H-bonding additives act like Lewis acid catalysts

A switch from anionic to bifunctional H-bonding catalyzed ring-opening polymerizations towards polyether–polyester diblock copolymers

A switch of an anionic ROP of epoxides into a bifunctional H-bonding ROP of cyclic esters paved a new avenue to one-pot, sequential, and block copolymerizations to previously rare polyether-block-polyester copolymers.

Catalytic synthesis of 3-aminooxindoles via addition to isatin imine: an update

3-Substituted-3-aminooxindoles have attracted the attention of organic chemist to develop different synthetic methodologies for their synthesis using catalysts. This review covers the recent developments.

Steric Effects of pH Switchable, Substituted (2-pyridinium)urea Organocatalysts: a Solution and Solid Phase Study

The study of hydrogen bonding organocatalysis is rapidly expanding. Much research has been directed at making catalysts more active and selective, with less attention on fundamental design strategies. This study systematically increases steric hindrance at the active site of pH switchable urea organocatalysts. Incorporating strong intramolecular hydrogen bonds from protonated pyridines to oxygen stabilizes the active conformation of these ureas thus reducing the entropic penalty that results from substrate binding. The effect of increasing steric hindrance was studied by single crystal X-ray diffraction and by kinetics experiments of a benchmark reaction.

The trans/cis photoisomerization in hydrogen bonded complexes with stability controlled by substituent effects: 3-(6-aminopyridin-3-yl)acrylate case study

The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments. The AA (acceptor, acceptor) hydrogen-bonding pattern present in the salt forms a complex readily with a DD (donor, donor) hydrogen-bonding pattern of the substituted ureas even in polar and competitive environment. The double carbon–carbon bond in the acrylic acid derivative is subjected to photoisomerization. This is dependent on the association with substituted urea derivatives. The substituent in ureas influences the trans/cis isomerization kinetics and position of the photostationary state. Two mechanisms that influence the photoisomerization were proposed. To the best of our knowledge, the trans/cis photoisomerization influenced by the substituent in such a hydrogen-bonding pattern has not observed previously. It was shown that interaction with urea derivatives causes lowering of the trans-to-cis photoreaction rates.

The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments.

Ionic liquid-mediated solvothermal synthesis of 4,4′-methylenediphenyl diisocyanate (MDI): an efficient and environment-friendly process

Mechanism for MDI synthesis via decomposition of MDC catalyzed by Zn(OAc)₂–[EAmim]BF₄.

Enantioselective Friedel-Crafts Alkylation between Nitroalkenes and Indoles Catalyzed by Charge Activated Thiourea Organocatalysts

A series of methylated and octylated pyridinium and quinolinium containing thiourea salts with a chiral 2-indanol substituent are reported. These organocatalysts are positively charged analogues of privileged bis(3,5-trifluoromethyl)phenyl substituted thioureas, and are found to be much more active catalysts despite the absence of an additional hydrogen bond donor or acceptor site (i.e., the presence of a heteroatom-hydrogen or heteroatom). Friedel-Crafts reactions of trans-β-nitorostyrenes with indoles are examined, and good yields and enantioselectivities are obtained. Mechanistic studies indicate that this is a second-order transformation under the employed conditions, and is consistent with the dimer of the thiourea being the active catalyst. Charged organocatalysts, consequently, represent an attractive design strategy for catalyst development.

Thiosemicarbazone organocatalysis: tetrahydropyranylation and 2-deoxygalactosylation reactions and kinetics-based mechanistic investigation

The first use of thiosemicarbazone-based organocatalysis was demonstrated on both tetrahydropyranylation and 2-deoxygalactosylation reactions. The organocatalysts were optimised using kinetics-based selection. The best catalyst outperformed previously reported thiourea catalysts for tetrahydropyranylation by 50-fold. Hammett investigations of both the organocatalyst and the substrate indicate an oxyanion hole-like reaction mechanism.

Reductive Etherification via Anion-Binding Catalysis

Reductive condensations of alcohols with aldehydes/ketones to generate ethers are catalyzed by a readily accessible thiourea organocatalyst that operates in combination with HCl. 1,1,3,3-tetramethyldisiloxane serves as a convenient reducing reagent. This strategy is applicable to challenging substrate combinations and exhibits functional group tolerance. Competing reductive homocoupling of the carbonyl component is suppressed.

Metal-organic and covalent organic frameworks as single-site catalysts

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

Asymmetric Synthesis of Rauhut-Currier type Products by a Regioselective Mukaiyama Reaction under Bifunctional Catalysis

The reactivity and the regioselective functionalization of silyl-diene enol ethers under a bifunctional organocatalyst provokes a dramatic change in the regioselectivity, from the 1,5- to the 1,3-functionalization. This variation makes possible the 1,3-addition of silyl-dienol ethers to nitroalkenes, giving access to the synthesis of tri- and tetrasubstituted double bonds in Rauhut-Currier type products. The process takes place under smooth conditions, nonanionic conditions, and with a high enantiomeric excess. A rational mechanistic pathway is presented based on DFT and mechanistic experiments.

Asymmetric Brønsted acid catalysis with chiral carboxylic acids

This review provides an overview of various catalytic enantioselective transformations that utilize chiral carboxylic acids as Brønsted acid catalysts.

Organocatalytic diastereoselective synthesis of spirooxindoles via [3+2] cycloadditions of azomethine ylides with α,β-unsaturated esters

Isatin-derived azomethine ylides react diastereoselectively with unsaturated esters to afford densely substituted spirooxindols.

Bis-Boron Compounds in Catalysis: Bidentate and Bifunctional Activation

The development of metal-free catalysts as an alternative to the use of transition metals has gained tremendous interest in the past. In catalysis, Lewis acidity is one of the major principles used for the activation of organic compounds. Improving the reactivity and selectivity of Lewis acids by utilizing bidentate interactions was already proposed 50 years ago. Nevertheless, product inhibition due to strong binding has made applications of bidentate Lewis acids challenging for many years. Recently, bis-boron compounds have been found to be very effective and several applications in Diels-Alder reactions, carbon dioxide reduction, and ammonia-borane dehydrogenation were reported. All three transformations are enabled by the catalyst at different stages during the course of the reaction. These new and useful examples illustrate the great potential of the concept.

On- and Off-Cycle Catalyst Cooperativity in Anion-Binding Catalysis

Chiral, neutral H-bond donors have found widespread use as catalysts in enantioselective reactions involving ion-pair intermediates. Herein, a systematic mechanistic study of a prototypical anion-binding reaction, the thiourea-catalyzed enantioselective alkylation of α-chloroethers, is detailed. This study reveals that the catalyst resting state is an inactive dimeric aggregate that must dissociate and then reassemble to form a 2:1 catalyst-substrate complex in the rate-determining transition structure. Insight into this mode of catalyst cooperativity sheds light on the practical limitations that have plagued many of the H-bond donor-catalyzed reactions developed to date and suggests design strategies for new, highly efficient catalyst structures.

A Chiral Thiourea as a Template for Enantioselective Intramolecular [2 + 2] Photocycloaddition Reactions

A chiral (1R,2R)-diaminocyclohexane-derived bisthiourea was found to exhibit a significant asymmetric induction in the intramolecular [2 + 2] photocycloaddition of 2,3-dihydropyridone-5-carboxylates. Under optimized conditions, the reaction was performed with visible light employing 10 mol % of thioxanthone as triplet sensitizer. Due to the different electronic properties of its carbonyl oxygen atoms, a directed binding of the substrate to the template is possible, which in turn enables an efficient enantioface differentiation.

1H-Imidazol-4(5H)-ones and thiazol-4(5H)-ones as emerging pronucleophiles in asymmetric catalysis

Asymmetric catalysis represents a very powerful tool for the synthesis of enantiopure compounds. In this context the main focus has been directed not only to the search for new efficient chiral catalysts, but also to the development of efficient pronucleophiles. This review highlights the utility and first examples of 1H-imidazol-4(5H)-ones and thiazol-4(5H)-ones as pronucleophiles in catalytic asymmetric reactions.