【作者】 王家勇；

【导师】 毕思玮；

【作者基本信息】 曲阜师范大学 ， 物理化学， 2011， 硕士

【摘要】 本论文在实验的基础上,我们用密度泛函理论(DFT)中的B3LYP方法,研究了以下三个方面的问题。我们的主要目的是研究下面所提到的金属有机化合物参与反应的反应机理和各个化合物的结构及成键特征。(1)在2009年,Abu-Omar及其合作者报道了用MeReO3 (缩写为MTO)为催化剂,用H2还原环氧化合物及双醇,使其脱氧脱水得到烯烃,并给出了H2先与催化剂MTO作用的反应机理。通过理论计算发现,第一步H2通过[2+3]机理与MTO作用不管是在热力学上还是动力学上都是不可行的。在本论文中,我们利用密度泛函理论(DFT)计算得出了较为合理的与Abu-Omar及其合作者提出的不同的反应机理。1)环氧化合物与催化剂MTO首先反应得到含金属五元环中间体2;2)H2以[2+3]方式进攻中间体2得到双羟基化合物中间体4;3)中间体4经过质子转移得到有水分子配位的含金属五元环中间体6;4)最后一步,直接从中间体6脱出产物烯烃,并伴随催化剂MTO再生,催化循环反应结束。在反应的开始有大量的双醇生成,是由于主反应生成的副产物水与中间体2作用,并且存在两个互相竞争的路径,通过理论计算发现,双醇的生成及双醇再转化为中间体2是一个快速的可逆过程,又从中间体2到产物烯烃是一个不可逆过程,随着时间的推移,生成的双醇最终全部转化为烯烃。同时也解释了双醇作为反应物时得到烯烃的反应机理,1)双醇与催化剂MTO反应,经过质子转移得到含金属五元环中间体2;2)H2以[2+3]方式进攻中间体2得到双羟基化合物中间体4;3)中间体4经过质子转移得到有水分子配位的含金属五元环中间体6;4)最后一步,直接从中间体6脱出产物烯烃,并伴随催化剂MTO再生,催化循环反应结束。本论文中提出的反应机理很好的验证了Abu-Omar及其合作者实验现象。(2)2005年,Jiang及其合作者首次利用五羰基铼化合物Re(CO)5Br或者CpRe(CO)3作为催化剂催化环氧化合物与CO2耦合反应合成环碳酸酯,反应在无溶剂高温条件下,产率最高能达到97%。Jiang及其合作者给出他们设想的可能机理,指出该反应机理可能类似于Ni催化剂作用机理。在2010年,Guo及其合作者依托Jiang及其合作者的实验利用密度泛函理论(DFT)研究了五羰基铼化合物Re(CO)5Br作为催化剂催化环氧化合物与CO2耦合反应合成环碳酸酯的反应机理。提出了Ⅰ和Ⅱ两种不同的反应机理,机理Ⅰ第一步是环氧化合物与催化剂Re(CO)5Br作用,而机理Ⅱ是催化剂首先活化CO2,通过理论计算,Guo及其合作者得出结论认为机理Ⅰ为可能的反应机理。我们认为,Guo及其合作者提出的机理Ⅰ总的反应势垒太高,可能不是真正的反应机理,在此基础上,我们利用同样的方法(DFT理论)研究了五羰基铼化合物Re(CO)5Br作为催化剂催化环氧化合物与CO2耦合反应合成环碳酸酯的反应机理,得出了其真正的反应机理V-b,即第一步,催化剂Re(CO)5Br脱去一个羰基得到一个16e的活性中间体Re(CO)4Br;第二步,环氧化合物配位到金属铼上,形成18e中间体2″；第三步,环氧化合物中C上有-CH2Cl取代基的C-O键与Re-Br键之间以[2σ+2σ]方式相互作用,形成一个5元环中间体a′；第四步,逐渐拉开Re-Br键形成16e中间体,使得C02可以进攻金属铼；第五步,C02插入到Re-O键之间,形成一个4元环中间体c′;第六步,断开Re←O配位键,使Br原子逐渐靠近金属中心得到中间体f；第七步,还原消除,形成C-O键同时伴随Br原子回到金属中心Re上,得到产物环碳酸酯与金属Re配位的中间体g′;最后一步产物环碳酸酯脱出同时伴随催化剂]Re(CO)4Br再生。总的来说,该反应机理包括三个过程,即环氧化合物氧化加成,C02插入,还原消除得到产物。值得注意的是,Br原子在此起到了很大作用。同时,我们还研究了CpRe(CO)3作为催化剂催化环氧化合物与C02耦合反应合成环碳酸酯的反应机理,找出了三种可能的反应机理Ⅰ′、Ⅱ′及Ⅲ′,最终分析得出机理Ⅲ′为其真实的反应机理,即第一步,催化剂CpRe(CO)3脱去一个羰基得到一个16e的活性中间体CpRe(CO)2;第二步,环氧化合物1配位到(CPRe(CO)2上得到中间体h；第三步,环氧化合物与CpRe(CO)2发生相互作用断开C上有-CH2Cl取代基的C-O键生成一个含金属的4元环中间体i;第四步,C02直接插入到Re-O键之间得到一个6元环中间体n′;第五步,中间体n′异构化得到中间体n；第六步,还原消除得到产物环碳酸酯与金属Re配位的中间体o;最后一步,得到产物并伴随催化剂CpRe(CO)2再生。(3)在Templeton实验(West, N. M.; et al. Organometallics 2008,27,5252)的基础上利用密度泛函理论,研究了(Cl-nacnac)Pt(H) (Cl-nacnac=bis(N-ary)-β-diiminate)与端炔的主、副反应机理。研究表明,叔丁基乙炔以1,2-方式插入到Pt—H键之间生成主产物,C一C键生成为决速步骤；以2,1-方式插入到Pt—H键之间生成副产物,炔烃插入为决速步骤。基于主副反应机理,合理的解释了主、副产物生成的原因。分析表明,主产物是热力学控制的产物,而副产物是动力学控制的产物。更多还原

【Abstract】 In this paper, density functional theory (DFT) calculations at the B3YLP level are carried out to study and analyze the following three problems on the basis of experiments. Our major intention is to investigate the molecular sturetures, bonding, and mechanisms on the organometallic reactions mentioned below.(1) In 2009, Abu-Omar and coworkers reported MeReO3-catalyzed deoxygenation of epoxides and diols and proposed a mechanism starting with addition of H2 to MeReO3 (abbreviated as MTO). Our calculations showed that addition of H2 to MTO via the [2+3] mechanism is significantly endothermic and the barrier is inaccessibly high. In this work, we proposed an alternative mechanism with the aid of density functional theory (DFT) calculations. In the alternative mechanism, MTO and epoxide react first to give a five-membered-ring rhenium diolate intermediate (2). To the intermediate, addition of H2 via a [2+3] mechanism gives an oxo-hydroxy species (4). A proton transfer in 4 results in formation of a rhenium diolate intermediate (6) having a water ligand. Subsequent extrusion of olefin from the rhenium diolate intermediate (6) completes the reaction and regenerates the catalyst MTO.Formation of diol at the beginning of the reaction observed experimentally is related to the hydrolysis of epoxide with the co-product water through the rhenium diolate intermediate (2) via two possible paths. Our calculation results confirmed that the rhenium diolate intermediate (2)+ H2O and the hydrolysis products (MTO+diol) are in fast equilibrium, resulting in the eventual conversion of diol to olefin. Meanwhile, we obtained the mechanism of the reactions about the diol as the reactant. MTO and diol react first to give a five-membered-ring rhenium diolate intermediate (2). To the intermediate, addition of H2 via a [2+3] mechanism gives an oxo-hydroxy species (4). A proton transfer in 4 results in formation of a rhenium diolate intermediate (6) having a water ligand. Subsequent extrusion of olefin from the rhenium diolate intermediate (6) completes the reaction and regenerates the catalyst MTO. This explained Abu-Omar’s experimental phenomenon successfully.(2) In 2005, Jiang and coworkers reported the first example of the Re(CO)5Br or CpRe(CO)3-catalyzed coupling of CO2 with epoxides to give cyclic carbonates at 383.15 K under solvent-free conditions and the yield up to 97%. Re(CO)5Br was shown to be an efficient, simple catalyst in this coupling reaction. They also postulated a possible mechanism for the reaction based on experimental observations, which is closely related to that of the Ni-catalyzed reactions. Most recently, Guo and coworkers carried out a DFT study on the mechanism of the coupling reaction between chloromethyloxirane and carbon dioxide catalyzed by Re(CO)5Br. Two possible mechanisms I and II were proposed. Mechanism I starts with epoxide oxidative addition, while mechanism starts with the CO2 activation by Re(CO)4Br. But we found that the overall barriers calculated for the two mechanisms are inaccessibly high, which promotes us to explore an alternative mechanism for the reaction on the basis of our DFT calculations. In this paper, we proposed an alternative mechanism in which the overall activation barrier height is much lower and the intermediates involved are found to be significantly more stable. In the alternative mechanism (V-b), initially, decarbonylation of Re(CO)5Br affords the active 16e intermediate, Re(CO)4Br. Oxidative addition of the C-0 bond of epoxides to Re-Br bond of Re(CO)4Br gives the metallaoxetane intermediate a’. Insertion of CO2 into the Re-O bond gives the metallaoxetane intermediate c’, and finally reductive elimination of C-0 bond from f produces the cyclic carbonate and regenerates the active form of the catalyst Re(CO)4Br. In summary, three main stages are involved:epoxide oxidative addition, CO2 insertion and reductive elimination of product. It is noteworthy that the Br atom played an important role in the catalyticcycle reaction.In this paper, we also investigated the mechanism of the reactions where the CpRe(CO)3 was employed as catalyst. We proposed three mechanismsⅠ’,Ⅱ’ andⅢ’. The mechanismⅢ’ is found to be reasonable. Initially, decarbonylation of CpRe(CO)3 affords the active 16e intermediate, CpRe(CO)2. Oxidative addition of the C-0 bond of epoxides to CpRe(CO)2 gives the metallaoxetane intermediate i. Insertion of CO2 into the Re-O bond gives the metallaoxetane intermediate n’, and then isomerization of n’results in the metallaoxetane intermediate n. Finally, reductive elimination of C-0 bond from n produces the cyclic carbonate and regenerates the active form of the catalyst CpRe(CO)2.(3) On the basis of Templeton’s experiment (West, N. M.; et al. Organometallics 2008,27, 5252), the mechanisms of the main and the side reactions between (Cl-nacnac)Pt(H) (Cl-nacnac: bis(N-aryl)-β-diiminate) and a terminal alkyne were investigated by density functional theory. Our study shows that the 1,2-insertion of t-BuC=CH into the Pt—H bond generates the main products and that C—C bond formation is the rate-determining step. The 2,1-insertion of t-BuC≡CH into the Pt—H bond generates the byproducts and alkyne insertion is the rate-determining step. Based on the mechanisms of the main and side reactions the presence of the main product and the by-product could be explained. We found that the main product is thermodynamically controlled while the side product is kinetically controlled.更多还原