SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Olefin metathesis

31

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

31

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

A novel chelated ruthenium-based metathesis catalyst bearing an N-2,6-diisopropylphenyl group is reported and displays near-perfect selectivity for the Z-olefin (>95%), as well as unparalleled TONs of up to 7400, in a variety of homodimerization and industrially relevant metathesis reactions. This derivative and other new catalytically-active species were synthesized using an improved method employing sodium carboxylates to induce the salt metathesis and C-H activation of these chelated complexes. All of these new ruthenium-based catalysts are highly Z-selective in the homodimerization of terminal olefins.

Concepts: Chemical reaction, Catalysis, Hydrogenation, Chemical equilibrium, Sodium chloride, Olefin metathesis, Richard R. Schrock, Grubbs' catalyst

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

AB diblock copolymers were prepared by sequential ring-opening metathesis polymerization of cyclooctenes catalyzed by a Ru-based Grubbs catalyst. The relatively slow polymerization of cis-3-phenylcyclooct-1-ene (3PC) or cis-cyclooct-2-en-1-yl acetate (3AC) was first carried out and then followed by the faster polymerization of unsubstituted cis-cyclooctene (COE) from the active Ru-alkylidene chain ends. In contrast, simultaneous polymerization of the two monomers provides copolymers with a statistical monomer distribution owing to extensive chain transfer. The resulting poly(3PC-b-COE) and poly(3AC-b-COE) diblock copolymers were subjected to hydrogenation to selectively saturate the backbone alkenes. The consequences of architectural variance between the materials from simultaneous vs. sequential polymerizations are reflected by the contrasting thermal characteristics.

Concepts: Hydrogen, Catalysis, Polymer, Copolymer, Polymer chemistry, Olefin metathesis, Monomer, Polymerization

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