Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Flow Photo-Nazarov Reactions of 2-Furyl Vinyl Ketones: Cyclizing a Class of Traditionally Unreactive Heteroaromatic Enones

Nazarov reactions of 2-furyl vinyl ketones and related heteroaromatic enones, to produce furan-fused cyclopentanones using a flow photochemical approach, are described. Compounds possessing this connectivity between heterocycle and ketone (2-furyl, 2-benzofuryl, 2-thiophene-yl, and 2-benzothiophene-yl) have traditionally proven difficult or impossible to cyclize with typical Brønsted and Lewis acid mediated methods. Using mild flow photochemistry conditions and acetic acid (AcOH) or hexafluoroisopropanol (HFIP) as solvent, these compounds were found to cyclize in 45-97% yields, with typical UV exposure times of 3.4-6.8 min. In all cases, 2-furyl and 2-thiophene-yl enones cyclized, whereas 2-benzofuryl and 2-benzothiophene-yl enones exhibited divergent properties with reactivity patterns tied to the identity of the vinyl group. This report discloses the first photo-Nazarov reactions of tetrahydropyridine-substituted 2-furyl ketones, providing a direct approach to the corresponding fused heterocyclic motifs built around a central cyclopentanone. These motifs constitute the core structures of biologically active natural products, including the marine alkaloid nakadomarin A.

Catalytic, Enantioselective Synthesis of Polycyclic Nitrogen, Oxygen, and Sulfur Heterocycles via Rh-Catalyzed Alkene Hydroacylation

Enantioselective synthesis of polycyclic nitrogen, oxygen, and sulfur heterocycles by rhodium-catalyzed intramolecular alkene hydroacylation is reported. The intramolecular hydroacylation reactions generate 1,4-dihydrocyclopenta[b]indol-3(2H)-ones and 3,4-dihydrocyclopenta[b]indol-1(2H)-one in moderate-to-high yields (65-99%) with good-to-excellent enantioselectivities (84-99% ee). The catalyst system also promotes alkene hydroacylation of 3-vinylfuran-, 3-vinylbenzothiophene-, and 3-vinylthiophene-2-carboxaldehydes to generate the corresponding ketone products in moderate-to-high yields (71-91% yield) with excellent enantioselectivities (97-99% ee).

Recent Progress on Nazarov Cyclizations: The Use of Iron Salts as Catalysts in Ionic Liquid Solvent Systems

Nazarov cyclization is an important and versatile method for the synthesis of five-membered carbocycles, and extensive studies have been conducted to optimize the reaction. Among recent studies, several trends are recognized. One is the combination of different reactions with Nazarov cyclization in a one-pot reaction system which enables the preparation of unique cyclization products. The second is the use of a transition-metal catalyst, though Lewis or Brønsted acids have generally been used for the reaction. The third is the realization of the asymmetric Nazarov cyclization. The fourth is the base-catalyzed Nazarov cyclization. Furthermore, several useful protocols for realizing Nazarov cyclization have also been developed. The recent progress on Nazarov cyclizations is summarized in Section 2. Section 3 is our chronicle in this field. We focused on the use of iron as the catalyst in Nazarov cyclizations and ionic liquids as solvents: Nazarov cyclization of thiophene derivatives using FeCl3 as the catalyst was accomplished and we succeeded in demonstrating the first example of an iron-catalyzed asymmetric Nazarov reaction. We next established Nazarov cyclization of pyrrole or indole derivatives using Fe(ClO4 )3 ·Al2 O3 as the catalyst with high trans selectivities in excellent yields. Since the cyclized product was reacted with a vinyl ketone in the presence of the same iron salt, the system allowed realization of the sequential type of Nazarov/Michael reaction of pyrrole derivatives. Furthermore, we demonstrated the recyclable use of the iron catalyst and obtained the desired Nazarov/Michael reaction products in good yields for five repetitions of the reactions without any addition of the catalyst using an ionic liquid, [bmim][NTf2 ], as the solvent. We expect that the iron-catalyzed Nazarov cyclization, in particular, in an ionic liquid solvent might become a useful method to synthesize functional molecules that include cycloalkene moieties.

The β-Silyl Effect on the Memory of Chirality in Friedel-Crafts Alkylation Using Chiral α-Aryl Alcohols

Iron salt-catalyzed Friedel-Crafts alkylation of chiral α-aryl alcohols with a trimethylsilyl group was found to proceed with retention of the configuration of the hydroxyl group as a leaving group. The memory of chirality of this system stems from the β-silyl effect of the trimethylsilyl group on the carbocation intermediate.