Artificial photosynthesis strategies for synthesis: Combined photoredox and transition metal-catalysed transfer hydrogenation of C-C multiple bonds

化学反应的可行性通常取决于是否存在足够的热力学驱动力来推动它们。当进行一系列反应以制备感兴趣的化合物时，通过在每个步骤中使用具有高能量含量的高反应性小分子反应物来确保这种驱动力，并且这种反应物的可用性和过程决定了化学方法的实用性和可持续性的限制。说明性的例子是氧化和还原，这是我们对化学反应进行分类的两个主要类别。在氧化反应的情况下，分子氧可以用作理想的氧化剂，因为它是丰富的和无害的，并且在理想条件下，它可以被消耗以仅产生水作为副产物。类似地，我们可以设想使用水作为理想的还原剂，这将导致仅产生氧气作为副产品。然而，这面临着水不是良好还原剂的问题，或者换句话说，缺乏推动反应所需的热力学驱动力。相反，有机化合物最常用的还原剂是分子氢，这在自然界中是找不到的，而绝大多数是由化石燃料在释放大量二氧化碳的过程中产生的。值得注意的是，光合生物使用水作为还原剂固定二氧化碳，形成碳水化合物并释放分子氧，阳光提供所需的能量。受此启发，在本提案中，我们的目标是开发一种“人工光合作用”方法，用于减少某些类型的具有工业重要性的有机化合物-即烯烃和炔烃。要做到这一点，我们需要开发两种催化剂协同工作的系统，一种利用光的能量氧化水（形成氧气并提供“还原力”），另一种还原有机化合物。我们已经知道催化剂能够发挥第一种作用，在本项目中，我们将开发第二种作用，从而弥合有机化学中真正的人工光合作用反应的关键差距。这项研究将导致更可持续的烯烃和炔烃还原方法，这是化学工业中最大规模的有机反应之一。因此，这一项目的成功将有助于总体上发展更可持续的化学工业，减少其对使用化石碳源的依赖。此外，这项调查将产生有价值的信息，所涉及的机械流形，从而提供促进发现其他有效的和可持续的反应在未来。

The feasibility of chemical reactions is generally governed by the existence of a sufficient thermodynamic driving force pushing them. When performing sequences of reactions to prepare compounds of interest, this driving force is ensured by using in each step highly reactive small molecule reactants with a high energy contents, and the availability and procedence of such reactants determine the limits of how practical and sustainable a chemical process can be. Illustrative examples are oxidations and reductions, two of the main categories in which we classify chemical reactions. In the case of oxidation reactions, molecular oxygen can be used an ideal oxidant, since it is abundant and innocuous and in ideal conditions it can be consumed to produce only water as a by-product. Similarly, we could conceive using water as an ideal reductant, which would result in production of only oxygen as by-product. However, this faces the problem of water not being a good reductant or, in other words, lacking the thermodynamic driving force needed to push the reaction. Instead, the most common reductant used for organic compounds is molecular hydrogen, which is not found in nature and is instead produced in an overwhelming majority from fossil fuels in a process that releases enormous amounts of carbon dioxide.Remarkably, photosynthetic organisms use water as the reductant in the fixation of carbon dioxide to form carbohydrates and release molecular oxygen, with sunlight providing the required energy. Taking inspiration from this, in this proposal we aim to develop an 'artificial photosynthesis' approach for the reduction of certain types of organic compounds of industrial importance -namely, alkenes and alkynes. To do this, we will need to develop systems where two catalysts operate in a concerted manner, with one using the energy from light to oxidise water (forming oxygen and providing the 'reductive power') and the other reducing the organic compound. Catalysts are already known capable of performing the first of these roles, and in this project we will develop the second, thus bridging the key gap to enable true artificial photosynthesis reactions in organic chemistry.This investigation will result in more sustainable methods for reduction of alkenes and alkynes which, importantly, are among the largest scale organic reactions performed in chemical industry. Thus, success in this project will contribute towards the development of a more sustainable chemical industry in general, reducing its dependence on the use of fossil sources of carbon. Also, this investigation will produce valuable information on the mechanistic manifolds involved, thus providing facilitating the discovery of other efficient and sustainable reactions in the future.