Acceleration of the DABCO-promoted Baylis–Hillman reaction using a recoverable H-bonding organocatalyst

Computational Identification of Potential Organocatalysts (CIPOC) Reveals a 2-aminoDMAP/Urea Catalyst Superior to Its Thiourea Analogue

Asymmetric organocatalysis by bifunctional acid- and base-type small organic molecules has emerged as a promising way to enhance stereoselective organic transformations since the beginning of this millennium. Takemoto's tert-amine/thiourea catalyst, an archetype in these endeavors, has encouraged many to design new multifunctional alternatives. However, the discovery of efficient catalysts in a library of thousands of candidates containing the desired functionalities in their structures remains a great challenge both synthetically and computationally. We, toward these ends, developed a computational protocol (CIPOC─Computational Identification of POtential (Organo)Catalysts), which discovered a chiral 2-aminoDMAP/urea catalyst among 1600 multifunctional catalyst candidates enabling conjugate addition of malonates to trans-β-nitroalkenes rapidly (in a few hours) with exquisite selectivities and yields, producing superior results than that of Takemoto's. The unique activity of this chiral 2-aminoDMAP/urea is attributed to the dual function of the 2-aminoDMAP unit (double H-bonding and π-stacking interactions) in addition to the exceptional performance of the urea unit compared to thiourea, as a result of a lower energetic penalty required to distort the catalyst to its active conformation to provide optimal catalytic interactions.

Morita-Baylis-Hillman Spirannulation under Phosphine- and Anion-Binding Catalysis

We introduce an advancement in Morita-Baylis-Hillman (MBH) chemistry that provides access to α-spirannulated enones. The treatment of enone-tethered azaarenium salts with catalytic amounts of organophosphines provides spiroindenyl dihydropyridines. It represents the α-spirannulation of enones via an intramolecular MBH (IMBH) reaction utilizing dual phosphine- and anion-binding catalysis. The IMBH adducts were subjected to several post-synthetic modifications to access highly functionalized molecules.

Thiourea as Bifunctional Hydrogen Bond Donor and Brønsted Base Catalyst for Green One-Pot Synthesis of 2-Aryl/Heteroaryl/Styryl Benzothiazoles in the Aqueous Medium under Ultrasound Irradiation

A green organocatalysis cascade strategy using thiourea in catalytic amounts as both a hydrogen bond donor and a Brønsted base bifunctional catalyst was utilized to synthesize a series of 2-aryl/heteroaryl/styryl benzothiazole derivatives. This strategy involved an ultrasound-irradiated one-pot two-component reaction between substituted aldehydes and 2-amino thiophenols in an aqueous medium at 60 °C, using air as an oxidant. At the gram-scale, this protocol yielded 87% of the desired product, making it suitable for production at a larger scale. This green and mild protocol offers excellent yields, cost-effectiveness, atom economy, step economy, and a simple operation that does not require extra purification steps. Furthermore, the catalyst is easily recoverable and can be used for up to five cycles without a significant loss of any activity.

An Artificial Cofactor Catalyzing the Baylis-Hillman Reaction with Designed Streptavidin as Protein Host*

An artificial cofactor based on an organocatalyst embedded in a protein has been used to conduct the Baylis-Hillman reaction in a buffered system. As protein host, we chose streptavidin, as it can be easily crystallized and thereby supports the design process. The protein host around the cofactor was rationally designed on the basis of high-resolution crystal structures obtained after each variation of the amino acid sequence. Additionally, DFT-calculated intermediates and transition states were used to rationalize the observed activity. Finally, repeated cycles of structure determination and redesign led to a system with an up to one order of magnitude increase in activity over the bare cofactor and to the most active proteinogenic catalyst for the Baylis-Hillman reaction known today.

O-Monoacyltartaric Acid/(Thio)urea Cooperative Organocatalysis for Enantioselective Conjugate Addition of Boronic Acid

(Thio)urea cocatalyst accelerates O-monoacyltartaric acid (MAT)-catalyzed enantioselective conjugate addition of boronic acid to unsaturated ketone. Kinetic studies of this reaction revealed first-order dependence of each substrate and catalyst and second-order dependence of (thio)urea, leading to reduction of the catalyst loading and development of more active and enantioselective MAT monoaryl ester catalyst.

Highly efficient Ir-catalyzed asymmetric hydrogenation of benzoxazinones and derivatives with a Brønsted acid cocatalyst

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, affording various chiral dihydrobenzoxazinones and derivatives with excellent results.

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, which may display a new activation mode with a single anion-binding interaction among the substrate, cocatalyst Brønsted acid and ligand. This synthetic approach afforded a series of chiral dihydrobenzoxazinones and derivatives with excellent results (>99% conversion, 88–96% yields, 91–>99% ee, up to 40 500 TON). A key to success is the utilization of a strong Brønsted acid as the cocatalyst, such as hydrochloric acid, to form a possible single anion-binding interaction with the substrate and catalyst, which greatly contributed to the improvement of reactivity and enantioselectivity. Importantly, a creative and efficient synthetic route was developed to construct the important intermediate for the potential IgE/IgG receptor modulator through our catalytic methodology system.

Urea/thiourea: efficient, inexpensive and reusable catalysts for the synthesis of pyrazole derivatives with 2-iminothiazolidin-4-one and 2,4-thiazolidinediones under solvent-free conditions

An efficient and green synthesis of pyrazolyl-2,4-thiazolidinediones/pyrazolyl-2-iminothiazolidine-4-ones 7(a-j) has been developed using urea/thiourea as catalyst. Two methods (A and B) have been introduced for the synthesis of these compounds. Method A performed well for the condensation of pyrazole-4-carboxaldehydes 4(a-e) with 2-iminothiazolidin-4-one 5a and with 2,4-thiazolidinedione 5b at 110 [Formula: see text] for 16-20 min to furnish (Z)-5-((1,3-diphenyl-1H-pyrazol-4-yl)methylene)thiazolidine-2,4-diones 7(a-j) in excellent yields. In method B, the pyrazole-4-carboxaldehydes 4(a-e) were condensed with urea/thiourea at 110 [Formula: see text] for 10 min to give key intermediates 6(a-j) which were later condensed with 5a and 5b to afford 7(a-j) via the elimination of urea/thiourea rather than the formation of a Biginelli product. These two protocols are inexpensive, eco-friendly and high yielding to provide the final products. The synthesis of compound 7f.

Cooperative Trifunctional Organocatalysts for Proficient Proton Transfer Reactions

Cooperativity is essential to proficient catalysis, and designing biomimetic, cooperative catalysis is a major avenue to finding new and efficient chemical reactions with practical applications. One challenge in designed cooperative catalysis is to access high catalytic proficiency (large enhancement in both rate and enantioselectivity) as seen in biocatalysis. Here described is an approach of developing and investigating trifunctional organocatalysts with three distinct catalytic functionalities, in order to understand how cooperativity could be organized for enantioselective activation that confers the observed proficiency in a tandem Michael-aldol-proton transfer elimination model reaction. This in future may assist in finding not just cooperative but also regulated catalysis to expand the level of catalytic complexity and efficiency in biomimetic systems.

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.

Highly effective design strategy for the heterogenisation of chemo- and enantioselective organocatalysts

The covalent heterogenisation of cinchonine and 1,4-diazabicyclo[2.2.2]octane within a range of mesoporous silicas affords highly selective and active organocatalysts.

N,N'-Bis[3,5-bis(trifluoromethyl)phenyl]thiourea: a privileged motif for catalyst development

Over the last decade, the use of (thio)urea derivatives as organocatalysts in organic chemistry has increased rapidly. One of the key features is their ability to activate substrates and subsequently stabilize partially developing negative charges (e.g., oxyanions) in the transition states employing explicit double hydrogen bonding. Among (thio)urea-based catalysts, N,N'-bis[3,5-bis(trifluoromethyl)phenyl]thiourea developed by Schreiner's group (abbreviated here as Schreiner's thiourea) has played a very important role in the development of H-bond organocatalysts. Nowadays it is used extensively in promoting organic transformations, and the 3,5-bis(trifluoromethyl)phenyl motif thereof is used ubiquitously in H-bond catalysts. This review summarizes the key developments of Schreiner's thiourea-mediated reactions with the aim to further expand the applications of (thio)urea-based catalysts.

A general route to 1,3'-bipyrroles

A general method is described for the synthesis of 1,3'-bipyrroles. The route involves constructing a pyrrole ring on the nitrogen of a substituted 1H-pyrrole, so as to generate the 1,3'-bipyrrole. In this approach the nitrogen of the starting pyrrole was alkylated with a special Michael acceptor having an allylic leaving group, and the product was then modified in such a way that the second pyrrole ring could be formed by a Paal-Knorr reaction. Two variants of this sequence were examined, one of which led to formation of a 3-hydroxypyridine instead of the second pyrrole ring; the other variant used phenacyl bromide instead of the special Michael acceptor.

Selective Organic Synthesis for Sustainable Development

The data on the development of selective organic synthesis suitable for obtaining multifunctional organic compounds both linear and cyclic structures using environmentally benign, inexpensive and renewable resources summarized. This article is an extended abstract of a communication presented at the Conference Ecological Chemistry 2012.

Synthesis and evaluation of new guanidine-thiourea organocatalyst for the nitro-Michael reaction: Theoretical studies on mechanism and enantioselectivity

A new guanidine-thiourea organocatalyst has been developed and applied as bifunctional organocatalyst in the Michael addition reaction of diethyl malonate to trans-β-nitrostyrene. Extensive DFT calculations, including solvent effects and dispersion corrections, as well as ab initio calculations provide a plausible description of the reaction mechanism.

Nitrogen-Centered Nucleophile Catalyzed Thiol-Vinylsulfone Addition, Another Thiol-ene "Click" Reaction

A new group of nitrogen-centered nucleophilic catalysts for the thiol-Michael addition "click" reactions is examined. These nucleophiles showed efficient catalytic activities as compared with traditional base catalysts, such as triethylamine, and are demonstrated to be a viable strategy for cross-linking polymerization reactions. Additionally, an experimental and computational mechanistic study was performed, suggesting a pathway for the nitrogen-centered catalyst to undergo the nucleophilic addition mechanism.

An Efficient and Recyclable DMAP-Based Ionic Liquid/Water System for Morita–Baylis–Hillman Reactions

The efficient and recyclable system, DMAP-based ionic liquid/water, has been developed and used in the Morita– Baylis–Hillman reaction of aromatic aldehydes with acrylates. The coupling reactions under the described basic reaction conditions proceed quickly with good to excellent yields. The ionic liquid could be reused at least five times without significant loss of activity.

Novel phenyl(thio)ureas bearing (thio)oxothiazoline group as potential BACE-1 inhibitors: synthesis and biological evaluation

We report the synthesis and the β-site amyloid precursor protein cleaving enzyme-1 inhibitory properties of novel phenyl(thio)ureas bearing 2-(thio)oxothiazoline derivatives. A library of analogues was prepared according to specific synthetic schemes and the inhibitory activity was monitored using a fluorescence resonance energy transfer assay. Several analogues show potent inhibitory activities ranging between 1 and 0.01 µM and the activity is related to the NH acidity of the (thio)urea motif. Our results illustrate once again the close relationship between molecular recognition, complexation of the active site in enzymatic system, and organocatalysis utilizing explicit hydrogen bonding.

Thiourea dioxide promoted efficient organocatalytic one-pot synthesis of a library of novel heterocyclic compounds

The utility of thiourea dioxide as an efficient organocatalyst for the library synthesis of novel heterocyclic compounds via one-pot multicomponent coupling reactions is disclosed. Thiourea dioxide is an inexpensive and readily accessible catalyst, resulting in better product yields as compared to the corresponding thiourea as catalyst. Thiourea dioxide is found to be insoluble in various organic solvents and therefore at the end of the reaction products can be separated by extraction with diethyl ether and the recovered catalyst can be used several times with consistent catalytic activity.

Direct carbon-carbon bond formation via soft enolization: a biomimetic asymmetric Mannich reaction of phenylacetate thioesters

An asymmetric Mannich reaction of phenylacetate thioesters and sulfonylimines using cinchona alkaloid-based amino (thio)urea catalysts is reported that employs proximity-assisted soft enolization. This approach to enolization is based on the cooperative action of a carbonyl-activating hydrogen bonding (thio)urea moiety and an amine base contained within a single catalytic entity to facilitate intracomplex deprotonation. Significantly, this allows thioesters over a range of acidity to react efficiently, thereby opening the door to the development of a general mode of enolization-based organocatalysis of monocarboxylic acid derivatives.