Copper-Catalyzed α-Arylation of Nitroalkanes with (Hetero)aryl Bromides/Iodides

α-Aryl substituted nitroalkanes are valuable synthetic building blocks that can be easily converted into α-aryl substituted aldehydes, ketones, carboxylic acids, as well as amines. Herein, an efficient Cu/oxalamide-catalyzed coupling between nitroalkanes and (hetero)aryl halides (Br, I) was developed to direct access highly diverse α-aryl substituted nitroalkanes. Compared with the current state of art, this protocol is more environmentally friendly and practical for synthetic chemists. This approach is characterized by a broad substrate scope on both nitroalkane part (primary nitroalkanes and nitromethane) and sp2 halide part ((hetero)aryl bromides/iodides and alkenyl bromides/iodides). The excellent functional group tolerance was observed, which would enable real world synthetic applications. More importantly, TON of current transformation reached to 3640, when some aryl iodides were used as coupling partners. This represents currently the highest catalyst turnover for transition-metal catalyzed α-arylation of nitroalkanes. Furthermore, the successful application in late-stage modification of complex molecules and synthesis of a known retinoid X receptor (RXR) antagonist exemplified its synthetic potential.

Fluorine-induced diastereodivergence discovered in an equally rare enantioselective syn-aza-Henry reaction

Attention to the aza-Henry reaction, particularly over the past two decades, has resulted in a wide range of effective catalysts for the enantio- and diastereoselective versions, driven by the versatility of the β-amino nitroalkane products as precursors to secondary amines and vic-diamines. Despite this broad effort, syn-diastereoselective variants are exceedingly rare. We have discovered a subset of α-fluoro nitroalkane additions that are characterized by an unusual crossover in diastereoselection, often delivering the products with high selectivities. We report here a rigorous comparative analysis of non-fluorinated and α-fluoro nitroalkanes in their additions to azomethines. Both homogeneous and heterogeneous catalysis were applied to probe the possibility that this phenomenon might be more widely operative in the enantioselective additions of fluorine-substituted carbon nucleophiles. A complete correlation within four categories is described that uncovered a clear trend, while revealing a dramatic and distinct reversal of diastereoselection that would normally go undetected.

Zwitterion-Catalyzed Isomerization of Maleic to Fumaric Acid Diesters

Fumaric acid diesters are important building blocks for organic synthesis. A class of zwitterionic organocatalysts based on an amide anion/iminium cation charge pair were found to be effective in catalyzing the isomerization of maleic acid diesters to give fumaric acid diesters. Comparison of the performance of different zwitterionic organocatalysts toward the reaction revealed that nonclassical hydrogen bonding was involved in the stabilization of the Michael adduct intermediate.

Computational insight into the cooperative role of non-covalent interactions in the aza-Henry reaction catalyzed by quinine derivatives: mechanism and enantioselectivity

Density functional theory (DFT) calculations were performed to elucidate the mechanism and the origin of the high enantioselectivity of the aza-Henry reaction of isatin-derived N-Boc ketimine catalyzed by a quinine-derived catalyst (QN). The C-C bond formation step is found to be both the rate-determining and the stereo-controlled step. The results revealed the important role of the phenolic OH group in pre-organizing the complex of nitromethane and QN and stabilizing the in situ-generated nitronate and protonated QN. Three possible activation modes for C-C bond formation involving different coordination patterns of catalyst and substrates were studied, and it was found that both the ion pair-hydrogen bonding mode and the Brønsted acid-hydrogen bonding mode are viable, with the latter slightly preferred for the real catalytic system. The calculated enantiomeric excess (ee) favouring the S enantiomer is in good agreement with the experimental result. The high reactivity and enantioselectivity can be ascribed to the cooperative role of the multiple non-covalent interactions, including classical and non-classical H bonding as well as anionπ interactions. These results also highlight the importance of the inclusion of dispersion correction for achieving a reasonable agreement between theory and experiment for the current reaction.

New Developments in Chiral Cooperative Ion Pairing Organocatalysis by Means of Ammonium Oxyanions and Fluorides: From Protonation to Deprotonation Reactions

This personal account summarizes our contribution to the ion pairing organocatalysis mainly by use of chiral quaternary or tertiary ammonium fluorides, aryloxides and carboxylates. Starting from an experimental observation, we were able to develop several approaches for the enantioselective protonation of silyl enolates and enol esters giving rise to chiral carbonyl compounds bearing a stereogenic center at the α-position. Moving from protonation to deprotonation reactions, chiral ammonium ion pair catalysts were successfully applied to several asymmetric transformations such as an Henry reaction or a direct vinylogous aldol reaction to cite a few. An outlook of further possible developments in this field of research will also be discussed.

Chiral ammonium betaine-catalyzed asymmetric Mannich-type reaction of oxindoles

A highly diastereo- and enantioselective Mannich-type reaction of 3-aryloxindoles with N-Boc aldimines was achieved under the catalysis of axially chiral ammonium betaines. This catalytic method provides a new tool for the construction of consecutive quaternary and tertiary stereogenic carbon centers on biologically intriguing molecular frameworks with high fidelity.

Cinchonium Betaines as Efficient Catalysts for Asymmetric Proton Transfer Catalysis: The Development of a Practical Enantioselective Isomerization of Trifluoromethyl Imines

We have developed a new class of cinchonium betaine catalysts bearing both a base moiety and an aromatic moiety as an N-substituent of the quinuclidine motif. These cinchonium betaines were found to promote proton transfer catalysis with 1000-5000 turnovers per 24 h, thereby enabling us to realize highly efficient enantioselective isomerization of trifluoromethyl imines to provide a practical access to optically active trifluoromethylated amines.

Vinylogy in nitronates: utilization of α-aryl conjugated nitroolefins as a nucleophile for a highly stereoselective aza-Henry reaction

Vinylogous nitronates generated from α,β-disubstituted nitroolefins undergo stereoselective bond formation with aldimines at the sterically encumbered α-position, establishing contiguous tertiary–quaternary stereocenters.

Enantioselective Addition of Bromonitromethane to Aliphatic N-Boc Aldimines Using a Homogeneous Bifunctional Chiral Organocatalyst

This report details the enantioselective synthesis of β-amino-α-bromo nitroalkanes with β-alkyl substituents, using homogeneous catalysis to prepare either antipode. Use of a bifunctional Brønsted base/acid catalyst allows equal access to either enantiomer of the products, enabling the use of Umpolung Amide Synthesis (UmAS) to prepare the corresponding L- or D-α-amino amide bearing alkyl side chains - overall, in only 4 steps from aldehyde. The approach also addresses an underlying incompatibility between bromonitromethane and solid hydroxide bases.