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

碳-碳和碳-杂原子（C=E）双键的还原在工业中是极其重要的催化反应，例如在除草剂、多种药物分子和化妆品的生产过程中使用。该反应目前由基于贵过渡金属（例如，Rh、Ir、Ru等）。用同样有效的无金属催化剂替代贵金属催化剂在成本和毒性方面极具吸引力，同时还消除了任何潜在的供应风险（与使用来自极少数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还原C=E部分：（i）使用有机还原剂（氢化物（H-）的碳基源）与有机催化剂（例如，磷酸），（ii）使用通常由硼基刘易斯酸组成的阻挫刘易斯对（不通过形成配价键而彼此猝灭的刘易斯酸和刘易斯碱）作为催化剂，H2或R3 Si-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点（特别是在制备规模上）。前者需要化学计量量的有机还原剂（具有明显的成本和废物影响），而后者使用不耐水性且具有官能团耐受性限制的硼刘易斯酸。本文提出的研究将利用FLP中的阳离子碳基刘易斯酸来活化H2 /R3 SiH。重要的是，这些刘易斯酸是耐水的，并且具有良好的官能团耐受性。在H2活化后，阳离子碳刘易斯酸转化为之前在C=E键的许多化学计量还原中使用的有机还原剂（或类似物）。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，从而使整个过程在刘易斯酸中具有催化性。这种新的催化循环针对与前两种无金属方法相关的关键缺点（i）在有机氢化物中进行催化还原，和（ii）产生H2O /官能团耐受的FLP催化剂。这种潜在的变革性方法是基于我们令人兴奋的初步结果，其中我们证明了基于阳离子碳刘易斯酸的FLP可以活化湿溶剂中的H2和R3 Si-H并还原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，从中长期来看，可以在一系列具有重要社会意义的催化还原中取代现有的过渡金属体系。

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.