Recent Advances in Water-Tolerance in Frustrated Lewis Pair Chemistry

Abstract

A water-tolerant frustrated Lewis pair (FLP) combines a sterically encumbered Lewis acid and Lewis base that in synergy are able to activate small molecules even in the presence of water. The main challenge introduced by water comes from its reversible coordination to the Lewis acid which causes a marked increase in the Brønsted acidity of water. Indeed, the oxophilic Lewis acids typically used in FLP chemistry form water adducts whose acidity can be comparable to that of strong Brønsted acids such as HCl, thus they can protonate the Lewis base component of the FLP. Irreversible proton transfer quenches the reactivity of both the Lewis acid and the Lewis base, precluding small molecule activation. This short review discusses the efforts to overcome water-intolerance in FLP systems, a topic that in less than five years has seen significant progress.1 Introduction2 Water-Tolerance (or Alcohol-Tolerance) in Carbonyl Reductions3 Water-Tolerance with Stronger Bases4 Water-Tolerant Non-Boron-Based Lewis Acids in FLP Chemistry5 Conclusions

Frustrated Lewis Pairs Bridged by Water and Immobilized in a Metal–Organic Framework as a Recyclable Transfer Hydrogenation Nanocatalyst

“…Based on our experimental observations and the previous reports, − the reaction mechanism of 1a transfer hydrogenation with FLP/MIL-101­(Cr) is proposed and presented in Scheme . Initially, the H + of [LB–H/MIL–101­(Cr)] + is transferred to the CC bond in 1a , giving the intermediate PhCHCH + (CO 2 Et) 2 .…”

Asymmetric catalysis with FLPs

“…3 A significant advancement in the rapid expansion of FLP catalysts and substrate scope for the catalytic hydrogenation has been achieved over the past 16 years. 4…”

Mechanistic Study of B(C6F5)3-Catalyzed Transfer Hydrogenation of Aldehydes/Ketones with PhSiH3 and Stoichiometric Water

“…Reduction of carbonyl compounds remains a significant research field of organic chemistry, and precious and transition metals such as Ni, Pd, Ru, Co, Fe, and others have often been used in the relevant catalytic systems. − Since the Stephan group reported the paradigm of metal-free reversible activation hydrogen in 2006, frustrated Lewis pair (FLP) chemistry has attracted great attention and been applied in a growing body of chemical problems, − in which Lewis acid (LA) boranes, especially (C 6 F 5 ) 3 B-based ones, can effectively activate H 2 or Si–H and have frequently been used in the catalytic reduction of imines, enamines, alkenes, alkynes, and others. Nevertheless, catalytic FLP reduction of carbonyls has been a longstanding problem for strong B–O bonds − until 2014, when the Stephan and Ashley groups reported their FLP-based carbonyl reduction, using weakly coordinating solvents ether and 1,4-dioxane, respectively, in which B­(C 6 F 5 ) 3 and ether behave as FLPs to activate H 2 and effect the reduction. − It is a priori assumed that any B­(C 6 F 5 ) 3 -involving reaction system requires strictly anhydrous conditions. − Generally B­(C 6 F 5 ) 3 -catalyzed aldehyde/ketone reduction with hydrosilanes is performed using a stepwise method, first siloxane is generated by hydrosilylation of carbonyl in anhydrous systems and then hydrolytic desilylation of siloxane is performed to produce the final alcohol. Piers, Sakata, et al proposed a S N 2-Si silane activation mechanism (Scheme , a) rather than carbonyl activation (Scheme , b). − …”

“…For systems involving water, the coordination of water to B­(C 6 F 5 ) 3 and irreversible O–H activation have usually been the main recent challenges, because water may prevent substrates from interacting with B­(C 6 F 5 ) 3 by forming the complex (C 6 F 5 ) 3 B–OH 2 , and deprotonation or protodeboronation of (C 6 F 5 ) 3 B–OH 2 yields inactive hydroxy species (Scheme ). − …”

“…Predominantly, avoiding moderate to strong Brønsted bases to prevent irreversible deprotonation of LA–OH 2 or altering electronic and steric factors of the active LA center to reduce the coordination propensity of water to LA, recently significant progress has been made to overcome water intolerance in FLP systems . Ashley and Stephan’s groups reported B­(C 6 F 5 ) 3 -catalyzed aldehyde hydrogenation in reagent grade polar solvents, − Soós and co-workers developed a highly water tolerant boron/nitrogen-centered FLP, , and Ingleson and Chang’s groups reported successful one-pot borane-catalyzed reductive amination or hydrosilylation. − B­(C 6 F 5 ) 3 was generally supposed to be water-tolerant and a small amount of water did not affect its catalysis activity, for the deprotonation of (C 6 F 5 ) 3 B–OH 2 was reversible in these systems.…”