Date of Degree
Adam B. Braunschweig
Rein V. Ulijn
Mark R. Biscoe
Chemistry | Materials Chemistry | Organic Chemistry
Photocatalysis, Supramolecular Chemistry, Self-assembly, Aqueous Catalysis, Photomechanochemistry
Visible-light photocatalysis in H2O provides an attractive, green alternative to typical organic synthesis, which often involves toxic solvents, metal catalysts, and large energy demands. Hence, there is a growing need for an efficient photocatalytic methods that use either aqueous media as a solvent or can proceed solvent-free. However, commercially available photocatalysts do not work well for aqueous photocatalysis. Supramolecular systems in particular have been explored recently to address these issues associated with aqueous photocatalysis. Chapter 1 will review recent advances in aqueous supramolecular photocatalysis with examples of different supramolecular systems and how they have addressed some of the challenges associated with aqueous photocatalysis.
We envisioned that supramolecular photocatalytic polymers provide a promising solution to this challenge as these polymers can recruit hydrophobic molecules inside the nanofiber network, and, upon exposure to light, induce a chemical change that would otherwise not occur in H2O. In Chapter 2, we show our design of new supramolecular photocatalytic nanofibers for an aqueous photooxidation reaction. These supramolecular nanofibers are composed of amino acid-functionalized derivatives of the organic chromophore diketopyrrolopyrrole (DPP). The monomeric units assemble into supramolecular nanofibers upon in situ enzymatic hydrolysis. In our design, the catalyst itself is an inseparable part of the monomer, forming high aspect ratio fibers, and, thereby, provides a high density of photoactive sites upon assembly. DPP molecules have been functionalized with three different amino acids (Y, F, or L) to modulate the supramolecular and photophysical properties of the assembly. In our first study, we explored the ability of these supramolecular polymers to produce 1O2 in H2O that under visible light irradiation. These nanofibers were then used for the photooxidation of thioanisole and cyclohexyl methyl sulfide with yields as high as 100% and without over-oxidation to the sulfone. With our initial success, we have started to explore the application of our aqueous supramolecular catalysts for other important photocatalytic organic reactions.
Amidation is one of the most important reactions in chemistry and the pharmaceutical industry. Amide bonds are prevalent in proteins, peptides, other natural products, pharmaceuticals, and polymers. However, traditional amidation methods have poor atom economy, produce toxic chemical waste, and often are moisture sensitive. Hence, there is a need for new methods for amidation that proceed under mild reaction conditions and produce less chemical waste. In Chapter 3, we show that, after systematic optimization of pH, light intensity, and molar concentration, amidation reactions can be performed under mild reaction conditions with quantitative yields in H2O using our photocatalyst. To further demonstrate the transformational nature of these nanofiber photocatalysts, we have incorporated them into a flow reactor for continuous amidation. Unlike other flow reactors, in our design, the catalyst is in a stationary state, which means that the reaction can run continuously without reloading the catalyst. Finally, I have shown the real and practical utility of these hydrogel photocatalysts by using these catalysts to amidate over 20 amines and two peptides.
In Chapter 4, we explored the concept of photomechanochemistry – a simultaneous combination of light and force, and how insolubility can tune the stereoselectivity of a [2+2] photocycloaddition reaction. The dimerization of acenaphthylene is frequently studied to understand how different reaction conditions affect the stereoselectivities and yields in [2+2] photochemical cycloadditions. In organic solvents, where this reaction is typically carried out, the products are a mixture of syn and anti dimers, and so the stereoselective formation of cyclobutanes is a problem that continues to vex organic chemists. To drive the reaction to the anti isomer, myriad conditions have been investigated by others, including the use of molecular cages or toxic solvents. To this end, we found that: A) running the reaction in H2O produces the anti product quantitatively and with among the highest anti stereoselectivity yet observed; and B) syn stereoselectivity can be obtained under photomechanochemical conditions, even though the anti product can be obtained when acenaphthylene crystals are irradiated in the absence of force. Therefore, we show that force can alter the stereoselectivity, and report solvent-free and environmentally benign conditions for forming the syn dimer as well.
Biswas, Sankarsan, "Visible Light Photocatalysis of Organic Reactions in H2O" (2022). CUNY Academic Works.