M. Mar Díaz-Requejo

Coinage metal catalyzed C-H bond functionalization of hydrocarbons.

3.1. The Reaction 3381 3.2. The Catalysts and the Carbene Source 3381 3.3. Achievements 3383 3.4. Selectivity and Mechanistic Considerations 3386 3.5. Future Development 3386 4. C-H Bond Functionalization by Nitrene Insertion 3387 4.1. The Reaction 3387 4.2. The Catalysts and the Nitrene Sources 3387 4.3. Development and Mechanistic Considerations 3388 4.4. Future Development 3389 5. C-H Bond Oxidations 3389 5.1. The Reaction and the Catalysts 3389 5.2. Development and Perspective 3390 6. Conclusions 3393 7. Acknowledgments 3393 8. References 3393

Silver-Catalyzed C-C Bond Formation Between Methane and Ethyl Diazoacetate in Supercritical CO2

Supercritical carbon dioxide solvent facilitates transformation of the generally inert carbon-hydrogen bonds in methane. Even in the context of hydrocarbons’ general resistance to selective functionalization, methane’s volatility and strong bonds pose a particular challenge. We report here that silver complexes bearing perfluorinated indazolylborate ligands catalyze the reaction of methane (CH4) with ethyl diazoacetate (N2CHCO2Et) to yield ethyl propionate (CH3CH2CO2Et). The use of supercritical carbon dioxide (scCO2) as the solvent is key to the reaction’s success. Although the catalyst is only sparingly soluble in CH4/CO2 mixtures, optimized conditions presently result in a 19% yield of ethyl propionate (based on starting quantity of the diazoester) at 40°C over 14 hours.

A General Mechanism for the Copper- and Silver-Catalyzed Olefin Aziridination Reactions: Concomitant Involvement of the Singlet and Triplet Pathways

The olefin aziridination reactions catalyzed by copper and silver complexes bearing hydrotris(pyrazolyl)borate (Tp(x)) ligands have been investigated from a mechanistic point of view. Several mechanistic probe reactions were carried out, specifically competition experiments with p-substituted styrenes, stereospecificity of olefins, effects of the radical inhibitors, and use of a radical clock. Data from these experiments seem to be contradictory, as they do not fully support the previously reported concerted or stepwise mechanisms. But theoretical calculations have provided the reaction profiles for both the silver and copper systems with different olefins to satisfy all experimental data. A mechanistic proposal has been made on the basis of the information that we collected from experimental and theoretical studies. In all cases, the reaction starts with the formation of a metal-nitrene species that holds some radical character, and therefore the aziridination reaction proceeds through the radical mechanism. The silver-based systems however hold a minimum energy crossing point (MECP) between the triplet and closed-shell singlet surfaces, which induce the direct formation of the aziridines, and stereochemistry of the olefin is retained. In the case of copper, a radical intermediate is formed, and this intermediate constitutes the starting point for competition steps involving ring-closure (through a MECP between the open-shell singlet and triplet surfaces) or carbon-carbon bond rotation, and explains the loss of stereochemistry with a given substrate. Overall, all the initially contradictory experimental data fit in a mechanistic proposal that involves both the singlet and the triplet pathways.

Catalytic insertion of diazo compounds into N-H bonds: the copper alternative.

The complexes TpxCu (Tpx = homoscorpionate) catalyse the insertion of diazo compounds into nitrogen-hydrogen bonds of amines and amides, under very mild conditions, with quantitative yields being obtained with equimolar ratios of reactants.

Introducing Copper as Catalyst for Oxidative Alkane Dehydrogenation

The dehydrogenation of n-hexane and cycloalkanes giving n-hexene and cycloalkenes has been observed in the reaction of such hydrocarbons with hydrogen peroxide, in the presence of copper complexes bearing trispyrazolylborate ligands. This catalytic transformation provides the typical oxidation products (alcohol and ketones) with small amounts of the alkenes, a novel feature in this kind of oxidative processes. Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. DFT studies support a copper-oxo active species, which initiates the reaction by H abstraction. Spin crossover from the triplet to the singlet state, which is required to recover the catalyst, yields the major hydroxylation and minor dehydrogenation products. Further calculations suggested that the superoxo and hydroperoxo species are less reactive than the oxo. A complete mechanistic proposal in agreement with all experimental and computational data is proposed.

Selective Synthesis of N-Substituted 1,2-Dihydropyridines from Furans by Copper-Induced Concurrent Tandem Catalysis

A novel transformation in which mono- or dialkyl-substituted furans are converted into 1,2-dihydropyridines upon reaction with PhI horizontal lineNTs at room temperature is reported. The reaction is catalyzed by complexes of general formula Tp(x)M (M = Cu, Ag) and consists of a one-pot procedure with four consecutive catalytic cycles. Furan aziridination is followed by aziridine ring-opening, transimination reaction, inverse-electronic-demand aza-Diels-Alder reaction, and a final hydrogen elimination reaction. The mechanism of the overall transformation is proposed where the metal complex displays a crucial role along the reaction pathway.

The carbene insertion methodology for the catalytic functionalization of unreactive hydrocarbons: No classical C–H activation, but efficient C–H functionalization

This contribution intends to highlight the use of the metal-catalyzed functionalization of unreactive carbon-hydrogen bonds by the carbene insertion methodology, that employs diazo compounds as the carbene source.

Direct, Copper-Catalyzed Oxidation of Aromatic C-H Bonds with Hydrogen Peroxide under Acid-Free Conditions

The direct oxidation of benzene into phenol using hydrogen peroxide has been achieved in the absence of any acid with Tp(x)Cu(NCMe) complexes as the catalysts. In the case of anthracenes as the substrates, valuable anthraquinones have been quantitatively obtained in the same manner.

Chemo-, Regio-, and Stereoselective Silver-Catalyzed Aziridination of Dienes: Scope, Mechanistic Studies, and Ring-Opening Reactions

Silver complexes bearing trispyrazolylborate ligands (Tp(x)) catalyze the aziridination of 2,4-diene-1-ols in a chemo-, regio-, and stereoselective manner to give vinylaziridines in high yields by means of the metal-mediated transfer of NTs (Ts = p-toluensulfonyl) units from PhI═NTs. The preferential aziridination occurs at the double bond neighboring to the hydroxyl end in ca. 9:1 ratios that assessed a very high degree of regioselectivity. The reaction with the silver-based catalysts proceeds in a stereospecific manner, i.e., the initial configuration of the C═C bond is maintained in the aziridine product (cis or trans). The degree of regioselectivity was explained with the aid of DFT studies, where the directing effect of the OH group of 2,4-diene-1-ols plays a key role. Effective strategies for ring-opening of the new aziridines, deprotection of the Ts group, and subsequent formation of β-amino alcohols have also been developed.

The use of polypyrazolylborate copper(I) complexes as catalysts in the conversion of olefins into cyclopropanes, aziridines and epoxides and alkynes into cyclopropenes

Abstract Polypyrazolylborate complexes of Cu(I) present catalytic activities towards the carbene, nitrene and oxo transfer reactions to unsaturated substrates to form cyclopropanes, cyclopropenes, aziridines and epoxides. Kinetic studies have led to the proposal of a general mechanism for the cyclopropanation reaction with copper catalysts, a 14-e species being responsible of the catalytic step. Heterogeneisation of those complexes on silica gel does not affect their catalytic capabilities under heterogeneous conditions when compared with those in the homogeneous phase.