Iron-facilitated oxidative dehydrogenative C-O bond formation by propargylic C sp 3-H functionalization

Gold-catalysed asymmetric net addition of unactivated propargylic C-H bonds to tethered aldehydes

The asymmetric one-step net addition of unactivated propargylic C-H bond to aldehyde leads to an atom-economic construction of versatile chiral propargylic alcohols but has not been realized previously. Here we show its implementation in an intramolecular manner under mild reaction conditions. Via cooperative gold catalysis enabled by a chiral bifunctional phosphine ligand, this chemistry achieves asymmetric catalytic deprotonation of propargylic C-H (pK_a > 30) by a tertiary amine group (pK_a ~ 10) of the ligand in the presence of much more acidic aldehydic α-hydrogens (pKa ~ 17). The reaction exhibits a broad scope and readily accommodates various functional groups. The 5-/6-membered ring fused homopropargylic alcohol products are formed with excellent enantiomeric excesses and high trans-selectivities with or without a preexisting substrate chiral center. DFT studies of the reaction support the conceived reaction mechanism and the calculated energetics corroborate the observed stereoselectivity and confirm an additional metal-ligand cooperation.

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.

Iron-catalyzed α-C-H functionalization of π-bonds: cross-dehydrogenative coupling and mechanistic insights

The deprotonation of propargylic C-H bonds for subsequent functionalization typically requires stoichiometric metal alkyl or amide reagents. In addition to the undesirable generation of stoichiometric metallic waste, these conditions limit the functional group compatibility and versatility of this functionalization strategy and often result in regioisomeric mixtures. In this article, we report the use of dicarbonyl cyclopentadienyliron(ii) complexes for the generation of propargylic anion equivalents toward the direct electrophilic functionalization of propargylic C-H bonds under mild, catalytic conditions. This technology was applied to the direct conversion of C-H bonds to C-C bonds for the synthesis of several functionalized scaffolds through a one-pot cross dehydrogenative coupling reaction with tetrahydroisoquinoline and related privileged heterocyclic scaffolds. A series of NMR studies and deuterium-labelling experiments indicated that the deprotonation of the propargylic C-H bond was the rate-determining step when a Cp*Fe(CO)2-based catalyst system was employed.

Divergent Mechanisms of the Banert Cascade with Propargyl Azides

Triazoles are privileged heterocycles for a variety of applications. The synthesis of 1H-triazoles can be accomplished by the Banert cascade from propargylic azides. Depending on the substrate and conditions, the Banert cascade can proceed by either a sigmatropic or prototropic mechanism. This report describes the first detailed kinetic analysis of the Banert cascade proceeding by both pathways including substituent effects and KIE. The analysis identified the inflection point in the divergent pathways, allowing future work to predict which Banert products are accessible.

Directing activator-assisted regio- and oxidation state-selective aerobic oxidation of secondary C(sp(3))-H bonds in aliphatic alcohols

The regioselective conversion of an unactivated C(sp(3))-H bond of a methylene carbon (CH2) into a C-O single bond is an attractive reaction in organic synthesis. Herein, we present a strategy for a regio- and oxidation state-selective aerobic C-H oxidation based on an N-hydroxyamide-derived directing activator (DA), which is attached to a hydroxy group in alcohol substrates. The DA reacts with NOx species generated in situ from NaNO2, a Brønsted acid, and aerobic oxygen, and effectively generates an amidoxyl radical from the N-hydroxy moiety of the DA. Then, the amidoxyl radical promotes site-selective intramolecular C-H abstraction from methylenes with γ- (or δ-) selectivity. The thus-generated methylene radicals are trapped by molecular oxygen and NO. This process results in the predominant formation of nitrate esters as products, which suppresses undesired overoxidation. The products can be easily converted into alcohols after hydrogenolysis.

Aerobic oxidative acylation of nitroarenes with arylacetic esters under mild conditions: facile access to diarylketones

A regioselectivet-BuONa-mediated aerobic oxidative acylation of nitroarenes under mild conditions is developed through a cascade CDC/oxidative decarboxylation process.

Palladium-Catalyzed Oxidative Synthesis of α-Acetoxylated Enones from Alkynes

We report a palladium-catalyzed oxidative functionalization of alkynes to generate α-acetoxylated enones in one step. A range of functional groups are well-tolerated in this reaction. Mechanistic studies, including the use of (18) O-labeled DMSO, revealed that the ketone oxygen atom in the product originates from DMSO.

Palladium-catalyzed direct C–H arylation of ferrocenecarboxamides with aryl halides

A simple and facile protocol for palladium-catalyzed ortho-arylation of ferrocenecarboxamides with aryl halides was developed with the assistance of the bidentate directing group.

Cross-dehydrogenative coupling for the intermolecular C-O bond formation

The present review summarizes primary publications on the cross-dehydrogenative C-O coupling, with special emphasis on the studies published after 2000. The starting compound, which donates a carbon atom for the formation of a new C-O bond, is called the CH-reagent or the C-reagent, and the compound, an oxygen atom of which is involved in the new bond, is called the OH-reagent or the O-reagent. Alcohols and carboxylic acids are most commonly used as O-reagents; hydroxylamine derivatives, hydroperoxides, and sulfonic acids are employed less often. The cross-dehydrogenative C-O coupling reactions are carried out using different C-reagents, such as compounds containing directing functional groups (amide, heteroaromatic, oxime, and so on) and compounds with activated C-H bonds (aldehydes, alcohols, ketones, ethers, amines, amides, compounds containing the benzyl, allyl, or propargyl moiety). An analysis of the published data showed that the principles at the basis of a particular cross-dehydrogenative C-O coupling reaction are dictated mainly by the nature of the C-reagent. Hence, in the present review the data are classified according to the structures of C-reagents, and, in the second place, according to the type of oxidative systems. Besides the typical cross-dehydrogenative coupling reactions of CH- and OH-reagents, closely related C-H activation processes involving intermolecular C-O bond formation are discussed: acyloxylation reactions with ArI(O2CR)2 reagents and generation of O-reagents in situ from C-reagents (methylarenes, aldehydes, etc.).

Oxidative cross-coupling of pyridine N-oxides and ethers between C(sp2)–H/C(sp3)–H bonds under transition-metal-free conditions

An oxidative cross-coupling of pyridine N-oxides and ethers between C(sp²)–H/C(sp³)–H bonds under transition metal free conditions was developed.

Iron-catalyzed aerobic oxidative functionalization of sp3 C–H bonds: a versatile strategy for the construction of N-heterocycles

Iron-catalyzed aerobic oxidative functionalization of sp³ C–H bonds using a biofriendly iron catalyst in combination with O₂ or air as the sole oxidant has been developed for the construction of N-heterocycles from easily available carboxylic acid derivatives and o-substituted anilines.

Direct approaches to nitriles via highly efficient nitrogenation strategy through C-H or C-C bond cleavage

Because of the importance of nitrogen-containing compounds in chemistry and biology, organic chemists have long focused on the development of novel methodologies for their synthesis. For example, nitrogen-containing compounds show up within functional materials, as top-selling drugs, and as bioactive molecules. To synthesize these compounds in a green and sustainable way, researchers have focused on the direct functionalization of hydrocarbons via C-H or C-C bond cleavage. Although researchers have made significant progress in the direct functionalization of simple hydrocarbons, direct C-N bond formation via C-H or C-C bond cleavage remains challenging, in part because of the unstable character of some N-nucleophiles under oxidative conditions. The nitriles are versatile building blocks and precursors in organic synthesis. Recently, chemists have achieved the direct C-H cyanation with toxic cyanide salts in the presence of stoichiometric metal oxidants. In this Account, we describe recent progress made by our group in nitrile synthesis. C-H or C-C bond cleavage is a key process in our strategy, and azides or DMF serve as the nitrogen source. In these reactions, we successfully realized direct nitrile synthesis using a variety of hydrocarbon groups as nitrile precursors, including methyl, alkenyl, and alkynyl groups. We could carry out C(sp(3))-H functionalization on benzylic, allylic, and propargylic C-H bonds to produce diverse valuable synthetic nitriles. Mild oxidation of C═C double-bonds and C≡C triple-bonds also produced nitriles. The incorporation of nitrogen within the carbon skeleton typically involved the participation of azide reagents. Although some mechanistic details remain unclear, studies of these nitrogenation reactions implicate the involvement of a cation or radical intermediate, and an oxidative rearrangement of azide intermediate produced the nitrile. We also explored environmentally friendly oxidants, such as molecular oxygen, to make our synthetic strategy more attractive. Our direct nitrile synthesis methodologies have potential applications in the synthesis of biologically active molecules and drug candidates.

Iron-catalyzed/mediated oxidative transformation of C–H bonds

Iron-catalyzed/mediated C–H bond oxidation has been demonstrated as one of practical and straightforward tools in synthetic chemistry.

FeCl3-diorganyl dichalcogenides promoted cyclization of 2-organochalcogen-3-alkynylthiophenes: synthesis of chalcogenophene[2,3-b]thiophenes

We report here our results on the FeCl3-diorganyl dichalcogenides intramolecular cyclization of 2-organochalcogen-3-alkynylthiophenes. The cyclization reaction proceeded cleanly under mild reaction conditions giving the (S)-Se-, (S)-S- and (S)-Te-heterocycles in good yields. In addition, the obtained chalcogenophenes were readily transformed into more complex products using the palladium cross-coupling reaction with boronic acids. Conversely, using a metal-halogen exchange reaction with n-BuLi, the chalcogenophenes produced the lithium-intermediate which was trapped with aldehyde furnishing the desired secondary alcohol in good yield.

Selective C(sp2)-C(sp) bond cleavage: the nitrogenation of alkynes to amides

Breakthrough: A novel catalyzed direct highly selective C(sp2)-C(sp) bond functionalization of alkynes to amides has been developed. Nitrogenation is achieved by the highly selective C(sp2)-C(sp) bond cleavage of aryl-substituted alkynes. The oxidant-free and mild conditions and wide substrate scope make this method very practical.