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International audience; Zeolite‐catalyzed alkene cracking is key to optimize the size of hydrocarbons. The nature and stability of intermediates and transition states (TS) are, however, still debated. We combine transition path sampling and blue moon ensemble density functional theory simulations to unravel the behavior of C7 alkenes in CHA zeolite. Free energy profiles are determined, linking π‐complexes, alkoxides and carbenium ions, for B1 (secondary to tertiary) and B2 (tertiary to secondary) β‐scissions. B1 is found to be easier than B2. The TS for B1 occurs at the breaking of the C−C bond, while for B2 it is the proton transfer from propenium to the zeolite. We highlight the dynamic behaviors of the various intermediates along both pathways, which reduce activation energies with respect to those previously evaluated by static approaches. We finally revisit the ranking of isomerization and cracking rate constants, which are crucial for future kinetic studies.

International audience; The discovery of carbocations and carbanions by Ion Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) instruments onboard the Cassini spacecraft in Titan׳s upper atmosphere is truly amazing for astrochemists and astrobiologists. In this paper we identify the reaction mechanisms for the growth of the complex macromolecules observed by the CAPS Ion Beam Spectrometer (IBS) and Electron Spectrometer (ELS). This identification is based on a recently published paper (Ali et al., 2013. Planet. Space Sci. 87, 96) which emphasizes the role of Olah׳s nonclassical carbonium ion chemistry in the synthesis of the organic molecules observed in Titan׳s thermosphere and ionosphere by INMS. The main conclusion of that work was the demonstration of the presence of the cyclopropenyl cation – the simplest Huckel׳s aromatic molecule – and its cyclic methyl derivatives in Titan׳s atmosphere at high altitudes. In this study, we present the transition from simple aromatic molecules to the complex ortho-bridged bi- and tri-cyclic hydrocarbons, e.g., CH2+ mono-substituted naphthalene and phenanthrene, as well as the ortho- and peri-bridged tri-cyclic aromatic ring, e.g., perinaphthenyl cation. These rings could further grow into tetra-cyclic and the higher order ring polymers in Titan׳s upper atmosphere. Contrary to the pre-Cassini observations, the nitrogen chemistry of Titan׳s upper atmosphere is found to be extremely rich. A variety of N-containing hydrocarbons including the N-heterocycles where a CH group in the polycyclic rings mentioned above is replaced by an N atom, e.g., CH2+ substituted derivative of quinoline (benzopyridine), are found to be dominant in Titan׳s upper atmosphere. The mechanisms for the formation of complex molecular anions are discussed as well. It is proposed that many closed-shell complex carbocations after their formation first, in Titan׳s upper atmosphere, undergo the kinetics of electron recombination to form open-shell neutral radicals. These radical species subsequently might form carbanions via radiative electron attachment at low temperatures with thermal electrons. The classic example is the perinaphthenyl anion in Titan׳s upper atmosphere. Therefore, future astronomical observations of selected carbocations and corresponding carbanions are required to settle the key issue of molecular anion chemistry on Titan. Other than earth, Titan is the only planetary body in our solar system that is known to have reservoirs of permanent liquids on its surface. The synthesis of complex biomolecules either by organic catalysis of precipitated solutes “on hydrocarbon solvent” on Titan or through the solvation process indeed started in its upper atmosphere. The most notable examples in Titan׳s prebiotic atmospheric chemistry are conjugated and aromatic polycyclic molecules, N-heterocycles including the presence of imino >CN–H functional group in the carbonium chemistry. Our major conclusion in this paper is that the synthesis of organic compounds in Titan׳s upper atmosphere is a direct consequence of the chemistry of carbocations involving the ion–molecule reactions. The observations of complexity in the organic chemistry on Titan from the Cassini–Huygens mission clearly indicate that Titan is so far the only planetary object in our solar system that will most likely provide an answer to the question of the synthesis of complex biomolecules on the primitive earth and the origin of life

This thesis details the use of aromatic ions, especially aminocyclopropenium ions, as empowering design elements in the development of new chemical reactions, organic catalysts and polymeric materials. A particular focus is placed throughout on understanding the relationship between the structure of aromatic ions and their performance in these novel applications. Additionally, the benefits that aromatic ions provide in these contexts are highlighted. The first chapter briefly summarizes the Lambert Group's prior efforts toward exploiting the unique reactivity profiles of aromatic ions in the context of new reaction design. Also provided in the first chapter is a comprehensive literature review of aminocyclopropenium ions, upon which the majority of advances described in this thesis are based. To set the stage for the first application of aminocyclopropenium ions, Chapter 2 provides an account of existing highly Brønsted basic functional groups, including guanidines, proazaphosphatranes and iminophosphoranes. The provided review on the synthesis and use in asymmetric catalysis of these bases indicates that there is a high need for conceptually new Brønsted basic functional groups. To address this need, the development of chiral 2,3-bis(dialkylamino)cyclopropenimines as a new platform for asymmetric Brønsted base catalysis is described in Chapter 3. This new class of Brønsted base is readily synthesized on scale, operates efficiently under practical conditions, and greatly outperforms closely related guanidine-based catalysts. Structure-activity relationship studies, mechanistic experiments and computational transition state modeling are all discussed in the context of asymmetric glycinate imine Michael reactions in order to arrive at a working model for cyclopropenimine chemistry. Cumulatively, this chapter provides a "user's guide" to understanding and developing further applications of 2,3-bis(dialkylamino)cyclopropenimines. The use of our optimal chiral 2,3-bis(dialkylamino)cyclopropenimine catalyst to promote asymmetric Mannich reactions of glycinate imines and N-Boc-aldimines is described in Chapter 4. The products of this transformation are optically enriched diamino acid derivatives, an important motif widely utilized in medicinal and synthetic chemistry. Importantly, unlike existing methods, our technology promotes reactions between tert-butyl glycinate and aliphatic N-Boc-aldimine substrates. A preparative-scale reaction is demonstrated and derivatization of its product to several valuable chiral compounds is shown. Chapter 5 describes the use of tris(dialkylamino)cyclopropenium (TDAC) ions as a new class of onium-like catalyst. A simple TDAC chloride salt is prepared on a 75-gram scale and its use as a phase transfer catalyst for a variety of reactions is demonstrated. This same salt is also utilized as an epoxide opening catalyst for a variety of transformations, including the fixation of carbon dioxide. Chapter 6 briefly highlights several continued applications of the chemistry advanced throughout this thesis. First, the work of other members of the Lambert Group toward the continued development of cyclopropenimine chemistry is described. Second, a broad initiative between the Lambert and Campos Groups at Columbia University focused on the synthesis and application of TDAC-based polymers is introduced. Lastly, the identification of a previously unknown equilibrium between fulvenes and imines/aldehydes in the context of a new mode of catalysis is presented.

Carbonium ions are carbocations with a pentacoordinated carbon atom, where the electronic octet is maintained. They possess a three center two electron bond in order to keep the tetravalence of the carbon atom. This paper reviews the concept of carbonium ions, their formation, stability and reactions.

Ab initio calculations for the protonation of the C-H and C-C bonds of methane, ethane, propane and isobutane by a superacid moiety was carried out. For the C-H protonation (H/H exchange) the transition state resembles an H-carbonium ion coordinated with the superacid. The activation energy for the H/H exchange was about 16 kcal.mol-1, at B3LYP/6-31++G** + RECP (Sb) level, regardless the type of C-H bond being protonated. For the C-C protonation the activation energy depends on the structure of the hydrocarbon and was always higher than the activation energy for C-H protonation, indicating a higher steric demand. Foram realizados cálculos ab initio para a protonação de ligações C-H e C-C do metano, etano, propano e isobutano com um agregado representando a estrutura de um superácido. Para a protonação na ligação C-H (troca H/H) o estado de transição se parece com um íon H-carbônio coordenado com o superácido. A energia de ativação para a troca H/H foi cerca de 16 kcal.mol-1 em nível B3LYP/6-31++G** + RECP (Sb), independentemente do tipo de ligação C-H sendo protonada. Para a protonação de ligações C-C a energia de ativação depende da estrutura do hidrocarboneto e foi sempre maior que a energia de ativação para a protonação na ligação C-H, indicando uma maior repulsão estérica.