LongLarf

Here, we demonstrate that polymethylhydrosiloxane (PMHS) is not only a sustainable terminal reductant and hydrogen atom donor in titanocene catalyzed epoxide hydrosilylations. Moreover, PMHS also results in higher diastereoselectivity of radical reduction. Therefore, Cp₂TiCl₂ can replace sterically demanding titanocene complexes that have to be used with other silanes, such as PhSiH₃, without significant loss of selectivity.

Abstract

Here, we describe that polymethylhydrosiloxane (PMHS) is a sustainable terminal reductant in the titanocene catalyzed hydrosilylation of epoxides. The products formally constitute the anti-Markovnikov products of H₂O-addition to alkenes. PMHS is not only a more sustainable and cheaper reductant than other silanes such as PhSiH₃ and Ph(Me)SiH₂. It also results in a higher diastereoselectivity of epoxide reduction.

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO₂ in an undivided cell to yield synthetically important β-hydroxy acids is described. CO₂ functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions.

Abstract

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO₂ in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate β-hydroxy acids. Mechanistic studies show that CO₂ functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Pyrimidopteridine organic photoredox catalysts are applied in a formal addition of carboxylic acids to activated alkenes as a dual photoredox and hydrogen atom transfer catalysts. Electron-rich alkenes counter the decarboxylation of carboxylic acids upon single-electron oxidation and a selective hydroacetoxylation is observed.

Abstract

Herein we report a photo-mediated formal addition of carboxylic acids to activated alkenes catalyzed by a pyrimidopteridine photoredox catalyst. The decarboxylation of aliphatic carboxylic acids upon single-electron oxidation is countered in the presence of electron-rich alkenes and a hydroacetoxylation is observed. Mechanistic proposals have been made based on CV measurements, competitive Stern-Volmer quenching and EPR experiments. Evidence that tetra-N-substituted pyrimidopteridines function as dual photoredox and hydrogen atom transfer catalyst was supported by spectroscopic means.

Due to the high affinity of phosphines for transition metals, C−H functionalization of aryl phosphines is dominated by phosphorous-directed reactions. Here we pair a sterically controlled borylation reaction of unprotected phosphines with phosphine self-assisted Suzuki-Miyaura reactions to grant ready access to libraries of functionalized phosphines with a positional selectivity that complements existing methods. More information can be found in the Research Article by C. N. Neumann et al. (DOI: 10.1002/chem.202202074).

Halodecarboxylation of α,β-unsaturated ferulic acid was achieved by combining photo-, bio- and phase transfer catalysis in a biphasic system. Reactive hypobromite was formed in situ by combining Blue-LED and vanadium chloroperoxidase, and efficiently transferred from the aqueous to the organic layer by a phase-transfer catalyst, improving overall performance and selectivity.

Abstract

A photochemoenzymatic halodecarboxylation of ferulic acid was achieved using vanadate-dependent chloroperoxidase as (bio)catalyst and oxygen and organic solvent as sole stoichiometric reagents in a biphasic system. Performance and selectivity were improved through a phase transfer catalyst, reaching a turnover number of 660.000 for the enzyme.

Palladium-catalyzed Negishi couplings rely typically on pre-formed organozinc reagents, dry organic solvents, and unsustainable loadings of a Pd catalyst (usually 1–5 mol %). In this report we describe heterogeneous nanoparticle-catalyzed couplings under mild conditions using water as the reaction medium, and only ppm levels of a Pd catalyst.

Abstract

New technology is reported that enables Negishi couplings to be run under sustainable, far greener conditions. Thus, ppm Pd-containing nanoparticles (NPs) have been developed that catalyze couplings in recyclable water under very mild conditions. These heterogeneous reactions involve loadings of Pd of typically only 2500 ppm (0.25 mol %). Highly functionalized aromatic and heteroaromatic bromides readily participate, including examples taken from the Merck Informer Library indicative of the functional group tolerance associated with these couplings. Direct comparisons with existing literature routes are made. Very low residual levels of Pd in newly formed products are to be expected, as determined by ICP-MS. The reagent involved has been extensively characterized via DLS, TEM, cryo-TEM, and EDX measurements.

The unprecedented construction of a C(sp²)-SCF₂CO₂Et bond was successfully achieved by Pd-catalyzed C−H bond activation. This robust protocol allowed the functionalization of a broad panel of aromatic and olefinic derivatives (37 examples, up to 87 % yield), a real advance as existing routes were restricted to the SCF₃ and SCF₂H residues. Post-functionalization reactions further illustrated the synthetic value of this fluorinated motif.

Abstract

The unprecedented Pd-catalyzed (ethoxycarbonyl)difluoromethylthiolation reaction of various unsaturated derivatives was studied. In the presence of the (ethoxycarbonyl)difluoromethylsulfenamide reagent I and under mild reaction conditions (60 °C), both 2-(hetero)aryl and 2-(α-aryl-vinyl)pyridine derivatives were smoothly functionalized with this methodology (37 examples, up to 87 % yield). Moreover, the synthetic interest of this fluorinated moiety was further showcased by its conversion into various original fluorinated residues. Finally, a plausible mechanism for this transformation was suggested.

Catalysis in a microporous polymer network: An iridium pincer complex was anchored in a multi-step post-synthetically modified microporous polymer with a highly hydrophobic character. The immobilized catalyst was applied in continuous-flow dehydrogenation of cyclohexane.

Abstract

An iridium pincer complex {p-KO-C₆H₂-2,6-[OP(t-Bu)₂]₂}Ir(C₂H₄) is immobilized in a propyl bromide-functionalized microporous polymer network using the concepts of surface organometallic chemistry. The support material enables the formation of isolated active metal sites embedded in a chemically robust and highly hydrophobic environment. The catalyst maintained high porosity and – without prior activation – exhibited high activity in the continuous-flow dehydrogenation of cyclohexane at elevated temperatures. The catalyst shows a stable performance for at least 7 days, even when additional H₂O was co-fed, owing to its hydrophobic nature.