Radical methods for the synthesis of aliphatic amines

Abstract

The aliphatic amine motif is a ubiquitous and uniquely important functional group in pharmaceutical agents and the development of ever more efficient synthetic methods for their synthesis is a continuous challenge. This thesis details the development of three new radical reactions for the synthesis of aliphatic amines. Following an introduction to radical chemistry and a summary of previous advances in amine synthesis, chapter 2 will describe a general carbonyl alkylative amination reaction, a successful version of which has been sought for over 70 years. Facilitated by visible light and a silane reducing agent, the operationally straightforward reaction forms tertiary amines via the coupling of aldehydes and secondary amines with alkyl halides. Combining an efficient radical-chain mechanism with the structural and functional diversity of readily available starting materials, the carbonyl alkylative amination provides a flexible strategy for the streamlined synthesis of complex tertiary amines. In chapter 3, an improved carbonyl alkylative amination was sought by replacing the alkyl halide with a more abundant radical precursor. This led to the development of a new carbonyl alkylative amination reaction using tetrachloro N-hydroxyphthalimide esters, prepared from carboxylic acids, in conjunction with inexpensive and environmentally friendly zinc dust instead of the costly silane reducing agent. The use of a carboxylic acid derived nucleophile enabled a broader scope in both amine and nucleophile components and the full potential of this reaction is currently being developed. In chapter 4, a visible light-mediated direct synthesis of N-heterospirocycles is reported. A photocatalyst was employed to reduce an aliphatic iminium ion to the corresponding -amino radical, which was cyclized to afford polar, heteroatom-rich spirocycles. The reaction tolerated a range of polar functional groups and the resulting scaffolds were shown to occupy a promising yet less exploited area of chemical space, making the reaction relevant for fragment-based drug discovery.