Pierre M. Esteves

The Three‐Component Biginelli Reaction: A Combined Experimental and Theoretical Mechanistic Investigation

Biginelli reactions have been monitored by direct infusion electrospray ionization mass spectrometry (ESI-MS) and key cationic intermediates involved in this three-component reaction have been intercepted and further characterized by tandem MS experiments (ESI-MS/MS). Density functional theory calculations were also used to investigate the feasibility of the major competing mechanisms proposed for the Biginelli reaction. The experimental and theoretical results were found to corroborate the iminium mechanism proposed by Folkers and Johnson, whereas no intermediates directly associated with either the more energy demanding Knoevenagel or enamine mechanisms could be intercepted.

Superacid-Promoted Reactions of α-Ketoamides and Related Systems

The superacid-promoted reactions of alpha-hydroxy and alpha-ketoamides have been studied. Ionization of these compounds leads to varied aryl-substituted oxyindole products. In some cases, electrocyclization can lead to substituted fluorene products. Dicationic, superelectrophilic intermediates are proposed as intermediates leading to the products from alpha-hydroxy and alpha-ketoamides.

Alkane Activation over Acidic Zeolites: The First Step

The heterogeneous acid-catalyzed activation step of alkanes leading to the reaction intermediates (carbocationic or alkoxy species) was up to now the matter of a longstanding controversy. Gas chromatography and online mass spectroscopy measurements show that H(2) and methane are formed over H-zeolites, whereas HD and CH(3)D are formed over D-zeolites as the primary products in the reaction with isobutane. These results indicate that sigma-bond protolysis by strong acid sites is the first step for hydrocarbon activation on these catalysts at mild temperatures (473 K), in analogy to the activation path occurring in liquid superacid media.

Trichloroisocyanuric acid in H2SO4: an efficient superelectrophilic reagent for chlorination of isatin and benzene derivatives

+ao anel aromatico mais eficientemente do que o TICA devido ao alivio da repulsao carga-carga. Trichloroisocyanuric acid (TICA) in H 2 SO 4 is an efficient reagent for regioselective chlorination of isatin at the 5-position and deactivated aromatic compounds, such as nitrobenzene. DFT calculations suggest formation of a superelectrophilic polyprotonated species that can transfer Cl + to the aromatic ring more efficiently than TICA due to charge-charge repulsion relief.

Trihaloisocyanuric acids/NaX: an environmentaly friendly system for vicinal dihalogenation of alkenes without using molecular halogen

The reaction of alkenes with easily handled trihaloisocyanuric acids / NaX in aqueous acetone produces the corresponding vicinal dihaloalkanes in moderate to excellent yields.

Heterogeneous Catalysis by Covalent Organic Frameworks (COF): Pd(OAc)2@COF‐300 in Cross‐Coupling Reactions

COF‐300, an imine‐linked, crystalline, and microporous covalent organic framework, modified by coordination of Pd(OAc)2 to its walls, afforded a hybrid material, Pd(OAc)2@COF‐300, which was used as an efficient heterogeneous catalyst for cross‐coupling reactions. This material showed excellent catalytic activity for the phosphine‐free Suzuki–Miyaura, Heck, and Sonogashira cross‐coupling reactions with low palladium loadings (0.1 mol % Pd). X‐ray photoelectron spectroscopy analysis of the catalyst after the reaction showed that PdII is converted to Pd0, which is trapped within the COFs nanopores. This was confirmed by high‐resolution transmission electron microscopy. Moreover, promising results were obtained using Pd(OAc)2@COF‐300 under continuous‐flow conditions for a Suzuki–Miyaura cross‐coupling reaction.

Chlorination of isatins with trichloroisocyanuric acid

Isatin and its derivatives have been extensively reported in the literature as having range of potential pharmacological compounds. The Sandmeyer method is the most widely used for isatin synthesis and furnishes different substituted isatins, usually with high yields. Although efficient, the Sandmeyer route has certain limitations, such as the formation of a mixture of regiosiomers and low yields depending on the type and position of the substituent. Thus, overcome these limitations, it is preferable that some derivative isatins be obtained by alternative methods. This article has investigated the chlorination of isatin derivatives using trichloroisocyanuric acid [1,3,5-trichloro1,3,5-triazine-2,4,6-(1H,3H,5H)-trione or TCCA] at different reaction conditions.

Understanding the planar tetracoordinate carbon atom: spiropentadiene dication.

The atoms in molecule theory shows that the spiropentadiene dication has a planar tetracoordinate carbon (ptC) atom stabilized mainly through the sigma bonds and this atom has a negative charge. The bonds to the ptC atom have less covalent character than the central carbon from neutral spiropentadiene. The total positive charge is spread along the structure skeleton. The analysis of the potential energy surface shows that the dication spiropentadiene has a 2.3 kcal/mol activation barrier for ring opening.

Density, Degeneracy, Delocalization-Based Index of Aromaticity (D 3 BIA)

A aromaticidade tem sido exaustivamente discutida e continua sendo um tema misterioso. Nesse trabalho e proposto um novo indice de aromaticidade chamado indice baseado na densidade-degenerescencia-deslocalizacao ou, simplesmente, D 3 BIA, numa tentativa de lancar nova introspeccao sobre esse tema. Esse indice e baseado na teoria de atomos em moleculas (AIM) e, de certa forma, e suportado pela teoria dos spins acoplandos (SC). A aromaticidade diminui com o numero de heteroatomos na molecula aromatica, pois a degenerescencia diminui, e diminui com o aumento do tamanho do anel do composto aromatico porque desfavorece a sobreposicao dos estados monoeletronicos. A relacao entre planaridade do anel, sua densidade eletronica e aromaticidade e tambem observada. A interacao atrativa da ressonância de 6 eletrons π no diânion ciclobutadieno compensa sua interacao repulsiva carbono-carbono enquanto no seu parente dicationico a ressonância de 2 eletrons π e insuficiente para contrabalancear sua interacao repulsiva e adota uma estrutura nao-plana. Aromaticity has been exhaustedly discussed for several years and it remains as a misterious issue. In this work it is proposed a new index of aromaticity named density, degeneracy and delocalization-based index of aromaticity or simply D 3 BIA in an attempt to cast new insight and perspective over this theme. This index is based on AIM (atoms in molecules) theory and it is somewhat supported by SC (spin-coupled) theory. Aromaticity decreases as the number of heteroatoms in the aromatic molecule increases since degeneracy decreases and it decreases as the ring size of an aromatic compound increases because it disfavors overlap of single-electron states. The relation between planar structures, electron density and aromaticity is also observed. The attractive interaction of 6π-electron resonance in cyclobutadiene dianion compensate its carbon-to-carbon repulsive interaction while in its dicationic parent the 2π-electron resonance is insufficient to counterbalance its carbon-to-carbon repulsive interaction and it adopts a puckered structure.

Enantioselective synthesis of substituted octalones

Abstract The octalones (R)-(−)- 1a and (S)-(−)- 1b were prepared from the Michael adducts 7a and 7b respectively. These intermediates were obtained by the “deracemizing alkylation” of cyclanones 3a and 3b .