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SummaryThe development of efficient Friedel–Crafts alkylations of arenes and heteroarenes using only catalytic amounts of a Lewis acid has gained much attention over the last decade. The new catalytic approaches described in this review are favoured over classical Friedel–Crafts conditions as benzyl-, propargyl- and allyl alcohols, or styrenes, can be used instead of toxic benzyl halides. Additionally, only low catalyst loadings are needed to provide a wide range of products. Following a short introduction about the origin and classical definition of the Friedel–Crafts reaction, the review will describe the different environmentally benign substrates which can be applied today as an approach towards greener processes. Additionally, the first diastereoselective and enantioselective Friedel–Crafts-type alkylations will be highlighted.

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The nitroaldol (Henry) reaction is an important carbonyl addition process that affords products that may be transformed into valuable building blocks. 1 Therefore, it is not surprising that recent efforts have focused on the development of catalytic enantioselective reaction variants. The current contributions to this area have been highlighted by the recent studies of Shibasaki and Trost. [2][3][4][5] The purpose of this Communication is to report a new catalyst system for the nitroaldol reaction (eqs 1,2; M ) Cu, X ) OAc). The basis for the current study was to identify a weakly Lewis acidic metal complex bearing moderately basic charged ligands (X) that would facilitate the deprotonation of nitroalkanes (eq 1) as a prelude to the aldol addition process (eq 2). It was felt that divalent metal acetate-ligand complexes of the general structure A might meet these requirements because acetate has been employed as a Brønsted base in the racemic nitroaldol reaction. 1 A series of divalent metal acetates in combination with chiral bidentate ligands were screened as enantioselective catalysts for the nitroaldol process. 6,7 From this survey, bis(oxazoline) 8 (box) complexes derived from Cu(OAc) 2 emerged as promising catalyst candidates. The results from the ligand survey with this metal acetate are summarized in Table 1. The five box ligands (1a-d, 2) with the illustrated absolute configurations that were evaluated with Cu(OAc) 2 ‚H 2 O afforded promising levels of enantioselection (entries 1-5). 8,9 In each instance, the reactions carried out at ambient temperature were complete within 24 h. From this comparison, the indabox ligand 2 proved to be the ligand of choice, 9 providing the nitro alcohol product in 74% ee (Table 1, entry 5). With ethanol as the solvent (Table 1, entry 6), the nitro alcohol product was isolated in 81% ee. Further optimization of this process showed that the reaction may be performed with lower catalyst loadings (1-5 mol %), while the use of 10 equiv of nitromethane was found to be sufficient for the reaction to proceed to completion. Reaction concentrations could also be increased to as high as 1.0 M with no change in enantioselectivity. Cu(II) carboxylate structure was also evaluated with ligand 2, and it was concluded that this catalyst variable is subordinate to ligand architecture. 10 In all instances, the only side reaction observed in these reactions was the accompanying dehydration product.With optimized conditions in hand, the scope of the reaction was explored (Table 2). In general, high enantiomeric excesses (87-94% ee) are observed at room temperature for aromatic aldehydes bearing either electron-withdrawing or electron-donating groups (entries 1-9). 11 Aliphatic branched and unbranched aldehydes are also acceptable substrates, affording nitro alcohol adducts in good yields and enantioselectivities (entries 10-15, 90-94% ee).Reaction enantioselectivity can be further improved by lowering the temperature at the accompanying expense of increasing the reaction time. In one insta...

The first enantioselective Brønsted acid catalyzed reduction of imines has been developed. This new organocatalytic transfer hydrogenation of ketimines with Hantzsch dihydropyridine as the hydrogen source offers a mild method to various chiral amines with high enantioselectivity. The stereochemistry of the chiral amines can be rationalized by a stereochemical model derived from an X-ray crystal structure of a chiral BINOL phosphate catalyst. [reaction: see text]

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Ketyl radical and amino radical anions, valuable reactive intermediates for C-C bond-forming reactions, are accessible through a C=O/C=NR umpolung. However, their utilization in catalysis remains largely underdeveloped owing to the high reduction potential of carbonyl compounds and imines. In the context of photoredox catalysis, tertiary amines are commonly employed as sacrificial co-reducing agents. Herein, an additional role of the amine is proposed, in which it is essential for the organocatalytic substrate activation. The combination of photoredox catalysis and carbonyl/imine activation enables the reductive coupling of aldehydes, ketones, and imines under mild reaction conditions.

The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that highly crystalline porous solids, metal-organic frameworks, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known metal organic framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal-organic frameworks, and for other applications.