Asymmetric Nazarov Cyclizations Catalyzed by Chiral‐at‐Metal Complexes

Abstract

The application of Lewis acidic chiral-atmetal complexes of iridium(III) and rhodium(III) as catalysts for the asymmetric polarized Nazarov cyclization of dihydropyran-and indole-functionalized a-unsaturated b-ketoesters is reported (overall 24 examples). For both substrate classes, catalyst loadings of 2 mol% were found to be sufficient for achieving high yields and high stereoselectivities. The cyclized dihydropyran products were isolated in 85-98% yield, with 89%-> 99% ee, and trans/cis ratios of 15:1-50:1 (9 examples). The cyclized indole products were typically isolated in more than 70% yield and in up to 93% yield, typically with more than 90% ee and in up to 97% ee, and trans/cis ratios of 12:1-28:1 (15 examples).

Planar Cyclopenten‐4‐yl Cations: Highly Delocalized π Aromatics Stabilized by Hyperconjugation

“…Recent interest on cyclopentenyl cations 1 stems from their intermediacy in important organic (Nazarov cyclization –formation of cyclopentenones) and industrial processes (methanol‐to‐hydrocarbon conversion) . Historically however, it was the presence of a C=C double bond in close proximity to a cationic sp 2 carbon center, and hence the possibility of gaining “extra stabilization” by allylic or homoaromatic delocalization, that put this system into the focus of carbocation chemistry .…”

Bis-Cyclometalated Indazole and Benzimidazole Chiral-at-Iridium Complexes: Synthesis and Asymmetric Catalysis

“…As a second model reaction, we chose an asymmetric Nazarov cyclization [ 49 , 50 ]. We recently demonstrated that Λ- IrS (2.0 mol%) is an excellent chiral Lewis acid for the conversion of ketoester 6 to the cyclopentenone (1 R ,2 S )- 7 ( Table 2 ) [ 51 ]. Using the same established reaction conditions, after a reaction time of 7 hours in hexafluoroisopropanol [ 52 ] at 50 °C, followed by base-induced isomerization to the thermodynamic trans diastereomer, (1 R ,2 S )- 7 was obtained in 75% yield with 15:1 dr and 93% ee.…”

A Review of some catalysts and promoters used in organic transformations

“…[86] Enantio selective reactions such as enantio selective Nazarov cyclisation is also performed by iridiumbased chiral catalyst. [87] Ir-based catalysts are particularly important in organic synthesis and oxidation reactions such as oxidation of primary, secondary alcohols, phenols, amines, phosphines, alkane, alkenes and aromatic compounds. [88] Importance of iridium as catalyst in organic synthesis is proven by various reactions viz.…”