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.

Nickel-Catalyzed Removal of Alkene Protecting Group of Phenols, Alcohols via Chain Walking Process

An efficient nickel-catalyzed removal of alkene protection group under mild condition with high functional group tolerance through chain walking process has been established. Not only phenolic ethers, but also alcoholic ethers can be tolerated with the retention of stereocenter adjacent to hydroxyl group. The new reaction brings the homoallyl group into a start of new type of protecting group.

Cross-Coupling Reaction of Allylic Ethers with Aryl Grignard Reagents Catalyzed by a Nickel Pincer Complex

A cross-coupling reaction of allylic aryl ethers with arylmagnesium reagents was investigated using β-aminoketonato- and β-diketiminato-based pincer-type nickel(II) complexes as catalysts. An β-aminoketonato nickel(II) complex bearing a diphenylphosphino group as a third donor effectively catalyzed the reaction to afford the target cross-coupled products, allylbenzene derivatives, in high yield. The regioselective reaction of a variety of substituted cinnamyl ethers proceeded to give the corresponding linear products. In contrast, α- and γ-alkyl substituted allylic ethers afforded a mixture of the linear and branched products. These results indicated that the coupling reaction proceeded via a π-allyl nickel intermediate.

The winding road of the uvaretin class of natural products: from total synthesis to bioactive agent discovery

Herein, we disclose the development of a synthetic route to gain access to the uvaretin class of chalcone natural products. In this, the construction of a small library was achieved, and the collection was evaluated for cytotoxicity and other biological properties. Uvaretin (1) was accessed via a seven-step route in an overall yield of 15.1%. Within this route, the unsaturated enone variant of uvaretin (2), also a natural product, was accessed in a 16.7% yield over six steps. This route provides a nearly three-fold increase in yields of 1 and 2 in comparison to the previous synthetic route accessing them in 5.8% and 3.0% overall yields, respectively. Evaluation of 1 and 2 revealed IC₅₀ values between 2.0 and 5.1 μM in the cancerous cell lines HeLa, U937, A549, and MIA PaCa-2. Screening of the whole chalcone library set led to the discovery of over 30 compounds, within six cancerous cell lines, possessing single digit μM IC₅₀ activity as sole agents. Furthermore, multiple library members were found to possess promising potentiating properties with known chemotherapeutic agents.

Iron-Catalyzed C-O Bond Activation: Opportunity for Sustainable Catalysis

Oxygen-based electrophiles have emerged as some of the most valuable cross-coupling partners in organic synthesis due to several major strategic and environmental benefits, such as abundance and potential to avoid toxic halide waste. In this context, iron-catalyzed C-O activation/cross-coupling holds particular promise to achieve sustainable catalytic protocols due to its natural abundance, inherent low toxicity, and excellent economic and ecological profile. Recently, tremendous progress has been achieved in the development of new methods for functional-group-tolerant iron-catalyzed cross-coupling reactions by selective C-O cleavage. These methods establish highly attractive alternatives to traditional cross-coupling reactions by using halides as electrophilic partners. In particular, new easily accessible oxygen-based electrophiles have emerged as substrates in iron-catalyzed cross-coupling reactions, which significantly broaden the scope of this catalysis platform. New mechanistic manifolds involving iron catalysis have been established; thus opening up vistas for the development of a wide range of unprecedented reactions. The synthetic potential of this sustainable mode of reactivity has been highlighted by the development of new strategies in the construction of complex motifs, including in target synthesis. The most recent advances in sustainable iron-catalyzed cross-coupling of C-O-based electrophiles are reviewed, with a focus on both mechanistic aspects and synthetic utility. It should be noted that this catalytic manifold provides access to motifs that are often not easily available by other methods, such as the assembly of stereodefined dienes or C(sp² )-C(sp³ ) cross-couplings, thus emphasizing the synthetic importance of this mode of reactivity.

Pincer-Nickel-Catalyzed Allyl-Aryl Coupling between Allyl Methyl Ethers and Arylzinc Chlorides

The P,N,N-pincer nickel complex [Ni(Cl){N(2-Ph2PC6H4)(2'-Me2NC6H4)}]-catalyzed allyl-aryl coupling was studied. The reaction of allyl methyl ethers, including (1-methoxyallyl)arenes and (3-methoxyprop-1-en-1-yl)arenes, with arylzinc chlorides afforded linear (E)-alkenes in high yields, whereas the reaction of (E)-1-methoxytridec-2-ene with p-Me2NC6H4ZnCl generated a mixture of linear and branched alkenes.

Iron-Catalyzed Cross-Coupling of Alkenyl Acetates

Stable C-O linkages are generally unreactive in cross-coupling reactions which mostly employ more electrophilic halides or activated esters (triflates, tosylates). Acetates are cheap and easily accessible electrophiles but have not been used in cross-couplings because the strong C-O bond and high propensity to engage in unwanted acetylation and deprotonation. Reported herein is a selective iron-catalyzed cross-coupling of diverse alkenyl acetates, and it operates under mild reaction conditions (0 °C, 2 h) with a ligand-free catalyst (1-2 mol%).

2-Arylcyclopropylmethanol as a substitute for homoallyl aryl alcohol in the construction of cis-2,6-disubstituted tetrahydropyran: synthesis of (±)-centrolobine

The application of 2-arylcyclopropylmethanols as substitutes to homoallyl aryl alcohols and their reactions with aliphatic aldehydes in the presence of SnCl4 in CH2Cl2 leads to an efficient Prins cyclization to generate cis-2,6-disubstituted tetrahydropyrans in high yields. The reaction is free from 2-oxonia-Cope rearrangement. This protocol was used to synthesize (±)-centrolobine in an overall 84% yield over three steps. The protocol holds promise for scaffold generation for medicinal chemistry exploitation.

Hydrazone-palladium-catalyzed allylic arylation of cinnamyloxyphenylboronic acid pinacol esters

Allylic arylation of cinnamyloxyphenylboronic acid pinacol esters 3, which have arylboronic acid moiety and allylic ether moiety, using a hydrazone 1d-Pd(OAc)2 system proceeded and gave the corresponding 1,3-diarylpropene derivatives 4 with a phenolic hydroxyl group via a selective coupling reaction of the π-allyl intermediate to the boron-substituted position of the leaving group.