Shining a New Light on Photoredox Catalysis and Small Molecule Activation

这个项目将提供对新兴的光氧化还原催化领域中基本反应机理的详细和迫切需要的理解。这将被用来实现人们期待已久但极具挑战性的新化学转变。其中包括史无前例地将惰性但丰富的氮素直接转化为有用的含氮化合物，绕过了现有的工业方法，这些方法需要极端条件、巨大的能量投入和低效的多步骤程序。化学合成--将简单和容易获得的化学前体转化为日益复杂、功能和价值越来越高的化合物的过程--是一门关键的科学学科和现代社会的关键基础，提供了在工业的每个分支中遇到的几乎无数合成化学品的途径。近年来，这一领域最引人注目和最令人兴奋的发展之一是所谓的光氧化还原催化(PRC)的出现。PRC反应使用可见光作为丰富的能源，在非常温和的条件下驱动复杂和具有挑战性的化学反应，使用易于操作和安全的试剂，并具有高选择性。因此，与现有的选择相比，它们通常显著更少的危险、更少的浪费和更可持续的选择。因此，这一领域的发展是现代合成化学的一个重要目标，它承诺显著改善各种化学过程的环境影响。为了促进PRC方法的应用，以实现这些具有挑战性的目标，有必要了解潜在的机制--即共同引起这些复杂反应的各个反应步骤--因为正是这种理解为进一步的进展提供了框架。不幸的是，对这些机制的调查还很不发达，这给进一步的进展造成了很大的障碍。随着对PRC反应的要求越来越严格(在产率、选择性、复杂性和底物方面)，这个问题变得越来越尖锐，这个项目将为PRC反应的研究提供一个强有力的新方法。通过仔细分离提出的催化剂中间态，以精确控制的方式研究单个反应步骤是可行的。通过直接询问催化循环的基本反应步骤，将有可能清楚和毫不含糊地建立PRC过程的全面机理图。由此产生的对现有催化剂和反应的深入了解将使它们能够快速优化，同时开发出全新的和前所未有的转化。作为一个引人注目的例子，PRC将被用于促进使用温和的试剂和在温和的条件下直接转化氮(N_2)。氮气是现代化学工业最重要的原料之一，是氮原子的丰富来源。然而，众所周知，高度惰性的氮气分子的转化具有挑战性，目前必须使用已有百年历史的Haber-Bosch工艺进行，该工艺在极高的温度和压力下产生NH3，并具有巨大的环境足迹(约占世界总能源消耗的2%)。然后，这种NH3可以进一步转化为其他含氮化合物，这通常需要多个反应步骤，进一步限制了整体效率。相比之下，PRC方法将允许在更温和的条件下执行氮气激活。此外，它们不仅可以将氮气转化为NH3，还可以直接转化为其他有用的含氮化合物(例如，有机发光二极管中使用的三芳胺空穴转运体)，从而省去了通过NH3生产它们所需的繁琐的多步骤程序。

This project will provide detailed and much-needed understanding of fundamental reaction mechanisms in the emerging field of photoredox catalysis. This will be used to achieve long-sought-after but highly challenging new chemical transformations. These include the unprecedented direct conversion of inert but abundant dinitrogen directly into useful nitrogen-containing compounds, bypassing existing industrial methods that require extreme conditions, enormous energy input and inefficient, multi-step procedures.Chemical synthesis - the process by which simple and readily-available chemical precursors are transformed into progressively more complex, functional and valuable compounds - is a crucial scientific discipline and a key foundation of modern society, providing access to the almost innumerable synthetic chemicals encountered throughout every branch of industry. In recent years, one of the most dramatic and exciting developments in this area has been the emergence of so-called "photoredox catalysis" (PRC). PRC reactions use visible light as an abundant energy source to drive complex and challenging chemical reactions under very mild conditions, using easy- and safe-to-handle reagents, and with high selectivity. As a result, they are typically significantly less hazardous, less wasteful, and more sustainable than existing options. The development of this field is therefore a crucial goal of modern synthetic chemistry that promises significant improvements to the environmental impact of a wide variety of chemical processes.In order to facilitate the application of PRC methods towards these challenging goals it is essential to understand the underlying mechanisms - that is, the individual reaction steps that combine to give rise to these complex reactions - as it is this understanding that provides the framework for further progress. Unfortunately, investigations into these mechanisms are significantly underdeveloped, creating a substantial barrier to further advances. As the demands on new PRC reactions become ever more stringent (in terms of yield, selectivity, complexity and, pertinently, substrates) this problem is becoming ever more acute.This project will provide a powerful new method for investigating PRC reactions. By carefully isolating proposed intermediate states of the catalyst it will become feasible to investigate individual reaction steps in a precisely controlled manner. By directly interrogating the elementary reaction steps of the catalytic cycle it will be possible to clearly and unambiguously establish a comprehensive mechanistic picture of PRC processes. The resulting deep understanding of established catalysts and reactions will permit their rapid optimisation, alongside the development of entirely new and unprecedented transformations.As a compelling example, PRC will be used to facilitate the direct transformation of dinitrogen (N2) using mild reagents and under mild conditions. N2 is one of the single most important feedstocks for the modern chemical industry, acting as an abundant source of nitrogen atoms. However, the transformation of the highly inert N2 molecule is notoriously challenging and must currently be performed using the century-old Haber-Bosch process, which generates NH3 under extremely high temperatures and pressures and has an enormous environmental footprint (being responsible for roughly 2% of total world energy consumption). This NH3 can then be further transformed into other nitrogen-containing compounds, which typically requires multiple reaction steps, further limiting overall efficiency. In contrast, PRC methods will allow N2 activation to be performed under much milder conditions. Moreover, they will allow N2 to be transformed not only into NH3, but also directly into other useful nitrogen-containing compounds (e.g. triarylamine hole transporters used in OLEDs), thus bypassing the need for the laborious multi-step procedures needed to produce them via NH3.