Sermadurai Selvakumar

Enantioselective Organo‐Photocatalysis Mediated by Atropisomeric Thiourea Derivatives

Can photocatalysis be performed without electron or energy transfer? To address this, organo-photocatalysts that are based on atropisomeric thioureas and display lower excited-state energies than the reactive substrates have been developed. These photocatalysts were found to be efficient in promoting the [2+2] photocycloaddition of 4-alkenyl-substituted coumarins, which led to the corresponding products with high enantioselectivity (77-96% ee) at low catalyst loading (1-10 mol%). The photocatalytic cycle proceeds by energy sharing via the formation of both static and dynamic complexes (exciplex formation), which is aided by hydrogen bonding.

Thermoset Coatings from Epoxidized Sucrose Soyate and Blocked, Bio-Based Dicarboxylic Acids

A new 100% bio-based thermosetting coating system was developed from epoxidized sucrose soyate crosslinked with blocked bio-based dicarboxylic acids. A solvent-free, green method was used to block the carboxylic acid groups and render the acids miscible with the epoxy resin. The thermal reversibility of this blocking allowed for the formulation of epoxy-acid thermoset coatings that are 100% bio-based. This was possible due to the volatility of the vinyl ethers under curing conditions. These systems have good adhesion to metal substrates and perform well under chemical and physical stress. Additionally, the hardness of the coating system is dependent on the chain length of the diacid used, making it tunable.

Organophotocatalysis: Insights into the Mechanistic Aspects of Thiourea-Mediated Intermolecular [2+2] Photocycloadditions.

Mechanistic investigations of the intermolecular [2+2] photocycloaddition of coumarin with tetramethylethylene mediated by thiourea catalysts reveal that the reaction is enabled by a combination of minimized aggregation, enhanced intersystem crossing, and altered excited-state lifetime(s). These results clarify how the excited-state reactivity can be manipulated through catalyst-substrate interactions and reveal a third mechanistic pathway for thiourea-mediated organo-photocatalysis.

Structural and Solubility Parameter Correlations of Gelation Abilities for Dihydroxylated Derivatives of Long‐Chain, Naturally Occurring Fatty Acids

Creating structure-property correlations at different distance scales is one of the important challenges to the rational design of molecular gelators. Here, a series of dihydroxylated derivatives of long-chain fatty acids, derived from three naturally occurring molecules-oleic, erucic and ricinoleic acids-are investigated as gelators of a wide variety of liquids. Conclusions about what constitutes a more (or less!) efficient gelator are based upon analyses of a variety of thermal, structural, molecular modeling, and rheological results. Correlations between the manner of molecular packing in the neat solid or gel states of the gelators and Hansen solubility data from the liquids leads to the conclusion that diol stereochemistry, the number of carbon atoms separating the two hydroxyl groups, and the length of the alkanoic chains are the most important structural parameters controlling efficiency of gel formation for these gelators. Some of the diol gelators are as efficient or even more efficient than the well-known, excellent gelator, (R)-12-hydroxystearic acid; others are much worse. The ability to form extensive intermolecular H-bonding networks along the alkyl chains appears to play a key role in promoting fiber growth and, thus, gelation. In toto, the results demonstrate how the efficiency of gelation can be modulated by very small structural changes and also suggest how other structural modifications may be exploited to create efficient gelators.

Novel alkyd-type coating resins produced using cationic polymerization

Novel, partially bio-based poly(vinyl ether) copolymers derived from soybean oil and cyclohexyl vinyl ether (CHVE) were produced by cationic polymerization and investigated for application as alkyd-type surface coatings. Three different polymers were produced and cured clear coatings and free films characterized as a function of CHVE comonomer content and curing conditions. CHVE comonomer content was varied at 0, 25, and 50xa0wt%, and the three different curing conditions were 1xa0week at room temperature, 120°C for 1xa0h, and 150°C for 1xa0h. The results of the study showed that the thermal, mechanical, and physical properties of the coatings produced from these novel polymers varied considerably as a function of polymer composition and cure temperature. Overall, the results suggest a good potential for these novel copolymers to be used for coatings cured by autoxidation. Compared to conventional alkyd resins, which are produced by high temperature melt condensation polymerization, these poly(vinyl ether)s provide several advantages. These advantages include milder, more energy efficient polymer synthesis, elimination of issues associated with gelation during polymer synthesis, production of polymers with well-defined composition and relatively narrow molecular weight distribution, and elimination of film formation and physical property issues associated with entrained monomers, dimers, trimers, etc.

3.21 Stoichiometric Auxiliary Ligands for Metals and Main Group Elements: Ligands for Magnesium and Calcium

This chapter highlights the use of magnesium and calcium salts in a variety of diastereo- and enantioselective transformations. The ready availability of magnesium and calcium salts along with their low toxicity makes them ideal catalysts for stereoselective reactions. The Lewis acidity of the metal salt can be tuned by the counterion. The magnesium (II) complexes show broad utility as Lewis acid catalysts and have been successfully applied in cycloadditions, radical reactions, aldol reactions, Mannich-type reactions, and conjugate additions. In contrast, the number of successful examples using chiral calcium complexes is rather sparse. The future looks promising for exploiting the useful characteristics of magnesium and calcium reagents for the development of new and novel stereoselective transformations.