New Approaches To Heterocycle Synthesis: A Greener Route To Structurally Complex Protonated Azomethine Imines, And Their Use In 1,3-Dipolar Cycloadditions
Date of Award
Doctor of Philosophy (PhD)
1-Aza-2-azoniaallene salts are reactive intermediates that undergo [3+2] cycloaddition with many different types of multiple bonds. For the past several years, the Brewer group has studied the reactivity of these intermediates in intramolecular reactions, and have discovered that these cationic heteroallenes can react through a variety of other, mechanistically distinct, pathways to give different classes of nitrogen heterocycles. For example, prior work in the Brewer group revealed that 1-aza-2-azoniaallene salts could react in an intramolecular [4+2] cycloaddition reaction to give protonated azomethine imine salts containing a 1,2,3,4-tetrahydrocinnoline scaffold. Further study of the scope and limitations of this Diels-Alder-like reaction are described herein. These studies primarily focused on how varying the N-aryl ring and alkene substituents affected the reaction. We discovered that in several instances, the metal mediated reaction did not facilitate the cycloaddition very well, so we searched for alternative ways to facilitate the reaction. We discovered that a non-metallic Lewis acid (TMSOTf) provided very clean products with α-chloroazo compounds. I hypothesized that changing the leaving group adjacent to the azo might further improve the reaction. With this in mind, I developed a technique to prepare α-trifluoroacetoxyazo compounds by treating aryl hydrazones with trifluoroacetoxy dimethylsulfonium trifluoroacetate. This technique is compatible with all types of functional groups including nitro aryl compounds, which gave low yields of the corresponding chloroazo derivatives. Importantly, these α-trifluoroacetoxyazo compounds gave even better cycloaddition results when treated with TMSOTf, and this method is more practical, more environmentally friendly, and greener than the metal mediated technique. This process even returned sterically hindered products in high yield, and provide a dearomatized non-protonated azomethine imine salt, which further verified the proposed mechanism of the [4+2] cycloaddition. Azomethine imines are well known to undergo 1,3-dipolar cycloadditions with alkenes. We wondered if the protonated azomethine imine salts generated by the [4+2] cycloaddition could be used in a subsequent base-mediated [3+2] cycloaddition to generate structurally complex tetra- or pentacyclic products. We were pleased to find that the protonated azomethine imines indeed reacted smoothly with a variety of π-system in the presence of triethylamine to give the corresponding cycloadducts in high yields with moderate to high diastereoselectivities. In an attempt to understand the diastereoselectivity of these [3+2] cycloadditions better, I modeled them computationally.
Number of Pages
Dhakal, Ram Chandra, "New Approaches To Heterocycle Synthesis: A Greener Route To Structurally Complex Protonated Azomethine Imines, And Their Use In 1,3-Dipolar Cycloadditions" (2017). Graduate College Dissertations and Theses. 777.