Effect of the enamine pyramidalization direction on the reactivity of secondary amine organocatalysts

Chiral secondary amines are valuable catalysts for reactions that proceed through an enamine intermediate. Here, we explored the importance of the pyramidalization direction of the enamine-N on the reactivity of chiral enamines with a combination of computational, NMR spectroscopic, and kinetic experiments. Studies with peptidic catalysts that bear cyclic amines with different ring sizes revealed that endo-pyramidalized enamines are significantly more reactive compared to exo-pyramidalized analogs. The results show that the pyramidalization direction can have a greater effect than n→π* orbital overlap on the reactivity of chiral enamines. The data enabled the development of a catalyst with higher reactivity compared to the parent catalyst.ISSN:2041-6520ISSN:2041-653

Amine Catalysis with Substrates Bearing N‐Heterocyclic Moieties Enabled by Control over the Enamine Pyramidalization Direction

Stereoselective organocatalytic C–C bond formations that tolerate N‐heterocycles are valuable since these moieties are common motifs in numerous chiral bioactive compounds. Such transformations are, however, challenging since N‐heterocyclic moieties can interfere with the catalytic reaction. Here, we present a peptide that catalyzes conjugate addition reactions between aldehydes and nitroolefins bearing a broad range of different N‐heterocyclic moieties with basic and/or H‐bonding sites in excellent yields and stereoselectivities. Tuning of the pyramidalization direction of the enamine intermediate enabled the high stereoselectivity. © WILEY 2020ISSN:0947-6539ISSN:1521-376

Tripeptide Organocatalysts for Stereoselective Conjugate Addition Reactions with N-Heterocyclic Substituents

N-heterocyclic moieties are abundant among pharmaceuticals and agrochemicals, but a challenge for metalorganic and organocatalytic transformations. We present tripeptides of the type H-Pro-Pro-Xaa as catalysts for stereoselective conjugate addition reactions between N-heterocyclic substituted aldehydes and electrophiles. Alkyl substituents at the N-terminal proline, the reactive site, were crucial for high chemo- and stereoselectivity. Different N-heterocyclic moieties, even at both reaction partners, were readily tolerated and products were obtained in yields of 61-95% and enantioselectivities of up to 98% ee.ISSN:1615-4150ISSN:1615-416

The impact of ammonia on particle formation in the Asian Tropopause Aerosol Layer.

Funder: ACCC Flagship funded by the Academy of Finland grant number 337549 (UH), 337552 (FMI), and 337550 (UEF); Academy professorship funded by the Academy of Finland (grant no. 302958); Academy of Finland projects no. 1325656, 311932, 334792, 316114, 325647, 325681, 347782, 346371,“Quantifying carbon sink, CarbonSink+ and their interaction with air quality”; INAR project funded by Jane and Aatos Erkko Foundation; “Gigacity” project funded by Wihuri foundation; European Research Council (ERC) project ATM-GTP Contract No. 742206; Research Council of Finland project no. 349659; US NSF AGS-2132089; German Federal Ministry of Education and Research project CLOUD-22 (01LK2201A); Federal Ministry of Education and Research (BMBF) funded project CLOUD-22 (01LK2201B); Swiss National Science Foundation (SNF): 200021 213071; NASA under grant 80NSSC19K0949.During summer, ammonia emissions in Southeast Asia influence air pollution and cloud formation. Convective transport by the South Asian monsoon carries these pollutant air masses into the upper troposphere and lower stratosphere (UTLS), where they accumulate under anticyclonic flow conditions. This air mass accumulation is thought to contribute to particle formation and the development of the Asian Tropopause Aerosol Layer (ATAL). Despite the known influence of ammonia and particulate ammonium on air pollution, a comprehensive understanding of the ATAL is lacking. In this modelling study, the influence of ammonia on particle formation is assessed with emphasis on the ATAL. We use the EMAC chemistry-climate model, incorporating new particle formation parameterisations derived from experiments at the CERN CLOUD chamber. Our diurnal cycle analysis confirms that new particle formation mainly occurs during daylight, with a 10-fold enhancement in rate. This increase is prominent in the South Asian monsoon UTLS, where deep convection introduces high ammonia levels from the boundary layer, compared to a baseline scenario without ammonia. Our model simulations reveal that this ammonia-driven particle formation and growth contributes to an increase of up to 80% in cloud condensation nuclei (CCN) concentrations at cloud-forming heights in the South Asian monsoon region. We find that ammonia profoundly influences the aerosol mass and composition in the ATAL through particle growth, as indicated by an order of magnitude increase in nitrate levels linked to ammonia emissions. However, the effect of ammonia-driven new particle formation on aerosol mass in the ATAL is relatively small. Ammonia emissions enhance the regional aerosol optical depth (AOD) for shortwave solar radiation by up to 70%. We conclude that ammonia has a pronounced effect on the ATAL development, composition, the regional AOD, and CCN concentrations