Cationic Carbon Lewis Acids in Frustrated Lewis Pairs as New Reduction Catalysts

碳碳和碳杂原子（C=E）双键的还原是工业上极为重要的催化反应，例如在除草剂、多种药物分子和化妆品的生产过程中。该反应目前主要由基于贵重过渡金属（如Rh、Ir、Ru等）的催化剂组成。用同样高效的无金属催化剂替代贵金属基催化剂在成本和毒性方面极具吸引力，同时也消除了任何潜在的供应风险（与利用来自极其有限的国家的稀缺金属有关）。近年来，已经开发了两种无金属的方法来减少C=E基团：(i)使用有机还原剂（氢化物（H-）的碳基来源）与有机催化剂（例如，磷酸）结合，（ii）使用失落的刘易斯对（路易斯酸和路易斯碱，不会通过形成还原键而相互淬灭），通常由硼基路易斯酸作为催化剂，H2或R3Si-H作为末端还原剂。虽然这些方法代表了重大的科学进步，但两者仍然存在相当大的缺陷（特别是在制备规模上）。前者需要化学计量量的有机还原剂（有明显的成本和浪费问题），而后者使用硼路易斯酸，不耐水，具有官能团耐受性限制。本文提出的研究将利用FLPs中的阳离子碳基路易斯酸来激活H2 / R3SiH。重要的是，这些路易斯酸具有耐水性和良好的官能团耐受性。H2活化后，阳离子碳Lewis酸转化为有机还原剂（或类似物），以前用于许多C=E键的化学计量还原。C=E还原使阳离子碳基FLP再生，使其能够用于进一步的H2活化，从而使整个过程在Lewis酸中具有催化作用。这种新型的催化循环针对的是与前两种无金属方法相关的主要缺陷(1)在有机氢化物中进行还原催化，以及(2)生成H2O /官能团耐受的FLP催化剂。这种潜在的变革方法是基于我们激动人心的初步结果，我们证明了基于阳离子碳Lewis酸的FLP可以在湿溶剂中激活H2和R3Si-H，并减少C=E键（尽管缓慢且在强制条件下）。该提案旨在将这一初步突破发展成为一种真正有用的催化方法，在中长期内可能取代现有的过渡金属系统，用于一系列具有社会意义的催化还原。

The reduction of carbon-carbon and carbon-heteroatom (C=E) double bonds is an extremely important catalysed reaction in industry used for example during the production of herbicides, multiple drug molecules and cosmetics. This reaction is currently dominated by catalysts based on precious transition metals (e.g., Rh, Ir, Ru etc.). The replacement of precious metal-based catalysts with equally efficient metal-free catalysts is highly attractive in terms of cost and toxicity, whilst also eliminating any potential supply risk (associated with utilising scarce metals sourced from an extremely limited number of countries). In recent years two metal-free methods have been developed to reduce C=E moieties: (i) using organic reductants (carbon based sources of hydride (H-)) in combination with organocatalysts (e.g., phosphoric acids), (ii) using Frustrated Lewis pairs (a Lewis acid and Lewis base that do not quench each other by formation of a dative bond) generally consisting of boron based Lewis acids as catalysts with either H2 or R3Si-H as the terminal reductant. Whilst these approaches represented significant scientific advances there are still considerable drawbacks to both (particularly on a preparative scale). The former requires stoichiometric quantities of organic reductant (with the obvious cost and waste implications) whilst the latter utilises boron Lewis acids that are not water tolerant and have functional group tolerance limitations. The research proposed herein will utilise cationic carbon based Lewis acids in FLPs to activate H2 / R3SiH. Importantly, these Lewis acids are water tolerant and have good functional group tolerance. Post H2 activation the cationic carbon Lewis acid is transformed into the organic reductant (or a close analogue of) previously used in many stoichiometric reductions of C=E bonds. C=E reduction regenerates the cationic carbon based FLP enabling it to be used for further H2 activation, thus making the overall process catalytic in Lewis acid. This novel catalytic cycle targets the key drawbacks associated with the two former metal-free approaches (i) making reductions catalytic in organic hydride, and (ii) generating a H2O / functional group tolerant FLP catalyst. This potentially transformative approach is based on our exciting preliminary results where we demonstrated that a cationic carbon Lewis acid based FLP can activate H2 and R3Si-H in wet solvent and reduce C=E bonds (albeit slowly and under forcing conditions). This proposal seeks to develop this initial breakthrough into a truly useful catalytic methodology that in the medium to long term may replace existing transition metal systems in a range of societally important catalytic reductions.