Direct synthesis of β-ketophosphonates and vinyl phosphonates from alkenes or alkynes catalyzed by CuNPs/ZnO

Copper nanoparticles (CuNPs) supported on ZnO have been shown to effectively catalyze the direct synthesis of β-ketophosphonates from alkenes or alkynes, and that of vinyl phosphonates from alkynes and diethylphosphite, under air and in the absence of any additive or ligand. When using alkynes as starting materials, the selectivity proved to be dependent on the nature of the alkyne. Thus, alkynes conjugated with an aromatic ring or a carbon–carbon double bond gave β-ketophosphonates as the main reaction products, whereas aliphatic alkynes or alkynes conjugated with a carbonyl group led to the formation of the corresponding vinyl phosphonates.

Copper Nanoparticles in Click Chemistry

Conspectus: The challenges of the 21st century demand scientific and technological achievements that must be developed under sustainable and environmentally benign practices. In this vein, click chemistry and green chemistry walk hand in hand on a pathway of rigorous principles that help to safeguard the health of our planet against negligent and uncontrolled production. Copper-catalyzed azide–alkyne cycloaddition (CuAAC), the paradigm of a click reaction, is one of the most reliable and widespread synthetic transformations in organic chemistry, with multidisciplinary applications. Nanocatalysis is a green chemistry tool that can increase the inherent effectiveness of CuAAC because of the enhanced catalytic activity of nanostructured metals and their plausible reutilization capability as heterogeneous catalysts. This Account describes our contribution to click chemistry using unsupported and supported copper nanoparticles (CuNPs) as catalysts prepared by chemical reduction. Cu(0)NPs (3.0 ± 1.5 nm) in tetrahydrofuran were found to catalyze the reaction of terminal alkynes and organic azides in the presence of triethylamine at rates comparable to those achieved under microwave heating (10–30 min in most cases). Unfortunately, the CuNPs underwent dissolution under the reaction conditions and consequently could not be recovered. Compelling experimental evidence on the in situ generation of highly reactive copper(I) chloride and the participation of copper(I) acetylides was provided. The supported CuNPs were found to be more robust and efficient catalyst than the unsupported counterpart in the following terms: (a) the multicomponent variant of CuAAC could be applied; (b) the metal loading could be substantially decreased; (c) reactions could be conducted in neat water; and (d) the catalyst could be recovered easily and reutilized. In particular, the catalyst composed of oxidized CuNPs (Cu2O/CuO, 6.0 ± 2.0 nm) supported on carbon (CuNPs/C) was shown to be highly versatile and very effective in the multicomponent and regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles in water from organic halides as azido precursors; magnetically recoverable CuNPs (3.0 ± 0.8 nm) supported on MagSilica could be alternatively used for the same purpose under similar conditions. Incorporation of an aromatic substituent at the 1-position of the triazole could be accomplished using the same CuNPs/C catalytic system starting from aryldiazonium salts or anilines as azido precursors. CuNPs/C in water also catalyzed the regioselective double-click synthesis of β-hydroxy-1,2,3-triazoles from epoxides. Furthermore, alkenes could be also used as azido precursors through a one-pot CuNPs/C-catalyzed azidosulfenylation–CuAAC sequential protocol, providing β-methylsulfanyl-1,2,3-triazoles in a stereo- and regioselective manner. In all types of reaction studied, CuNPs/C exhibited better behavior than some commercial copper catalysts with regard to the metal loading, reaction time, yield, and recyclability. Therefore, the results of this study also highlight the utility of nanosized copper in click chemistry compared with bulk copper sources.

Ligand-Free Palladium-Catalyzed Oxyarylation of Dihydronaphthal­enes and Chromenequinone with o-Iodophenols and 3-Iodolawsone in PEG-400: An Efficient Synthesis of 5-Carbapterocarpans and Pterocarpanquinones

Dihydronaphthalenes were oxyarylated with o-iodophenols, in PEG-400 at 140 or 170 °C, leading regio- and stereoselectively to 5-carbapterocarpans. By using Pd(OAc)2 (5–10 mol%) as precatalyst and Ag2CO3 (1.1 equiv) as base (conditions A), products were obtained in good to excellent chemical yields, in 5–30 minutes, irrespective of the pattern of substitution the starting materials. Alternatively, when p-hydroxyacetophenone oxime derived palladacycle (1 mol%) was used as precatalyst, and dicyclohexylamine (2 equiv) was used as base (silver-free, conditions B), the corresponding adducts were obtained in moderate to good yields, in 0.5 to 4 hours. Finally, the oxyarylation of dihydronaphthalenes­ and chromenquinone with o-iodophenols and 3-iodolawsone in PEG-400 under conditions A led regio- and stereoselectively to the formation of carbapterocarpanquinones and pterocarpanquinones in moderate yield.

Remote Substituent Effects on the Stereoselectivity and Organocatalytic Activity of Densely Substituted Unnatural Proline Esters in Aldol Reactions

The organocatalytic activities of highly substituted proline esters obtained through asymmetric [3+2] cycloadditions of azomethine ylides derived from glycine iminoesters have been analyzed by 19F NMR and through kinetic isotope effects. Kinetic rate constants have been determined for unnatural proline esters incorporating different substituents. It has been found that exo-L and endo-L unnatural proline methyl esters yield opposite enantiomers in aldol reactions between cyclic ketones and aromatic aldehydes. The combined results reported in this study show subtle and remote effects that determine the organocatalytic behavior of these synthetic but readily available amino acid derivatives. These data can be used as design criteria for the development of new pyrrolidine-based organocatalysts.

Catalytic asymmetric transfer hydrogenation of ketones: recent advances

In this review, we consider the main processes for the asymmetric transfer hydrogenation of ketones from 2008 up today. The most effective organometallic compounds (derived from Ru, Rh, Ir, Fe, Os, Ni, Co, and Re) and chiral ligands (derived from amino alcohols, diamines, sulfur- and phosphorus-containing compounds, as well as heterocyclic systems) will be shown paying special attention to functionalized substrates, tandem reactions, processes under non-conventional conditions, supported catalysts, dynamic kinetic resolutions, the use of water as a green solvent, theoretical and experimental studies on reaction mechanisms, enzymatic processes, and finally applications to the total synthesis of biologically active organic molecules.

Indium-Mediated Cleavage of the Trityl Group from Protected 1H-Tetrazoles

On treatment with indium metal in MeOH–THF, trityl groups undergo reductive removal from 1H-protected tetrazoles (including aliphatic, aromatic, and heteroaromatic substituents), affording the corresponding free tetrazoles in excellent yields, without any decomposition of the tetrazole ring or reduction of any other group.

Catalytic Asymmetric Transfer Hydrogenation of Imines: Recent Advances

In this review article recent developments in the asymmetric transfer hydrogenation of imines from 2008 up to today are presented. The main methodology involves either metal-catalyzed procedures in the presence of a chiral ligand or organocatalyzed technologies using a Hantzsch ester and a chiral BINOL-derived phosphoric acid. The most important procedures are collected, paying special attention to the application of this methodology in synthetic organic chemistry.

Recent Advances in the Direct Nucleophilic Substitution of Allylic Alcohols through SN1-Type Reactions

Direct nucleophilic substitution reactions of allylic alcohols are environmentally friendly, since they generate only water as a byproduct, allowing access to new allylic compounds. This reaction has, thus, attracted the interest of the chemical community and several strategies have been developed for its successful accomplishment. This review gathers the latest advances in this methodology involving SN1-type reactions.

Synthesis of 5-Carbapterocarpens by α-Arylation of Tetralones Followed by One-Pot Demethylation/Cyclization with BBr3

5-Carbapterocarpens, one of them displaying estrogenic activity, were prepared from α-aryltetralones in high yields through a one-pot, BBr3-promoted O-demethylation and cyclization sequence. The key α-aryltetralone intermediates were obtained by direct α-arylation of tetralones with o-alkoxybromoarenes in the presence of Pd2(dba)3 (2.5 mol-%) and tBu3PHBF4 (10 mol-%) as catalysts, together with 2.5 equiv. of KOH in dioxane/H2O (4:1), under microwave irradiation conditions (80 W, 100 °C, 40 min), leading to α-monoaryltetralones in good yields.

Electrolito sólido polimérico: desde las pilas de combustible hasta la electrosíntesis orgánica

En este trabajo, en primer lugar, se presenta una nueva técnica desarrollada en nuestro laboratorio para el estudio electroquímico de las capas catalíticas de las pilas de combustible en células de tres electrodos, centrándonos en el proceso de electroxidación de ácido fórmico como reacción de test. Gracias a esta técnica se han estudiado parámetros de construcción como % en peso del metal, relación Nafion / sólidos totales y recubrimiento catalítico comprobando como la adsorción irreversible de adátomos de Bi sobre Pt soportado sobre Vulcan XC-72 favorece este proceso y como puede caracterizarse la capa catalítica de una pila de combustible de ácido fórmico (DFAFC) de forma integral utilizando estudios de sistemas nanoparticulados de Pt-Pd soportados sobre Vulcan XC-72 en el seno de ésta. En segundo lugar se ha introducido el concepto de PEMER (Polymer Electrolyte Membrane Electrochemical Reactor). De esta forma, una configuración electródica propia de las pilas de combustible se utiliza en electrosíntesis orgánica. Como reacciones test se han testeado la formación de 1-feniletanol como producto mayoritario por hidrogenación electrocatalítica de la acetofenona sobre nanopartículas de Pd soportadas sobre Vulcan XC-72 como electrocatalizador y, utilizando Pb (catalizador no noble) soportado sobre Vulcan XC-72, se ha estudiado la ruptura del puente disulfuro de L-cistina y N,N-diacetil-L-cistina (NNDAC) para obtener L-cisteína y N-acetil- L-cisteína (NAC). En ambas reacciones, hidrogenación y ruptura del puente disulfuro, se han analizado tanto parámetros constructivos de la capa catalítica como parámetros de proceso tanto a escala laboratorio con el uso de un reactor comercial de 25 cm² como a escala pre-piloto con la construcción de un reactor de 100 cm².