Unravelling [3+2] Cycloaddition Reactions through the Molecular Electron Density Theory

Después de la primera clasificación de las reacciones 32CA en reacciones de tipo zw y pr, establecidas en el año 2014, la estructura y reactividad de los TACs más importantes utilizados en las reacciones 32CA ha sido completamente caracterizado en base a la MEDT propuesta recientemente. Entre la gran cantidad de trabajo desarrollado a lo largo de la presente tesis doctoral, se han seleccionado y discutido ocho publicaciones representativas, que permitieron caracterizar dos nuevos tipos de reactividad y consolidar la reactividad original de tipo zw. Así, dependiendo de las cuatro estructuras electrónicas diferentes de los TACs, es decir, pseudodiradical, pseudoradical, carbenoide y zwitteriónica, las reacciones 32CA se han clasificado en reacciones de tipo pdr, pmr, cb y zw. Mientras que las reacciones 32CA de tipo pdr son las más rápidas, las reacciones de tipo zw son las más lentas. Las diferentes estructuras electrónicas en el estado fundamental de los reactivos explican esta tendencia de reactividad y revelan que la reactividad de los TACs carbenoides es diferente. Además, ningún TAC puede considerarse una estructura 1,2-zwitteriónica, tal y como se propone para los “1,3-dipolos”. El carácter polar de la reacción, medido por el valor de la GEDT calculado en la estructura del TS, afecta a los cuatro tipos de reactividad, de tal forma que cuanto más fuertes sean las interacciones nucleofílicas / electrofílicas que tienen lugar en el TS, más rápida es la reacción, e incluso puede cambiar el mecanismo molecular de acuerdo con las funciones de Parr definidas dentro de la CDFT. Esta racionalización basada en la MEDT de las reacciones 32CA esclarece las propuestas mecanísticas de Huisgen y Firestone establecidas en los años 60. Independientemente del tipo de reactividad y el carácter polar de la reacción, el análisis topológico de la ELF a lo largo de las reacciones 32CA que tienen lugar en un solo paso sugiere que los cambios de enlace no son “concertados” sino secuenciales, descartando así la clasificación de estas reacciones como “pericíclicas”. En la presente tesis doctoral, la teoría clásica de las reacciones 32CA, establecida en los años 60 del siglo pasado y que aún prevalece en la actualidad, es revisitada y reinterpretada en base a la MEDT. Se establece un nuevo y sólido modelo de reactividad para las reacciones 32CA, enfatizando que la visión actual de la química orgánica necesita replantearse en base al análisis de la densidad electrónica.After the first classification of [3+2] cycloaddition (32CA) reactions into zw-type and pr-type reactions, established in 2014, the structure and reactivity of the most important three-atom-components (TACs) used in 32CA reactions has been completely characterised within the recently proposed Molecular Electron Density Theory (MEDT). Among the huge amount of work developed along the present Ph.D thesis, eight representative publications have been selected and discussed herein, which allowed characterising two new reactivity types as well as consolidating the original zw-type reactivity. Thus, depending on the four different electronic structures of TACs, i.e. pseudodiradical, pseudoradical, carbenoid and zwitterionic, 32CA reactions have been classified into pdr-, pmr-, cb- and zw-type reactions. While pdr-type 32CA reactions are the fastest, zw-type reactions are the slowest. The different electronic structures at the ground state of the reagents account for this reactivity trend and reveal that the reactivity of carbenoid TACs is different. In addition, no TAC can be considered a 1,2-zwitterionic structure as proposed for “1,3-dipoles”. The polar character of the reaction, measured by the global electron density transfer value computed at the transition state structure (TS), affects the four reactivity types in such a manner that the stronger the nucleophilic/electrophilic interactions taking place at the TS, the faster the reaction, and may even change the molecular mechanism according to the Parr functions defined within Conceptual DFT. This MEDT rationalisation of 32CA reactions unravels classical Huisgen’s and Firestone’s mechanistic proposals established in the 60’s. Regardless of the reactivity type and polar character of the reaction, topological analysis of the electron localisation function along one-step 32CA reactions suggests that the bonding changes are not “concerted”, but sequential, thus ruling out the classification of these reactions as “pericyclic”. In the present thesis, the classical theory of 32CA reactions, established in the 60’s of the past century and still prevailing today, is revisited and reinterpreted based on MEDT. A solid new reactivity model for 32CA reactions is established, emphasising that the way that organic chemists conceive organic chemistry demands a contemporary revision aimed towards the analysis of electron density

Synthesis and applications of materials and polymers containing graphenic and/or triptycene moities

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, February 2013."November 2012." Cataloged from PDF version of thesis.Includes bibliographical references.In this thesis, molecular design is used to synthesize novel materials with specific properties. The materials presented herein are based on two motifs. In part I, new methods of functionalizing graphenic materials and the applications of those materials are presented. In part II, new triptycene-based polymers are synthesized and their performance is explored. Part I: Graphenic Materials Chapter 1: Three different types of epoxy-functionalized multiwalled carbon nanotubes (EpCNTs) were prepared via multiple covalent functionalizaton methods. The effect of the different chemistries on the adhesive properties of a nanocomposite prepared from commercial epoxy at 0.5, 1, 2, 3, 5, and 10 weight % CNT was studied. It was found that a covalently functionalized EpCNT (EpCNT2) at 1 weight % increased the lap shear strength, by 36 % over the unfilled epoxy formulation and by 27 % over a 1 weight % unmodified MWNT control sample. SEM images revealed a fracture surface morphology change with the incorporation of EpCNT and a deflection of the crack fronts at the site of embedded CNTs, as the mechanism accounting for increased adhesive strength. Chapter 2: In this chapter, the hydroxyl functionalities in graphene oxide (GO, a highly oxidized analogue of graphite), the vast majority that must be allylic alcohols, are subjected to Johnson-Claisen rearrangement conditions. In these conditions, a [3, 3] sigmatropic rearrangement after reaction with triethyl orthoacetate gives rise to an ester functional group, attached to the graphitic framework via a robust C-C bond. The resultant functional groups were found to withstand reductive treatments for the deoxygenation of graphene sheets and a resumption of electronic conductivity is observed. The chemical versatility of the ester groups allows for a variety of functional graphenes to be synthesized, and several of these have been used to successfully build layer-by-layer (LBL) constructs. Chapter 3: The effects of quantity of graphene and carbon nanotube-based fillers and their pendant functional groups on the shear properties of a thermoset epoxy were investigated. Two novel functionalized graphenes, one with epoxy functionality and the other with an amine, are synthesized for this purpose. The properties of the epoxy nanocomposites containing epoxy- and amine-functionalized graphene are compared with those containing GO, Claisen-functionalized graphene, MWNT, the EpCNTs, and the unfilled epoxy. One of the EpCNT (EpCNT3) was found to increase the plateau shear storage modulus by 136 % (1.67 MPa to 3.94 MPa) and the corresponding loss modulus by almost 400 % at a concentration of 10 weight %. A hybrid system of EpCNT3 and graphite was also studied, which improved the storage modulus by up to 51 %. Part II: Triptycene-Based Polymers Chapter 4: A series of soluble, thermally stable aromatic polyimides were synthesized using commercially available five and six membered ring anhydrides and 2, 6- diaminotriptycene derivatives. All of these triptycene polyimides (TPIs) were soluble in common organic solvents despite their completely aromatic structure, due to the threedimensional triptycene structure that prevents strong interchain interactions. Nanoporosity in the solid state gives rise to high surface areas (up to 430 m2/g) and low refractive indices (1.19- 1.79 at 633 nm), which suggest very low dielectric constants at optical frequencies. The decomposition temperature (Td) for all of the polymers is above 500 'C, indicating excellent prospects for high temperature applications. Chapter 5: Several new triptycene-containing polyetherolefins were synthesized via acyclic diene metathesis (ADMET) polymerization. Two types of triptycene-based monomer with varying connectivities were used in the synthesis of homopolymers, block copolymers, and random copolymers. In this way, the influence of the triptycene architecture and concentration in the polymer backbone on the thermal behavior of the polymers was studied. Inclusion of increasing amounts of triptycene were found to increase the glass transition temperature, from -44 °C in polyoctenamer to 59 °C in one of the hydrogenated triptycene homopolymers (H-PT2). Varying the amounts and orientations of triptycene was found to increase the stiffness (H-PT1), toughness (PT1₁- b-PO₁) and ductility (PT1₁-ran-PO₃) of the polymer at room temperature. Chapter 6: A novel all-hard block polyurea containing triptycene units was synthesized from N,N-carbonyldiimidizole and 2,6-diaminotriptycene. The incorporation of triptycene along the backbone prohibits the polymer from hydrogen-bonding with itself and leave the sites of the urea open for the capture of H-bond accepting analytes. Targeted analytes include cyclohexanone, a signature of the powerful explosive RDX (1 ,3,5-trinitro- 1,3,5- triazacyclohexane) and organophosphate nerve agents. This triptycene-polyurea (TPU) was found to be fluorescent in solution in the presence of H-bonding solvents. TPU was cast into thin films and a 12% increase in the fluorescent emission at 443 nm was observed in the presence of saturated cyclohexanone vapor. The sensitivity and selectivity of this response is enhanced by creating a hybrid system with squaraine (Hbond acceptor) and a HFIP-dypyrrin based dye (H-bond donor).by Stefanie A. Sydlik.Ph.D