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The stereoselective functionalization of C–H bonds represents a central challenge in modern organic synthesis. Despite decades of innovation in C–H activation chemistry, methods for Z-selective functi...

Discovering and optimizing reactions is central to synthetic chemistry. However, chemical reactions are traditionally screened using relatively low-throughput methods, prohibiting exploration of diver...

A fully-funded 3-year PhD position in organic chemistry, C-H functionalization, and homogeneous enantioselective metal catalysis under the supervision of Associate Professor Søren Kramer is available ...

Queen’s University is a globally engaged, research-intensive institution dedicated to attracting and supporting exceptional PhD students who will significantly advance our research mission.

4-Substituted 7-(4′-alkenyloxy)-1-indanones were prepared from the respective substituted aryl propanoic acids and subjected to UV-A irradiation (λ = 350 or 366 nm). While the 4-chloro compound was directly converted at λ = 366 nm into a pentacyclic product (47% yield) by a three-photon cascade process, the oxygenated substrates reacted in trifluoroethanol at λ = 350 nm by a two-photon cascade, involving an ortho photocycloaddition, a thermal disrotatory ring opening, and a [4π] photocyclization (six examples, 67–82% yield). An ensuing photochemical di-π-methane rearrangement of the latter products was achieved by irradiation at λ = 350 nm in toluene (five examples, 36–70% yield). The diastereoselectivity of the reaction was probed employing a chiral 1-indanone with a stereogenic center at carbon atom C3. 1-Indanones with a 4-hexenyloxy side chain [(E)- or (Z)-configured] at carbon atom C7 served to interrogate the stereospecifity of the reaction.

A fascinating exploration of microbes that can travel through the air reveals how the pandemic marked a turning point for a crucial research field.

ConspectusMillions of chiral compounds contain a stereogenic sp3-hybridized carbon center with a hydrogen atom as one of the four different substituents. The stereogenic center can be edited in an increasing number of cases by selective hydrogen atom transfer (HAT) to and from a photocatalyst. This Account describes the development of photochemical deracemization reactions using chiral oxazole-annulated benzophenones with a bonding motif that allows them to recognize chiral lactam substrates by two-point hydrogen bonding. The backbone of the catalysts consists of a chiral azabicyclo[3.3.1]nonan-2-one with a U-shaped geometry, which enables substrate recognition to occur parallel to the benzoxazole part of the aromatic ketones. The photocatalysts facilitate a catalytic photochemical deracemization of several compound classes including hydantoins, N-carboxyanhydrides, oxindoles, 2,5-diketopiperazines, and 4,7-diaza-1-isoindolinones. In addition, if more than one stereogenic center is present, the editing delivers a distinct diastereoisomer upon the appropriate selection of the respective photocatalyst enantiomer. The chiral photocatalysts operate via the benzophenone triplet that selectively abstracts a properly positioned hydrogen atom in exclusively one of the two substrate enantiomers. The photochemical step creates a planar carbon-centered radical and erases the absolute configuration at this position. While returning HAT to the same position would likely recreate the stereogenic center with the same absolute configuration, spectroscopic and quantum chemical studies suggest that the hydrogen atom is delivered from the photocatalyst to a heteroatom that is in conjugation to the radical center. Two scenarios can be distinguished for the hydrogen atom shuttling process. For hydantoins, N-carboxyanhydrides, and 4,7-diaza-1-isoindolinones, the back HAT occurs to a carbonyl oxygen atom or an imine-type nitrogen atom which is not involved in binding to the catalyst. For oxindoles and 2,5-diketopiperazines, a single lactam carbonyl group in the substrate is available to accept the hydrogen atom. It is currently assumed that back HAT occurs to this group, although the carbonyl oxygen atom is involved in hydrogen bonding to the catalyst. In comparison to the former reaction pathway, the latter process appears to be less efficient and more prone to side reactions. For both cases, an achiral enol or enamine is formed, which delivers upon dissociation from the catalyst statistically either one of the two stereoisomers of the substrate. Since only one substrate enantiomer (or diastereoisomer) is processed, a high enantioselectivity (or diastereoselectivity) results. Even though the editing is a contra-thermodynamic process, the described decoupling of a photochemical and a thermal step allows the usage of a single catalyst in loadings that vary between 2.5 and 10 mol % depending on the specific mode of action.

This fellowship is for non-UK scientists who are at an early stage of their research career and wish to conduct research in the UK.