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Scalable and economical methods for the production of optically pure amino acids, both natural and unnatural, are essential for their use as synthetic building blocks. Currently, enzymatic dynamic kinetic resolution (DKR) underpins some of the most effective processes. Here we report the development of enantioselective extraction coupled with racemization (EECR) for the chirality conversion of underivatized amino acids. In this process, the catalytic racemization of amino acids in a basic aqueous solution is coupled with the selective extraction of one enantiomer into an organic layer. Back-extraction from the organic layer to an acidic aqueous solution then completes the deracemization of the amino acid. The automation of the EECR process in a recycling flow reactor is also demonstrated. Continuous EECR is made possible by the sterically hindered chiral ketone extractant 5, which prevents the coextraction of the copper racemization catalyst because of its nonplanar geometry. Furthermore, the extractant 5 unexpectedly forms imines with amino acids faster and with greater enantioselectivity than less bulky derivatives, even though 5 cannot participate in intramolecular resonance-assisted hydrogen bonding. These features may allow EECR to challenge the preponderance of enzymatic DKR in the production of enantiomerically enriched amino acids., Dynamic kinetic resolution is a common approach for the preparation of optically pure amino acids using enzymes. Here the authors report an alternative method based on enantioselective extraction coupled with racemization, in which a bulky extractant affords hydrophobic extractable imines with amino acids rapidly, reversibly and enantioselectively.

Despite the common use of quinones as oxidizing agents in N-heterocyclic carbene (NHC)-based organocatalysis and transition-metal catalysis, the direct reactivity of quinones with NHCs remains underexplored. In this paper, we report the reactivity of NHCs with common p- and o-quinones, uncovering three unreported reactions involving contractions of the quinone ring that lead to push-pull furanolactone chromophores, NHC fulvalenes, and α-acylimidazolium cyclopentenone derivatives. These experiments also provide a rationale for the superior compatibility of tetra- tert-alkylated diphenoquinones in NHC-based oxidative transformations.

Recent advances in transition metal-catalyzed C–H bond functionalization have profoundly impacted synthetic strategy. Since organic substrates typically contain several chemically distinct C–H bonds, controlling the regioselectivity of C–H bond functionalization is imperative to harness its full potential. Moreover, the ability to alter reaction pathways to selectively functionalize different C–H bonds in a substrate represents a greater opportunity and challenge. The choice of catalysts, ligands, solvents, and even more subtle variations of the reaction conditions have been shown to allow the formation of regioisomeric C–H functionalization products starting from the same precursors. This review describes recent advances in transition metal-catalyzed divergent C–H bond functionalization that highlight its potential in organic synthesis.

A rhodium(II)-catalyzed denitrogenative coupling of N-sulfonyl-1,2,3-triazoles with ambiphilic β-enamino esters affords 2,5-dihydro-1H-imidazoles (3-imidazolines) with broad functional group tolerance. Mechanistic studies using a deuterium-labeled triazole suggest that the reaction proceeds in a cascade through the N-H insertion of an α-imino rhodium-carbene, followed by enamine-imine tautomerization and conjugate addition. Moreover, the reaction proceeds with high diastereoselectivity for α-substituted β-enamino esters (R(3) = Me, Ph) to give a single diastereomer.

The Pd-catalyzed oxidative olefin amination of the zinc bromide complex intermediate, formed by the sequential reaction of nitriles with a Reformatsky reagent and 1-alkynes, affords pyrrole derivatives in good to excellent yields. This tandem protocol provides a simple, efficient, and atom- and pot-economical way to quickly build polysubstituted NH-pyrroles starting from readily available reagents in a regiocontrolled manner with a broad substrate scope and high functional group tolerance. In contrast, the Pd-catalyzed oxidative olefin amination of an isolated α-vinyl-β-enamino ester did not proceed effectively, but the reaction efficiency can be restored by addition of n-BuZnBr or Zn(OAc)2, indicating the crucial role of the zinc complex in this transformation. The synthetic utility of this protocol is exemplified by the rapid synthesis of pyrrolophenanthrenes and pyranopyrrolones through selective Pd- and Cu-catalyzed C-C and C-O bond-forming reactions.

A rhodium(II)-catalyzed coupling of 1-sulfonyl-1,2,3-triazoles, prepared from 1-alkynes and sulfonyl azides, with Morita–Baylis–Hillman (MBH) adducts afforded highly functionalized α-methylene-δ-oxo-γ-amino esters in excellent yields with broad functional group tolerance. This transformation can also be successfully accomplished as a multicomponent all-in-one-pot reaction of 1-alkynes, sulfonyl azides and MBH adducts in the presence of Cu(I) and Rh(II) catalysts.

Precursor polymers prepared from 1,4-dialkoxy-1,4-diarylcyclohexane monomers are converted to the conjugated poly(arylene dienylene)s during the course of an acid-catalyzed deprotection−elimination reaction. These materials are employed as combined active materials and photoresists in the surface patterning of luminescent conjugated polymers.

A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C60 with an arylboronic acid in the presence of a catalytic amount of Pd(2-PyCH=NPh)(OCOC6F5)2 in H2O/1,2-Cl2C6H4 at room temperature furnishes the hydroarylation product (Ar-C60-H) in good yield with high selectivity. This complex possesses high catalytic activity paired with bench stability in the solid state.

Iptycene-type quinoxaline and thienopyrazine monomers were successfully synthesized via a condensation between 10-dihydro-9,10-ethanoanthracene-11,12-dione and the corresponding diamines. Copolymers based on fluorene and three iptycene monomers were prepared via Suzuki coupling reaction, and they exhibited good solubility in appropriate organic solvents. These copolymers are fluorescent in both solution and the solid state, emitting blue, greenish-blue, and red color due to the different electronic properties of the iptycene comonomers. The difference in their absorption and emission spectra was attributed to the donor−acceptor charge transfer interactions and/or polymer backbone conformational changes induced by steric effects. Moreover, the spectroscopic data clearly demonstrated the insulating effect of iptycene units, which prevented the aggregation of the polymer chains and the formation of excimers in the solid state.

Heterocyclic monomers based on 2,1,3-benzooxadiazole and 2,1,3-benzothiadiazole bearing solubilizing side chains have been synthesized in high yields over three steps from readily available starting materials. The monomers are efficiently cross-coupled with diynes and bis(boronates) to afford high molecular weight luminescent poly(arylene ethynylene)s and polyfluorenes that exhibit red-shifted absorption and emission maxima, greater solubility, and reduced aggregation.

A new fluorescent turn-on probe (3) for the selective sensing and bioimaging of thiols is reported. In aqueous buffer solutions at physiological pH, thiols cleave the 2,4-dinitrobenzenesulfonyl group to release the red-emissive donor-acceptor fluorophore (4). The probe displays excellent immunity to interference from nitrogen and oxygen nucleophiles and the imaging of thiols in living cells is demonstrated.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives bearing varied substituents at the meso position (i.e., CF3 , CH3 , COOR, CHO, CN, Cl, iPr) were synthesized to elucidate the structure-property relationships that give rise to emissive J-aggregates. Several new BODIPY derivatives can be added to the previously reported 1,3,5,7-tetramethyl-8-trifluoromethyl derivative to the list of those forming J-aggregates, in addition to other dyes that are emissive in the solid state without forming J-aggregates.

The design and implementation of tandem reactions provides organic chemists with numerous challenges, in particular that of undesired cross-reactivity between substrates. Among organometallics, the use of organozinc reagents in tandem reactions provides several advantages as a result of their broad functional group tolerance and compatibility with transition metals. This review highlights prominent examples of recent advances in tandem reactions with organozinc reagents that illustrate their potential in organic synthesis.

Soluble anthryl-based conjugated poly( phenylene ethynylene)s (PPEs) have been synthesized using palladium-catalyzed Sonogashira-Hagihara cross-coupling polymerization reactions. Molecular weights up to 3.5x10(4) g center dot mol(-1) were obtained, making them suitable for spectroscopic solution characterizations and thin film processing. The selective reactivity of these polymers as multidienes has been successfully demonstrated with strong dienophiles. Diels-Alder reactions proceed cleanly to completion with unhindered dienophiles such as N-alkylated maleimide derivatives. TGA analysis revealed thermal retro-Diels-Alder reactions at modest temperatures around 210 degrees C. Compared with their parent polymers, the cycloadduct polymers exhibited dramatic hypsochromic shifts of their emission and absorption maxima up to 80nm along with a considerable quantum yield enhancement. These original anthryl-based polymers appear attractive as reactive conjugated materials whose optical properties can easily be tuned with quantitative Diels-Alder reactions.

A zinc bromide complex, formed by the sequential reaction of nitriles with a Reformatsky reagent and terminal alkynes, is used as an intermediate for divergent palladium-catalyzed reactions. The reaction pathway of the intermediate is precisely controlled by the choice of the reaction solvent or the palladium catalyst to quickly form four different aromatic scaffolds--arylamines, aminoindenes, pyrroles, and quinolines--starting from readily available nitriles.

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds.

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions.

Direct metalation of bis(1,2,3-triazolium) salts affords mononuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to that of 2,2′-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-ylidenes) (bitz). As the former, but in contrast to the latter, the free i-bitz can be isolated, which paves the way for various applications.

The synthesis of new heterocyclic oligo(phenylene) analogues based on soluble, nonaggregating 1,2-diazines is reported. Improved palladium-catalyzed reductive coupling methods were developed to allow for the preparation of large quantities of iptycene-derived bipyridazines and biphthalazines, and the controlled synthesis of well-defined oligomers up to sexipyridazine. Crystallographic, spectroscopic, and computational evidence indicate that in these analogues, hindrance at the ortho position is relaxed relative to poly(phenylenes). The resulting building blocks are promising for incorporation in conjugated electronics materials, and as new iptycene-derived ligands for transition metals.

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Bronsted acid results in protonolysis of the Ru–MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts.