Robby Vroemans

Crystals of a BOIMPY dianion made the exploration of a catalytic process that relies on consecutive multi-photoinduced electron transfer (multi-PET) possible. Irradiation of the dye molecule leads to an excited species that allows the reduction of strong carbon-halide bonds and arenes. The synthetic value is showcased by hydrodehalogenations, C−C, and C−P bond formations, and Birch-type reductions. Thorough mechanistic investigation is presented.

Abstract

An established concept to create radical intermediates is photoexcitation of a catalyst to a higher energy intermediate, subsequently leading to a photoinduced electron transfer (PET) with a reaction partner. The known concept of consecutive photoinduced electron transfer (con-PET) leads to catalytically active species even higher in energy by the uptake of two photons. Generally speaking, increased photon uptake leads to a more potent reductant. Here, we report the concept of multi-photoinduced electron transfer catalysis (>2 photons), termed multi-PET, which is enabled by photoinduced one-electron reductions of an organic dye. Further irradiation of the doubly reduced species leads to a photoexcited dianionic super-reductant, which is more potent than Li metal – one of the strongest chemical reductants known. This multi-photon process which is enabled by 390 nm LEDs allows the cleavage of strong carbon-fluorine bonds and reduction of other halides even in very electron-rich substrates. The resulting radicals are quenched by hydrogen atoms or engaged in carbon-carbon and carbon-phosphorus bond formations, highlighting the utility of multi-PET for organic chemistry. In addition, multi-PET enabled Birch-type reductions. Spectroscopic, chemical and computational investigations are presented to gain mechanistic insights.

In the Cover Feature, three green-glowing nickel-porphyrins surround a central red-glowing hexa-peri-hexabenzocoronene (HBC). The aromatic fragments have tesla coils attached to their periphery through which they interact with each other, highlighted by electrical discharges. The tesla coils in the porphyrins emphasize the β-meso five-ring fusion motif in the HBC–porphyrin conjugates presented in the paper, where up to three porphyrins are coupled to a single hexabenzocoronene. More information can be found in the Research Article by B. Meyer, N. Jux and co-workers (DOI: 10.1002/chem.202403250). The cover art was designed and made by Christoph Oleszak.

Synlett
DOI: 10.1055/a-2456-9333

The quest for general and highly efficient and enantioselective catalytic route to chiral alcohols remains a formidable challenge in asymmetric synthesis. Here, we highlight our recent work of asymmetric transfer hydrogenation (ATH) of N-methyliminodiacetyl (MIDA) acylboronates, showcasing a versatile platform for the efficient synthesis of enantiomerically enriched secondary alcohols. Acyl-MIDA-boronates harboring diverse (het)aryl, alkyl, alkynyl, alkenyl, and carbonyl substituents can be hydrogenated, yielding various α-borylated alcohols with high ee values. Crucially, the boron moiety can be easily transformed into other groups, allowing access to previously unattainable carbinols adorned with two structurally similar substituents. The enantioselectivity-directing role of BMIDA is elucidated by computational analyses, which stems from the CH–O electrostatic attraction between the η6-arene-CH of the catalyst and the σ-bonded oxygen atoms within BMIDA. This work represents the first asymmetric transformation on acylboronates and expands the domain of asymmetric transfer hydrogenation.
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Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text

A light-powered molecular motor featuring a specifically engineered helicene moiety is presented. The inversion of the helicene fragment is mechanically coupled to the unidirectional rotation of the motor in a 6-step rotation cycle, which involves 8 isomers. This coupled motion results in the dynamic modulation of the handedness of the helicene by the motor via a central to helical to helical chiral transmission mechanism.

Abstract

Towards complex coupled molecular motions, the remote handedness inversion of a helicene moiety was achieved by a rotary molecular motor. The use of a specifically engineered dynamic helicene stator in a novel overcrowded-alkene second-generation molecular motor based on a fluorinated dibenzofluorene fragment allows for an unprecedented control over helicity inversion. This is achieved by the mechanical coupling of the rotation of the rotor to the helicene inversion of the stator half via a remote chirality transmission process. Thus, the unidirectional rotary motion generated upon irradiation is used to invert the dynamic stereochemistry of a helicene, leading to a 6-step cycle with eight intermediates. In this cycle, both alternation between P and M configurations of the helicene stator and dynamic thermal interconversion (paddling motion) can be achieved. In-depth computational and spectroscopic studies were performed to support the associated mechanism. The control over coupled motion and dynamic helicity offers prospects for the development of complex responsive systems.

Nature Sustainability, Published online: 22 November 2024; doi:10.1038/s41893-024-01479-4

We present a protocol for a solvent-free mechanically induced Wittig olefination under ambient conditions often without the need of ylide pre-formation with a 30 seconds reaction time under high-energy ball milling conditions.

Abstract

30 Seconds to success!—The Wittig reaction, a fundamental and extensively utilized reaction in organic chemistry, enables the efficient conversion of carbonyl compounds to olefins using phosphonium salts. Traditionally, meticulous reaction setup, including the pre-formation of a reactive ylide species via deprotonation of a phosphonium salt, is crucial for achieving high-yielding reactions under classical solution-based conditions. In this report, we present an unprecedented protocol for an ultra-fast mechanically induced Wittig reaction under solvent-free and ambient conditions, often eliminating the need for tedious ylide pre-formation under strict air and moisture exclusion. A range of aldehydes and ketones were reacted with diverse phosphonium salts under high-energy ball milling conditions, frequently giving access to the respective olefins in only 30 seconds.

Singly linked and fused dimers were synthesized for aromatic [22]smaragdyrin BF₂ complex and antiaromatic [20]smaragdyrin free base. The fused dimers show larger electronic interactions but the respective antiaromatic and aromatic characters are well preserved.

Abstract

Singly-linked aromatic [22]smaragdyrin BF₂ complex dimer was synthesized by the reductive coupling of 16-brominated [22]smaragdyrin BF₂ complex, which was oxidized to a stable diradical with PbO₂. As the first example of fused smaragdyrin dimer, a fused [22]smaragdyrin BF₂ complex dimer was synthesized by the oxidation of a CuCl-BF₂ complex dimer with FeCl₃ and subsequent reduction with NaBH₄. After removal of the BF₂ group, the singly-linked and fused aromatic dimers were oxidized to the corresponding antiaromatic [20]smaragdyrin free base dimers. The first oxidation and reduction potentials of these dimers are split depending upon the intramolecular electronic interactions, which are larger for the fused dimers. Despite the large electronic interactions, the aromatic and antiaromatic characters are well preserved in the fused dimers.

Nature Catalysis, Published online: 21 November 2024; doi:10.1038/s41929-024-01270-w

Nature Synthesis, Published online: 22 November 2024; doi:10.1038/s44160-024-00693-4

Even after Woodward and Hoffmann had determined the orbital symmetry mechanism of electrocyclizations and cycloadditions, it was not evident to either of them that a further mechanistic problem awaited. But Hoffmann's curiosity about the mechanism of the Cope reaction led him down a winding, nonlinear path that eventually resulted in the definition of what entailed „sigmatropic reacitons“ and its mechanism. Along the way, Hoffmann used five different molecular orbital tools (correlation diagrams, the HOMO overlap interaction method, extended Hückel calculations, interaction diagrams with perturbation theory, and three-center bond analyses) before he convinced himself of the orbital symmetry mechanism.

Abstract

On June 1, 1965, R. B. Woodward and Roald Hoffmann published their third communication in the Journal of the American Chemical Society in which they applied orbital symmetry control to explain the mechanism of a wide variety of valence isomerizations that they termed “sigmatropic reactions.” This publication reveals the research trajectory taken by Hoffmann from which this portion of the no-mechanism problem was solved. Hoffmann used five different quantum chemical tools, all based on either extended Hückel theoretical calculations or frontier molecular orbital theory, in his research. Hoffmann′s laboratory notebooks and his three draft manuscripts along with Woodward′s four subsequent drafts have survived the past 59 years and provide an excellent window into the thinking and manuscript-writing processes used by these Nobel laureates in February-April 1965.

The Cover Feature depicts a dynamic superhero who embodies the elements of carbon, phosphorus, oxygen and nitrogen, thus symbolizing the metal-free catalyst central to our research. Armed with scissors, the superhero is shown cleaving lignin model dimers into valuable monomers. The scene is set on a road that transitions from a lush forest on the left, representing the natural source of lignin, to a modern industry on the right, illustrating the sustainable transformation of lignin into industrially valuable compounds such as phenol. This image highlights the innovative approach of using a P,N co-doped carbon catalyst to achieve efficient lignin valorization, offering an eco-friendly solution for producing industrial chemicals and fuels. More information can be found in the Research Article by R. Srivastava and co-workers.