Meyer-Schuster rearrangement of propargylic alcohols mediated by phosphorus-containing Brønsted acid catalysts.

Commercially available (aqueous) hypophosphorus acid is an efficient catalyst for the synthesis of α,ß-unsaturated carbonyl compounds from their corresponding propargylic alcohols. Reactions were carried out in technical toluene in the presence of air and in several instances the desired products were isolated analytically pure after a simple work-up.

Expedient metal-free preparation of aryl aziridines via thermal cycloaddition reactions.

A straightforward synthesis of aryl aziridines is reported from readily available azides and alkenes and using technical solvents in the presence of air. This methodology does not require any additives and the obtained compounds can be employed in ring-opening and ring-expansion reactions.

Taming Brønsted Acid Reactivity: Nucleophilic Substitutions of Propargylic Alcohols with N-Nucleophiles Mediated by Phosphorus-Based Brønsted Acid Catalysts.

The activity of diethyl phosphite and diphenyl phosphate in propargylation reactions with N-nucleophiles of varying basicity is presented. A careful choice of the reaction conditions minimized undesired rearrangements and arylation processes, typical side reactions with Brønsted acid catalysis. These systems are compatible with technical solvents and presence of air, and they are also applicable to C-, O-, and S-nucleophiles.

Ring-Expanded N-Heterocyclic Carbenes for Copper-Mediated Azide-Alkyne Click Cycloaddition Reactions.

A series of well-defined copper(I) complexes bearing ring-expanded N-heterocyclic carbene (NHC) ligands has been applied to the azide-alkyne cycloaddition reaction. The obtained results notably showed that the six-membered NHC ligands outperform well-established five-membered ones. [CuI(Mes-6)] displayed a remarkable catalytic activity while respecting the strict criteria for click reactions.

Copper(I)-Phosphinite Complexes in Click Cycloadditions: Three-Component Reactions and Preparation of 5-Iodotriazoles.

The remarkable activity displayed by copper(I)-phosphinite complexes of general formula [CuBr(L)] in two challenging cycloadditions is reported: a) the one-pot azidonation/cycloaddition of boronic acids, NaN3, and terminal alkynes; b) the cycloaddition of azides and iodoalkynes. These air-stable catalysts led to very good results in both cases and the expected triazoles could be isolated in pure form under 'Click-suitable' conditions.

HBF4-Catalysed Nucleophilic Substitutions of Propargylic Alcohols.

The activity of HBF4 (aqueous solution) as a catalyst in propargylation reactions is presented. Diverse types of nucleophiles were employed in order to form new C-O, C-N and C-C bonds in technical acetone and in air. Good to excellent yields and good chemoselectivities were obtained using low acid loading (typically 1 mol-%) under simple reaction conditions.

Chiral transition-metal complexes as Brønsted-acid catalysts for the asymmetric Friedel-Crafts hydroxyalkylation of indoles.

The Friedel-Crafts reaction between 3,3,3-trifluoropyruvates and indoles is efficiently catalysed by the iridium complex [(η(5)-C5Me5)Ir{(R)-Prophos}(H2O)][SbF6]2 (1) with up to 84% ee. Experimental data and theoretical calculations support a mechanism involving the Brønsted-acid activation of the pyruvate carbonyl by the protons of the coordinated water molecule in 1. Water is not dissociated during the process and, therefore, the catalytic reaction occurs with no direct interaction between the substrates and the metal.

Reusable and highly active supported copper(I)-NHC catalysts for Click chemistry.

Immobilised [Cu(NHC)] catalysts are reported for the preparation of 1,2,3-triazoles. In addition to showing outstanding catalytic activity, the catalyst systems are easy to prepare and can be recycled many times.

Well-defined diimine copper(I) complexes as catalysts in click azide-alkyne cycloaddition reactions.

A series of 1,4-disubstituted 1,2,3-triazoles have been prepared in high yields while respecting the stringent Click criteria. In these reactions, highly stable pre-formed complexes bearing diimine ligands were used.

[CuBr(PPh3)3] for azide-alkyne cycloaddition reactions under strict Click conditions.

A careful methodological study revealed a true Click catalytic system based on commercially available [CuBr(PPh(3))(3)]. This system is active at room temperature, with 0.5 mol % [Cu] (or less), in the absence of any additive, and it does not require any purification step to isolate pure triazoles.

[(NHC)CuX] complexes: synthesis, characterization and catalytic activities in reduction reactions and Click chemistry. On the advantage of using well-defined catalytic systems.

The preparation of three series of [(NHC)CuX] complexes (NHC = N-heterocyclic carbene, X = Cl, Br, or I) is reported. These syntheses are high yielding and only use readily available starting materials. The prepared complexes were spectroscopically and structurally characterized. Notably, two of them present a bridging NHC ligand between two copper centers in the solid state, an extremely rare coordination mode for these ligands. These complexes were then applied to two distinct organic reactions: the hydrosilylation of ketones and the 1,3-dipolar cycloaddition of azides and alkynes. In both transformations, outstanding catalytic systems were found for preparing the corresponding products in excellent yields and short reaction times. Most remarkably, the screening of well-defined systems in the hydrosilylation reaction allowed for the identification of a pre-catalyst previously overlooked since, originally, catalytic species were in situ generated. Under such conditions, major formation of [(NHC)(2)Cu](+) species, inactive in this reduction reaction, occurred instead of the expected copper hydride. These results highlight one of the most important advantages of employing well-defined complexes in catalysis: gaining an improved control of the nature of the catalytically relevant species in the reaction media.

Au/Ag-cocatalyzed aldoximes to amides rearrangement under solvent- and acid-free conditions.

The gold/silver-cocatalyzed conversion of aldoximes into primary amides is reported. The reaction, which proceeds under neat and acid-free conditions, allows for the conversion of a range of aldoximes, and is a rare example of cooperative catalysis involving well-defined gold species.

A [(NHC)CuCl] complex as a latent Click catalyst.

A latent catalyst for the [3+2] cycloaddition reaction of azides and alkynes has been developed in accordance with the principles of Click chemistry.

Copper, silver, and gold complexes in hydrosilylation reactions.

The reduction of diverse functional groups is an essential protocol in organic chemistry. Transition-metal catalysis has been successfully applied to the reduction of olefins, alkynes, and many carbonyl compounds via hydrogenation or hydrosilylation; the latter presenting several advantages over hydrogenation. Notably, hydrosilylation generally occurs under mild reaction conditions, and consequently over-reduced products are rarely detected. Moreover, the great majority of hydrosilanes employed in this reaction are easily handled, inexpensive, or both. A large number of multiple bonds can be involved in this context, and the hydrosilylation reaction can be regarded as a useful method for the synthesis of silicon-containing organic molecules or a convenient way of reducing organic compounds. Furthermore, the silyl group can also be retained as a protecting group, a strategy that can be of great usefulness in organic synthesis. Since the first Wilkinson's catalyst-mediated hydrosilylation of ketones in 1972, metals such as rhodium and iridium have attracted most of the attention in this area. A wide array of catalytic systems for hydrosilylation reactions is nowadays available, which has allowed for a great expansion of the synthetic scope of this transformation. After having been overlooked in the early years, group 11 metals (Cu, Ag, and Au), especially copper, have emerged as appealing alternatives for hydrosilylation. The use of a stabilized form of copper hydride, the hexameric [(Ph3P)CuH]6, by Stryker represented a breakthrough in copper-catalyzed reduction reactions. Nowadays, several copper-based catalytic systems compare well with a variety of reported rhodium-based catalysts, which generally suffer from the high cost of the catalyst. Tertiary phosphine ligands are the most widely used in these transformations. However, other families such as N-heterocyclic carbenes (NHCs) have shown promising activities. Compared with copper, little attention has been paid to silver- or gold-based catalysts. Silver salts have been considered inert towards hydrosilylation, and they are often employed as innocent anion exchange reagents for the in situ generation of cationic transition metal catalysts. Despite the rare reports available, they have already shown interesting reactivity profiles, for example, in the chemoselective reduction of aldehydes in the presence of ketones. Furthermore, 1,2-hydride delivery is favored over 1,4-reductions for alpha,beta-unsaturated carbonyl compounds, in contrast with most copper-based systems.

Synthesis and characterization of [Cu(NHC)2]X complexes: catalytic and mechanistic studies of hydrosilylation reactions.

The preparation of two series of [Cu(NHC)2]X complexes (NHC=N-heterocyclic carbene, X=PF6 or BF4) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurally characterized. The activity of these cationic bis-NHC complexes in the hydrosilylation of ketones was examined, and both the ligand and the counterion showed a significant influence on the catalytic performance. Moreover, when compared with related [Cu(NHC)]-based systems, these cationic complexes proved to be more efficient under similar reaction conditions. The activation step of [Cu(NHC)2]X precatalysts towards hydrosilylation was investigated by means of 1H NMR spectroscopy. Notably, it was shown that one of the N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) ligands in [Cu(IPr)2]BF4 is displaced by tBuO(-) in the presence of NaOtBu, producing the neutral [Cu(IPr)(OtBu)]. This copper alkoxide is known to be a direct precursor of an NHC-copper hydride, the actual active species in this transformation. Furthermore, reagent loading and counterion effects have been rationalized in light of the species formed during the reaction.

N-heterocyclic carbenes as organocatalysts.

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.