The concerted action of a Lewis acid and base can activate H2 and other small molecules. Such frustrated Lewis pairs (FLPs) have garnered much attention and prompted many investigations into the activation of small molecules and catalysis. Although the nature, mechanism of action, and range of FLP systems continues to expand, this concept has also inspired ever-widening chemistry. Applications in hydrogenation and polymerization catalysis, as well as in synthetic chemistry, have provided selective processes and metal-free protocols. Heterogeneous FLP catalysts are emerging, and polymeric FLPs offer avenues to unique materials and strategies for sensing and carbon capture. The prospects for further impact of this remarkably simple reaction paradigm are considered.

New Directions for Frustrated Lewis Pair Chemistry

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or...

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs.

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H2, CO, alkynes, alkenes and even...

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Metallomimetic Chemistry of Boron

Polymers have become one of the largest and most important materials we use in daily life. Their popularity has stemmed from their wide range of material properties combined with their low cost of ...

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Recent Trends in Catalytic Polymerizations

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual c...

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts.

A metal-free B(C6F5)3-catalyzed approach is developed for the disproportionation reaction of a series of indoles with various hydrosilanes, without any additives such as base and production of any small molecule such as dihydrogen. This boron catalyst system also exhibits excellent catalytic performance for practical application, such as catalyst loading as low as 0.01 mol % under solvent-free conditions, and a long-life catalytic performance highlighted by a constant catalytic activity being maintained and excellent yields being achieved for the desired products over 10 sequential additions of starting materials. On the basis of characterization of key intermediates through a series of in situ NMR reactions and detailed experimental data, we proposed a reaction mechanism which illustrated pathways for the formation of different products, including both major products and byproducts. Additional control experiments were conducted to support our proposed mechanism. Understanding the mechanism enables us to ...

B(C6F5)3-Catalyzed (Convergent) Disproportionation Reaction of Indoles

The living polymerization of conjugated polar alkenes such as methacrylates by a noninteracting, authentic frustrated Lewis pair (FLP) has remained elusive ever since the report on FLP-promoted polymerization in 2010. Here we report that the polymerization of alkyl methacrylates by a FLP system based on a strongly nucleophilic N-heterocyclic olefin (NHO) Lewis base and sterically encumbered but modestly strong Lewis acid MeAl(4-Me-2,6-tBu2-C6H2O)2 is not only rapid but also living. This living polymerization was indicated by the formation of a linear, living chain, capped with NHO/H chain ends, without backbiting-derived cyclic chain ends. The true livingness of this FLP-promoted polymerization has been unequivocally verified by five lines of evidence, including the predicted polymer number-average molecular weight (Mn, up to 351 kg·mol–1) coupled with low dispersity (Đ = 1.05) values; obtained high to quantitative initiation efficiencies; an observed linear increase of polymer Mn vs monomer conversion an...

Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs

Without the addition of any additives and production of any small molecules, C3-borylated indoles and transfer hydrogenated indolines have been simultaneously achieved by a B(C6F5)3-catalyzed disproportionation reaction of a broad range of indoles with catecholborane. This catalyst system exhibits excellent catalytic performance for practical applications, such as easy scale-up under solvent-free conditions and long catalytic lifetime over ten sequential additions of starting materials. A combined mechanistic study, including isolation and characterization of key reaction intermediates, analysis of the disproportionation nature of the reaction, in situ NMR of the reaction, and analysis of detailed experimental data, has led to a possible reaction mechanism which illustrates pathways for the formation of both major products and byproducts. Understanding the reaction mechanism enables us to successfully suppress side reactions by choosing appropriate substrates and adjusting the amount of catecholborane nee...

B(C6F5)3–catalyzed C3-selective C-H Borylation of Indoles: Synthesis, Intermediates, and Reaction Mechanism

It is a challenging task to achieve regioselective C–H silylation of indoles with Ph2SiH2, as bis(indol-3-yl)-substituted silanes are always obtained along with the C3-silylated indoles. Without adding any additive, we developed an Al(C6F5)3-based catalyst system for the synthesis of C3-selective silylated indoles under mild conditions and propose here the reaction mechanism on the basis of systematic studies including detailed experimental data, characterization of key reaction intermediates, and isotope-labeled experiments. The adduct 8a generated from the coordination of indoline with Al(C6F5)3 at room temperature could activate silanes such as 2a to form a thermally induced frustrated Lewis pair (FLP) under heated conditions, which could efficiently catalyze the regioselective C–H silylation. The direct use of 8a as catalyst enabled us to successfully inhibit the formation of different indoline byproducts without the specific requirement of indole substrates and any additive, achieving up to 99% yield...

Sutao Zhang

Papers

B(C6F5)3-Catalyzed (Convergent) Disproportionation Reaction of Indoles

Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs

B(C6F5)3–catalyzed C3-selective C-H Borylation of Indoles: Synthesis, Intermediates, and Reaction Mechanism

Switchable C–H Silylation of Indoles Catalyzed by a Thermally Induced Frustrated Lewis Pair

Application of Mutualism in Organic Synthetic Chemistry: Mutually Promoted C-H Functionalization of Indole and Reduction of Quinoline