金属硫配合物催化还原不饱和化合物反应机理的理论研究

The stereo- and region-selectivity in reduction of C=O and C=N bonds are very useful in synthetic organic chemistry. The hydrosilylation and hydroboration for pyridine, carbonyl, and imine compounds have been found great interests in the literature. Many efficient catalytic systems were developed for the hydrosilylation and hydroboration of these compounds, but most of these methods required a strong base and generated byproducts which are difficult to be separated. Hence, to develop a useful method in mild condition is significant. Firstly, Complexes with metal sulfur bond showed remarkable capacity in catalyzing hydrosilylation of pyridine and its derivatives, leading to the synthesis of six-membered nitrogen-containing cyclic compounds directly, and only 1, 4-dihydropyridines product was found. As we all know, NAD(P)H/NAD(P)+ redox cycle with its 1, 4-dihydropyridine/pyridinium ion interconversion is one of the fundamental transformations in biological systems, however, a remarkable challenge, and to date, is that there is no general method available for the partial reduction of pyridines. Therefore, it is worth to note that, the 1, 4-selective hydrosilylation catalyzed by metal sulfur complex is straightforward, high-efficiency, highly regioselective in room temperature, which leading a new method in this field. Secondly, transition metal complexes with metal sulfur bonds are useful in catalytic cycle of carbonyl reduction. M-S bond displayed its outstanding aptitude in activating Si-H bond, dehydrogenative couplings of ketones and imines with excellent yield and exceeding selectivity, no any external base required, and the reaction rates are impressive at room temperature with low cost of catalyst. These experimental results are charming and provide basis for further study in order to understand the reaction insight well and find more general methods for these reactions. Thirdly, chiral amines are finding applications in an ever-increasing number of fields, such as pharmaceuticals, fragrances and agrochemicals. Thus developing efficient catalytic methodologies for the selective reduction of C=N bonds is urgent and important. Interestingly, because Si-N or B-N bond serves as a placeholder for an N-H bond, the asymmetric hydrosilylation and hydroboration of imines catch increasing attention among chemists and theorists. Despite the recent significant advances in metal catalyzed hydrosilylation and hydroboration of pyridine, carbonyl and imine compounds, the methods are still far from being general. Hence, our ongoing interest in reaction mechanisms with transition metal catalysis with M-S bond has led us to investigate metal dithiolene catalyzed hydrosilylation and hydroboration of pyridine, carbonyl and imine compounds as well as carbon dioxide. And in this protocol we will employ density function theory (DFT) calculations to study the reaction mechanisms and explore new general method for hydrosilylation and hydroboration of those compounds.

由于含N和含P配体优异的配位能力和良好的给电子性能，金属含氮配合物和金属含磷配合物长久以来是有机金属催化剂中的“明星”。众所周知，与过渡金属成键的硫原子同样具有良好的给电子能力，不仅能稳定配合物，而且增强其催化能力。实验上，金属硫配合物催化二氧化碳、吡啶、羰基、亚胺类化合物的加氢化、硅氢化和硼氢化的催化反应已纷纷被报道。本课题组对这类还原反应的如下几个方面很感兴趣。第一，反应无须加碱，因为催化剂本身就是碱；第二，反应条件温和但反应却快速高效；第三，选择性好；第四，一些金属硫配合物表现出光催化特征。所有这些反应特点都与催化剂和反应物的内在本质息息相关。因此，本项目将采用理论计算化学的方法研究各个催化还原反应的机理，反应路径，反应产物选择性的成因，比较不同催化剂对各反应的催化性能差异，通过理解各个催化反应的机理，从理论上预测新型高效的催化剂，为实现高效、高选择性、条件温和的反应提供理论指导。

金属硫配合物有着与其他金属配合物不同的反应活性和催化活性，主要有两方面的决定因素：第一，金属硫配合物的前线轨道主要由硫原子的p轨道组成，使得金属硫配合物与有机分子的反应往往发生在硫原子上；第二，硫原子可以使用孤对电子与金属中心形成配位键，也可以使用单电子的杂化轨道与金属中心形成共价键，这种配体不确定性使得这类配合物有良好的氧化还原性，可以在光催化或者电催化中实现还原反应。这些特征引起了人们的广泛关注，但是对这些反应机理的理解仍然有不足之处。我们使用密度泛函理论方法研究了1）金属二硫配合物与烯烃的加成； 2）金属二硫配合物催化制氢；3）金属单硫配合物催化羰基、亚胺、炔烃类化合物的还原；4）二氧化碳的催化还原； 5）不对称氢化机理：碱金属离子在不对称氢化中的作用。我们对金属硫配合物在化学反应中的作用进行了详细且系统的研究，揭示金属硫配合物的反应选择性和在催化过程中硫的作用原理。我们的研究证实了金属二硫配合物与烯烃的反应发生在硫原子上，而且不同的取代基对反应的动力学倾向性有着不可忽视的影响。发展了新的“MoS2配合物的催化机理”，实现了极具挑战性的PCET过程的详细研究，从理论上优化出各个步骤的中间体并比较各个路径的还原电势和pKa值，认为反应过程中没有其他金属配合物催化还原质子制备氢气过程中的金属氢配合物。同时也发现金属单硫配合物在催化不饱和化合物还原时硫原子的协同催化作用。通过该项目的研究，发表了16篇高水平的通讯作者论文，申请了一个专利，培养了12名硕士研究生并有一名毕业，1名博士后，获得广东省“青年珠江学者”称号。该项目的研究也是多核金属硫配合物和有机硫化物催化反应机理研究的基础，为硫化学的研究开辟了一条新路。