Ruthenium-Catalyzed Oxidative Cleavage of Alkynes to Carboxylic Acids

Transformative reactions in nitroarene chemistry: C-N bond cleavage, skeletal editing, and N-O bond utilization

Nitroarenes are highly versatile building blocks in organic synthesis, playing a pivotal role in various reactions. Common transformations involving nitroarenes include nucleophilic aromatic substitution (SNAr) reactions, where the nitro group functions both as a potent electron-withdrawing group that activates the aromatic ring and as a leaving group facilitating the substitution. Additionally, the direct transformation of nitro groups, such as reduction-driven syntheses of amines and carboxylic acids, as well as ipso-substitution SNAr reactions, have been extensively explored. Interactions between ortho-nitro groups and neighboring substituents also provide unique opportunities for selective transformations. However, beyond these well-established processes, direct transformations of nitro groups have been relatively limited. In recent years, significant advancements have been made in alternative methodologies for nitro group transformations. This review focuses on the latest progress in novel transformations of nitroarenes, with emphasis on three major categories: (i) functional group transformations involving C-N bond cleavage in nitroarenes, (ii) skeletal editing via nitrene intermediates generated by N-O bond cleavage, and (iii) the utilization of nitroarenes as an oxygen source through N-O bond cleavage. These developments under-score the expanding utility of nitroarenes in modern organic synthesis.

Organic Dye Photocatalyzed Synthesis of Functionalized Lactones and Lactams via a Cyclization-Alkynylation Cascade

An organic dye photocatalyzed lactonization-alkynylation of easily accessible homoallylic cesium oxalates using ethynylbenziodoxolone (EBX) reagents has been developed. The reaction gave access to valuable functionalized lactones and lactams in up to 88% yield via the formation of two new C-C bonds. The transformation was carried out on primary, secondary, and tertiary homoallylic alcohols and primary homoallylic amines and could be applied to the synthesis of spirocyclic compounds as well as fused and bridged bicyclic lactones.

Electrochemical Oxidative Cleavage of Alkynes to Carboxylic Acids

A sustainable method for converting terminal alkynes into their corresponding carboxylic acids is reported using synthetic electrolysis in an undivided cell at room temperature. This protocol, avoiding transition metal catalysis and stoichiometric chemical oxidants, tolerates a variety of aryl, heteroaryl, and alkyl akynes. Preliminary mechanistic studies demonstrate that sodium nitrite serves a triple role as the electrolyte, nitryl radical precursor, and a nitrosating reagent.

Photochemical Organocatalytic Aerobic Cleavage of C═C Bonds Enabled by Charge-Transfer Complex Formation

A novel visible-light-driven organocatalytic protocol to access aerobic oxidative cleavage of olefins, promoted by sodium benzene sulfinate, is described herein. An array of alkenes smoothly delivered the corresponding aldehydes and ketones under transition-metal-free conditions. Notably, α-halo-substituted styrenes proceeded with photoinduced oxidation to finally afford α-halo-acetophenones with halogen migration. Crucial to this oxidation was the formation of charge-transfer complexes between sodium benzene sulfinate with molecular O₂ to ultimately deliver the carbonyl products.

Catalyst-Free and Transition-Metal-Free Approach to 1,2-Diketones via Aerobic Alkyne Oxidation

A catalyst-free and transition-metal-free method for the synthesis of 1,2-diketones from aerobic alkyne oxidation was reported. The oxidation of various internal alkynes, especially more challenging aryl-alkyl acetylenes, proceeded smoothly with inexpensive, easily handled, and commercially available potassium persulfate and an ambient air balloon, achieving the corresponding 1,2-diketones with up to 85% yields. Meanwhile, mechanistic studies indicated a radical process, and the two oxygen atoms in the 1,2-diketons were most likely from persulfate salts and molecular oxygen, respectively, rather than water.

Oxidation of Alkynyl Boronates to Carboxylic Acids, Esters, and Amides

A general efficient protocol was developed for the synthesis of carboxylic acids, esters, and amides through oxidation of alkynyl boronates, generated directly from terminal alkynes. This protocol represents the first example of C(sp)-B bond oxidation. This approach displays a broad substrate scope, including aryl and alkyl alkynes, and exhibits excellent functional group tolerance. Water, primary and secondary alcohols, and amines are suitable nucleophiles for this transformation. Notably, amino acids and peptides can be used as nucleophiles, providing an efficient method for the synthesis and modification of peptides. The practicability of this methodology was further highlighted by the preparation of pharmaceutical molecules.

Kolbe-Schmitt type reaction under ambient conditions mediated by an organic base

The combined use of an organic base for resorcinols realized a Kolbe-Schmitt type reaction under ambient conditions. When resorcinols (3-hydroxyphenol derivatives) were treated with DBU under a carbon dioxide atmosphere, nucleophilic addition to carbon dioxide proceeded to afford the corresponding salicylic acid derivatives in high yields.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

Ruthenium-ion-catalyzed oxidation (RICO) of polyaromatic hydrocarbons (PAHs) has been studied in detail using experimental and computational approaches to explore the reaction mechanism. DFT calculations show that regioselectivity in these reactions can be understood in terms of the preservation of aromaticity in the initial formation of a [3+2] metallocycle intermediate at the most-isolated double bond. We identify two competing pathways: C-C bond cleavage leading to a dialdehyde and C-H activation followed by H migration to the RuO_x complex to give diketones. Experimentally, the oxidation of pyrene and phenanthrene has been carried out in monophasic and biphasic solvent systems. Our results show that diketones are the major product for both phenanthrene and pyrene substrates. These diketone products are shown to be stable under our reaction conditions so that higher oxidation products (acids and their derivatives) are assigned to the competing pathway through the dialdehyde. Experiments using ¹⁸ O-labelled water do show incorporation of oxygen from the solvents into products, but this may take place during the formation of the reactive RuO₄ species rather than directly during PAH oxidation. When the oxidation of pyrene is carried out using D₂ O, a kinetic isotope effect (KIE) is observed implying that water is involved in the rate-determining step leading to the diketone products.

Transformations of alkynes to carboxylic acids and their derivatives via C[triple bond, length as m-dash]C bond cleavage

Alkynes are building blocks of high synthetic value and their usefulness as precursors to many chemical and biological systems is widely established. Amongst several transformations of alkynes, cleavage of the C[triple bond, length as m-dash]C bond for obtaining diverse compounds is considered to be important. This review, in particular, comprehensively assimilates the transformations of alkynes to carboxylic acids including esters, amides and nitriles resulting from the cleavage of the C[triple bond, length as m-dash]C bond either under the influence of a metal catalyst or via a metal-free approach.

Oxidation of Polynuclear Aromatic Hydrocarbons using Ruthenium-Ion-Catalyzed Oxidation: The Role of Aromatic Ring Number in Reaction Kinetics and Product Distribution

Oxidation of aromatic hydrocarbons with differing numbers of fused aromatic rings (2-5), have been studied in two solvent environments (monophasic and biphasic) using ruthenium-ion-catalyzed oxidation (RICO). RICO reduces the aromaticity of the polyaromatic core of the molecule in a controlled manner by selective oxidative ring opening. Moreover, the nature of the solvent system determines the product type and distribution, for molecules with more than two aromatic rings. Competitive oxidation between substrates with different numbers of aromatic rings has been studied in detail. It was found that the rate of polyaromatic hydrocarbon oxidation increases with the number of fused aromatic rings. A similar trend was also identified for alkylated aromatic hydrocarbons. The proof-of-concept investigation provides new insight into selective oxidation chemistry for upgrading of polyaromatic molecules.

Ruthenium tetroxide and perruthenate chemistry. Recent advances and related transformations mediated by other transition metal oxo-species

In the last years ruthenium tetroxide is increasingly being used in organic synthesis. Thanks to the fine tuning of the reaction conditions, including pH control of the medium and the use of a wider range of co-oxidants, this species has proven to be a reagent able to catalyse useful synthetic transformations which are either a valuable alternative to established methods or even, in some cases, the method of choice. Protocols for oxidation of hydrocarbons, oxidative cleavage of C-C double bonds, even stopping the process at the aldehyde stage, oxidative cleavage of terminal and internal alkynes, oxidation of alcohols to carboxylic acids, dihydroxylation of alkenes, oxidative degradation of phenyl and other heteroaromatic nuclei, oxidative cyclization of dienes, have now reached a good level of improvement and are more and more included into complex synthetic sequences. The perruthenate ion is a ruthenium (VII) oxo-species. Since its introduction in the mid-eighties, tetrapropylammonium perruthenate (TPAP) has reached a great popularity among organic chemists and it is mostly employed in catalytic amounts in conjunction with N-methylmorpholine N-oxide (NMO) for the mild oxidation of primary and secondary alcohols to carbonyl compounds. Its use in the oxidation of other functionalities is known and recently, its utility in new synthetic transformations has been demonstrated. New processes, synthetic applications, theoretical studies and unusual transformations, published in the last eight years (2006-2013), in the chemistry of these two oxo-species, will be covered in this review with the aim of offering a clear picture of their reactivity. When appropriate, related oxidative transformations mediated by other metal oxo-species will be presented to highlight similarities and differences. An historical overview of some aspects of the ruthenium tetroxide chemistry will be presented as well.

Cyclohexyne cycloinsertion in the divergent synthesis of guanacastepenes

The guanacastepenes are a family of 15 diterpenes that share a common 5-6-7 tricyclic core, which is decorated with quaternary centers, unsaturation, hydroxyl and carbonyl groups. Some of these natural products show interesting antimicrobial potency. Their collective structural and biological features have stirred up vibrant activity among organic chemists. Herein, we disclose an account of our studies toward the synthesis of a number of guanacastepenes. The synthetic strategy relies on the use of cyclohexyne in a cycloinsertion reaction to rapidly construct the guanacastepene core. Isolation of a cyclobutenol as intermediate in the cyclohexyne cycloinsertion provided us with the possibility to study further the reactivity of this metastable compound, and we uncovered novel rearrangements and ring-opening reactions. Stereoselective, late-stage oxidative diversification of the carbon scaffold allowed the synthesis of guanacastepenes N and O and paved the way for the synthesis of guanacastepenes H and D.

Catalytic oxidative cleavage of olefins promoted by osmium tetroxide and hydrogen peroxide

Hydrogen peroxide was employed as the terminal oxidant in the osmium tetroxide mediated oxidative cleavage of olefins, producing the corresponding aldehyde and ketone products. Aryl olefins are cleaved in good to excellent yield regardless of arene electronics. Alkyl olefins cleave in moderate to good yield for di- and tri-substituted alkenes.

(S)-2-pentyl (R)-3-hydroxyhexanoate, a banana volatile and its olfactory recognition by the common fruit fly, Drosophila melanogaster

The volatile organic compounds emitted from ripening bananas that elicit an antennal response from the common fruit fly, Drosophila melanogaster, were analyzed by a combination of gas chromatographic-electroantennographic detection, mass spectrometry, and (1)H NMR spectroscopy. These analyses revealed that the headspace of ripening bananas contains a number of EAD-active components including the new ester (S)-2-pentyl (R)-3-hydroxyhexanoate, the structural assignment of which was confirmed by chemical synthesis.

Ruthenium-Catalyzed Oxidation of Alkenes, Alkynes, and Alcohols to Organic Acids with Aqueous Hydrogen Peroxide

A protocol that adopts aqueous hydrogen peroxide as a terminal oxidant and [(Me3tacn)(CF3CO2)2Ru(III)(OH2)]CF3CO2 (1; Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) as a catalyst for oxidation of alkenes, alkynes, and alcohols to organic acids in over 80% yield is presented. For the oxidation of cyclohexene to adipic acid, the loading of 1 can be lowered to 0.1 mol %. On the one-mole scale, the oxidation of cyclohexene, cyclooctene, and 1-octanol with 1 mol % of 1 produced adipic acid (124 g, 85% yield), suberic acid (158 g, 91% yield), and 1-octanoic acid (129 g, 90% yield), respectively. The oxidative C=C bond-cleavage reaction proceeded through the formation of cis- and trans-diol intermediates, which were further oxidized to carboxylic acids via C-C bond cleavage.

Generation of mono- and bis-dioxiranes from 2,3-butanedione

Biacetyl reacts with oxone to give bis-dioxirane [3,3'-dimethyl-3,3'-bidioxirane, 3B] and mono-dioxirane [1-(3-methyl-dioxiran-3-yl)ethanone, 3A)]. Bis-dioxirane 3B is formed when two oxygens are incorporated into biacetyl, while mono-dioxirane 3A incorporated only one. A greater stability is observed in 3B compared to 3A, which is attributed to an alpha-dioxiranyl (anomeric) effect in the former. In contrast, 3A suffers from a destabilizing pi-electron withdrawing effect from the adjacent carbonyl group.