Oximes are valuable synthetic building blocks with reactivity modes that enable their use in diverse methodologies, from cycloadditions to bioconjugation. Their reactivity towards photocatalysis and transition metals makes them ideal starting materials for N-containing heterocycles, amino alcohols and amines. Developments in oxime reactivity since 2016 have enabled transformations such as the addition of iminyl radicals to alkenes to generate functionalized imines, and [2 + 2]-cycloadditions to access azetidines. The unique properties imparted by the oxime N-O bond have also been used to integrate dynamic chemistries into materials. In this Review, we discuss the innovative use of this powerful functional group, with a focus on N-O bond fragmentation and cycloadditions, along with applications including dynamic materials, energetic materials and biocatalytic oxime reductions. We conclude by highlighting methodologies based on oxime starting materials, along with the challenges of using oximes for diverse applications, and offer insight into future directions in these areas.
One of the most efficient ways to synthesize oxetanes is the light-enabled [2+2] cycloaddition reaction of carbonyls and alkenes, referred to as the Paternò-Büchi reaction. The reaction conditions for this transformation typically require the use of high energy UV light to excite the carbonyl, limiting the applications, safety, and scalability. We herein report the development of a visible light-mediated Paternò-Büchi reaction protocol that relies on triplet energy transfer from an iridium-based photocatalyst to the carbonyl substrates. This mode of activation is demonstrated for a variety of aryl glyoxylates and negates the need for both, visible light-absorbing carbonyl starting materials or UV light to enable access to a variety of functionalized oxetanes in up to 99% yield.
<div> <div> <p>Herein, we describe the application of Lewis acid-catalyzed carbonyl-olefin metathesis towards the synthesis of chiral, substituted tetrahydropyridines from commercially available amino acids as chiral pool reagents. This strategy relies on FeCl<sub>3</sub> as an inexpensive and environmentally benign catalyst and enables access to a variety of substituted tetrahydropyridines under mild reaction conditions. The reaction proceeds with complete stereoretention and is viable for a variety of natural and unnatural amino acids to provide the corresponding tetrahydropyridines in up to 99% yield.</p> </div> </div> <br>
Despite their favorable properties, azetidines are often overlooked as lead compounds across multiple industries. This is often attributed to the challenging synthesis of densely functionalized azetidines in an efficient manner. In this work, we report the scalable synthesis and characterization of seven azetidines with varying regio- and stereochemistry and their application as novel azetidine-based energetic materials, enabled by the visible-light-mediated aza Paternò–Büchi reaction. The performance and stark differences in the physical properties of these new compounds make them excellent potential candidates as novel solid melt-castable explosive materials, as well as potential liquid propellant plasticizers. This work highlights the scalability and utility of the visible-light aza Paternò–Büchi reaction and demonstrates the impact of stereochemical considerations on the physical properties of azetidine-based energetics. Considering the versatility and efficiency of the presented synthetic strategies, we expect that this work will guide the development of new azetidine-based materials in the energetics space as well as other industries.
Carbonyl–ene, Prins, and carbonyl–olefin metathesis reactions represent powerful strategies for carbon–carbon bond formation relying on Lewis acid catalysts. Although common Lewis acids are able to provide efficient activation, the reactions often proceed with low regio- or chemoselectivity, while high selectivity frequently requires the use of well-designed metal–ligand complexes. Here we demonstrate that simple Lewis acids including Me2AlCl, FeCl3, and SnCl4 can show remarkable selectivity in differentiating between distinct transformations of carbonyl and olefin functional groups, resulting in either carbonyl–ene or carbonyl–olefin metathesis products. Specifically, we report the development of predictive multivariate linear regression models that rely on kinetic and thermodynamic information obtained in DFT calculations to gain important insights into the complex potential energy surfaces (PES) of these competing reaction paths. The presented results further our understanding of Lewis acid reactivity and suggest that even simple Lewis acids have the potential to function as highly selective catalysts.
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