A new class of chiral
iron catalysts is introduced that contains
exclusively achiral ligands with the overall chirality being the result
of a stereogenic iron center. Specifically, iron(II) is cis-coordinated to two N-(2-pyridyl)-substituted N-heterocyclic
carbene (PyNHC) ligands in a bidentate fashion in addition to two
monodentate acetonitriles, and the dicationic complex is complemented
by two hexafluorophosphate ions. Depending on the helical twist of
the PyNHC ligands, the metal center adopts either a Λ or Δ
absolute configuration. Importantly, the two PyNHC ligands are constitutionally
and configurationally inert, while the two acetonitriles are labile
and allow asymmetric transition metal catalysis. This is demonstrated
with an enantioselective Cannizzaro reaction (96% yield, 88% ee) and
an asymmetric Nazarov cyclization (89% yield, >20:1 dr, 83% ee).
Chiral phosphoric acid catalyzed the formal [4+2]-cycloaddition of 2-benzothiazolimines with enecarbamates to provide benzothiazolopyrimidines with up to 99% yield and >99% ee.
A highly efficient catalytic enantioselective intramolecular Povarov reaction was developed with primary anilines as 2-azadiene precursors. A wide variety of angularly fused azacycles were obtained without column chromatography in high to excellent yields and with excellent diastereo- and enantioselectivity (d.r.>99:1 and up to e.r. 99:1). Furthermore, the catalyst loading could be lowered to 1 mol %, and the obtained azacycles could be used as key intermediates for further transformations to generate additional molecular diversity.
A readily available catalyst consisting of iron dichloride in combination with 1,10‐phenanthroline catalyzes the ring‐closing C−H amination of N‐benzoyloxyurea to form imidazolidin‐2‐ones in high yields. The C−H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non‐activated aliphatic C(sp3)−H bonds, and it also works for C(sp2)−H bonds. The surprisingly simple method can be performed under open flask conditions.
A novel photoredox-mediated tandem three-component process afforded a wide variety of CF3-containing phthalans and isoindolines in respectable yields and with moderate to excellent diastereoselectivity.
As table asymmetrici ntramolecular Povarovr eaction has been established to provide an efficient method to access structurally diverse trans,trans-trisubstituted tetrahydrochromeno[4,3-b]quinolines in high stereoselectivities of up to > 99:1 diastereomeric ratio and 99 %e nantiomeric excess,w ithout any purification step.A dditionally,t of acilitate large-scalea pplication of this method, al ow catalyst loadingp rotocol was employed, 0.2mol %c hiral phosphoric acid, which provided the cycloadducts withouta ny loss in yield and enantioselectivity.T heoretical studies revealed that the reaction occurred throughasequential Mannich reaction and an intramolecular Friedel-Craftsr eaction, wherein the phosphoric acid acted as ab ifunctional catalystt oa ctivate the para-phenolicd ienophile and N-2-hydroxy-2-azadiene simultaneously.
A simple, straightforward strategy for the synthesis of N-substituted azoles is reported that involves a visible-light photoredox-catalyzed coupling reaction of azoles with α-carbamoyl sulfides. A variety of heterocyclic units, including pyrazoles, benzopyrazoles, benzoimidazoles, and purines, can be efficiently incorporated under mild reaction conditions in respectable yields.
A readily available catalyst consisting of iron dichloride in combination with 1,10‐phenanthroline catalyzes the ring‐closing C−H amination of N‐benzoyloxyurea to form imidazolidin‐2‐ones in high yields. The C−H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non‐activated aliphatic C(sp3)−H bonds, and it also works for C(sp2)−H bonds. The surprisingly simple method can be performed under open flask conditions.
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