Synthesis of Nitrones and Nitroalkanes via Chiral Cyclic Imines

A series of structurally chiral cyclic imines efficiently yields chiral nitrones and nitroalkanes. This is the first report of the synthesis of nitro groups by C═N bond cleavage of imines through a nitrone intermediate.

Synthesis of Bridged Cycloisoxazoline Scaffolds via Rhodium-Catalyzed Coupling of Nitrones with Cyclic Carbonate

Bridged isoxazolidines were synthesized via Rh(III)-catalyzed C-H allylation of α-aryl nitrones with 5-methylene-1,3-dioxan-2-one. The nitrone group serves as a directing group and 1,3-dipole in the C-H activation/[3 + 2] cycloaddition cascade, exhibiting excellent chemo- and stereoselectivity along with good functional group compatibility. The resulting skeletal structure was conveniently modified to produce a range of important chemical frameworks, and the protocol was applied to biologically active molecules.

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Synthesis of α-Keto Aldehydes via Selective Cu(I)-catalyzed Oxidation of α-Hydroxy Ketones

An efficient approach to synthesize α-keto aldehydes was established through selective oxidation of α-hydroxy ketones catalyzed by Cu(I) using oxygen as oxidant. A wide array of α-keto aldehydes was prepared...

Palladium-Catalyzed Regiospecific peri- and ortho-C-H Oxygenations of Polyaromatic Rings Mediated by Tunable Directing Groups

An efficient divergent approach of Pd-catalyzed C-H oxygenation of polyaromatic rings is described. Reversible directing groups enable regiospecific peri- and ortho-oxygenation to readily access a wide array of polyaromatic phenols without pre- and postmanipulation of directing groups. The systematic mechanistic investigation, including deuterium-labeling experiments, palladacycle trapping, and DFT calculations, reveals that the tunable ligand-assisted C-H bond cleavage played a crucial role during the reaction process.

Ruthenium-catalyzed coupling of α-carbonyl phosphoniums with sulfoxonium ylides via C–H activation/Wittig reaction sequences

A Ru(ii)-catalyzed coupling of various α-carbonyl phosphoniums with sulfoxonium ylides has been realized for the facile synthesis of 1-naphthols in good to excellent yields.

Synthesis of polycyclic naphthols and naphthalenes via tandem Ti(Oi-Pr)4-promoted photoenolization Diels–Alder reaction and aromatization

An efficient approach to naphthol and naphthalene scaffolds has been developed using a sequence involving tandem Ti(Oi-Pr)₄-promoted photoenolization Diels–Alder (PEDA) and aromatization reactions.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Rhodium(iii)-catalyzed unreactive C(sp3)-H alkenylation of N-alkyl-1H-pyrazoles with alkynes

The first example of pyrazole-directed rhodium(iii)-catalyzed unreactive C(sp³)-H alkenylation with alkynes has been described, which showed a relatively broad substrate scope with good functional group compatibility. Moreover, we demonstrated that the transitive coordinating center pyrazole could be easily removed under mild conditions.

Synthesis and Transformations of Nitrones for Organic Synthesis

Nitrones are important compounds and are highly useful in many aspects. The first part describes the methods for synthesis of nitrones, which are useful and environmentally friendly. Catalytic oxidations, condensations, and other useful reactions are described. The nitrones thus obtained are key intermediates for the synthesis of biologically important nitrogen compounds. The second part describes the fundamental transformations of nitrones, which will provide the strategies and means for the construction of nitrogen compounds. The reactions with nucleophiles or radicals, C-H functionalization, and various addition reactions are described. The last reactions are particularly important for highly selective carbon-carbon bond formations. 1,3-Dipolar cycloaddition reactions are excluded because the size of the review is limited and excellent reviews have been published in Chemical Reviews.

Synthesis of 1-naphthols via Cp*Co(iii)-catalyzed C–H activation and cyclization of sulfoxonium ylides with alkynes

A highly practical synthesis of 1-naphthols was developed via Cp*Co(iii)-catalyzed C–H activation and cyclization between sulfoxonium ylides and alkynes.

Access to 2-naphthols via Ru(ii)-catalyzed C–H annulation of nitrones with α-diazo sulfonyl ketones

Efficient synthesis of 2-naphthols was realized by Ru(ii)-catalyzed C–H activation of aryl nitrones and intermolecular [3+3] annulation with α-diazo sulfonyl ketones under redox-neutral conditions.

Formal Total Synthesis of Hybocarpone Enabled by Visible-Light-Promoted Benzannulation

The formal total synthesis of hybocarpone was achieved in eight steps from commercially available 1,2,4-trimethoxybenzene. Key transformations include a visible-light-promoted benzannulation to construct the key α-naphthol intermediate and a modified CAN-mediated dimerization/hydration cascade sequence to generate the vicinal all-carbon quaternary centers in a stereocontrolled manner. The total synthesis of boryquinone was also achieved in seven steps.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Reactivity of Morita-Baylis-Hillman Adducts in C-H Functionalization of (Hetero)aryl Nitrones: Access to Bridged Cycles and Carbazoles

The rhodium(III)-catalyzed cross-coupling of (hetero)aryl nitrones with Morita-Baylis-Hillman (MBH) adducts is described. An allylated intermediate derived from aryl nitrones and MBH adducts allows the formation of bridged cyclic compounds via an exotype [3 + 2] cycloaddition. In sharp contrast, electron-rich indolinyl or aniline substrates were found to couple with MBH adducts to generate naphthalene or carbazole derivatives, respectively.

Rh(iii)-Catalyzed regioselective C–H [4 + 2] C-annulation of vinyl enaminones with alkynes to form polysubstituted salicylaldehydes

Sterically congested polysubstituted salicylaldehydes are accessed through a Rh(iii)-catalyzed regioselective vinylic C–H [4 + 2] C-annulation reaction of vinyl enaminones with alkynes.

One-pot synthesis of 2-naphthols from nitrones and MBH adducts via decarboxylative N–O bond cleavage

The unprecedented one-pot synthesis of 2-naphthols through an acid-mediated decarboxylative N–O bond cleavage of bridged benzoxazepine intermediates is described.

Synthesis of 2-aminobenzaldehydes by rhodium(iii)-catalyzed C–H amidation of aldehydes with dioxazolones

Various aldehydes were directly amidated with dioxazolones to afford 2-aminobenzaldehydes through rhodium(iii)-catalyzed CHO-directed C–H activation.