Acid-Catalyzed Oxy-aminomethylation of Styrenes.

We report a strong Brønsted acid-catalyzed three-component oxy-aminomethylation of styrenes with sym-trioxane and sulfonamides or carbamates. This transformation provides a variety of 1,3-oxazinanes in moderate to good yields under mild reaction conditions. The obtained heterocycles can be readily transformed into the corresponding 1,3-amino alcohols, which are useful building blocks for the synthesis of pharmaceutically relevant molecules. Mechanistic studies suggest the intermediacy of an in situ formed 1,3,5-dioxazinane and a subsequent reaction with the olefin.

Formation of Breslow Intermediates from N-Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal.

Under aprotic conditions, the stoichiometric reaction of N-heterocyclic carbenes (NHCs) such as imidazolidin-2-ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2-C-to-O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on the mechanism of this kinetically disfavored H-translocation. Variable time normalization analysis (VTNA) revealed that the kinetic orders of the reactants vary for different NHC-to-aldehyde ratios, indicating different and ratio-dependent mechanistic regimes. We propose that for high NHC-to-aldehyde ratios, the H-shift takes place in the primary, zwitterionic NHC-aldehyde adduct. With excess aldehyde, the zwitterion is in equilibrium with a hemiacetal, in which the H-shift occurs. In both regimes, the critical H-shift is auto-catalyzed by the BI. Kinetic isotope effects observed for R-CDO are in line with our proposal. Furthermore, we detected an H-bonded complex of the BI with excess NHC (NMR).

Additions of Aldehyde-Derived Radicals and Nucleophilic N-Alkylindoles to Styrenes by Photoredox Catalysis.

The consecutive addition of acyl radicals and N-alkylindole nucleophiles to styrenes was established, as well as some additional radical-nucleophile combinations. Both aryl and aliphatic aldehydes give reasonable yields. The reaction proceeds best for α-substituted styrenes, effectively creating a quaternary all-carbon center. Some iridium-based photoredox systems are catalytically active; furthermore, a base is needed in this transformation. Radicals are formed by reductive perester cleavage and hydrogen atom transfer.

Acid promoted radical-chain difunctionalization of styrenes with stabilized radicals and (N,O)-nucleophiles.

A difunctionalization of alkenes through sequential addition of a radical and a nucleophile has been developed, which is suggested to proceed by a radical chain mechanism not requiring a catalyst. An electron transfer step to the oxidant benzoyl peroxide is facilitated by protonation with a strong acid.

Investigating the Oxidation Step in the CuCl2-Catalyzed Aerobic Oxidative Coupling Reaction of N-Aryl Tetrahydroisoquinolines.

The oxidative coupling of N-aryl tetrahydroisoquinolines with nucleophiles has inspired the development of novel C-H functionalization reactions as well as mechanistic studies. Here, we investigate the oxidation step that forms iminium ions as key intermediates in the method using CuCl2 as the catalyst and oxygen as the terminal oxidant. A strong electronic effect of substituents in the N-aryl ring was found by synthetic studies and a Hammett plot analysis, supporting initial electron transfer from the amine to Cu(II). The importance of the mechanism of oxidation on the substrate scope with differently substituted tetrahydroisoquinolines is discussed.

Aldehydes as Alkylating Agents for Ketones.

Common and non-toxic aldehydes are proposed as reagents for alkylation of ketones instead of carcinogenic alkyl halides. The developed reductive alkylation reaction proceeds in the presence of the commercially available ruthenium catalyst [(cymene)RuCl2 ]2 (as low as 250 ppm) and carbon monoxide as the reducing agent. The reaction works well for a broad substrate scope, including aromatic and aliphatic aldehydes and ketones. It can be carried out without a solvent and often gives nearly quantitative yields of the products. This straightforward and cost-effective method is promising not only for laboratory application but also for industry, which produces carbon monoxide as a large-scale waste product.

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study.

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+ ) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.

Molecular Radical Chain Initiators for Ambient- to Low-Temperature Applications.

An overview of methods for the initiation of radical chain reactions by specific initiator compounds, which generate radicals, is given. These can be utilized to initiate any kind of radical chain reaction by transforming substrates into the desired radical intermediates. Azo initiators, peroxides, nitroxides, trialkylboranes, dialkyl zinc compounds, and type I photoinitiators are discussed, as well as methods of redox- and sonochemical initiation. Methods of direct radical formation from the substrates, such as photoredox catalysis or high-energy irradiation, are not included. The focus of this review lies on rather "low" temperatures in the range of 50 °C down to -78 °C, which can be useful to achieve more selective reactions. Illustrative applications of such radical chain initiators in a variety of reactions are discussed, including stereoselective ones and polymerizations.

Alkenyl and Aryl Peroxides.

Alkenyl and aryl peroxides are a special class of organic peroxides. Under ambient conditions, they are usually short-lived, rapidly decomposing into radicals by homolytic O-O bond cleavage. They play an important role in the chemistry of the lower atmosphere, where they are formed through ozonolysis of alkenes. In the dark, this pathway is considered the major source of hydroxyl radicals, the "detergent of the atmosphere". In solution, alkenyl and aryl peroxides can be formed by various methods and their decomposition can be harnessed synthetically. For example, reactions involving alkenyl peroxides can be used to introduce ketones through radical additions, and nucleophilic aromatic substitution reactions generating aryl peroxides have been used to synthesize phenols. The radicals can also initiate radical polymerization reactions or chain reactions and mediate oxidative coupling by C-H bond functionalization. Knowledge of their chemistry could be helpful for projects generating or utilizing peroxides.

Reaction Mechanism of Iodine-Catalyzed Michael Additions.

Molecular iodine, an easy to handle solid, has been successfully employed as a catalyst in different organic transformations for more than 100 years. Despite being active even in very small amounts, the origin of this remarkable catalytic effect is still unknown. Both a halogen bond mechanism as well as hidden Brønsted acid catalysis are frequently discussed as possible explanations. Our kinetic analyses reveal a reaction order of 1 in iodine, indicating that higher iodine species are not involved in the rate-limiting transition state. Our experimental investigations rule out hidden Brønsted acid catalysis by partial decomposition of I2 to HI and suggest a halogen bond activation instead. Finally, molecular iodine turned out to be a similar if not superior catalyst for Michael additions compared with typical Lewis acids.

Acid-Catalyzed Activation of Peroxyketals: Tunable Radical Initiation at Ambient Temperature and Below.

This study details how peroxyketals, commercially available thermal initiators, and structurally related peroxides are activated in the presence of an acid catalyst to generate radicals at room temperature and below. This simple combination of two substrates was shown to efficiently initiate a variety of radical processes. This phenomenon is rationalized by the acid-catalyzed in situ formation of highly unstable alkenyl peroxides which readily decompose into initiating radical species.

Asymmetric counteranion-directed Lewis acid organocatalysis for the scalable cyanosilylation of aldehydes.

Due to the high versatility of chiral cyanohydrins, the catalytic asymmetric cyanation reaction of carbonyl compounds has attracted widespread interest. However, efficient protocols that function at a preparative scale with low catalyst loading are still rare. Here, asymmetric counteranion-directed Lewis acid organocatalysis proves to be remarkably successful in addressing this problem and enabled a molar-scale cyanosilylation in quantitative yield and with excellent enantioselectivity. Also, the catalyst loading could be lowered to a part-per-million level (50 ppm: 0.005 mol%). A readily accessible chiral disulfonimide was used, which in combination with trimethylsilyl cyanide, turned into the active silylium Lewis acid organocatalyst. The nature of a peculiar phenomenon referred to as a "dormant period", which is mainly induced by water, was systematically investigated by means of in situ Fourier transform infrared analysis.

Acid-Mediated Formation of Radicals or Baeyer-Villiger Oxidation from Criegee Adducts.

The acid-mediated reaction of ketones with hydroperoxides generates radicals, a process with reaction conditions similar to those of the Baeyer-Villiger oxidation but with an outcome resembling the formation of hydroxyl radicals via ozonolysis in the atmosphere. The Baeyer-Villiger oxidation forms esters from ketones, with the preferred use of peracids. In contrast, alkyl hydroperoxides and hydrogen peroxide react with ketones by condensation to form alkenyl peroxides, which rapidly undergo homolytic O-O bond cleavage to form radicals. Both reactions are believed to proceed via Criegee adducts, but the electronic nature of the peroxide residue determines the subsequent reaction pathways. DFT calculations and experimental results support the idea that, unlike previously assumed, the Baeyer-Villiger reaction is not intrinsically difficult with alkyl hydroperoxides and hydrogen peroxide but rather that the alternative radical formation is increasingly favored.

Experimental and computational studies on the C-H amination mechanism of tetrahydrocarbazoles via hydroperoxides.

The acid-catalyzed reactions of photochemically generated tetrahydrocarbazole peroxides with anilines have been studied experimentally and computationally to identify the underlying reaction mechanism. The kinetic data indicate a reaction order of one in the hydroperoxide and zero in the aniline. Computational investigations using density functional theory support the experimental findings and predict an initial tautomerization between an imine and enamine substructure of the primarily generated tetrahydrocarbazole peroxide to be the rate controlling step. The enamine tautomer then loses hydrogen peroxide upon protonation, generating a stabilized allylic carbocation that is reversibly trapped by solvent or aniline to form the isolated products.

Reaction progress kinetic analysis of a copper-catalyzed aerobic oxidative coupling reaction with N-phenyl tetrahydroisoquinoline.

The results from a kinetic investigation of a Cu-catalyzed oxidative coupling reaction between N-phenyl tetrahydroisoquinoline and a silyl enol ether using elemental oxygen as oxidant are presented. By using reaction progress kinetic analysis as an evaluation method for the obtained data, we discovered information regarding the reaction order of the substrates and catalysts. Based on this information and some additional experiments, a refined model for the initial oxidative activation of the amine substrate and the activation of the nucleophile by the catalyst was developed. The mechanistic information also helped to understand why silyl nucleophiles have previously failed in a related Cu-catalyzed reaction using tert-butyl hydroperoxide as oxidant and how to overcome this limitation.

Synthesis of antiviral tetrahydrocarbazole derivatives by photochemical and acid-catalyzed C-H functionalization via intermediate peroxides (CHIPS).

The direct functionalization of C-H bonds is an important and long standing goal in organic chemistry. Such transformations can be very powerful in order to streamline synthesis by saving steps, time and material compared to conventional methods that require the introduction and removal of activating or directing groups. Therefore, the functionalization of C-H bonds is also attractive for green chemistry. Under oxidative conditions, two C-H bonds or one C-H and one heteroatom-H bond can be transformed to C-C and C-heteroatom bonds, respectively. Often these oxidative coupling reactions require synthetic oxidants, expensive catalysts or high temperatures. Here, we describe a two-step procedure to functionalize indole derivatives, more specifically tetrahydrocarbazoles, by C-H amination using only elemental oxygen as oxidant. The reaction uses the principle of C-H functionalization via Intermediate PeroxideS (CHIPS). In the first step, a hydroperoxide is generated oxidatively using visible light, a photosensitizer and elemental oxygen. In the second step, the N-nucleophile, an aniline, is introduced by Brønsted-acid catalyzed activation of the hydroperoxide leaving group. The products of the first and second step often precipitate and can be conveniently filtered off. The synthesis of a biologically active compound is shown.

Acid-catalyzed oxidative radical addition of ketones to olefins.

Based on a mechanistic study, we have discovered a Brønsted acid catalyzed formation of ketone radicals. This is believed to proceed via thermally labile alkenyl peroxides formed in situ from ketones and hydroperoxides. The discovery could be utilized to develop a multicomponent radical addition of unactivated ketones and tert-butyl hydroperoxide to olefins. The resulting γ-peroxyketones are synthetically useful intermediates that can be further transformed into 1,4-diketones, homoaldol products, and alkyl ketones. A one-pot reaction yielding a pharmaceutically active pyrrole is also described.

Aerobic C-H amination of tetrahydrocarbazole derivatives via photochemically generated hydroperoxides.

A Brønsted acid catalyzed C-H functionalization via Intermediate PeroxideS (CHIPS), generated photochemically, allows the oxidative coupling of indole derivatives with a variety of nitrogen nucleophiles. The reaction can be performed in one pot and requires only visible light, elemental oxygen, a Brønsted acid and a photosensitizer. The method can be applied to an efficient synthesis of some biologically active compounds.