Pentacoordinated carboxylate π-allyl nickel complexes as key intermediates for the Ni-catalyzed direct amination of allylic alcohols

Direct amination of allylic alcohols with primary and secondary amines catalyzed by a system made of [Ni(1,5-cyclooctadiene)2 ] and 1,1'-bis(diphenylphosphino)ferrocene was effectively enhanced by adding nBu4 NOAc and molecular sieves, affording the corresponding allyl amines in high yield with high monoallylation selectivity for primary amines and high regioselectivity for monosubstituted allylic alcohols. Such remarkable additive effects of nBu4 NOAc were elucidated by isolating and characterizing some nickel complexes, manifesting the key role of a charge neutral pentacoordinated η(3) -allyl acetate complex in the present system, in contrast to usual cationic tetracoordinated complexes earlier reported in allylic substitution reactions.

Intramolecular etherification and polyene cyclization of π-activated alcohols promoted by hot water

Hot water, acting as a mildly acidic catalyst, efficiently promoted intramolecular direct nucleophilic substitution reactions of unsaturated alcohols with heteroatom or carbon nucleophiles. In a mixed solvent of water and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), polyene cyclizations using allylic alcohols as initiators gave the desired cyclized products, and in neat HFIP, a tricyclization reaction gave a tetracyclic product in 51% chemical yield.

Iron- and indium-catalyzed reactions toward nitrogen- and oxygen-containing saturated heterocycles

A myriad of natural and/or biologically active products include nitrogen- and oxygen-containing saturated heterocycles, which are thus considered as attractive scaffolds in the drug discovery process. As a consequence, a wide range of reactions has been developed for the construction of these frameworks, much effort being specially devoted to the formation of substituted tetrahydropyrans and piperidines. Among the existing methods to form these heterocycles, the metal-catalyzed heterocyclization of amino- or hydroxy-allylic alcohol derivatives has emerged as a powerful and stereoselective strategy that is particularly interesting in terms of both atom-economy and ecocompatibility. For a long time, palladium catalysts have widely dominated this area either in Tsuji-Trost reactions [Pd(0)] or in an electrophilic activation process [Pd(II)]. More recently, gold-catalyzed formation of saturated N- and O-heterocycles has received growing attention because it generally exhibits high efficiency and diastereoselectivity. Despite their demonstrated utility, Pd- and Au-complexes suffer from high costs, toxicity, and limited natural abundance, which can be barriers to their widespread use in industrial processes. Thus, the replacement of precious metals with less expensive and more environmentally benign catalysts has become a challenging issue for organic chemists. In 2010, our group took advantage of the ability of the low-toxicity and inexpensive FeCl3 in activating allylic or benzylic alcohols to develop iron-catalyzed N- and O-heterocylizations. We first focused on N-heterocycles, and a variety of 2,6-disubstituted piperidines as well as pyrrolidines were synthesized in a highly diastereoselective fashion in favor of the cis-compounds. The reaction was further extended to the construction of substituted tetrahydropyrans. Besides triggering the formation of heterocycles, the iron salts were shown to induce a thermodynamic epimerization, which is the key to reach the high diastereoselectivities observed in favor of the most stable cis-isomers. It is worth noting that spiroketals could be prepared by using this method, which was successfully applied to a synthetic approach toward natural products belonging to the bistramide family. We then turned our attention to heterocycles incorporating two heteroatoms such as isoxazolidines. These frameworks can be found in biologically active natural products, and in addition, they can be transformed into 1,3-amino alcohols, which are of importance in organic chemistry. The use of FeCl3·6H2O allowed the access to a large variety of 3,5-disubstituted isoxazolidines from δ-hydroxylamino allylic alcohol derivatives with good yields and diastereoselectivities in favor of the cis-isomer. Recently, a Lewis acid-catalyzed synthesis of six- and five-membered ring carbonates starting from linear tert-butyl carbonates was reported. In some cases, the mild and chemoselective InCl3 was preferred over FeCl3·6H2O to avoid side-product formation. The resulting cyclic carbonates were easily transformed into 1,3- or 1,2-diols, and a total synthesis of (3S,5S)-alpinikatin was achieved.

Palladium-catalyzed glycosylation: novel synthetic approach to diverse N-heterocyclic glycosides

An efficient and highly stereoselective method for the construction of N-heterocyclic glycosides is reported. This method is based on a palladium-catalyzed allylation which proceeded to provide N-heterocyclic glycosyl compounds in good-to-excellent yields with β- or α-selectivity. Various N-nucleophiles were examined for this reaction and selected N-glycosyl isatin substrates were further elaborated to bis-indole sugars which have potential as antiproliferative drugs.

Direct use of allylic alcohols for palladium-catalyzed synthesis of 3-allylbenzo[b]thiophenes, benzofurans and indoles in aqueous media

Palladium-catalyzed allylative cyclization of 1, 2 and 3 using simple allylic alcohols in aqueous media afforded 3-allylbenzo[b]thiophenes 4, 3-allylindoles 5 and 3-allylbenzofurans 6 in good yields.

A palladium NNC-pincer complex: an efficient catalyst for allylic arylation at parts per billion levels

Allylic arylation of allylic acetates by sodium tetraarylborates in the presence of ppb to ppm (molar) loadings of a palladium NNC-pincer complex catalyst in methanol at 50 °C gave the corresponding arylated products in excellent yields.

3-Aza π-allyl palladium derived from imino migration in palladium-carbene: MCRs toward multiple substituted indole skeleton

Palladium-catalyzed multi-component reactions (MCRs) between 2-iodoaniline, aryl isonitrile, N-tosylhydrazones and solvent were developed.

Transition metal-catalyzed allylic substitution reactions with unactivated allylic substrates

This review highlights recent developments in the area of transition metal-catalyzed allylic substitution reactions with unactivated allylic substrates.

Mild rhodium(III)-catalyzed C-H allylation with 4-vinyl-1,3-dioxolan-2-ones: direct and stereoselective synthesis of (E)-allylic alcohols

A rhodium(III)-catalyzed C-H direct allylation reaction with 4-vinyl-1,3-dioxolan-2-ones has been developed. The reaction provides a facile and stereoselective access to substituted-(E)-allylic alcohols under mild and redox-neutral reaction conditions. Olefinic C-H activation is applicable, giving multifunctionalized skipped dienes in good yields. Minimal double-bond migration was observed.

Novel palladium-catalyzed cascade carboxylative annulation to construct functionalized γ-lactones in ionic liquids

A novel palladium-catalyzed, one-pot, four-step cascade method has been developed to afford functionalized γ-lactones in moderate to good yields. This novel and general methodology represents a rare instance of carbonylation of the C(sp(3))-palladium bond.

Progress towards bioorthogonal catalysis with organometallic compounds

The catalysis of bioorthogonal transformations inside living organisms is a formidable challenge--yet bears great potential for future applications in chemical biology and medicinal chemistry. We herein disclose highly active organometallic ruthenium complexes for bioorthogonal catalysis under biologically relevant conditions and inside living cells. The catalysts uncage allyl carbamate protected amines with unprecedented high turnover numbers of up to 270 cycles in the presence of water, air, and millimolar concentrations of thiols. By live-cell imaging of HeLa cells and with the aid of a caged fluorescent probe we could reveal a rapid development of intense fluorescence within the cellular cytoplasm and therefore support the proposed bioorthogonality of the catalysts. In addition, to illustrate the manifold applications of bioorthogonal catalysis, we developed a method for catalytic in-cell activation of a caged anticancer drug, which efficiently induced apoptosis in HeLa cells.

Activation of alcohols with carbon dioxide: intermolecular allylation of weakly acidic pronucleophiles

The direct coupling of allyl alcohols with nitroalkanes, nitriles, and aldehydes using catalytic Pd(PPh3)4 has been accomplished via activation of C-OH bonds with CO2. The in situ formation of carbonates from alcohols and CO2 facilitates oxidative addition to Pd to form reactive π-allylpalladium intermediates. In addition, the formation of a strong base activates nucleophiles toward the reaction with the π-allylpalladium electrophile. Overall, this atom economical reaction provides a new C-C bond without the use of an external base and generates water as the only byproduct.

18F-labelled intermediates for radiosynthesis by modular build-up reactions: newer developments

This brief review gives an overview of newer developments in (18)F-chemistry with the focus on small (18)F-labelled molecules as intermediates for modular build-up syntheses. The short half-life (<2 h) of the radionuclide requires efficient syntheses of these intermediates considering that multistep syntheses are often time consuming and characterized by a loss of yield in each reaction step. Recent examples of improved synthesis of (18)F-labelled intermediates show new possibilities for no-carrier-added ring-fluorinated arenes, novel intermediates for tri[(18)F]fluoromethylation reactions, and (18)F-fluorovinylation methods.

A diastereoselective one-pot, three-step cascade toward α-substituted allylboronic esters

A new highly diastereoselective synthesis of chiral α-substituted allylboronic esters, based on a one-pot, three-step cascade, is presented. The palladium- and acid-cocatalyzed reaction cascade involves a desilylation of a TBS-protected allylic alcohol, borylation, and addition of an allyl group to an aldehyde. Herein we present the first application of a TBS-protected allylic alcohol in a palladium-catalyzed borylation/allylation reaction.

Pd-Catalyzed stereospecific allyl-aryl coupling of allylic alcohols with arylboronic acids

An efficient method for Pd-catalyzed stereospecific allyl-aryl coupling of allylic alcohols with arylboronic acids has been described. The reactions proceeded smoothly in the presence of Pd2(dba)3·CHCl3 and racemic BINAP under mild and neutral conditions, affording allyl-aryl coupling products in moderate to high yields with excellent stereospecificities.

Hydrogen-bond-assisted activation of allylic alcohols for palladium-catalyzed coupling reactions

We report direct activation of allylic alcohols using a hydrogen-bond-assisted palladium catalyst and use this for alkylation and amination reactions. The novel catalyst comprises a palladium complex based on a functionalized monodentate phosphoramidite ligand in combination with urea additives and affords linear alkylated and aminated allylic products selectively. Detailed kinetic analysis show that oxidative addition of the allyl alcohol is the rate-determining step, which is facilitated by hydrogen bonds between the alcohol, the ligand functional group, and the additional urea additive.