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

碳-碳和碳-杂原子(C=E)双键的还原在工业上是一种非常重要的催化反应，例如在除草剂、多种药物分子和化妆品的生产中。目前，该反应主要由基于贵金属(如Rh、Ir、Ru等)的催化剂进行。用同样有效的无金属催化剂取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(与使用来自极其有限的国家的稀有金属有关)。近年来，已经开发了两种无金属方法来还原C=E部分：(I)使用有机还原剂(碳基氢化物(H-)源)与有机催化剂(例如磷酸)相结合，(Ii)使用受挫的Lewis对(Lewis酸和Lewis碱不会通过形成缩合键彼此淬灭)作为催化剂，通常由以H2或R3Si-H为末端还原剂的硼基Lewis酸组成。虽然这些方法代表着重大的科学进步，但这两种方法仍然存在相当大的缺陷(特别是在准备规模上)。前者需要化学计量的有机还原剂(具有明显的成本和浪费影响)，而后者使用的是不耐水且具有官能团耐受性限制的硼路易斯酸。本文提出的研究将利用Flp中的阳离子碳基Lewis酸来激活H2/R3SiH。重要的是，这些Lewis酸是耐水性的，具有良好的官能团耐受性。氢气活化后，阳离子碳路易斯酸转变为有机还原剂(或类似物)，以前在许多化学计量比的C=E键还原中使用。C=E还原可再生基于阳离子碳的FLP，使其能够用于进一步的氢活化，从而使整个过程在Lewis酸中具有催化作用。这种新的催化循环针对以前的两种无金属方法的主要缺点：(I)在有机氢化物中进行还原催化，以及(Ii)生成耐水/官能团的FLP催化剂。这一潜在的转变方法是基于我们令人兴奋的初步结果，其中我们证明了阳离子碳Lewis酸基FLP可以在湿溶剂中激活H2和R3Si-H，并还原C=E键(尽管速度很慢，在强迫条件下)。这项建议旨在将这一初步突破发展成一种真正有用的催化方法，在中长期内可以在一系列具有社会重要性的催化还原中取代现有的过渡金属系统。

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.