UC San Diego

This dissertation involves the study and manipulation of molecular orbitals on ambiphilic molecules to obtain unusual properties and develop new applications. Carbenes, which can simultaneously serve as powerful Brønsted bases and Lewis acids, undergo further amplification of those properties as the bond angle at the carbene center changes (chapter 2). Such profound reactivity fluctuations from seemingly simple geometric modifications highlight the need for accurate measurement of carbene frontier orbitals. However, X-ray and NMR studies show that carbene-selenium adducts are susceptible to H-Se non-classical hydrogen bonds which cause large downfield shifts in 77Se NMR, disrupting a commonly used technique for assessing carbene π-acidity (chapter 3). Furthermore, common techniques used to characterize carbene donor abilities are unable to avoid the confounding of carbene basic and acidic orbitals. To directly see how carbene lone pairs compare to each other, we ranked different carbene families by basicity through a proton exchange method (chapter 4). The basicity ranking highlights several surprising qualitative errors that occur throughout many theoretical calculations. Finally, exploring the reactivity of aminoboranes, a class of 1,2-ambiphiles, we developed a new methodology for the synthesis of aldimines (chapter 5). Preliminary mechanistic studies suggest that the mechanism is orthogonal to traditional condensation methods and allows access to products that are conventionally difficult to obtain. This dissertation will hopefully serve as another example of how we can advance main-group synthetic chemistry by taking a molecular orbital perspective.