Convenient synthesis of [3]catenane by olefin metathesis dimerizations

[3]Catenane 5a and 5b were synthesized conveniently by olefin metathesis dimerization of pseudorotaxanes 3a and 3b. The yields of 5a and 5b were influenced by concentrations of 3a and 3b, and a ring size of a center wheel of [3]catenane.     

New synthesis of 3-cyclopentenecarboxylic acid esters by olefin metathesis

Olefin metathesis of diallylacetic acid esters is proposed for the synthesis of 3-cyclopentenecarboxylic acid esters. WCl₆-organosilicon compounds are used as homogeneous catalysts.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1503–1504, August, 1994.     

Enantioselective synthesis of [7]helicene: dramatic effects of olefin additives and aromatic solvents in asymmetric olefin metathesis

The asymmetric synthesis of [7]helicene was accomplished in good ee (80%) by kinetic resolution by means of asymmetric olefin metathesis. Three key factors contributed to the success of the kinetic resolution: the use of new Ru-based olefin metathesis catalysts bearing C1-symmetric N-heterocyclic carbene ligands, simple olefins as additives to control the nature of the propagating alkylidene and hexafluorobenzene as a solvent.     

The synthesis of cyclic polymers by olefin metathesis: Achievements and challenges

Cyclic polymers have drawn considerable interest for their peculiar physical properties in comparison to linear polymers, despite their equivalent compositions. Synthetically, cyclic polymers can be accessed through either macrocyclic ring‐closure or by ring‐expansion polymerization, but the main challenge with either method is the production of highly pure cyclic polymer samples. This highlight describes advances in the area of cyclic polymer synthesis, with a particular focus on ring‐expansion metathesis polymerization. Methods for characterizing cyclic polymers and assessing their purity are also discussed in order to emphasize the need for additional robust and reliable methods for synthesizing and studying topologically complex macromolecules. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 228–242     

Synthesis of 7-deoxypancratistatin from carbohydrates by the use of olefin metathesis

The stereocontrolled synthesis of (+)-7-deoxypancratistatin is described. The convergent synthesis has been achieved by two different strategies, both of which commence from a pentose and piperonal. The latter is converted into allylic bromide 7, which is then coupled with a protected methyl 5-deoxy-5-iodo-D-ribofuranoside in the presence of zinc metal. The first strategy involves a total of only 13 steps from D-ribose and piperonal, but suffers from a low yield in the zinc-mediated reaction between ribofuranoside 9, benzylamine, and bromide 7. The second strategy involves a total of 18 steps from D-xylose and piperonal. The former is converted into ribofuranoside 28, which is coupled with bromide 7 in the presence of zinc, and this is followed by ring-closing olefin metathesis. Subsequent Overman rearrangement, dihydroxylation, and deprotection then affords the natural product.     

Synthesis of 2,6-dioxabicyclo[3.3.0]octenes by tandem ring-rearrangement/cross metathesis

A facile synthesis of stereodefined 2,6-dioxabicyclo[3.3.0]octene derivatives from the vinyl ether of endo-7-oxanorbornen-2-ol via tandem ring-opening/ring-closing(ring-rearrangement)/cross metathesis is reported.     

Some aspects of the synthesis and characterization of special polymers and oligomers by olefin metathesis

Homopolymers of 2-norbornene and 2,3-bis(trifluoromethyl)-2,5-norbornadiene and copolymers of these bicyclic olefins with 1,5-cyclooctadiene and cyclopentene were prepared via ring opening metathesis polymerization. The molecular weight distributions of the polymers were estimated by gel permeation chromatography.The polymers were degraded in a cross metathesis reaction with E-4-octene; only poly[2,3-bis(trifluoromethyl)-2,5-norbornadiene] was not degradable.All reactions were carried out with WCl₆/(CH₃)₄Sn as the catalyst system. The low molecular cyclic oligomers in the polymerization mixtures and the degradation products were analyzed by gas chromatography and identified using a gas chromatography/mass spectrometry coupling.The degradation experiments show reactivity differences for the double bonds situated in the polymer backbone. On the basis of these differences and the fact that this is the first time that a metathesis degradation of such polymers has been reported, the consequences on the ring opening metathesis copolymerization of cycloolefins are discussed in general terms, leading to some new aspects in the planning of the synthesis of special copolymers and oligomers.With reference to a lecture presented at the 4th International Symposium on Olefin Metathesis (ISOM 4), Belfast, 1–4 Sept. 1981.     

Macrocyclization by ring-closing metathesis in the total synthesis of natural products: reaction conditions and limitations

The construction of macrocycles by ring-closing metathesis (RCM) is often used as the key step in the synthesis of natural products containing large rings. This reaction is attractive because of its high functional group compatibility and the possibility for further transformations. The finding of suitable reaction conditions is critical for the success of the synthesis. In this Minireview we summarize the efforts of many research groups to develop efficient RCM reactions on their way towards the total synthesis of natural macrocyclic products. Their findings should help in future synthesis to reduce the time-consuming phase of the optimization of the reaction conditions.     

Acyclic diene metathesis (ADMET) depolymerization: Ethenolysis of 1,4-polybutadiene using a ruthenium complex

α,ω-Vinyl-terminated butadiene oligomers can be generated through the cross-metathesis of ethylene and high molecular weight 1,4-polybutadiene catalyzed by the complex RuCl₂(CHPh)(PCy₃)₂. The effect of varied ethylene pressure is studied in order to obtain the highest conversion to the monomer 1,5-hexadiene. A dramatic increase in yield of this monomeric diene is realized relative to previous published attempts utilizing a well-defined group VI metal alkylidene in the same chemistry. Traces of side products are observed, and alternate mechanisms for their formation were proposed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1857–1861, 1999     

Concise stereoselective synthesis of cis-3-alkoxy-2-carbomethoxy medium-ring oxacycles from (R)-3-(3-butenyl)-4-propynoyloxazolidin-2-one

A concise process for the stereoselective synthesis of chiral cis-3-alkoxy-2-carbomethoxy medium-ring oxacycles from (R)-3-(3-butenyl)-4-propynoyloxazolidin-2-one (1) was developed. The process includes five major steps: (i) hetero-Michael reaction between an alcohol and 1, (ii) stereoselective reduction of the resulting ketone, featuring stereochemical assistance of the neighboring oxazolidin-2-one group, (iii) esterification with an alkoxy acetic acid, (iv) chirality-transferring Ireland-Claisen rearrangement of the resulting 3-alkoxyallyl glycolate ester to provide a syn-2,3-dialkoxy carboxylate ester, and (v) relay ring-closing olefin metathesis to form a medium-ring ether along with the simultaneous removal of the oxazolidin-2-one moiety.     

Ligand topology effects on olefin oxidations by bio-inspired [FeII(N2Py2)] catalysts

Linear tetradentate N2Py2 ligands can coordinate to an octahedral FeII center in three possible topologies (cis-alpha, cis-beta, and trans). While for the N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) complex, only the cis-alpha topology has been observed, for N,N'-bis(2-pyridylmethyl)-1,2-diaminocyclohexane (bpmcn) both cis-alpha and cis-beta isomers have been reported. To date, no facile interconversion between cis-alpha and cis-beta topologies has been observed for ironII complexes even at high temperatures. However, this work provides evidence for facile interconversion in solution of cis-alpha, cis-beta, and trans topologies for [Fe(bpmpn)X2] (bpmpn=N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane; X=triflate, CH3CN) complexes. As reported previously, the catalytic behavior of cis-alpha and cis-beta isomers of [Fe(bpmcn)(OTf)2] with respect to olefin oxidation depends dramatically on the geometry adopted by the iron complex. To establish a general pattern of the catalysis/topology dependence, this work presents an extended comparison of the catalytic behavior for oxidation of olefins of a family of [Fe(N2py2)] complexes that present different topologies. 18O labeling experiments provide evidence for a complex mechanistic landscape in which several pathways should be considered. Complexes with a trans topology catalyze only non-water-assisted epoxidation. In contrast, complexes with a cis-alpha topology, such as [Fe(bpmen)X2] and [Fe(alpha-bpmcn)(OTf)2], can catalyze both epoxidation and cis-dihydroxylation through a water-assisted mechanism. Surprisingly, [Fe(bpmpn)X2] and [Fe(beta-bpmcn)(OTf)2] catalyze epoxidation via a water-assisted pathway and cis-dihydroxylation via a non-water-assisted mechanism, a result that requires two independent and distinct oxidants.     

Synthesis of a [2]rotaxane incorporating a "magic sulfur ring" by the thiol-ene click reaction

The mild and highly efficient thiol-ene click reaction has been used to construct a rotaxane incorporating dibenzo-24-crown-8 (DB24C8) and a dibenzylammonium-derived thread in high yield under the irradiation of UV light. A rotaxane containing a disulfide linkage in the macrocycle was also synthesized by the thiol-ene click reaction. It has been demonstrated that the formation of the [2]rotaxane with the disulfide bond in the macrocycle occurs by a mechanism that is different to the threading-followed-by-stoppering process. The successful construction of a rotaxane directly from its constituent components, the macrocycle containing a disulfide linkage and the dibenzylammonium hexafluorophosphate salt, suggests that the space within the macrocycle incorporating the disulfide linkage is smaller than the phenyl unit and a plausible reaction mechanism has been proposed as follows: A small amount of the initiator forms two radicals upon the absorption of UV irradiation; the radicals act as a "key" to "unlock" the disulfide bond in the macrocycle. The resulting crown ether like moiety in the macrocycle is clipped around the ammonium ion center in the dumb-bell-shaped compound. The [2]rotaxane is generated upon recombination of the disulfide linkage.     

Nickel(II) and Copper(II) Complexes of β‐Unsubstituted 5,15‐Diazaporphyrins and Pyridazine‐Fused Diazacorrinoids: Metal–Template Syntheses and Peripheral Functionalizations

The present paper reports the first comprehensive study on the synthesis, structures, optical and electrochemical properties, and peripheral functionalizations of nickel(II) and copper(II) complexes of β‐unsubstituted 5,15‐diazaporphyrins (M‐DAP; M=Ni, Cu) and pyridazine‐fused diazacorrinoids (Ni‐DACX; X=N, O). These two classes of compounds were constructed starting from mesityldipyrromethane by a metal–template method. Ni‐DAP and Cu‐DAP were prepared in high yields by the reaction of the respective metal–bis(dibromodipyrrin) complexes with NaN₃–CuX (X=I, Br), whereas Ni‐DACN and Ni‐DACO were formed as predominant products by the reaction with NaN₃. In both cases, the metal centers change their geometry from tetrahedral to square planar during the aza‐annulation; X‐ray crystallographic analyses of M‐DAPs showed highly planar diazaporphyrin π planes. The Q band of Ni‐DAP was redshifted and intensified compared with that of a nickel–porphyrin reference, due to the involvement of electronegative nitrogen atoms at the meso positions. It was found that the peripheral bromination of Ni‐DAP and Ni‐DACO occurred regioselectively to afford Ni‐DAP‐Br₄ and Ni‐DACO‐Br, respectively. These brominated derivatives underwent Stille reactions with tributyl(phenyl)stannane to give the corresponding phenylated derivatives, Ni‐DAP‐Ph₄ and Ni‐DACO‐Ph. On the basis of the absorption spectra and X‐ray analysis, it has been concluded that the attached phenyl groups efficiently conjugate with the diazaporphyrin π system. The present results unambiguously corroborate that the β‐unsubstituted DAPs and DACXs are promising platforms for the development of a new class of π‐conjugated azaporphyrin‐based materials.