SciCombinator

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Concept: Olefin metathesis

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Transition metal-catalyzed transfers of carbenes, nitrenes and oxenes are powerful methods for functionalizing C=C and C-H bonds. Nature has evolved a diverse toolbox for oxene transfers, as exemplified by the myriad monooxygenation reactions catalyzed by cytochrome P450 enzymes. The isoelectronic carbene transfer to olefins, a widely used C-C bond forming reaction in organic synthesis, has no biological counterpart. Here, we report engineered variants of cytochrome P450(BM3) that catalyze highly diastereo- and enantioselective cyclopropanation of styrenes from diazoester reagents via putative carbene transfer. This work highlights the capacity to adapt existing enzymes for catalysis of synthetically important reactions not previously observed in Nature.

Concepts: Metabolism, Enzyme, Chemical reaction, Catalysis, Hydrogenation, Ribozyme, Cytochrome P450, Olefin metathesis

30

Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino)pyridine ligands, show promise for industrial application.

Concepts: Oxygen, Chemical reaction, Hydrogen, Catalysis, Hydrogenation, Catalytic converter, Olefin metathesis, Alkene

28

We have observed by NMR spectroscopy that the diffusive movement of a ruthenium-based Grubbs' catalyst increases during ring-closing metathesis as a function of the substrate concentration. This is one of the smallest single molecule motors to exhibit catalytically driven motion.

Concepts: DNA, Enzyme, Chemistry, Olefin metathesis, Molecular motor, Motor, Grubbs' catalyst, Robert H. Grubbs

28

A formal total synthesis of the spiroketal containing cytotoxic myxobacteria metabolite spirangien A (1) is described. The approach utilizes a late introduction of the C20 alcohol that mirrors the biosynthesis of this compound. The key steps involved a high yielding cross metathesis reaction between enone 6 and alkene 7 to give E-enone 4 and a Mn-catalyzed conjugate reduction α-oxidation reaction to introduce the C20 hydroxyl group. Acid treatment of the α-hydroxyketone 4 gave spiroketal 19 which was converted into known spirangien A (1) advanced intermediate spiroketal 3.

Concepts: Alcohol, Functional group, Total synthesis, Olefin metathesis, Functional groups, Richard R. Schrock, Hydroxyl, Alkane metathesis

28

Olefin metathesis catalysts for controlling the formation of trisubstituted macrocyclic Z alkenes have been developed. The most effective complexes are Mo alkylidenes with a pentafluorophenylimido group and two large aryloxide ligands. The macrocyclic lactone precursor to anticancer agents epothilones B and D is obtained in 73 % yield and 91 % Z selectivity in less than 6 hours at room temperature.

Concepts: Amine, Chemotherapy, Hydrogenation, Olefin metathesis, Alkene, Blood transfusion, Triphenylphosphine, Grubbs' catalyst

28

A series of anthracene and acridine derivatives were hydrogenated under mild reaction conditions (80 °C, 3 bar of H(2)) using the bis(dihydrogen) complex [RuH(2)(η(2)-H(2))(2){P(C(6)H(11))(3)}(2)] (1) as a catalyst precursor. The influence of a methyl substituent on the substrate was studied. In all our systems, hydrogenation was only observed at the external rings leading to the corresponding 4H- or 8H-derivatives of anthracene and acridine. Three complexes resulting from the η(4)(C,C)-coordination of the substrate to the unsaturated fragment [RuH(2){P(C(6)H(11))(3)}(2)] were characterized. In the case of 9-methyl acridine, the corresponding complex [RuH(2)(η(4)-C(14)H(11)N){P(C(6)H(11))(3)}(2)] (4) turned out to be an active catalyst precursor leading to 1,2,3,4,5,6,7,8-octahydro-9-methylacridine as the sole product after 24 h. Regeneration of 1 from 4 supports the role of complex 4 in the catalytic cycle. Three hydrogenated products, 1,2,3,4-tetrahydroanthracene (4H-Anth), 1,2,3,4-tetrahydro-9-methylanthracene (4H-9-Me-Anth) and 1,2,3,4-tetrahydroacridine (4H-Acr), were characterized by X-ray diffraction.

Concepts: Enzyme, X-ray, Chemical reaction, Hydrogen, Catalysis, Hydrogenation, Ribozyme, Olefin metathesis

28

An efficient total synthesis of (-)-epothilone B has been achieved in ca. 8% yield over 11 steps from 9 (or 10 steps from 7/8), which features a bissiloxane-tethered ring closing metathesis reaction to approach the trisubstituted (Z) double bond and forms a new basis for further development of an industrial process for epothilone B and ixabepilone.

Concepts: Total synthesis, Paclitaxel total synthesis, Olefin metathesis, Chemical synthesis, Epothilone, Process management, Ixabepilone, Sorangium cellulosum

28

Three keys to success: A concise method for the construction of a tricyclic substructure (2) of haliclonin A (1) in racemic form is described. This synthesis features a new Pd-mediated chemoselective carbonyl-enone coupling reaction, an organocatalytic reaction, and a ring-closing metathesis reaction for the construction of the macrocyclic ring as key steps.

Concepts: Catalysis, Total synthesis, Olefin metathesis, Key

28

Novel ruthenium(ii) complexes were obtained as a result of a stoichiometric reaction of Grubbs' benzylidene second generation catalysts with 3-nitropropene. These stable complexes, formally ruthenaisoxazole N-oxide derivatives, display activity in both metathesis and non-metathetic processes such as cycloisomerisation, isomerisation and transfer hydrogenation.

Concepts: Chemical reaction, Hydrogen, Catalysis, Hydrogenation, Chemical equilibrium, Olefin metathesis, Alkene, Richard R. Schrock

28

The thorium(iv) tetraiodide complex ThI(4)(DME)(2) () (DME = 1,2-dimethoxyethane) has been prepared in high yield by reacting the corresponding chloride complex ThCl(4)(DME)(2) with an excess of trimethylsilyl iodide (Me(3)SiI) in toluene. This new route avoids the use of thorium metal as a reagent. ThI(4)(DME)(2) () exhibits excellent thermal stability compared to ThI(4)(THF)(4) (), which undergoes rapid ring-opening of THF at ambient temperature to yield the iodobutoxide complex ThI(3)[O(CH(2))(4)I](THF)(3) (). Subsequent ligand-exchange between and DME affords ThI(3)[O(CH(2))(4)I](DME)(2) (), which can be converted to with Me(3)SiI. Salt metathesis between and K(L(Me)) (L(Me) = (2,6-(i)Pr(2)C(6)H(3))NC(Me)CHC(Me)N(2,6-(i)Pr(2)C(6)H(3))) cleanly gives (L(Me))ThI(2)[O(CH(2))(4)I](THF) (), which is a rare example of a thorium β-diketiminate complex. Complexes , , and represent the first reported examples of THF ring-opening mediated by thorium. The synthetic utility of ThI(4)(DME)(2) () is demonstrated by preparation of thorium(iv) alkoxide, amide, and organometallic compounds.

Concepts: Olefin metathesis, Complex, Complexity, Solvent, Chloride, Tetrahydrofuran, Organometallic chemistry, Thorium