Catalytic Z-selective olefin cross-metathesis for natural product synthesis

Practical, efficient, and broadly applicable synthesis of readily differentiable vicinal diboronate compounds by catalytic three-component reactions

A practical, efficient and broadly applicable catalytic method for synthesis of easily differentiable vicinal diboronate compounds is presented. Reactions are promoted by a combination of PCy3 or PPh3, CuCl and LiOt-Bu and may be performed with readily accessible alkenyl boronate substrates. Through the use of an alkenyl-B(pin) (pin = pinacolato) or alkenyl- B(dan) (dan = naphthalene-1,8-diaminato) starting material and commercially available (pin)B- B(dan) or B2(pin)2 as the reagent, a range of vicinal diboronates, including those that contain a B-substituted quaternary carbon center, may be prepared in up to 91% yield and with >98% site selectivity. High enantioselectivities can be obtained (up to 96:4 er) through the use of commercially available chiral bis-phosphine ligands for reactions that afford mixed diboronate products.

Stereoretentive Olefin Metathesis: An Avenue to Kinetic Selectivity

Olefin metathesis is an incredibly valuable transformation that has gained widespread use in both academic and industrial settings. Lately, stereoretentive olefin metathesis has garnered much attention as a method for the selective generation of both E- and Z-olefins. Early studies employing ill-defined catalysts showed evidence for retention of the stereochemistry of the starting olefins at low conversion. However, thermodynamic ratios E/Z were reached as the reaction proceeded to equilibrium. Recent studies in olefin metathesis have focused on the synthesis of catalysts that can overcome the inherent thermodynamic preference of an olefin, providing synthetically useful quantities of a kinetically favored olefin isomer. These reports have led to the development of stereoretentive catalysts that not only generate Z-olefins selectively, but also kinetically produce E-olefins, a previously unmet challenge in olefin metathesis. Advancements in stereoretentive olefin metathesis using tungsten, ruthenium, and molybdenum catalysts are presented.

Synthesis of 2-Alkenyl-Tethered Anilines

Three general routes for the synthesis of (E)-2-alkenyl-tethered anilines have been developed. The first route involves a 3-aza-Cope rearrangement of N-allylic anilines in the presence of a Lewis acid. The requisite N-allylic anilines were prepared by the addition of vinyl-magnesium reagents to the corresponding aldimines. The second route details a direct cross-metathesis of 2-allylic or 2-homoallylic anilines with styrenes. The third route involves a palladium-catalyzed C-N cross-coupling of aryl halides. Taken together, these three strategies allowed access to the requisite aniline substrates with pendant alkenes at the 2-position with excellent trans selectivities.

Olefin Cross-Metathesis in Polymer and Polysaccharide Chemistry: A Review

Olefin cross-metathesis, a ruthenium-catalyzed carbon-carbon double bond transformation that features high selectivity, reactivity, and tolerance of various functional groups, has been extensively applied in organic synthesis and polymer chemistry. Herein, we review strategies for performing selective cross-metathesis and its applications in polymer and polysaccharide chemistry, including constructing complex polymer architectures, attaching pendant groups to polymer backbones and surfaces, and modifying polysaccharide derivatives.

Molybdenum chloride catalysts for Z-selective olefin metathesis reactions

The development of catalyst-controlled stereoselective olefin metathesis processes has been a pivotal recent advance in chemistry. The incorporation of appropriate ligands within complexes based on molybdenum, tungsten and ruthenium has led to reactivity and selectivity levels that were previously inaccessible. Here we show that molybdenum monoaryloxide chloride complexes furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the commercially available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene. Furthermore, otherwise inefficient and non-stereoselective transformations with Z-1,2-dichloroethene and 1,2-dibromoethene can be effected with substantially improved efficiency and Z selectivity. The use of such molybdenum monoaryloxide chloride complexes enables the synthesis of representative biologically active molecules and trifluoromethyl analogues of medicinally relevant compounds. The origins of the activity and selectivity levels observed, which contradict previously proposed principles, are elucidated with the aid of density functional theory calculations.

Synthesis of Z-alkenes via visible light promoted photocatalytic E → Z isomerization under metal-free conditions

The synthesis of thermodynamically less stable Z-alkenes via visible light promoted photocatalytic E-to-Z isomerization has been reported.

Synthesis of vanadium–alkylidene complexes and their use as catalysts for ring opening metathesis polymerization

Synthesis of vanadium–alkylidene complexes and some reactions have been reviewed; highly active, thermally robust, cis specific ROMP of cyclic olefins has been attained by ligand modification in (imido)vanadium(v)–alkylidene catalysts.

Hoveyda–Grubbs catalyst analogues bearing the derivatives of N-phenylpyrrol in the carbene ligand – structure, stability, activity and unique ruthenium–phenyl interactions

N-Phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative are active in ring-closing metathesis at 80 °C, but inactive at room temperature.

Synthesis of Linear (Z)-α,β-Unsaturated Esters by Catalytic Cross-Metathesis. The Influence of Acetonitrile

Kinetically controlled catalytic cross-metathesis reactions that generate (Z)-α,β-unsaturated esters selectively are disclosed. A key finding is that the presence of acetonitrile obviates the need for using excess amounts of a more valuable terminal alkene substrates. On the basis of X-ray structure and spectroscopic investigations a rationale for the positive impact of acetonitrile is provided. Transformations leading to various E,Z-dienoates are highly Z-selective as well. Utility is highlighted by application to stereoselective synthesis of the C1-C12 fragment of biologically active natural product (-)-laulimalide.

Tandem Olefin Metathesis/Oxidative Cyclization: Synthesis of Tetrahydrofuran Diols from Simple Olefins

A tandem olefin metathesis/oxidative cyclization has been developed to synthesize 2,5-disubstituted tetrahydrofuran (THF) diols in a stereocontrolled fashion from simple olefin precursors. The ruthenium metathesis catalyst is converted into an oxidation catalyst in the second step and is thus responsible for both catalytic steps. The stereochemistry of the 1,5-diene intermediate can be controlled through the choice of catalyst and the type of metathesis conducted. This olefin stereochemistry then controls the THF diol stereochemistry through a highly stereospecific oxidative cyclization.

Kinetically controlled E-selective catalytic olefin metathesis

A major shortcoming in olefin metathesis, a chemical process that is central to research in several branches of chemistry, is the lack of efficient methods that kinetically favor E isomers in the product distribution. Here we show that kinetically E-selective cross-metathesis reactions may be designed to generate thermodynamically disfavored alkenyl chlorides and fluorides in high yield and with exceptional stereoselectivity. With 1.0 to 5.0 mole % of a molybdenum-based catalyst, which may be delivered in the form of air- and moisture-stable paraffin pellets, reactions typically proceed to completion within 4 hours at ambient temperature. Many isomerically pure E-alkenyl chlorides, applicable to catalytic cross-coupling transformations and found in biologically active entities, thus become easily and directly accessible. Similarly, E-alkenyl fluorides can be synthesized from simpler compounds or more complex molecules.

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Olefin metathesis has had a large impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is limited. Here we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of a catalyst that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents, and proceed to high conversion at ambient temperature within four hours. We obtain many alkenyl chlorides, bromides and fluorides in up to 91 per cent yield and complete Z selectivity. This method can be used to synthesize biologically active compounds readily and to perform site- and stereoselective fluorination of complex organic molecules.

New indenylidene-type metathesis catalysts bearing unsymmetrical N-heterocyclic ligands with mesityl and nitrobenzyl substituents

ABSTRACT

New indenylidene-type second generation catalysts bearing modified unsymmetrically substituted N-heterocyclic carbene ligands were synthesized. The complexes contain an N-mesityl and N'-nitrobenzyl substituted NHC ligand. The precursors of free carbenes-imidazolinium salts-were obtained in an easy and environment-friendly way (under aqueous or neat conditions). The new catalysts were prepared by reaction of in situ generated carbenes with a 1st generation indenylidene catalyst, containing pyridine ligands instead of tricyclohexylphosphine. The complexes were tested in RCM, CM, and ene-yne metathesis model reactions in commercial-grade solvents in air. Their activities were compared with that of commercially available indenylidene catalyst. The structures of complexes and their stability were investigated using static DFT calculations with mixed basis set.

GRAPHICAL ABSTRACT

Amphiphilic hydroxyalkyl cellulose derivatives for amorphous solid dispersion prepared by olefin cross-metathesis

Olefin CM followed by transfer hydrogenation is an efficient method for synthesizing amphiphilic hydroxypropyl cellulose derivatives.

Aminoquinoline-assisted vinylic C–H arylation of unsubstituted acrylamide for the selective synthesis of Z olefins

A method for Pd-catalyzed, aminoquinoline-directed arylation of vinylic C–H bonds with aryl iodides has been developed. This reaction represents a rare example of Pd-catalyzed vinylic C–H functionalization of unsubstituted acrylamide, allowing for preparation of Z-olefins.

Amino acids as chiral anionic ligands for ruthenium based asymmetric olefin metathesis

Several amino acid ligands were introduced into the Hoveyda-Grubbs 2nd generation complex by a facile anionic ligand exchange. The chiral pre-catalysts obtained displayed enantioselectivity in asymmetric ring-closing and ring-opening cross-metathesis reactions. Reduction of the lability of the carboxylate ligands was found to be cardinal for improving the observed enantiomeric product enrichment.

Charge control of the inverse trans-influence

The synthesis and characterization of uranium(VI) mono(imido) complexes, by the oxidation of corresponding uranium(V) species, are presented. These experimental results, paired with DFT analyses, allow for the comparison of the electronic structure of uranium(VI) mono(oxo) and mono(imido) ligands within a conserved ligand framework and demonstrate that the magnitude of the ground state stabilization derived from the inverse trans-influence (ITI) is governed by the relative charge localization on the multiply bonded atom or group.

Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf)2] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)2] (4), [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf){OCH(CF3)2}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH2)(CHCMe2Ph)(OTf)](+)BArF(-) (6), [Mo(N-2,6-Cl2C6H3)(IMesH2)(CHCMe3)(OTf)2] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)2] (8) are reported (IMesH2=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF(-)=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3(-)). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω-diynes. In the RCM of α,ω-dienes, in the homometathesis of 1-alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100,000, 210,000 and 30,000, respectively, were achieved. With I1 and I2, virtually Mo-free products were obtained (<3 ppm Mo). With 1,6-hepta- and 1,7-octadiynes, catalysts 3, 4, and 5 allowed for the regioselective cyclopolymerization of 4,4-bis(ethoxycarbonyl)-1,6-heptadiyne, 4,4-bis(hydroxymethyl)-1,6-heptadiyne, 4,4-bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne, 4,4,5,5-tetrakis(ethoxycarbonyl)-1,7-octadiyne, and 1,6-heptadiyne-4-carboxylic acid, underlining the high functional-group tolerance of these novel Group 6 metal alkylidenes.

Quantitatively analyzing metathesis catalyst activity and structural features in silica-supported tungsten imido-alkylidene complexes

A broad series of fully characterized, well-defined silica-supported W metathesis catalysts with the general formula [(≡SiO)W(═NAr)(═CHCMe2R)(X)] (Ar = 2,6-iPr2C6H3 (AriPr), 2,6-Cl2C6H3 (ArCl), 2-CF3C6H4 (ArCF3), and C6F5 (ArF5); X = OC(CF3)3 (OtBuF9), OCMe(CF3)2 (OtBuF6), OtBu, OSi(OtBu)3, 2,5-dimethylpyrrolyl (Me2Pyr) and R = Me or Ph) was prepared by grafting bis-X substituted complexes [W(NAr)(═CHCMe2R)(X)2] on silica partially dehydroxylated at 700 °C (SiO2-(700)), and their activity was evaluated with the goal to obtain detailed structure-activity relationships. Quantitative influence of the ligand set on the activity (turnover frequency, TOF) in self-metathesis of cis-4-nonene was investigated using multivariate linear regression analysis tools. The TOF of these catalysts (activity) can be well predicted from simple steric and electronic parameters of the parent protonated ligands; it is described by the mutual contribution of the NBO charge of the nitrogen or the IR intensity of the symmetric N-H stretch of the ArNH2, corresponding to the imido ligand, together with the Sterimol B5 and pKa of HX, representing the X ligand. This quantitative and predictive structure-activity relationship analysis of well-defined heterogeneous catalysts shows that high activity is associated with the combination of X and NAr ligands of opposite electronic character and paves the way toward rational development of metathesis catalysts.