Evaluation of Silica Sol-Gel Microcapsule for the Controlled Release of Insect Repellent, N,N-Diethyl-2-methoxybenzamide, on Cotton

N,N-diethyl-2-methoxybenzamide, an insect repellent compound, was incorporated into cotton textile using the sol-gel method. Evaluation of the results of four different sol-gel formulations showed that a lower condensation rate resulted in significantly better encapsulation. Also investigated was the encapsulation in the presence of other polymers like polymethylmethacrylate (PMMA), polystyrene (PS) and polyvinyl alcohol (PVA) that prolong the release of the repellent. The sol-gel formulation, SiO2 : polymethylmethacrylate (PMMA) (1:0.8), gave smooth fabric and good encapsulation of the insect repellent compound. PS and PVA resulted in cloths which are aesthetically undesirable. Cloths coated with sol-gel PVA gave the best repellent release profile

Transition Metal Catalyzed Transformations of Strained Heterocycles

Heterocycles are present in more than half of organic compounds. For organic chemists, they are valued as synthetic targets or scaffolds to construct valuable products. For the past two decades, the Howell group has made contributions towards the synthesis and applications of 4-membered heterocyclic compounds, such as oxetanes and beta-lactones. Most of the previously reported transformations that involve strained heterocyclic compounds rely on traditional methods in which rendering the reaction with good and predictable selectivity (regio-, chemo- and stereoselectivity) is challenging. The works described took advantage of the intrinsic reactivity of strained heterocycles and combined that with the highly selective transformations promoted by transition metal (TM) catalysts. Three successful methodologies were developed. Chapter 1 describes the discovery and scope of a novel Pt-catalyzed expansion of spirocyclopropyl oxetanes to synthetically useful 3-methylenetetrahydrofurans. This unprecedented oxetane expansion was realized via cyclopropane activation under platinum catalysis. Mechanistic studies, through 13C-labelling and 13C-DEPT NMR analyses, suggested that the oxetane expansion was promoted by a regioselective carbon-carbon bond activation of cyclopropane with platinum. Chapter 2 describes two transition metal catalyzed transformations of alpha-methylene-beta-lactones. First is a Rh-catalyzed conjugate addition with aryl boronic acids to access various beta-lactones. beta-Lactones are highly privileged synthetic products and intermediates. They have been shown to elicit serine hydrolase inhibition. They are also used as intermediates to obtain difunctionalized acyclic compounds, and this was the goal in the next method. The second method developed involves a chemoselective opening of beta-lactones to form beta-hydroxy amides. Ring opening of beta-lactones with several nucleophiles typically provide a mixture of two major products; opening via the (a) alkyl C–O bond, or (b) the acyl C–O bond. The selective ring-opening was realized via activation of acyl carbon-oxygen bond under palladium catalysis. Under the developed conditions, several beta-lactones were selectively opened with various amine nucleophiles and gave beta-hydroxy amides as sole product. This method was also translated to a Pd-catalyzed asymmetric kinetic resolution of racemic to enantioenriched beta-lactones

Pd-Catalyzed Decarbonylative Cross-Couplings of Aroyl Chlorides

This report describes a method for Pd-catalyzed decarbonylative cross-coupling that enables the conversion of carboxylic acid derivatives to biaryls, aryl amines, aryl ethers, aryl sulfides, aryl boronate esters, and trifluoromethylated arenes. The success of this transformation leverages the Pd⁰/Brettphos-catalyzed decarbonylative chlorination of aroyl chlorides, which can then participate in diverse cross-coupling reactions <i>in situ</i> using the same Pd catalyst

Palladium- and Nickel-Catalyzed Decarbonylative C–S Coupling to Convert Thioesters to Thioethers

This Letter describes the development of a catalytic decarbonylative C–S coupling reaction that transforms thioesters into thioethers. Both Pd- and Ni-based catalysts are developed and applied to the construction of diaryl, aryl alkyl, and heterocycle-containing thioethers

Pt-Catalyzed Rearrangement of Oxaspirohexanes to 3‑Methylenetetrahydrofurans: Scope and Mechanism

A novel Pt-catalyzed rearrangement of oxaspirohexanes to 3-methylenetetrahydrofurans is reported. Mechanistic studies by ¹³C-labeling experiments confirm oxidative addition of Pt­(II) regioselectively to the least substituted carbon–carbon bond of the cyclopropane to form a platinacyclobutane intermediate. To our knowledge, this is the first alkoxy-substituted platinacyclobutane that has been observed spectroscopically. The scope and a proposed mechanism of this new Pt-catalyzed transformation are described

Pd-Catalyzed Acyl C–O Bond Activation for Selective Ring-Opening of α‑Methylene-β‑lactones with Amines

A Pd-catalyzed ring-opening of β-lactones with various types of amines (primary, secondary, and aryl) to provide β-hydroxy amides with excellent selectivity toward acyl C–O bond cleavage is reported. The utility of this protocol is demonstrated in an asymmetric kinetic resolution providing enantioenriched α-methylene-β-lactones

Pt-Catalyzed Rearrangement of Oxaspirohexanes to 3‑Methylenetetrahydrofurans: Scope and Mechanism

A novel Pt-catalyzed rearrangement of oxaspirohexanes to 3-methylenetetrahydrofurans is reported. Mechanistic studies by ¹³C-labeling experiments confirm oxidative addition of Pt­(II) regioselectively to the least substituted carbon–carbon bond of the cyclopropane to form a platinacyclobutane intermediate. To our knowledge, this is the first alkoxy-substituted platinacyclobutane that has been observed spectroscopically. The scope and a proposed mechanism of this new Pt-catalyzed transformation are described

Rh-Catalyzed Conjugate Addition of Aryl and Alkenyl Boronic Acids to α‑Methylene-β-lactones: Stereoselective Synthesis of <i>trans</i>-3,4-Disubstituted β‑Lactones

A one-step preparation of 3,4-disubstituted β-lactones through Rh-catalyzed conjugate addition of aryl or alkenyl boronic acids to α-methylene-β-lactones is described. The operationally simple, stereoselective transformation provides a broad range of β-lactones from individual α-methylene-β-lactone templates. This methodology allowed for a direct, final-step C-3 diversification of nocardiolactone, an antimicrobial natural product

Electroreductive Olefin-Ketone Coupling

A user-friendly approach to sidestep the venerable Grignard addition to unactivated ketones to access tertiary alcohols by reversing the polarity of the disconnection. In this work a ketone instead acts as a nucleophile when adding to simple unactivated olefins to accomplish the same overall transformation. The scope of this coupling is broad as enabled using an electrochemical approach and the reaction is scalable, chemoselective, and requires no precaution to exclude air or water. Multiple applications demonstrate the simplifying nature of the reaction on multi-step synthesis and mechanistic studies point to an intuitive mechanism reminiscent of other chemical reductants such as SmI2 (which cannot accomplish the same reaction).</div

Electrochemical cobalt-catalyzed selective carboxylation of benzyl halides with CO2 enabled by low-coordinate cobalt electrocatalysts

The direct, transition metal-catalyzed carboxylation of organohalides with carbon dioxide is a highly desirable transformation in organic synthesis as it utilizes feedstock chemicals and delivers carboxylic acids –among the most utilized class of organic molecules. Phenyl acetic acids, in particular, are privileged motifs that appear in many pharmaceuticals and biologically active compounds. This article reports the development of a sustainable and selective cobalt-catalyzed electrochemical carboxylation of benzyl halides with CO2 to generate phenyl acetic acids. The success of this transformation is enabled by the development of low-coordinate cobalt/pyrox complexes as electrocatalysts to convert various benzyl chlorides and bromides to their corre-sponding phenyl/heteroaryl acetic acids with high selectivity over undesired homocoupling of the benzyl halides. The combina-tion of electroanalytical methods, simulation studies, control reactions, and first-principles density functional theory (DFT) calculations informed the mechanistic analysis of this reaction. An EC’C-type activation mechanism of benzyl halides, which is unique to Co(II)/pyrox electrocatalysts, provides the rationalization of the exceptional observed selectivity for carboxylation. Specifically, the Co(II)/pyrox catalyst undergoes reduction to Co(I) followed by halogen abstraction and a favorable radical rebound to Co(II)/pyrox to form alkyl–Co(III) intermediates. Although voltammetry only shows a single electron transfer step, bulk electrolysis shows a two electron process and using DFT calculations, the intermediates are proposed to undergo two-electron reduction to alkyl–Co(I) followed by a ZnCl2-assisted CO2 insertion to form the carboxylated adducts with regenera-tion of Co(I)/pyrox