Computational Study of Metathesis Degradation of Rubber, 3. Distribution of Cyclic and Linear Oligomers via Intermolecular Degradation of cis‐Poly(butadiene)

The molecular modeling of the product distributions for the intermolecular metathesis degradation of cis‐poly(butadiene) (cis‐PB) in the presence of ethylene as chain‐transfer agent (CTA) at 298.15 K using the B3LYP/6‐31G (d, p) level of theory reveals that chain–ring and chain‐chain equilibria are shifted toward the formation of 1,5‐hexadiene. The amount of cyclic oligomers at equilibrium with linear molecules is negligible. The α,ω‐vinyl‐terminated butadiene oligomers–1,5‐hexadiene equilibrium constant depends on the cis/trans isomer ratio in linear butadiene molecules. While the concentration of 1,5‐hexadiene at equilibrium with cis‐butadiene oligomers is 86 mol‐%, this value for trans‐butadiene oligomers corresponds to 50 mol‐% of 1,5‐hexadiene. The results of calculations are in reasonable agreement with recent experimental data on the intermolecular metathesis of 1,4‐cis‐PB with ethylene using a well‐defined ruthenium alkylidene catalyst. The calculations predict that cis‐butene as a CTA is more efficient in the metathesis depolymerization of cis‐PB compared with ethylene.     

Computational study of metathesis degradation of rubber, 1. Distribution of cyclic oligomers via intramolecular metathesis degradation of cis‐polybutadiene

The results of the molecular modelling of the intramolecular metathesis degradation of cis‐polybutadiene (cis‐PB) into cyclic oligomers at PM3 level of theory showed that the chain‐ring equilibrium is completely shifted towards the all‐trans cyclic isomers. The formation of cyclic products, containing from three to six butadiene units, from larger rings is thermodynamically favoured with cyclic butadiene tetramers and pentamers being the main products. These results are in reasonable agreement with most of the available experimental data. The discrepancy observed in some cases between found and calculated ring distributions for the intramolecular metathesis degradation of cis‐PB suggests that the reaction is kinetically controlled under certain conditions.     

Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis‐diiodo/sulfur‐chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis‐cyclooctene by the RCM reaction of 1,9‐undecadiene. Moreover, cis‐1,4‐polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5‐cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.     

Mechanistic Studies of Hoveyda–Grubbs Metathesis Catalysts Bearing S‐, Br‐, I‐, and N‐coordinating Naphthalene Ligands

Derivatives of the Hoveyda–Grubbs complex bearing S‐, Br‐, I‐, and N‐coordinating naphthalene ligands were synthesized and characterized with NMR and X‐ray studies. Depending on the arrangement of the coordinating sites on the naphthalene core, the isomeric catalysts differ in activity in model metathesis reactions. In particular, complexes with the Ru?CH bond adjacent to the second aromatic ring of the ligand suffer from difficulties experienced on their preparation and initiation. The behavior most probably derives from steric hindrance around the double bond and repulsive intraligand interactions, which result in abnormal chemical shifts of benzylidene protons observed with ¹H NMR. Furthermore EXSY studies revealed that the halogen‐chelated ruthenium complexes display an equilibrium, in which major cis‐Cl₂ structures are accompanied with small amounts of isomeric forms. In general, contents of the minor forms, measured at 80 °C, correlate with the observed activity trends of the catalysts, although some exceptions complicate the mechanistic picture. We assume that for the family of halogen‐chelated metathesis catalysts the initiation mechanism starts with the cis‐Cl₂?trans‐Cl₂ isomerization, although further steps may become rate‐limiting for selected systems.     

FeCl3‐Catalyzed Ring‐Closing Carbonyl–Olefin Metathesis

Exploiting catalytic carbonyl–olefin metathesis is an ongoing challenge in organic synthesis. Reported herein is an FeCl₃‐catalyzed ring‐closing carbonyl–olefin metathesis. The protocol allows access to a range of carbo‐/heterocyclic alkenes with good efficiency and excellent trans diastereoselectivity. The methodology presents one of the rare examples of catalytic ring‐closing carbonyl–olefin metathesis. This process is proposed to take place by FeCl₃‐catalyzed oxetane formation followed by retro‐ring‐opening to deliver metathesis products.     

Photocyclization reactions. Part 8. Synthesis of 2‐quinolone,quinoline and coumarin derivatives using trans‐cis isomerization by photoreaction

2‐Quinolone 2 , quinoline 3 , coumarin (2H‐1‐benzopyran‐ 2 ‐one) 5 , and 2H‐1‐benzopyran hemiacetal 6 were synthesized by photocyclization reaction of traans‐o‐aminocinnamoyl derivatives trans‐ 1 and trans‐o‐hydroxycinnamoyl derivatives trans‐ 4 . The reaction proceeds through trans‐cis isomerization followed by intramolecular cyclization.     

Molecular weight and configurational stability of poly[(fluorophenyl)acetylene]s prepared with metathesis and insertion catalysts

Series of high‐cis and cis/trans poly[(fluorophenyl)acetylene]s (PFPhA) have been prepared by polymerization of (2‐fluorophenyl)acetylene, (3‐fluorophenyl)acetylene, and (4‐fluorophenyl)acetylene with catalysts: [Rh(1,5‐cyclooctadiene) OCH₃]₂ (high‐cis PFPhAs) and tungsten(VI) oxychloride/tetraphenyltin (cis/trans PFPhAs). The molecular weight and configurational stability under various conditions at room temperature were studied for both PFPhAs series by means of size exclusion chromatography, ¹H‐NMR, and UV‐vis techniques. All samples exhibited slow degradation when exposed to the atmosphere in the solid state; the rate of degradation was independent on the F‐position on the Ph ring. The rate of degradation increased up to three orders of magnitude in the tetrahydrofuran solution where it was higher for high‐cis polymers compared with their cis/trans counterparts. The degradation of high‐cis PFPhAs was accompanied by significant cis‐to‐trans isomerization in aerated tetrahydrofuran solution. Rate of degradation and isomerization exhibited the same dependence on the F‐position on the Ph ring. The hypothesis was postulated that the degradation of high‐cis PFPhAs in solution was accelerated by cis‐to‐trans isomerization due to which the content of unpaired electrons on the main chains is enhanced. In both high‐cis and cis/trans series of polymers the ortho‐substituted isomers exhibited an enhanced stability compared with meta‐ and para‐substituted isomers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4296–4309, 2010     

The Key Role of the Nonchelating Conformation of the Benzylidene Ligand on the Formation and Initiation of Hoveyda–Grubbs Metathesis Catalysts

Experimental studies of Hoveyda–Grubbs metathesis catalysts reveal important consequences of substitution at the 6‐position of the chelating benzylidene ligand. The structural modification varies conformational preferences of the ligand that affects its exchange due to the interaction of the coordinating site with the ruthenium center. As a consequence, when typical S‐chelated systems are formed as kinetic trans‐Cl₂ products, for 6‐substituted benzylidenes the preference is altered toward direct formation of thermodynamic cis‐Cl₂ isomers. Activity data and reactions with tricyclohexylphosphine (PCy₃) support also a similar scenario for O‐chelated complexes, which display fast trans‐Cl₂?cis‐Cl₂ equilibrium observed by NMR EXSY studies. The presented conformational model reveals that catalysts, which cannot adopt the optimal nonchelating conformation of benzylidene ligand, initiate through a high‐energy associative mechanism.     

Nickel Fluorocarbene Metathesis with Fluoroalkenes

Alkene metathesis with directly fluorinated alkenes is challenging, limiting its application in the burgeoning field of fluoro‐organic chemistry. A new nickel tris(phosphite) fluoro(trifluoromethyl)carbene complex ([P₃Ni]=CFCF₃) reacts with CF₂=CF₂ (TFE) or CF₂=CH₂ (VDF) to yield both metallacyclobutane and perfluorocarbene metathesis products, [P₃Ni]=CF₂ and CR₂=CFCF₃ (R=F, H). The reaction of [P₃Ni]=CFCF₃ with trifluoroethylene also yields metathesis products, [P₃Ni]=CF₂ and cis/trans‐CFCF₃=CFH. However, unlike reactions with TFE and VDF, this reaction forms metallacyclopropanes and fluoronickel alkenyl species, resulting presumably from instability of the expected metallacyclobutanes. DFT calculations and experimental evidence established that the observed metallacyclobutanes are not intermediates in the formation of the observed metathesis products, thus highlighting a novel variant of the Chauvin mechanism enabled by the disparate four‐coordinate transition states.     

A Unified Strategy Targeting the Thiodiketopiperazine Mycotoxins Exserohilone,Gliotoxin, the Epicoccins,the Epicorazines,Rostratin A and Aranotin

A unified synthetic strategy directed towards mycotoxins belonging to the thiodiketopiperazine family is reported. The building blocks for a number of natural products—including exserohilone, gliotoxin, the epicoccins, the epicorazines, rostratin A and aranotin—have been synthesised stereoselectively from a common precursor. This key intermediate was constructed through an efficient and highly diastereoselective [2+2] cycloaddition between a ketene and an enecarbamate derived from L ‐pyroglutamic acid. The annelation of the second ring was accomplished through ring‐closing metathesis and enol ether–olefin ring‐closing metathesis to provide both cis‐ and trans‐annelated azabicyclic cyclohexenones, as well as an annelated seven‐membered cyclic enol ether. A Pd‐catalysed elimination of allyl acetate gave rise to the cyclohexadienol structure of gliotoxin. Dimerisation of one building block to afford the diketopiperazine core was demonstrated.     

Thermisches Verhalten von o-Dipropenylbenzol,Beispiel einer aromatischen [1,7]-sigmatropischen H-Verschiebung. Vorläufige Mitteilung

cis, cis-, cis, trans- and trans, trans-o-Dipropenylbenzene (cis, cis-, cis, trans- and trans, trans- 1 ) were prepared. At 225° cis, cis- 1 isomerises to give cis, trans- 1 and vice versa. The isomerisation follows 1. order kinetics. At equilibrium 89% cis, trans- and 11% cis, cis- 1 are present. It is shown by deuterium labelling that the isomerisation is due to aromatic [1, 7 a] sigmatropic H-shifts. trans, trans- 1 rearranges at 225° to yield 2, 3-dimethyl-1, 2-dihydronaphthalene ( 3 ). This can be visualized by disrotatory ring closure of trans, trans- 1 followed by an aromatic [1, 5 s] H-shift. When cis, cis- or cis, trans- 1 are heated for 153 hrs at 225° a small amount (3%) of 1-ethyl-1,2-dihydronaphthalene ( 5 ) is formed.     

Donor/Acceptor‐Stabilized 1‐Silaketene: Reversible [2+2] Cycloaddition with Pyridine and Evolution by an Olefin Metathesis Reaction

The reaction of silacyclopropylidene 1 with benzaldehyde generates a 1‐silaketene complex 2 by a formal atomic silicon insertion into the C=O bond of the aldehyde. The highly reactive 1‐silaketene 2 undergoes a reversible [2+2] cycloaddition with pyridine to give sila‐β‐lactam 3 . Of particular interest, in the presence of 4‐dimethylaminopyridine (DMAP), 1‐silaketene complex 2 evolves through an intramolecular olefin metathesis reaction, generating a new 1‐silaketene complex 8 and cis‐stilbene. Theoretical studies suggest that the reaction proceeds through the formation of a transient silacyclobutanone, a four‐membered‐ring intermediate, similar to that proposed by Chauvin and co‐workers for the transition‐metal‐based olefin metathesis.     

X‐ray analysis of the bisphenol‐A‐type macrocyclic carbonate trimer

Collection of the single crystal X‐ray refraction data of the Bisphenol‐A‐type Macrocyclic oligocarbonate trimer (c‐3mer) at room temperature was carried out. The single crystal of the cyclic trimer that is recrystallized from ethyl acetate showed solvent molecule in the center of macro ring. Similarly, cyclic tetramer (c‐4mer) contained two p‐xylene molecules. Smaller dimer (c‐2mer) did not afford co‐crystal with solvent. Conformation of the carbonate in c‐3mer was s‐cis and s‐cis as in c‐4mer. A relationship between the conformation of carbonate and noncatalyst polymerization activity was not found. Copyright © 2000 John Wiley & Sons, Ltd.     

Synthesis of Novel Pyran β-Amino Acid and 5,6-Dihydro-2H-pyran β-aminoxy Acid from Carbohydrate Derivatives

Synthesis of two new amino acids, containing pyran rings, is reported from carbohydrate derivatives. The cis-3-amino-pyran-2-carboxylic acid (cis-APyC) was prepared from (R)-glyceraldehyde derivative, using nucleophilic substitution reaction for pyran ring formation. Similarly, the trans-3-aminoxy-5,6-dihydro-2H-pyran-2-carboxylic acid (trans-AmPyC) was prepared from diacetone glucose (DAG), using ring closing metathesis (RCM) reaction for the ring formation.     

Intermolecular Carbonyl–olefin Metathesis with Vinyl Ethers Catalyzed by Homogeneous and Solid Acids in Flow

The carbonyl–olefin metathesis reaction has experienced significant advances in the last seven years with new catalysts and reaction protocols. However, most of these procedures involve soluble catalysts for intramolecular reactions in batch. Herein, we show that recoverable, inexpensive, easy to handle, non‐toxic, and widely available simple solid acids, such as the aluminosilicate montmorillonite, can catalyze the intermolecular carbonyl–olefin metathesis of aromatic ketones and aldehydes with vinyl ethers in‐flow, to give alkenes with complete trans stereoselectivity on multi‐gram scale and high yields. Experimental and computational data support a mechanism based on a carbocation‐induced Grob fragmentation. These results open the way for the industrial implementation of carbonyl–olefin metathesis over solid catalysts in continuous mode, which is still the origin and main application of the parent alkene–alkene cross‐metathesis.     

A Conformational Study of Cyclohexane‐1,3,5‐tricarbonitrile Derivatives

Cyclohexane‐1,3,5‐tricarbonitrile reached equilibrium having 1,3‐cis‐1,5‐cis and 1,3‐cis‐1,5‐trans isomers in a ratio of 3:7. The cis, cis‐isomer preferred the conformation with three equatorial cyano groups, where as the cis, trans‐isomer displayed two cyano groups on equatorial positions and another cyano group on axial position. Condensation of cis, cis‐cyclohexane‐1,3,5‐tricarbonitrile with L‐(S)‐valinol by the catalysis of ZnCl₂ in refluxing 1,2‐dichlorobenzene afforded two isomeric cyclohexane‐1,3,5‐trioxazolines in favor of the 1,3‐cis‐1,5‐trans isomer. Metalation of cis, cis‐cyclohexane‐1,3,5‐tricarbonitrile, followed by alkylations with dimethyl sulfate, benzyl bromide or allyl bromide, gave the cor responding trialkylation products with predominance of 1,3‐cis‐1,5‐trans isomers. The cis, trans‐isomer showed two cyano groups on axial positions and another cyano group on equatorial position, where as the cis, cis‐isomer exhibited three axial cyano groups. Treatment of trimethyl cis, cis‐cyclohexane‐1,3,5‐tricarboxylate with lithium diisopropylamide and dimethyl sulfate afforded mainly the trimethyl ester of Kemp's triacid, which showed three axial carboxylate groups. Two competitive factors, i.e. the steric effect of in coming electrophiles and the dipole‐dipole inter actions of the cyano or carboxylate groups, might inter play to give different stereoselectivities in these reaction systems.     

ESIMS Studies and Calculations on Alkali‐Metal Adduct Ions of Ruthenium Olefin Metathesis Catalysts and Their Catalytic Activity in Metathesis Reactions

Electrospray ionization mass spectrometry (ESIMS) and subsequent tandem mass spectrometry (MS/MS) analyses were used to study some important metathesis reactions with the first‐generation ruthenium catalyst 1 , focusing on the ruthenium complex intermediates in the catalytic cycle. In situ cationization with alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) using a microreactor coupled directly to the ESI ion source allowed mass spectrometric detection and characterization of the ruthenium species present in solution and particularly the catalytically active monophosphine–ruthenium intermediates present in equilibrium with the respective bisphosphine–ruthenium species in solution. Moreover, the intrinsic catalytic activity of the cationized monophosphine–ruthenium complex 1 a ?K⁺ was directly demonstrated by gas‐phase reactions with 1‐butene or ethene to give the propylidene Ru species 3 a ?K⁺ and the methylidene Ru species 4 a ?K⁺, respectively. Ring‐closing metathesis (RCM) reactions of 1,6‐heptadiene ( 5 ), 1,7‐octadiene ( 6 ) and 1,8‐nonadiene ( 7 ) were studied in the presence of KCl and the ruthenium alkylidene intermediates 8 , 9 , and 10 , respectively, were detected as cationized monophosphine and bisphosphine ruthenium complexes. Acyclic diene metathesis (ADMET) polymerization of 1,9‐decadiene ( 14 ) and ring‐opening metathesis polymerization (ROMP) of cyclooctene ( 18 ) were studied analogously, and the expected ruthenium alkylidene intermediates were directly intercepted from reaction solution and characterized unambiguously by their isotopic patterns and ESIMS/MS. ADMET polymerization was not observed for 1,5‐hexadiene ( 22 ), but the formation of the intramolecularly stabilized monophosphine ruthenium complex 23 a was seen. The ratio of the signal intensities of the respective with potassium cationized monophosphine and bisphosphine alkylidene Ru species varied from [I _4a ]/[I ₄ ]=0.02 to [I _23a ]/[I ₂₃ ]=10.2 and proved to be a sensitive and quantitative probe for intramolecular π‐complex formation of the monophosphine–ruthenium species and of double bonds in the alkylidene chain. MS/MS spectra revealed the intrinsic metathesis catalytic activity of the potassium adduct ions of the ruthenium alkylidene intermediates 8 a , 9 a , 10 a , 15 a , and 19 a , but not 23 a by elimination of the respective cycloalkene in the second step of RCM. Computations were performed to provide information about the structures of the alkali metal adduct ions of catalyst 1 and the influence of the alkali metal ions on the energy profile in the catalytic cycle of the metathesis reaction.     

Ring‐opening metathesis polymerization of cis‐cyanocyclooct‐4‐ene: Search for active catalysts,variation of monomer to catalyst ratios and monomer concentrations

The ring‐opening metathesis polymerization (ROMP) of cis‐cyanocyclooct‐4‐ene initiated by ruthenium‐based catalysts of the first, second, and third generation was studied. For the polymerization with the second generation Grubbs catalyst [RuCl₂(?CHPh)(H₂IMes)(PCy₃)] (H₂IMes = N,N′‐bis(mesityl)‐4,5‐dihydroimidazol‐2‐ylidene), the critical monomer concentration at which polymerization occurs was determined, and variation of monomer to catalyst ratios was performed. For this catalyst, ROMP of cis‐cyanocyclooct‐4‐ene did not show the features of a living polymerization as M_n did not linearly increase with increasing monomer conversion. As a consequence of slow initiation rates and intramolecular polymer degradation, molar masses passed through a maximum during the course of the polymerization. With third generation ruthenium catalysts (which contain 3‐bromo or 2‐methylpyridine ligands), polymerization proceeded rapidly, and degradation reactions could not be observed. Contrary to ruthenium‐based catalysts of the second and third generation, a catalyst of the first generation was not able to polymerize cis‐cyanocyclooct‐4‐ene. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Ring Closing Metathesis Using Chiral Template Consisting of Hard and Soft Parts

Summary: Macrocyclic compounds having 18-, 19-, 22-, and 23-membered rings were prepared quantitatively by ring closing metathesis of diene substrates consisting of a rigid o- or m-phthalate group and flexible but geometrically regulated 2,4-pentanediol tethers. From the p-phthalate analogues, the ring closing metathesis and the cyclic dimmer formation were observed to give 24-membered ring (70% yield) and 40-membered ring (65% yield) respectively.     

Phenylene vinylene oligomers studied by theoretical methods: Joint analysis of computational and x-ray results of the configurational isomers of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene

The configurational isomers of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene have been investigated by ab initio and MOPAC-AM1 semiempirical methods. The calculations were guided by and compared with single crystal X-ray results of the trans, trans-isomer (taken from the literature) and of the cis,cis-isomer (reported here). Using 4-21G-based ab initio calculations, free state geometries, deviations from coplanarity, and barriers to rotation of the central and peripheral rings were evaluated. Such barriers were also enumerated for the solid state of the cis,cis- and trans,trans-isomers. A single-molecule cluster surrounded by point charges sufficed to rationalize observed solid state properties in the trans,trans-isomer, including the quasi-free rotation of the central ring. A multimolecule cluster, however, was required to rationalize the restricted rotation of the rings in the cis,cis-isomer. MOPAC-AM1 methods were used to calculate geometries and energies of rotameric forms on the singlet photoisomerization path cis,cis → cis,trans → trans,trans. Finally, UV absorption wavelengths and oscillator strengths were calculated and the electronic structure of the states discussed. © 1996 by John Wiley & Sons, Inc.