Assessing the activity of different plant-derived molecules and potential biological nitrification inhibitors on a range of soil ammonia- and nitrite-oxidizing strains.

IMPORTANCE: Synthetic nitrification inhibitors are routinely used with nitrogen fertilizers to reduce nitrogen losses from agroecosystems, despite having drawbacks like poor efficiency, cost, and entry into the food chain. Plant-derived BNIs constitute a more environmentally conducive alternative. Knowledge on the activity of BNIs to soil nitrifiers is largely based on bioassays with a single Nitrosomonas europaea strain which does not constitute a dominant member of the community of ammonia-oxidizing microorganisms (AOM) in soil. We determined the activity of several plant-derived molecules reported as having activity, including the recently discovered maize-isolated BNI, zeanone, and its natural analog, 2-methoxy-1,4-naphthoquinone, on a range of ecologically relevant AOM and one nitrite-oxidizing bacterial culture, expanding our knowledge on the intrinsic inhibition potential of BNIs toward AOM and highlighting the necessity for a deeper understanding of the effect of BNIs on the overall soil microbiome integrity before their further use in agricultural settings.

Tandem Dearomatization/Enantioselective Allylic Alkylation of Pyridines.

Herein, we report a multistep one-pot reaction of substituted pyridines leading to N-protected tetrahydropyridines with outstanding enantioselectivity (up to 97% ee). An iridium(I)-catalyzed dearomative 1,2-hydrosilylation of pyridines enables the use of N-silyl enamines as a new type of nucleophile in a subsequent palladium-catalyzed asymmetric allylic alkylation. This telescoped process overcomes the intrinsic nucleophilic selectivity of pyridines to synthesize enantioenriched, C-3-substituted tetrahydropyridine products that have been otherwise challenging to access.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts.

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Enantioselective Formation of Quaternary Centers by Allylic Alkylation with First-Row Transition-Metal Catalysts.

Asymmetric allylic alkylation mediated by transition metals provides an efficient strategy to form quaternary stereogenic centers. While this transformation is dominated by the use of second- and third-row transition metals (e.g., Pd, Rh, and Ir), recent developments have revealed the potential of first-row transition metals, which provide not only a less expensive and potentially equally efficient alternative but also new mechanistic possibilities. This review summarizes examples for the assembly of quaternary stereocenters using prochiral allylic substrates and hard, achiral nucleophiles in the presence of copper complexes and highlights the complementary approaches with soft, prochiral nucleophiles catalyzed by chiral cobalt and nickel complexes.

Enantioselective Nazarov Cyclizations Catalyzed by an Axial Chiral C6 F5 -Substituted Boron Lewis Acid.

A chiral variant of B(C6 F5 )3 with a 3,3'-disubstituted binaphthyl backbone is shown to catalyze Nazarov cyclizations with high levels of enantio- and diastereocontrol. The parent B(C6 F5 )3 also promotes these ring closures efficiently. This electrocyclization is another example of the still small family of C-C bond formations mediated by B(C6 F5 )3 as the catalyst.

The Asymmetric Piers Hydrosilylation.

An axially chiral, cyclic borane decorated with just one C6F5 group at the boron atom promotes the highly enantioselective hydrosilylation of acetophenone derivatives without assistance of an additional Lewis base (up to 99% ee). The reaction is an unprecedented asymmetric variant of Piers' B(C6F5)3-catalyzed carbonyl hydrosilylation. The steric congestion imparted by the 3,3'-disubstituted binaphthyl backbone of the borane catalyst as well as the use of reactive trihydrosilanes as reducing agents are keys to success.