X-H Bond Insertion Promoted by Heterogeneous Dirhodium Metal-Organic Cage with Alkynes as Safe Carbene Precursors.

A facile and efficient methodology for the generation of the C-X (X = Si, B) bond through a carbene insertion process was demonstrated using a dirhodium metal-organic cage, MOC-Rh-1, as a heterogeneous catalyst. A series of functionalized alkynes were utilized as safe carbene precursors to furnish Si-H and B-H insertion products in moderate to excellent yields. These reactions featured a high atom-economy, a broad substrate scope, and mild reaction conditions. Moreover, the as-prepared MOC-Rh-1 catalyst was recovered easily from the reaction system by simple centrifugation and reused for ten runs without a significant loss in activity, which made good use of the valuable precious metal rhodium.

Formal Allylation and Enantioselective Cyclopropanation of Donor/Acceptor Rhodium(II) Azavinyl Carbenes.

A highly efficient formal allylation of dihydronaphthotriazoles with alkenes under rhodium(II) catalysis is reported. Various allyl dihydronaphthalene derivatives were furnished via rhodium(II) azavinyl carbenes with moderate to good yields and excellent chemoselectivity. When monosubstituted alkenes are used, cyclopropanation occurs and good to excellent enantioselectivities have been achieved. Particularly noteworthy is the allylic C(sp2)-H activation instead of traditional C(sp3)-H activation in the formal allylation process.

Recent progress on donor and donor-donor carbenes.

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Construction of polycyclic bridged indene derivatives by a tandem 1,3-rearrangement/intramolecular Friedel-Crafts cyclization of propargyl acetates.

An unprecedented Lewis acid-catalyzed cascade 1,3-rearrangement/Friedel-Crafts cyclization of propargyl acetates is developed for the construction of polycyclic bridged indene derivatives in moderate to good yields. This practical procedure features mild conditions, broad substrate scope, and easy operation.

Rapid Access to Oxa-Bridged Bicyclic Skeletons through Gold-Catalyzed Tandem Rearrangement Reaction.

A gold (I)-catalyzed reaction of enyne-ethers to rapidly construct oxa-bridged compounds via a tandem 1,2-acyloxy migration/intramolecular oxonium formation/1,2-rearrangement process was reported. The reaction was shown to be robust with a wide range of substitution patterns tolerated to provide the corresponding oxygen-containing bridged products in good to excellent yields.

Mechanism-Guided Scaffold Diversification: Perturbing and Trapping the Intermediates of Maltol-Type Cascade Claisen Rearrangement.

A mechanism-guided study to harness different kinds of active species in the cascade Claisen rearrangement for scaffold diversification has been developed. These robust and practical processes furnished a series of architectures with a large chemical space, varying from planar to three-dimensional. In addition, several interesting reactions were observed, such as [3 + 3] dimerization, quinone-based vinylogous Nazarov-type cyclization, and a rare 12e [σ2a + π2s + π2a + π2s + (π2a + π2s)] Mobius aromatic transition state mediated rearrangement.

Donor- and acceptor-enynals/enynones.

Enynals and enynones have emerged as focal substrates in organic synthesis in view of their structure diversity, high reactivity and intermediate variety. Transition metal- or Lewis acid-promoted protocols starting from enynals/enynones could provide efficient and direct access to functionalized homo- or heterocyclic compounds. In consideration of the different electronic properties of substituents on the alkyne group of enynals/enynones, they are classified into two kinds in this review, namely donor-enynals/enynones with non-electron-withdrawing substituents and acceptor-enynals/enynones with electron-withdrawing moieties. Herein we mainly introduced three kinds of transformations based on benzo-fused donor- and acceptor-enynals/enynones, including their reactions with alkenes, alkynes, and H2O, respectively.

Highly Chemo- and Stereoselective Catalyst-Controlled Allylic C-H Insertion and Cyclopropanation Using Donor/Donor Carbenes.

The highly chemo-, enantio-, and diastereoselective catalyst-controlled intramolecular allylic C-H insertion and cyclopropanation of donor/donor carbenes are reported. The RuII /Pybox complex selectively catalyzed the intramolecular allylic C-H insertion, providing vinyl-substituted dihydroindoles with greater than 20:1 chemoselectivity and up to greater than 99 % ee. Chiral dirhodium(II) tetracarboxylates, however, selectively promoted the intramolecular cyclopropanation, giving rise to cyclopropane-fused tetrahydroquinoline derivatives in excellent yields with greater than 99:1 chemoselectivity and up to 97 % ee.

Cascade Claisen Rearrangement: Rapid Synthesis of Polysubstituted Salicylaldehydes and Total Syntheses of Hemigossypol and Gossypol.

A cascade Claisen rearrangement of a well-organized maltol propargyl ether for the construction of polysubstituted salicylaldehydes is reported. This reaction features high atom economy (100 %), as well as catalyst-free and gram-scale conditions. Based on this novel methodology, the total synthesis of hemigossypol, gossypol, and their analogues has been realized.

Catalytic [1,3]†O-to-C Rearrangement: Rapid Access to Bridged Bicyclic Systems.

A catalytic [1,3] O-to-C rearrangement from enyne-ethers was developed for the rapid synthesis of diverse bridged bicyclic systems. In this reaction, a vinyl oxonium intermediate, generated in situ from enyne-ether, was the precursor for the [1,3] O-to-C rearrangement. This versatile protocol represents the first example of catalytic [1,3] O-to-C rearrangement based on ring-expansion strategy, enabling efficient access to bridged bicyclic scaffolds.

Selectivity-switchable construction of benzo-fused polycyclic compounds through a gold-catalyzed reaction of enyne-lactone.

A gold-catalyzed selectivity-switchable reaction of enyne-lactone is reported. Different products, including naphthalenes and benzo-fused polycyclic compounds, can be selectively obtained from the same starting material. The choice of the gold complex is the key for the chemoselectivity of this system.

Carbene insertion into N-H bonds with size-selectivity induced by a microporous ruthenium-porphyrin metal-organic framework.

A stable and porous porphyrinic metal-organic framework, Ru-PMOF-1(Hf), has been prepared through the self-assembly of [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato](monocarbonyl)ruthenium (Ru(TCPP)(CO)) and HfCl4. Single-crystal X-ray diffraction analysis reveals that Ru-PMOF-1(Hf) possesses a three-dimensional (3D) structure with orthogonal 1D open channels of 1.9 × 1.9 nm2. The porous nature has been confirmed by gas adsorption measurements. Its catalytic activity for the carbene insertion into N-H bonds has been investigated. The catalytic results indicate that Ru-PMOF-1(Hf) is efficient for the insertion reactions of ethyl 2-diazoacetate (EDA) into a range of secondary amines with up to 92% yield, 938 TON and 2475 h-1 TOF. As an excellent heterogeneous catalyst, Ru-PMOF-1(Hf) can be recovered and reused for at least ten runs with negligible loss of catalytic activity. Due to its uniform microporosity in three orthogonal directions, Ru-PMOF-1(Hf), of which the particle sizes were carefully controlled by sieving, can induce size selectivity regarding the amine substrates. The reactivities of different unbranched dialkylamines, such as diethylamine (NHEt2), dibutylamine (NHBu2) and dipentylamine (NHPent2), have been compared, and the results display an apparent decreasing trend along the chain lengthening. For comparison, the corresponding nonporous and homogeneous catalyst Ru(TMCPP)(CO) (TMCPP = tetrakis(4-methoxycarbonylphenyl)porphyrin) displays negligible difference towards the reactions with these three amines.

Gold-Catalyzed Ring Expansion of Enyne-Lactone: Generation and Transformation of 2-Oxoninonium.

An efficient gold-catalyzed ring-expansion reaction of enyne-lactones to form 2-oxoninonium intermediates is reported. The 2-oxoninonium generated in this work could undergo further 6π electrocyclization and aromatization reaction to produce different aromatic compounds.

CuCl/Et3N-Catalyzed Synthesis of Indanone-Fused 2-Methylene Pyrrolidines from Enynals and Propargylamines.

An efficient CuCl/Et3N-catalyzed tandem reaction for the synthesis of indanone-fused pyrrolidine was developed. In this process, two rings and four bonds are generated in one pot with high atom-economy and step-efficiency. The addition of Et3N was found as the key factor for the success of the tandem reaction.

Homochiral coordination cages assembled from dinuclear paddlewheel nodes and enantiopure ditopic ligands: syntheses, structures and catalysis.

A series of homochiral metal-organic cages (MOCs) have been obtained from self-assembly of Cu(II) salts with chiral N,N'-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-amino acids. Single-crystal X-ray diffraction analyses reveal that these compounds show a lantern-type cage structure, in which one pair of Cu2(CO2)4 paddlewheels is linked by four diacid ligands. The resulting homochiral cages have been fully characterized by EA, TOF-MS, TGA, VTPXRD, IR, UV, and CD measurements. The catalytic tests reveal that these Cu(II) cages are effective in cyclopropanation with excellent diastereoselectivity (up to 99 : 1 E/Z). In addition, the cage catalysts can promote the aziridination reaction with PhI=NNs.

Applications of metal-organic frameworks in heterogeneous supramolecular catalysis.

This review summarizes the use of metal-organic frameworks (MOFs) as a versatile supramolecular platform to develop heterogeneous catalysts for a variety of organic reactions, especially for liquid-phase reactions. Following a background introduction about catalytic relevance to various metal-organic materials, crystal engineering of MOFs, characterization and evaluation methods of MOF catalysis, we categorize catalytic MOFs based on the types of active sites, including coordinatively unsaturated metal sites (CUMs), metalloligands, functional organic sites (FOS), as well as metal nanoparticles (MNPs) embedded in the cavities. Throughout the review, we emphasize the incidental or deliberate formation of active sites, the stability, heterogeneity and shape/size selectivity for MOF catalysis. Finally, we briefly introduce their relevance into photo- and biomimetic catalysis, and compare MOFs with other typical porous solids such as zeolites and mesoporous silica with regard to their different attributes, and provide our view on future trends and developments in MOF-based catalysis.

Two ligand-functionalized Pb(II) metal-organic frameworks: structures and catalytic performances.

A microporous Pb(II) metal-organic framework (MOF) [PbL(2)]·2DMF·6H(2)O (1) has been assembled from a N-oxide and amide doubly functionalized ligand HL (= N-(4-carboxyphenyl)isonicotinamide 1-oxide). Complex 1 features a three-dimensional (3D) framework possessing one-dimensional (1D) rhombic channels with dimensions of 13 × 13 Å(2). The 3D framework is built up from 1D PbO(2) chains that link ligands in parallel fashion to construct single-wall channels. When recrystallizing 1 in a DMSO-DMF mixture (3 : 5 v/v), a new coordination polymer, [PbL(2)]·DMF·2H(2)O (2), was obtained. Complex 2 is also a 3D framework containing 1D rectangular channels, but the channel dimensions become reduced in size to 13 × 8 Å(2) due to reorganization of the Pb(ii) coordination environment. The PbO(2) chains in 2 are reformed to link ligands in a double-wall fashion, significantly reducing the channel size. Even though, the guest exchange study indicates that the DMF molecules in 2 could be replaced with benzene molecules when immersing in benzene solvent, showing single-crystal-to-single-crystal (SC-SC) guest exchange in the solid state and leading to a daughter crystal [PbL(2)]·0.5C(6)H(6)·2H(2)O (2'). Desolvated 1 and 2 display preferential adsorption behaviors of water vapour over CO(2) due to the hydrophilic nature of the channels and the strong host-guest interactions. Catalytic tests indicate that desolvated 1 and 2 have size-selective catalytic activity towards the Knoevenagel condensation reaction.