Showing papers in "Organic chemistry frontiers in 2017"

Recent advances in positional-selective alkenylations: removable guidance for twofold C–H activation

Abstract: The transition metal-catalyzed transformation of otherwise inert C–H bonds into substituted alkenes offers a versatile tool for the synthesis of value added olefinic molecules. Recent developments in the directing group assisted C–H activation approach ensured high levels of positional selectivity. A vast number of coordinating groups have been utilized in directed C–H alkenylation, which are often not removable after the desired transformation. However, the concept of easily removable or traceless directing group strategy overcomes this limitation and enables site-selective C–H alkenylation of relevance to academia and the practitioners in industry. Various easily removable or transformable directing groups utilized in the transition metal-catalyzed oxidative C–H alkenylations are discussed in this review until February 2017.

Lu's [3 + 2] cycloaddition of allenes with electrophiles: discovery, development and synthetic application

Abstract: Lu's [3 + 2] cycloaddition reaction has experienced explosive development due to its versatile and powerful ability to access highly functionalized carbo- and heterocycles. After the first example reported by Prof. Xiyan Lu's group, many research groups expanded its substrate scope, and developed its asymmetric variants, and demonstrated its synthetic applications as well. This review briefly introduces the history of Lu's [3 + 2] cycloaddition reaction and highlights the important developments and synthetic applications with respect to Lu's [3 + 2] cycloaddition reaction.

Catalytic C–H amination at its limits: challenges and solutions

Abstract: Catalytic C–H amination reactions enable direct functionalization of non-activated C(sp3)–H bonds with high levels of regio-, chemo- and stereoselectivity. As a powerful tool to unlock the potential of inert C–H bonds, C–H amination chemistry has been applied to the preparation of synthetically challenging targets since major simplification of synthetic sequences are expected from this approach. Pushing C–H amination to its limits has led to a deeper understanding of the reaction mechanism and scope. In this review, we present a description of the specific challenges facing catalytic C–H amination in the synthesis of natural products and related compounds, as well as innovative tactics created to overcome them. By identifying and discussing the major insights gained and strategies designed, we hope that this review will stimulate further progress in C–H amination chemistry and beyond.

Methodology and applications of the hexadehydro-Diels–Alder (HDDA) reaction

Abstract: The hexadehydro-Diels–Alder (HDDA) reaction between an alkyne and a 1,3-diyne has recently become a rapidly growing area in the field of aryne chemistry. Both concerted and stepwise mechanisms for HDDA are energetically and geometrically feasible. The formation of a reactive benzyne intermediate under thermal conditions has been coupled with a wide variety of intra- and intermolecular trapping reactions to access highly functionalised aromatic compounds. With this in mind, reagents can be tailored to generate compounds for desired applications such as the synthesis of bioactive molecules or optoelectronic materials. This review presents a comprehensive overview of the HDDA reaction to date.

Synthesis of β-keto sulfones via a multicomponent reaction through sulfonylation and decarboxylation

Abstract: A copper(I)-catalyzed synthesis of β-keto sulfones through a multicomponent reaction of aryldiazonium tetrafluoroborates, 3-arylpropiolic acids, sulfur dioxide, and water was developed. This reaction proceeds through a tandem radical process, and the sulfonyl radical, generated from the combination of aryldiazonium tetrafluoroborates with DABCO·(SO2)2, acts as the key intermediate. The transformation involves sulfonylation and decarboxylation, which allows for the efficient synthesis of the desired β-keto sulfones.

Cobalt(III)-catalyzed cross-coupling of enamides with allyl acetates/maleimides

Abstract: Cp*Co(III)-catalyzed direct allylation of enamides has been accomplished with the exclusive formation of allylated Z-enamides with high efficiency. In addition, the employment of maleimides as the reaction partner under the same catalytic conditions provides a series of succinimide-substituted Z-enamides.

Synthesis of 3-((arylsulfonyl)methyl)indolin-2-ones via insertion of sulfur dioxide using anilines as the aryl source

Abstract: Facile assembly of 3-((arylsulfonyl)methyl)indolin-2-ones via insertion of sulfur dioxide starting from anilines, N-arylacrylamides and DABCO·(SO2)2 is developed. This one-pot reaction proceeds efficiently in dichloroethane, leading to sulfonated oxindoles in good yields. Anilines serve as the aryl source, and the in situ generated sulfonyl radicals from sulfur dioxide as the key intermediates are trapped by N-arylacrylamides to provide the corresponding products. Additionally, a broad reaction scope is observed under mild conditions.

A benzothiadiazole-based fluorescent sensor for selective detection of oxalyl chloride and phosgene

Abstract: Oxalyl chloride and phosgene are highly toxic chemicals but are widely used in industrial processes, which have been regarded as a serious threat to our health and public security. Thus, it is imperative to develop a convenient and reliable detection method for these noxious agents. In this research, we report the rational design of a fluorescent sensor (denoted as BTA) for simultaneously detecting oxalyl chloride and phosgene, in which o-phenylenediamine (OPD) as the reactive recognition site and benzothiadiazole (BTD) as the fluorophore moiety are coupled into one single benzo-fused motif. The probe works in a “turn-on” fluorescence mode due to the formation of piperazine-2,3-dione and 2-imidazolidinone rings by intramolecular cyclization reactions between the OPD part with oxalyl chloride and phosgene, respectively, which greatly inhibits the intramolecular charge transfer (ICT) process from two amines to the BTD core in the sensor BTA. The detection limit of oxalyl chloride and phosgene is as low as 3 nM and 20 nM in solutions, respectively. Moreover, the sensor exhibits outstanding selectivity toward oxalyl chloride and phosgene over the nerve-agent mimic diethyl chlorophosphate and various acyl chlorides. Furthermore, a portable test paper with BTA has been facilely fabricated for visual on-site detection of the two toxic chemicals in the gas phase.

Metal-mediated C–O bond forming reactions in natural product synthesis

Abstract: Metal catalyzed reactions for the formation of C–O bonds have had a dramatic impact in natural product synthesis over the past few decades. Various metals have been reported to efficiently catalyze cross-coupling reactions for the formation of various C(sp2)–O bonds from aryl/alkenyl halides or synthetic equivalents and phenols, aliphatic alcohols and water. The implementation of such reactions in natural product synthesis enabled the emergence of new bond disconnections, which notably resulted in remarkably efficient and short synthetic pathways. The use of these reactions for the formation of C–O bonds in natural product synthesis is overviewed in this critical review, with an emphasis on copper and palladium catalysts which are the most efficient ones to date.

Metal-catalyzed enyne cycloisomerization in natural product total synthesis

Abstract: Since the seminal work of B. M. Trost et al. in 1985, metal catalyzed enyne cycloisomerization has become a fast growing and fascinating field. Due to its diversified chemistry and ability to increase molecular complexity in an efficient, atom economical, step economic and redox economic way, enyne cycloisomerization has become a powerful and attractive strategy for the construction of cyclic compounds, and thus has great potential for applications in total synthesis of natural products and pharmaceuticals. In this review, a brief summary of the state-of-the-art mechanism studies, the classification of structures accessed through enyne cycloisomerizations, and recent completed total synthesis of representative natural products showcasing creative and ingenious incorporation of enyne cycloisomerization as a strategic manoeuver are included, with the aim of providing a complement to the existing reviews to inspire future developments.

Reductive insertion of sulfur dioxide for the synthesis of trifluoromethyl thiolsulphonates through a one-pot reaction of aniline and trifluoromethanesulfanylamide

Abstract: A bismuth(III) chloride-mediated one-pot reaction of anilines, sulfur dioxide, and trifluoromethanesulfanylamide is reported herein, leading to antifungal trifluoromethyl thiolsulphonates. This transformation employs N-aminomorphine as an additive and provides the final products in good yields with a broad functional group tolerance. It is believed that bismuth(III) chloride facilitates the formation of “CF3S+”, and N-aminomorphine plays a critical role in providing electrons to catalyse the cross-coupling of in situ generated aryldiazonium, sulfur dioxide and trifluoromethanesulfanylamide.

Asymmetric synthesis of trifluoromethyl-substituted 3,3′-pyrrolidinyl-dispirooxindoles through organocatalytic 1,3-dipolar cycloaddition reactions

Abstract: Unprecedented asymmetric exo′-selective [3 + 2] cycloaddition reactions of CF3-containing isatin-derived azomethine ylides with methyleneindolinones have been disclosed. Under bifunctional organocatalysis by cinchona-derived squaramide catalysts, a series of potential biologically important trifluoromethylated 3,3′-pyrrolidinyl-dispirooxindoles were efficiently constructed in a highly stereoselective manner (84%–99% yields, up to >20 : 1 dr and >99% ee). The reaction leads to the formation of four contiguous stereogenic centers, including two adjacent spiro quaternary stereocenters.

Metal-free molecular iodine-catalyzed direct sulfonylation of pyrazolones with sodium sulfinates leading to sulfonated pyrazoles at room temperature

Abstract: A simple and convenient molecular iodine-catalyzed direct sulfonylation of pyrazolones with sodium sulfinates has been developed in the presence of TBHP at room temperature. This methodology can allow for the synthesis of a series of valuable sulfonated pyrazoles in good to excellent yields simply using readily-available starting materials without requiring any metal or cryogenics.

Transition-metal-free electrophilic trifluoromethylthiolation with sodium trifluoromethanesulfinate at room temperature

Abstract: A transition-metal-free direct trifluoromethylthiolation of indole derivatives under mild conditions has been developed. Commercially available sodium trifluoromethanesulfinate (CF3SO2Na) acts as the trifluoromethylthiolating reagent, which is cheap, stable and easy to handle. The substrate scope of this protocol could also be expanded to pyrroles and enamines.

Transition-metal-free oxidative reaction of hydrazines and potassium metabisulfite for preparation of sulfonohydrazides

Abstract: A transition-metal-free oxidative coupling reaction for the synthesis of sulfonohydrazides from two types of hydrazines and potassium metabisulfite under air has been developed. This highly selective reaction used one arylhydrazine as an aryl coupling partner and potassium metabisulfite as a sulfur dioxide precursor, which provides a new and green strategy to sulfonohydrazides.

Fe-Catalyzed oxidative spirocyclization of N-arylpropiolamides with silanes and TBHP involving the formation of C–Si bonds

Abstract: A new iron-catalyzed oxidative spirocyclization of N-arylpropiolamides with silanes and TBHP (tert-butyl hydroperoxide) for the synthesis of 3-silyl spiro[4,5]trienones is presented. The reaction proceeds via a sequence of radical addition, spirocyclization and dearomatization, and offers a practical and straightforward route to introduce silicon-centered radicals into the C–C triple bonds.

Palladium catalyzed C(sp3)–H acetoxylation of aliphatic primary amines to γ-amino alcohol derivatives

Abstract: It still remains a major challenge to apply free primary amino groups as the directing group for aliphatic C–H functionalization. In this article, we used the protonation strategy to control the binding ability of primary amines and realized free amino group directed inert aliphatic C–H acetoxylation in good chemo- and regio-selectivity. This methodology provided a straightforward approach from primary amines to γ-amino alcohols.

Aroyl chlorides as novel acyl radical precursors via visible-light photoredox catalysis

Abstract: Abundant and inexpensive aroyl chlorides have been employed for the first time as novel acyl radical precursors in visible-light photocatalysis. This radical cascade reaction building new C–C bonds is an effective way to synthesize valuable heterocyclic compounds. This approach mainly features a concise redox-neutral mechanism, radical cascade reaction, mild conditions and convenient operations.

A novel functionalized pillar[5]arene-based selective amino acid sensor for L-tryptophan

Abstract: A novel L-tryptophan (L-Trp) fluorescent sensor (BTAP5) based on a functionalized pillar[5]arene has been developed The sensor BTAP5 exhibits high selectivity and sensitivity (283 × 10−7 M) towards L-Trp in H2O/DMSO (7 : 3, v/v) solution The recognition mechanism was studied by 1H NMR, electrospray ionization mass spectrometry and 2D NOESY

Generation of benzosultams via a radical process with the insertion of sulfur dioxide

Abstract: An efficient route to diverse 3-sulfonated-2,3-dihydrobenzo[d]isothiazole 1,1-dioxides is achieved through a three-component reaction of 2-ethynylbenzenesulfonamides, DABCO-bis(sulfur dioxide), and aryldiazonium tetrafluoroborates. The corresponding sulfonated benzosultams are produced in moderate to good yields. During the reaction process, the in situ generated arylsulfonyl radical via addition of an aryl radical to sulfur dioxide and the subsequent single electron transfer would be the key steps for the final outcome. DABCO acts as the carrier for single electron transfer, as well as a base to promote the C–N bond formation.

Visible light-induced tandem oxidative cyclization of 2-alkynylanilines with disulfides (diselenides) to 3-sulfenyl- and 3-selenylindoles under transition metal-free and photocatalyst-free conditions

Abstract: A simple and efficient strategy for the synthesis of 3-sulfenyl and 3-selenyl indoles via a visible light-induced tandem cyclization of 2-alkynylanilines with disulfides (diselenides) was developed. The reaction generated the corresponding products in good yields at room temperature under transition metal-free and photocatalyst-free conditions.

Pincer cobalt complex-catalyzed Z-selective hydrosilylation of terminal alkynes

Abstract: A phosphine-iminopyridine (PCNN) cobalt-catalyzed Z-selective hydrosilylation of terminal alkynes with Ph2SiH2 has been developed for the synthesis of (Z)-β-vinylsilanes with high regio- and stereoselectivity and wide functional group tolerance. Furthermore, the Co-catalyzed hydrosilylations of unsymmetrical arylalkyl disubstituted internal alkynes afford syn-addition products with unique regioselectivity: the silyl group is added to the alkyl-substituted carbon, instead of the aryl-substituted carbon. The (Z)-β-vinylsilane products are further applied to Pd-catalyzed Hiyama–Denmark cross-couplings for stereoselective synthesis of (Z)-disubstituted alkenes.

Direct thiocyanation of ketene dithioacetals under transition-metal-free conditions

Abstract: The first direct thiocyanation of ketene dithioacetals has been accomplished in the presence of N-chlorosuccinimide (NCS) and NH4SCN under transition-metal-free conditions. The operationally simple one-pot procedure, which is insensitive to air, was found to be well-tolerated by a wide range of substrates. Terminal alkenes and hydrazine were identified to be qualified candidates for the process as well. Furthermore, the product could be successfully transformed into synthetically and biologically interesting –SCF3 substituted N-heteroaromatic compounds and thiotetrazole containing compounds.

Site-selective Cp*Rh(III)-catalyzed C–H amination of indolines with anthranils

Abstract: The pyrimidinyl-directed C–H functionalization of indolines with anthranils as amination sources under rhodium(III) catalysis is described. This transformation efficiently provides a range of C7-aminated indoline derivatives with excellent site-selectivity and functional group compatibility. The resulting framework containing amino and carbonyl groups provides facile access to an important synthetic building block, which can be readily converted to biologically interesting heterocycles.

Catalytic asymmetric synthesis of hetero-substituted oxindoles

Abstract: Chiral disubstituted oxindoles represent an important family of bioactive molecules. The construction of asymmetric quaternary carbon in the cyclopentane ring is therefore a challenging problem for organic chemists. In the last few years, many papers have appeared on asymmetric catalytic methods for the synthesis of these compounds and the literature on spirocyclic derivatives, 3-substituted-3-hydroxyoxindoles and 3-substituted-3-aminooxindoles has been largely reviewed. The literature on other hetero-substituted oxindoles has not been fully reviewed after 2011. This review aims to fill this gap by covering the literature on these topics until the end of 2016. A special section is also devoted to indoline-thiones.

Catalytic and catalyst-free diboration of alkynes

Abstract: The diboration of alkynes has become the most straightforward and powerful method for the synthesis of functionalized 1,2-diborylalkenes, which are widely used as building blocks in organic synthesis. The present review provides a comprehensive summary of the diboration of alkynes, including platinum-, palladium-, cobalt-, copper-, iridium-, gold-, iron-, base- and small organic molecule-catalyzed diboration, direct and metal-free diboration, and base-promoted diboration of alkynes up to early 2017. The reaction conditions, regio- and stereoselectivities, and mechanisms are summarized in detail. Moreover, synthetic applications and perspectives on the diboration of alkynes are also discussed.

Cobalt-catalyzed C–H activation and regioselective intermolecular annulation with allenes

Abstract: The cyclization reaction of allene has attracted considerable attention in recent years because of the unique reactivity and interesting modes of action in the process. However, rare examples have been reported on the annulation with allene via a C–H activation strategy. Here, we report a novel and efficient intermolecular annulation of N-(quinolin-8-yl)benzamide with allenes via cobalt-catalyzed C–H activation. This method provides a shortcut for the preparation of novel isoquinolin-1(2H)-one scaffolds, and both endo-cyclic and exo-cyclic isoquinolinones were respectively prepared in the presence of different bases. In addition, this C–H functionalization proceeds under mild conditions and tolerates a wide range of benzamide substrates, and reactions with terminal as well as internal allenes afforded the corresponding products in up to 88% yield. Furthermore, a new annulation pathway with allene is also presented.

Transition-metal-free direct perfluoroalkylation of quinoline amides at C5 position through radical cross-coupling under mild conditions

Abstract: A simple and efficient method is established for the synthesis of perfluoroalkylated quinolines in the absence of any transition metal catalysts. The eco-friendly reaction reveals lower energy and time consumption. Desired products are isolated in good to excellent yields. A number of control experiments are carried out to illustrate that a radical cross-coupling process plays a vital role in the perfluoroalkylation.

Merging photoredox catalysis with transition metal catalysis: site-selective C4 or C5-H phosphonation of 8-aminoquinoline amides

Abstract: A simple and efficient protocol for the transition metal-catalyzed site-selective C–H phosphonation of 8-aminoquinolines with diarylphosphine oxide or H-phosphonate diesters was developed via a photoredox process. This reaction features high regioselectivity at the C4 or C5 position under mild and simple conditions (with a catalytic amount of silver salt at room temperature). Note that the C4 position of 8-aminoquinoline is an unusual reactive site.

Copper-catalyzed C5-selective thio/selenocyanation of 8-aminoquinolines

Abstract: Copper-catalyzed direct C5-position thio/selenocyanation of quinolines using commercially available, inexpensive KSCN/SeCN as the thio/selenocyanation reagent was developed, which had good tolerance toward various aliphatic or aromatic 8-aminoquinoline derivatives Importantly, the synthetic application of this protocol led to some useful compounds