Boris J. Nachtsheim

A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis

Summary The 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.

A metal-free amination of benzoxazoles--the first example of an iodide-catalyzed oxidative amination of heteroarenes.

An efficient transition-metal-free amination of benzoxazoles has been developed. With catalytic amounts of tetrabutylammoniumiodide (TBAI), aqueous solutions of H(2)O(2) or TBHP as co-oxidant and under mild reaction conditions, highly desirable 2-aminobenzoxazoles were isolated in excellent yields of up to 93%. First mechanistic experiments indicate the in situ iodination of the secondary amine as the putative mode of activation.

Modulating the Acidity: Highly Acidic Brønsted Acids in Asymmetric Catalysis

Recently, chiral highly acidic Brønsted acids have emerged as powerful catalysts for enantioselective C-C and C-X bond-forming reactions. Their strong acidity renders them valuable tools for the activation of imines, carbonyl compounds, and other weakly basic substrates. As a result, new perspectives are opened and highly stereoselective transformations based on the concept of chiral contact-ion-pair catalysis can be realized. This Minireview gives an overview of the design and application of these new organocatalysts and presents recent results in this rapidly growing field.

OH‐Directed Alkynylation of 2‐Vinylphenols with Ethynyl Benziodoxolones: A Fast Access to Terminal 1,3‐Enynes

The first direct alkynylation of 2-vinylphenols was developed. The rationally optimized hypervalent iodine reagent TIPS-EBX* in combination with [(Cp*RhCl2)2] as a C-H-activating transition metal catalyst enables the construction of a variety of highly substituted 1,3-enynes in high yields of up to 98%. This novel C-H activation method shows excellent chemoselectivity and exclusive (Z)-stereoselectivity, and it is also remarkably mild and tolerates a variety of functional groups. Furthermore, synthetic modifications of the resulting 1,3-enynes were demonstrated. To our knowledge, this is the first example for an OH-directed C-H alkynylation with hypervalent iodine reagents.

TBAI-Catalyzed Oxidative Cross-Coupling of Phenols and 2-Aminoacetophenones

An iodide-catalyzed oxidative cross-coupling between phenols and 2-aminoacetophenones has been developed. Using catalytic amounts of tetrabutylammoniumiodide (TBAI) as an iodine-containing catalyst and aqueous solutions of tert-butyl hydro-peroxide (TBHP) as the stoichiometric co-oxidant, a variety of α-phenoxylated 2-aminoacetophenones could be obtained in yields of up to 92% after remarkably short reaction times (20 min). This is a very rare example for an iodide-catalyzed intermolecular cross-coupling utilizing phenols. However, this efficient methodology could be further extended toward an intramolecular variant which gives direct access to a range of dihydro-4H-benzo[e][1,3]oxazin-4-ones.

Iodide-Catalyzed Halocyclization/Cycloaddition/Elimination Cascade Reaction

An iodocyclization reaction of o-alkynylphenyl carboxaldehydes is reported that is truly catalytic with respect to the electrophilic iodine source. With a combination of tetrabutylammonium iodide (TBAI), Oxone as non-nucleophilic and easy to handle co-oxidant, and fluorinated protic solvents, highly substituted 1-naphthalenones could be prepared in high yields of up to 91%.

Alkynyliodonium Salt Mediated Alkynylation of Azlactones: Fast Access to Cα-Tetrasubstituted α-Amino Acid Derivatives

An efficient electrophilic alkynylation of azlactones (oxazol-5(4H)-ones) is developed using alkynyl(phenyl)iodonium salts as the electrophilic alkyne source. After remarkably short reaction times, the desired alkyne functionalized azlactones are obtained in 60-97% yield and can be transformed easily into a variety of quaternary α-amino acid derivatives.

Direct Synthesis of 2,5-Disubstituted Oxazoles through an Iodine-Catalyzed Decarboxylative Domino Reaction

An efficient iodine-catalyzed synthesis of highly substituted oxazoles is presented. Starting from readily available aryl methyl ketones, β-keto esters, or styrenes, in combination with α-amino acids as amine-containing coupling partners, the corresponding 2-alkyl-5-aryl- substituted oxazoles were obtained in up to 80% yield via a decarboxylative domino reaction.

Palladium-catalyzed synthesis of N-arylated carbazoles using anilines and cyclic diaryliodonium salts

Summary The direct synthesis of N-arylated carbazoles through a palladium-catalyzed amination of cyclic iodonium salts with anilines is described. In particular, electron-poor aniline derivatives reacted smoothly with only 5 mol % of Pd(OAc)2 as catalyst to give the desired products in up to 71% yield. Furthermore, the reactivity of cyclic iodonium salts is compared with the reactivity of the corresponding cyclic bromonium analogues.

Creating antioxidants by oxidation catalysis

A key component of vitamin E can be synthesized without use of expensive transition metal catalysts. [Also see Report by Uyanik et al.] Tocopherols are naturally occurring fat-soluble antioxidants that protect living cells against highly reactive free radicals. The best-known example is D-α-tocopherol, the most biologically active ingredient in vitamin E. Their potent antioxidative properties result from the 6-hydroxy chroman (6-chromanol) motif, which is a hidden hydroquinone. The 6-chromanol unit readily reacts with fatty acid peroxide radicals to give stabilized chromanoxyl radicals and far less reactive hydroperoxides (1); both intermediates can then be degraded by the cell safely. Because of their unique antioxidative properties, tocopherols are widely used components in human and animal nutrition additives. On page 291 of this issue, Uyanik et al. (2) report a low-cost synthesis method for making tocopherols with high biological activity.