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Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N′-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93 % e.e. using a catalytic amount of NiCl2(glyme) in the presence of the chiral aminoindanol‐derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible‐light irradiation (blue LEDs, 456 nm) at 8–12 °C. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air‐sensitive Ni(COD)2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction. [ABSTRACT FROM AUTHOR]

In recent years, the field of organocatalysis has extensively explored the use of carbenium ions for practical C−C bond forming reactions. In order to control the formation of new stereocenters, the generation of carbenium ions was investigated by exploring asymmetric Brønsted acid catalysis. Oxocarbenium ions, iminium ions, and other carbenium ions stabilized by heteroatoms could also be generated by Brønsted acid catalysis, through the departure of suitable leaving groups. The aforementioned reactivity has recently been exploited leading to remarkable achievements. The application of chiral Brønsted acids in the generation of heteroatom stabilized carbenium ions has recently been investigated, with the demonstration of important results. Although diverse architectural arrays of Brønsted acid catalysts were conceived, BINOL‐derived phosphoric acids have played a dominant role in this area of research. Their low pKa values combined with the possibility to alter their architecture, and their ability to assemble ordered transition states was exploited in different SN1‐type reactions allowing the formation of a variety of products with high enantiomeric excesses. In addition, the generation of highly stabilized chiral counter anions allowed the exploitation of counterion asymmetric catalysis with various nucleophiles. Go fish! Carbenium ions generated under Brønsted acids conditions are suitable activated precursors for C−C and C−X (X=O, N, S) bond‐formation reactions. Through cooperativity, control of pKa, and by secondary interactions it is possible to control the stereoselective formations of new bonds to a remarkable level, by using chiral Brønsted acids. [ABSTRACT FROM AUTHOR]

Stereoselective alkylation of enolates represents a valuable and important procedure for accessing carbon-carbon-bond frameworks in natural and nonnatural product synthesis. Usually, activated electrophilic partners that react through an SN2 mechanism are employed. To overcome the limitations due to reduced reactivity and steric hindrance, SN1-type reactions can be considered a valid and practical alternative. Accessible enolates can be used in stereoselective (diastereo- or enantioselective) reactions with electrophilic carbenium ions, either used as stable reagents or generated in situ from suitable precursors. The results achieved in this active field are summarized in this review. 1 Introduction 2 Alcohols in SN1-Type Reactions with Enolates 2.1 Enantioselective Reactions with Metal Complexes 2.2 Organocatalytic Enantioselective Reactions 3 Alcohols and Alcohol Derivatives in SN1-Type Reactions with Enolates: Enantioselective Reactions with Metal Enolates 4 Isolated Carbenium Ions in SN1-Type Reactions with Enolates: Enantioselective Reactions with Metal Enolates 5 Miscellaneous 6 Conclusion. [ABSTRACT FROM AUTHOR]

A direct and simple activation of quinolines, without isolating unstable intermediates, or using isolated N,O-acetals in the presence of Lewis or Brønsted acids, is described. The procedure is quite straightforward and allows the addition in a stereoselective manner of different aldehydes to various differently substituted quinolines. The desired products were obtained in 28-76 % yields, with dr values up to 83:17 in favor of the syn isomer, and up to 99 % ee. Studies towards the use of acetaldehyde were also performed with different catalysts and the addition was promoted affording the desired product in 62 % yield with 46 % ee. Finally, deprotection and chemical transformations of the enantioenriched adducts were performed. [ABSTRACT FROM AUTHOR]

The combination of a practical and highly enantioselective organocatalytic reaction, which allows the stereoselective introduction of a benzodithiol group, with a fluorination step, gives a new and effective strategy for the stereoselective synthesis of difluorinated building blocks. The benzodithiol group is a versatile and chameleonic group that can be further functionalized before fluorination, giving customized and tailored useful synthetic strategies. As an example of the application of this facile strategy, the effective enantioselective synthesis of difluoroarundic acid is described. [ABSTRACT FROM AUTHOR]

Stable diarylcarbenium salts, obtained by the direct coupling of indole or indole derivatives with aryl (or heteroaryl) aldehydes in the presence of a strong organic Brønsted acid, have been employed in the direct alkylation of aldehydes. Excellent enantiomeric excesses and good diastereomeric ratios were obtained with a number of aryl or heteroaryl(3-indolyl)carbenium ions as the highly stable o-benzenedisulfonimide (OBS) salts and with the reaction promoted by the Hayashi-Jørgensen catalyst. A one-pot, three-component, stereoselective alkylation of aldehydes affording the same compounds was also investigated with various aldehydes, indole derivatives, and organocatalysts. The results obtained with the isolated carbenium ions were superior in terms of yields and stereoselectivity. [ABSTRACT FROM AUTHOR]

The frequent occurrence of chiral 1-substituted-1,2,3,4-tetrahydroisoquinoline ring systems in a large number of alkaloids, possessing a broad spectrum of biological and pharmaceutical properties, has prompted out a considerable interest in their synthesis. Economical and valuable stereoselective processes based on organocatalytic transformations represent a new avenue for approaching isoquinoline alkaloids with efficiency and creativity. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

ABSTRACT Recently, the direct substitution of allylic, benzylic, and tertiary alcohols has been achieved via SN1-type reactions with catalytic amounts of Brønsted or Lewis acids. When a new stereogenic center is formed most of these transformations produce the desired product as a racemate, as these reactions proceed through carbenium ions. The arsenal of activation modes available in organocatalysis can be used to set up suitable reaction conditions in which chiral nucleophiles (enamine ) or chiral electrophiles (iminium , chiral counterion ) can easily be generated. Recently, we have used stabilized carbenium ions, directly available or obtained from the corresponding alcohols, in new organocatalytic SN1-type reactions. The commercially available carbenium ion benzodithiolylium tetrafluoroborate 1 can be used for the straightforward organocatalytic . In this account we will illustrate the application of this methodology in the total synthesis of natural products and the preparation of valuable starting materials. Chirality 26:607-613, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The compatibility of indium(III) Lewis acids with water and amines allows their employment in synergistic and cooperative catalysis. Stereoselective organocatalytic SN1-type reactions, in which carbenium ions are generated, are promoted by the presence of indium salts. The peculiar properties of indium salts can be exploited in organocatalysis for the design of water-compatible, benign, green processes. The development of such indium(III)-promoted organocatalytic procedures is the focus of our recent re- search, a summary of which is presented in this article. [ABSTRACT FROM AUTHOR]

In recent years there has been an accelerated rate of development in the field of organocatalysis, with asymmetric organocatalysis now reaching full maturity. The invention of new organocatalytic reactions and the exploration of new concepts now appear in tandem with the application of organocatalytic techniques in the synthesis of natural products and active pharmaceutical ingredients (APIs). After a 'golden rush' in organocatalysis, researchers are now starting to combine different methods, thereby taking advantage of the significant benefits of synergy. Metals are used in combination with organocatalytic processes, thus reaching complexity that is found in nature, where enzymes take advantage of the presence of certain metals to increase the arsenal of organic transformations available. In this Focus review, we illustrate the possibility of a 'happy marriage' between Lewis acids and organocatalytic stereoselective processes. Questions have been raised about the combination of Lewis acids and organocatalysis owing to the presence of water and/or strong bases in these processes. Some Lewis acids have been shown to be compatible with organocatalysis and concepts relating to their use will be illustrated herein. To summarize the fruitful use of Lewis acids in stereoselective organocatalytic processes, we will draw attention to the advantages and selectivity achieved using this method. [ABSTRACT FROM AUTHOR]

The stereoselective alkylation of aldehydes with benzodithiolylium tetrafluoborate gave a straightforward access to key compounds for the synthesis of fragrances and flavors. [ABSTRACT FROM AUTHOR]

Diastereoselective SN1-type reactions have been known for many years and have been used in the synthesis of many natural products. It has recently been shown that chiral nucleophiles, obtained using organocatalytic methodologies, can react with highly reactive carbenium ions in a stereoselective fashion. Based on this, carbenium ions can be applied in asymmetric catalytic reactions. [ABSTRACT FROM AUTHOR]

The article discusses the organocatalytic stereoselective formation of ketones by the use of N-methylbenzothiazolylium iodide. The benzothiazolylium salt is electrophilic in nature and acts as a precursor of carbenes. The salt shows moderate yield and high stereoselectivity with different types of ketones.

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N′-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published. [ABSTRACT FROM AUTHOR]

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