Monthly Archives: September 2023

Metal‐Free Defluoroborylation of 3,3‐Difluoropropenes

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Metal-Free Defluoroborylation of 3,3-Difluoropropenes

A metal-free defluoroborylation of 3,3-difluoropropenes has been developed. The reaction involves the convenient borylation agent B2pin2 and a combination of both TBAF and HMDS as an activating system. The fluorinated allylboronates were used as a platform to prepare functionalized monofluoroalkenes.


Abstract

The defluorinative functionalization of allylic fluorides represents an attractive approach for the preparation of molecules containing a monofluoroalkene core. In that sense, introducing a “boryl nucleophile” is a powerful strategy to obtain polyvalent borylated intermediates as versatile synthetic precursors. To perform this reaction without the use of transition metals, the nucleophilic character of a diborane/fluoride system was exploited in a SN2′ type-substitution reaction of gem-difluoropropenes to install a pinacolborane group. The use of HMDS as a silylated additive is necessary to improve the reactivity. A direct oxidation of the intermediate boronates allowed the isolation of the corresponding β-fluoroallyl alcohols in low to good yields (9–81 %). Other synthetic transformations of a (2-fluoroallyl)boronate are also illustrated.

Ecotoxicity of Lead to a Phytoplankton Community: Effects of pH and Phosphorus Addition and Implications for Risk Assessment

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Abstract

Ecological risk assessment and water quality criteria for lead (Pb) are increasingly making use of bioavailability-based approaches to account for the impact of toxicity-modifying factors, such as pH and dissolved organic carbon. For phytoplankton, which are among the most Pb-sensitive freshwater species, a Pb bioavailability model has previously been developed based on standard single-species exposures at a high phosphorus (P) concentration and pH range of 6.0 to 8.0. It is well known that P can affect metal toxicity to phytoplankton and that the pH of many surface waters can be above 8.0. We aimed to test whether the single-species bioavailability model for Pb could predict the influence of pH on Pb toxicity to a phytoplankton community at both low and high P supply. A 10-species phytoplankton community was exposed to Pb for 28 days at two different pH levels (7.2 and 8.4) and two different P supply levels (low and high, i.e., total P input 10 and 100 µg/L, respectively) in a full factorial 2 × 2 test design. We found that the effects of total Pb on three community-level endpoints (biodiversity, community functioning, and community structure) were highly dependent on both pH and P supply. Consistent lowest-observed-effect concentrations (LOECs) ranged between 21 and >196 µg total Pb/L and between 10 and >69 µg filtered Pb/L. Long-term LOECs were generally higher, that is, 69 µg total Pb/L (42 µg filtered Pb/L) or greater, across all endpoints and conditions, indicating recovery near the end of the exposure period, and suggesting the occurrence of acclimation to Pb and/or functional redundancy. The highest toxicity of Pb for all endpoints was observed in the pH 7.2 × low P treatment, whereas the pH 8.4 × low P and pH 8.4 × high P treatment were the least sensitive treatments. At the pH 7.2 × high P treatment, the algal community showed an intermediate Pb sensitivity. The effect of pH on the toxicity of filtered Pb could not be precisely quantified because for many endpoints no effect was observed at the highest Pb concentration tested. However, the long-term LOECs (filtered Pb) at low P supply suggest a decrease in Pb toxicity of at least 1.6-fold from pH 7.2 to 8.4, whereas the single-species algal bioavailability model predicted a 2.5-fold increase. This finding suggests that bioavailability effects of pH on Pb toxicity cannot be extrapolated as such from the single species to the community level. Overall, our data indicate that, although the single-species algal Pb bioavailability model may not capture pH effects on Pb ecotoxicity in multispecies systems, the bioavailability-based hazardous concentration for 5% of the species was protective of long-term Pb effects on the structure, function, and diversity of a phytoplankton community in a relevant range of pH and P conditions. Environ Toxicol Chem 2023;00:1–17. © 2023 SETAC

Catalytic Enantioselective Synthesis of Inherently Chiral Molecules: Recent Advances

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Catalytic Enantioselective Synthesis of Inherently Chiral Molecules: Recent Advances

Inherent chirality represents a distinct form of molecular chirality that does not fit into the four traditional types of molecular chiral elements, which has been observed in a diverse range of molecular structures. This review summarizes the recent advances in the catalytic enantioselective synthesis of inherently chiral molecules, including chiral calixarenes, saddle-shaped chiral cycles, mechanically planar chiral rotaxanes and others.


Abstract

Inherent chirality represents a distinct category of molecular chirality that does not fall within the traditional classification of four chiral elements: central, axial, planar, and helical chirality. While extensive research has been conducted on the catalytic enantioselective construction of these conventional chiralities, the corresponding synthesis of inherently chiral molecules has remained largely unexplored. This minireview provides a comprehensive summary of recent advancements in this field, focusing on the catalytic asymmetric synthesis of inherently chiral calixarenes, saddle-shaped tetraphenylenes and their heterocycle derivatives, mechanically planar chiral rotaxanes and chiral multilayer 3D frameworks, as well as our perspective in this field.

CO2‐Based Carbamate Synthesis Utilizing Reusable Polymer‐Supported DBU

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CO2-Based Carbamate Synthesis Utilizing Reusable Polymer-Supported DBU

A novel and advantageous protocol for accessing carbamates through the known three-component coupling reaction involving carbon dioxide, amines, and alkyl halides is described. Employing mild conditions, simple experimental set-up, and immobilized DBU, this protocol addresses several drawbacks from the previous described methods. No classical purification procedures are necessary and the immobilized DBU can be recycled and reused several times.


Abstract

The present study highlights a novel and advantageous protocol for accessing carbamates through the well-established three-component coupling reaction involving CO2, amines, and alkyl halides. By employing an immobilized organic base, operating under mild reaction conditions, an array of alkyl carbamates in yields of up to 95 % could be isolated. This approach offers a broad and versatile product scope, allowing for the facile modification of both the amine and alkyl halide reactants. Notably, the pioneering use of an immobilized organic base, specifically the polymer-supported 1,8-diazabicyclo[5.4.0]undec-7-ene (PS-DBU), in this three-component reaction eliminates the need for classical purification steps, streamlining the process. To ensure practicality and sustainability, extensive studies were conducted to verify the recovery and reusability of the polymer-supported DBU catalyst, which consistently maintained the high chemical yield of the carbamates across multiple cycles. Overall, this innovative protocol represents a significant advancement in carbamate synthesis, combining efficiency, generality, and the potential for DBU recycling.

Three‐Component Reactions of Quinoxalin‐2(1H)‐ones: Recent Advances

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Three-Component Reactions of Quinoxalin-2(1H)-ones: Recent Advances

This Review focuses on the very recent advances (from 2021 to the beginning of 2023) in the field of three-component reactions of quinoxalin-2(1H)-ones at the C3 position. According to the kind of radical types involved, some representative examples and detailed reaction mechanism have been categorized and discussed.


Abstract

The multicomponent reactions of quinoxalin-2(1H)-ones has attracted considerable interest due to their significant biological and chemical activities. The very recent advances (from 2021 to the beginning of 2023) on the radical three-component cascade reaction of quinoxalin-2(1H)-one derivatives at the C3 position were summarized in this mini-review. According to the kind of radical types involved, some representative examples and detailed reaction mechanism have been categorized and discussed.

The red front was covered by Figure 1.

Strategies for the Controlled Hydrostannylation of Alkynes

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Strategies for the Controlled Hydrostannylation of Alkynes

This Concept article summarises recent advances in approaches to the synthesis of important vinyl stannane building blocks in a stereocontrolled and regiocontrolled manner. Particular emphasis is put on those routes in which judicious choice of ligand affords a switchable reaction, providing routes to stereodefined olefins from a single starting material.


Abstract

Organostannanes have represented one of the most widely applied reagents in modern cross-coupling chemistry and represent a key reagent in the synthesis of a range of pharmaceutically relevant scaffolds. This Concept article reviews recent advances in approaches to the synthesis of these building blocks in a stereocontrolled and regiocontrolled manner. Particular focus is paid to methods which allow for divergent synthesis of alkenylstannanes and developments in methods which present opportunities for sustainable synthesis.

Manganese‐Catalysed Transfer Hydrogenation of Quinolines under Mild Conditions

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Manganese-Catalysed Transfer Hydrogenation of Quinolines under Mild Conditions

A PNP pincer manganese complex was applied as catalyst for transfer hydrogenation of quinolines with ammonia borane as hydrogen source. 1,2,3,4-Tetrahydroquinolines were obtained under mild conditions with good to excellent yields. 1,2-Dihydroquinolines were detected by 1H NMR in the progress and isotopic labelling experiments were performed to determine the destination of ammonia-borane hydrogen atoms.


Abstract

Herein, an efficient methodology for the homogeneous manganese-catalysed transfer hydrogenation of N-heterocycles by using ammonia-borane as a hydrogen source under mild reaction conditions is reported. Good to excellent isolated yields are achieved by applying a PNP manganese pincer complex. In the reaction, 1,2-dihydroquinoline is detected as intermediate by NMR spectra analysis and deuterium labelling experiment. The catalytic reaction likely proceeded by an outer-sphere pathway based on the bifunctional pincer complex.

Diastereoselective Dearomatization of Chalcone‐Based Quinolinium Salts to Assemble Bridged Quinobenzazepine Polycycles

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Diastereoselective Dearomatization of Chalcone-Based Quinolinium Salts to Assemble Bridged Quinobenzazepine Polycycles

Herein, we designed and synthesized a range of multi-functional chalcone-based quinolinium salts and their synthetic application in the rapid and straightforward construction of oxa-bridged quinobenzazepine polycycles was successfully explored.


Abstract

Quinobenzazepines are useful in medicinal chemistry, but their synthesis is very challenging. Herein, we designed and synthesized a range of multi-functional chalcone-based quinolinium salts; their synthetic application in the rapid and straightforward construction of quinobenzazepines was successfully explored. A wide range of oxa-bridged quinobenzazepine polycycles were afforded serendipitously through a dearomative cascade reaction of our newly developed quinolinium salts and acetylacetone. This synthetic strategy features high bond- and ring-forming efficiency and complete regio- and diastereoselective control.

Amine Adsorbents Stability for Post‐Combustion CO2 Capture: Determination and Validation of Laboratory Degradation Rates in a Multi‐staged Fluidized Bed Pilot Plant

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Amine Adsorbents Stability for Post-Combustion CO2 Capture: Determination and Validation of Laboratory Degradation Rates in a Multi-staged Fluidized Bed Pilot Plant

A robust method to determine degradation rates of an amine functionalized polystyrene adsorbent in a laboratory setup is developed and validated with a continuous multi-staged fluidized bed pilot plant. The very good agreement between experimental 1000-hour laboratory and 2200-hour pilot degradation showcases how small scale experiments can be extrapolated for scale-up and adsorbent screening.


Abstract

Alternative to current liquid amine technologies for post-combustion CO2 capture, new technologies such as adsorbent-based processes are developed, wherein material lifetime and degradation is important. Herein a robust method to determine degradation rates in a laboratory setup is developed, which was validated with a continuous multi-staged fluidized bed pilot plant designed to capture 1 ton CO2 per day. An amine functionalized polystyrene adsorbent showed very good agreement between the experimental 1000-hour laboratory degradation rates and 2200 hours of degradation in a pilot plant. This validates how laboratory experiments can be extrapolated for sorbent screening and for scale-up. Resulting, the oxidative degradation in the desorber at high temperatures (120 °C) and low O2 concentrations (150 ppmv) is 3 times higher compared to the adsorber at low temperatures and high O2 (56 °C, 7 vol %). Laboratory degradation experiments can hence be used to further optimize process operations to limit degradation or screen for potential new adsorbents.

Diaminocyclopentadienone Ruthenium Complex Catalyzed Alkylation of Indoles and Ketones with Primary Alcohols

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Diaminocyclopentadienone Ruthenium Complex Catalyzed Alkylation of Indoles and Ketones with Primary Alcohols

The borrowing hydrogen approach allows the direct utilization of non-activated alcohols as alkylating agents. A readily available, air and moisture stable ruthenium complex proves to be a particularly effective hydrogen autotransfer catalyst for the alkylation of various indoles or ketones with poorly reactive alcohols.


Abstract

A diaminocyclopentadienone ruthenium complex has proven to be a highly effective catalyst for the alkylation of indoles or ketones with poorly reactive alcohols. The catalyst is readily available, air and moisture stable and exhibits wide functional group tolerance. The environmentally benign procedure follows a borrowing hydrogen mechanism and requires no excess of either component. Various indoles and ketones are selectively monoalkylated in high yield.