Enantioselective Diels–Alder Reaction of α-Acyloxyacroleins Catalyzed by Chiral 1,1′-Binaphthyl-2,2′-diammonium Salts

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

Novel asymmetric aminocatalytic cycloadditions are described between formyl cycloheptatrienes and 6,6-dimethylfulvene that lead to [4 + 2], [6 + 2], and [4 + 6] cycloadducts. The unprecedented reaction course is dependent on the position of the formyl functionality in the cycloheptatriene core, and each formyl cycloheptatriene isomer displays a distinct reactivity pattern. The formyl cycloheptatriene isomers are activated by a chiral primary diamine catalyst, and the activation mode is dependent on the position of the formyl functionality relative to the cycloheptatriene core. The [4 + 2] and [6 + 2] cycloadducts are formed via rare iminocatalytic inverse electron-demand cycloadditions, while the [4 + 6] cycloadduct is formed by a normal electron-demand cycloaddition. The reactivity displayed by the different formyl cycloheptatrienes was investigated by DFT calculations. These computational studies account for the different reaction paths for the three isomeric formyl cycloheptatrienes. The aminocatalytic [4 + 2], [6 + 2], and [4 + 6] cycloadditions proceed by stepwise processes, and the interplay between conjugation, substrate distortion, and dispersive interactions between the fulvene and aminocatalyst mainly defines the outcome of each cycloaddition.

Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells

P-i-n geometry perovskite solar cells (PSCs) offer simplified fabrication, greater amenability to charge extraction layers, and low-temperature processing over n-i-p counterparts. Self-assembled monolayers (SAMs) can enhance the performance of p-i-n PSCs but ultrathin SAMs can be thermally unstable. We report a thermally robust hole-selective layer comprised of nickel oxide (NiOx) nanoparticle film with a surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC) SAM that can improve and stabilize the NiOx/perovskite interface. The energetic alignment and favorable contact and binding between NiOx/MeO-4PADBC and perovskite reduced the voltage deficit of PSCs with various perovskite compositions and led to strong interface toughening effects under thermal stress. The resulting 1.53-electron-volt devices achieved 25.6% certified power conversion efficiency and maintained >90% of their initial efficiency after continuously operating at 65 degrees Celsius for 1200 hours under 1-sun illumination.

Stereoselective Construction of Fluorinated Quaternary Stereogenic Centers via an Organocatalytic Asymmetric exo-Selective Diels-Alder Reaction in the Presence of Water

A catalytic, asymmetric Diels-Alder reaction of α-fluoro α,β-unsaturated aldehydes and cyclopentadiene was developed using diarylprolinol silyl ether as an organocatalyst. The reaction proceeds in toluene with trifluoroacetic acid as an additive (condition A). Perchloric acid salt of diarylprolinol silyl ether also promotes the reaction using water as a reaction medium (condition B). In both cases, excellent exo-selectivity and enantioselectivity were obtained with generation of a fluorinated quaternary chiral center.

Expanding the Chiral Monoterpene Pool: Enantioselective Diels-Alder Reactions of α-Acyloxy Enones

An enantioselective Diels-Alder (DA) reaction of α-acyloxy enones has been developed to synthesize chiral oxidized cyclohexenes. Yttrium(III) triflate, in conjunction with a chiral pyridinebisimidazoline (PyBim) ligand, was found to catalyze the asymmetric [4 + 2] cycloaddition with a variety of dienes and α-acyloxy enone dienophiles. Using this method, terpinene-4-ol, a key intermediate in the synthesis of commercial herbicide cinmethylin, can be prepared in four steps from isoprene. A combination of kinetic data and NMR studies support a mechanism involving reversible binding of a dienophile to a yttrium catalyst followed by cycloaddition with a diene as the rate-determining step.

Supramolecular polymers with reversed viscosity/temperature profile for application in motor oils

We report novel supramolecular polymers, which possess a reversed viscosity/temperature profile. To this end, we developed a series of ditopic monomers featuring two self-complementary binding sites, either the guanidiniocarbonyl pyrrole carboxylic acid (GCP) or the aminopyridine carbonyl pyrrole carboxylic acid (ACP). At low temperatures, small cyclic structures are formed. However, at elevated temperatures, a ring–chain transformation leads to the formation of a supramolecular polymer. We demonstrate that this effect is dependent on the concentration of the solution and on the polarity of the solvent. This effect can counteract the loss of viscosity of the solvent at elevated temperatures, thus opening an application of our systems as viscosity index improvers (VIIs) in working fluids. This was tested for different motor oils and led to the identification of one compound as a promising VII.

Asymmetric Diels-Alder Reaction of α-Substituted and β,β-Disubstituted α,β-Enals via Diarylprolinol Silyl Ether for the Construction of All-Carbon Quaternary Stereocenters

The asymmetric Diels-Alder reaction of α-substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl-substituted diarylprolinol silyl ether to afford the exo-isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels-Alder reaction of a β,β-disubstituted α,β-unsaturated aldehyde, good exo-selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all-carbon quaternary stereocenters are constructed enantioselectively.

Catalytic discrimination between formyl groups in regio- and stereoselective intramolecular cross-aldol reactions

Catalytic discrimination between inequivalent formyl groups was achieved using an aniline-type acid-base catalyst for the regio-, diastereo-, and enantioselective intramolecular cross-aldol reactions of enolizable dials. Although l-proline gave a mixture of the regio- and stereoisomeric products in the presence of an N-containing 1,6-dial, the aniline-type catalyst afforded anti-3,4-disubstituted pyrrolidine in high regio-, and stereoselectivity beyond the background reaction, which led to the regioisomeric 2,3-disubstituted products. The mild reactivity of the aniline-type amine facilitated catalytic discrimination between the inequivalent formyl groups. Kinetic isotope effect studies and reductive amination experiments suggested that the regioselectivity was controlled under the enamine-forming steps.

An enantioselective Diels-Alder reaction of 1,2-dihydropyridines with α-acyloxyacroleins catalyzed by a chiral primary ammonium salt

The first enantioselective Diels-Alder reaction of 1,2-dihydropyridines with α-acyloxyacroleins catalyzed by a chiral primary ammonium salt has been developed and it offers more efficient routes to key synthetic intermediates of alkaloids, for which the direct preparations were unavailable before. The asymmetric induction can be understood through the optimized geometry of an iminium salt aqua complex derived from the catalyst and the dienophile.

New insights into the asymmetric Diels–Alder reaction: the endo- and S-selective retro-Diels–Alder reaction

The endo- and S-selective retro-Diels–Alder reactions have been proposed and verified in an imidazolethione-catalyzed asymmetric Diels–Alder reaction.

Trienamine catalysis for asymmetric Diels–Alder reactions of 2,4-dienones: a theoretical investigation

In the Diels–Alder reactions of 2,4-dienones with two dienophiles, cinchona alkaloid acts as an efficient bifunctional catalyst by generating an extended trienamine π-conjugated system and by orienting the dienophile at an appropriate position for a cycloaddition reaction.

Enantioselective Diels-Alder reaction of α-(acylthio)acroleins: a new entry to sulfur-containing chiral quaternary carbons

A catalytic and enantioselective Diels-Alder reaction of α-(carbamoylthio)acroleins induced by an organoammonium salt of chiral triamine is described. α-(Carbamoylthio)acroleins are designed and synthesized as new sulfur-containing dienophiles for the first time. The Diels-Alder reaction affords chiral tertiary thiol precursors with up to 91% ee.

Synthesis of axially chiral amino acid and amino alcohols via additive-ligand-free Pd-catalyzed domino coupling reaction and subsequent transformations of the product amidoaza[5]helicene

Novel optically active axially chiral amino acid and amino alcohols have been synthesized efficiently via lactam ring-opening, with the aid of an optically active alcohol, amidoaza[5]helicene 5, which has been readily prepared by an additive-ligand-free Pd catalyzed domino coupling reaction in a single step. The stereostructures of these chiral molecules have also been clarified.

Secondary and primary amine catalysts for iminium catalysis

Formation of iminium ions from the condensation of chiral secondary or primary amines with alpha,beta-unsaturated aldehydes or ketones can be used as an effective platform for the acceleration of a wide variety of catalytic asymmetric cycloaddition and conjugate addition reactions. The reversible formation of the active iminium ion species simulates the pi-electronics and equilibrium dynamics traditionally associated with Lewis acid activation of alpha,beta-unsaturated carbonyl compounds lowering the energy level of the LUMO associated with the pi-system and activating subsequent reaction. Importantly, these iminium ion catalysed processes offer the opportunity to conduct reactions in the presence of both moisture and air greatly adding to the practicality and general applicability of the chemistry described. Proposed catalytic cycles and transition state models for the induction of asymmetry provide reliable and robust predictive tools for the outcome of reactions and high functional group tolerance suggests this class of transformation will have broad application in the arena of synthetic organic chemistry as the area matures. This review describes the rapid expansion of iminium ion catalysis over recent years from its conceptual introduction to the development of a whole new arsenal of highly practical and effective methods with which to approach challenging and fundamental bond construction processes.

Steric modulation of chiral biaryl diamines via Pd-catalyzed directed C-H arylation

Palladium-catalyzed directed arylation of 2,2'-diacetamidobiaryls with aryl iodides provides efficient access to chiral ortho-substituted biaryl diamines. Aryl iodides with para- and meta-substituents are tolerated. Deprotection of the acetyl groups under basic conditions furnishes the free diamines, which should find broad utility in asymmetric catalysis.