Ring-chain Equilibrium Research Articles

Ring-Chain Tautomerism of 1,2,3,4-Tetrahydroquinazolines. The Products of Reaction of 1,3-Dicarbonyl Compounds with 2-Aminomethylaniline

The reaction of 2-aminomethylaniline with 1,3-dicarbonyl compounds gives a series of 1,2,3,4-tetrahydroquinazoline derivatives. A ring-chain tautomeric equilibrium of the type enamine-1,2,3,4-tetrahydroquinazoline is observed in solutions of these compounds and its position depends of the structure of the starting dicarbonyl component and the solvent polarity.

Conformational control in the cyclization of hydrogen-bonded supramolecular polymers.

Bifunctional 2-ureido-4[1H]-pyrimidinone (UPy) derivatives can form small cyclic oligomers as well as long supramolecular polymers in chloroform solutions using the quadruple hydrogen-bonding motif. Ring-chain equilibria of a set of supramolecular monomers containing methyl-substituted alkyl linkers between the hydrogen-bonding UPy moieties were investigated by (1)H NMR spectroscopy and viscometry. The data were characterized in terms of critical concentration (CC, denoting the onset of polymerization) and equilibrium cyclic dimer concentration (EDC, representing preorganization of the monomer toward selective formation of cyclic dimer). Methyl substituents in the monomer were found to promote conformations favorable for cyclic dimerization, leading to an increase in both the EDC and the CC with respect to unsubstituted monomer. Furthermore, we observed an odd-even effect in the CC and EDC with increasing length of the linker between the hydrogen-bonding units. The combined results allow tuning of the critical concentration over a broad range and offer detailed information on the correlation between monomer structure, conformation, and polymerizability which may provide new insights for the study and design of other ring-chain equilibria or helix-random coil transitions.

Molecular modeling of ring-chain equilibria for the ring-opening cross-metathesis of cis, cis-1,5-dimethyl-cycloocta-1,5-diene with ethylene at T=298.15 K

The molecular modeling of ring-opening cross-metathesis of cis, cis-1,5-dimethyl-cycloocta-1,5-diene (1,5-DM-COD), cis, cis-1,6-dimethyl-cycloocta-1,5-diene (1,6-DM-COD) and cis, cis-cycloocta-1,5-diene (COD) with ethylene (ethenolysis) at T=298.15 K using the B3LYP/6-31G(d,p) level of theory reveals that ring-chain equilibrium constants are dependent on the nature of cyclic diene. The ring-chain equilibria for the ethenolysis of 1,5-DM-COD is completely shifted to the formation of monomeric 2-methyl-hexa-1,5-diene.

Formation and characterization of a multicomponent equilibrium system derived from cis- and trans-1-aminomethylcyclohexane-1,2-diol.

Both cis and trans isomers of amino diols 3-6 were prepared stereoselectively. In the reactions between 3-6 and phenyl isothiocyanate, the ring closure proceeded regioselectively and resulted only in spiro derivatives of 2-phenyliminooxazolidines 9, 10, 13, and 14. The reaction of cis- (or trans-)1-aminomethylcyclohexane-1,2-diol 4 (or 6) with 1 equiv of an aromatic aldehyde 15a-g in EtOH at room temperature resulted in a complex, multicomponent equilibrium mixture of 16a-g and 18a-g (or 17a-g and 19a-g), in each case consisting of a five-component, ring-chain tautomeric system 16A-E (or 17A-E), involving the Schiff base, two epimeric spirooxazolidines, two epimeric condensed 1,3-oxazines, and some of the four tricyclic compounds 18A-D (or 19A-D). The five-component, ring-chain equilibria were found to be adequately described by the Hammett-Brown linear free energy equation.

Ring[bond]chain tautomerism of 2-Aryl-substituted cis- and trans-decahydroquinazolines.

In CDCl(3) at 300 K, 2-aryl-substituted cis- and trans-3-isopropyldecahydroquinazolines and trans-3-phenyldecahydroquinazolines proved to be three-component (r(1)[bond]o[bond]r(2)) ring[bond]chain tautomeric mixtures, whereas only ring-closed tautomers could be detected for the 3-methyl-substituted analogues. The proportions of the ring-chain tautomeric forms at equilibrium were strongly influenced by the N-substitutents and the cis-trans ring junction and could be described by the equation log K(X) = rho sigma(+) + log K(X=H). These are the first examples among 2-aryl-1,3-N,N-heterocycles of a three-component ring-chain tautomeric equilibrium characterized by a Hammett-type equation. The stabilities of the ring-closed forms of cis- and trans-2-aryldecahydroquinazolines and the corresponding 3,1-benzoxazines were found to increase in the following sequence of the heteroatom at position 3: NPh < N-i-Pr < O < NMe.

Molecular design of tautomeric interconversions of heterocycles

A classification and methodology of molecular design of ring—chain—ring (ring—ring) tautomeric systems by superposition of ring—chain equilibria in a structure of one molecule are considered.

Ring-Closing Polycondensations

The role of cyclization in polycondensations is discussed for two different scenarios: thermodynamically-controlled polycondensation (TCPs) on the one hand and kinetically-controlled polycondensations (KCPs) on the other. The classical Carothers–Flory theory of step-growth polymerization does not include cyclization reactions. However, TCPs involve the formation of cycles via ‘back-biting degradation’, and when the ring–chain equilibrium is on the side of the cycles the main reaction products of the TCP will be cyclic oligomers. Two groups of examples are discussed: polycondensations of salicyclic acid derivatives (e.g. aspirin) and polycondensations of dibutyltin derivatives with long α-, ω-diols or dicarboxylic acids. Furthermore, various kinetically-controlled syntheses of polyesters and polyamides were studied and carefully optimized in the direction of high molecular weights. High fractions of cyclic oligomers and polymers were found by MALDI-TOF mass spectrometry, and their fractions increased with optimization of the process for molecular weight. These results disagree with the Carothers–Flory theory but agree with the theoretical background of the Ruggli–Ziegler dilution method (RZDM). When poly(ether-sulfone)s were prepared from 4,4′-difluorodiphenylsulfone and silylated bisphenol-A two different scenarios were found. With CsF as catalyst at a temperature of more than 145°C cyclic oligoethers were formed under thermodynamic control. When the polycondensation was promoted with K2CO3 in N-methylpyrolidone at ≤145°C the formation of cyclic oligoethers and polyethers occurred under kinetic control. A new mathematical formula is presented correlating the average degree of polymerization with the conversion and taking into account the competition between cyclization and propagation.

Organoantimony Ring-Chain Equilibria: Trapping of catena-Tetrastibanes in cyclo-[Cr(CO)4(R‘2Sb−SbR−SbR−SbR‘2)] (R‘ = Me, Ph; R = Me3SiCH2) and Related Complexes

Ring-chain equilibria between cyclo-(Me3SiCH2Sb)n(n = 4, 5) and R‘4Sb2 (R‘ = Me, Ph) contain the catena-tristibanes R‘2Sb−SbR−SbR‘2 (1, R‘ = Me; 2, R‘ = Ph, R = Me3SiCH2) as the most abundant chain components. Addition of [nbdCr(CO)4] (nbd = norbornadiene) to the equilibrium mixture formed from cyclo-(Me3SiCH2Sb)n(n = 4, 5) and R‘4Sb2 (R‘ = Me, Ph) gives cyclo-[Cr(CO)4(R‘2Sb−SbR−SbR−SbR‘2)] (3, R‘ = Me; 4, R‘ = Ph, R = Me3SiCH2), two complexes with catena-tetrastibane ligands. Reaction of 3 with [W(CO)5(thf)] (thf = tetrahydrofuran) leads to cyclo-[Cr(CO)4(Me2Sb−SbR−SbR−SbMe2)W(CO)5] (R = Me3SiCH2) (5). The crystal structures of 3 and 5 have been determined by X-ray diffraction.

Ab initio and Density Functional Calculations on the Ring-Chain Tautomerism of γ-Oxocarboxylic Acids

The results of ab initio (HF, MP2) and density functional (B3LYP) calculations on a series of γ-oxocarboxylic acids [(Z)-3-acetyl or benzoylacrylic and 2-acetyl or benzoylbenzoic acids] A and their cyclic tautomers B are presented. For the open-chain tautomers, different rotamers were considered. Bulk solvent effects were considered by using the polarised continuum model (PCM) and were found to be essential for a proper description of the ring-chain tautomeric equilibrium. Acetyl derivatives generally prefer the cyclic form, whereas for benzoyl derivatives the preference depends on the nature of the link. Introduction of substituents (methyl, phenyl) at C2 and/or C3 for acrylic acid derivatives substantially shifts the equilibrium towards the cyclic form B.

Cyclization in reversible and irreversible step-growth polymerizations

Several aspects of cyclization reactions in step-growth polymerizations are considered. The Jacobson–Stockmayer ring-chain equilibrium theory provides the basis for understanding these systems, but there are circumstances where some modifications to the theory are needed. In particular, for ring expansion polymerizations, i.e. those for which there is never a significant fraction of open chains, the statistics are a bit different from those appropriate for ring-chain equilibrium. Also, if the reactions are irreversible the occurrence of rings is significantly different from the reversible case. Rings are more prevalent in irreversible reactions. The implications of these observations for the processing of polymers are briefly considered.

Equilibrium ring concentrations and the statistical conformations of polymer chains: Part 16. Theoretical and experimental investigations of cyclics prepared from poly(decamethylene fumarate) and poly(decamethylene maleate) ring-chain equilibrium reactions

The molar cyclisation equilibrium constants K x for cyclics [OCOCHCHCOO(CH 2) 10] x (with x=1–7) in two closely related polyester systems, poly(decamethylene maleate) and poly(decamethylene fumarate), were measured experimentally using gel permeation chromatography. The corresponding theoretical K x values were calculated by two methods. The first applied a rotational isomeric state model to describe the statistical conformations of chains forming rings and it assumed that such chains obey Gaussian statistics. The second approach used MSI's program Insight II to perform the Monte Carlo Metropolis calculations. When the expected chain expansion effects in a good solvent media was considered, a good agreement between experiment and theory for all the cyclics with x≥2 was found.

Stereoregular Polysiloxanes via Ring-Opening Polymerization, a Review

Although polysiloxanes are not ordinarily considered stereoregular, a few references describe ring-opening polymerizations of cyciosiloxanes that can produce polymers enriched in stereoregularity. Most are made from the cis-isomers of unsymmetrically substituted strained cyclosiloxanes by methods where ring–chain and chain–chain equilibria are suppressed. The monomers insert randomly at the reactive chain ends with equal probabilities of forming new meso (m) or raceme (r) siloxane links while preserving the stereoconfiguration of the original monomer. The resulting polysiloxanes are enriched in isotactic configurations but are limited in their run lengths by the new randomly configured siloxane links formed between the original monomer units. One citation, however, describes a ring–chain equilibrium where the isotactic conformation was the preferred conformation. Another illustrates the profound influence of stereoregularity on the mechanical properties of a silicone elastomer.

Cyclic polyesters: Part 8. Preparation and characterization of cyclic oligomers in six aromatic ester and ether-ester systems

Cyclic oligomers of tetraethylene glycol terephthalate (TEGT), tetraethylene glycol isophthalate (TEGI), tetraethylene glycol orthophthalate (TEGO), decamethylene terephthalate (DMT), decamethylene isophthalate (DMI) and decamethylene orthophthalate (DMO) were prepared successfully and cleanly from a ring-chain equilibrium reaction in dilute solution of chlorobenzene. The reactions were monitored by gel permeation chromatography (GPC) and proton nmr and 13C NMR spectroscopy, and the molar cyclic concentrations [M x ] deduced ( x = 1–10). The cyclic oligomers were also analysed by fast atom bombardment mass spectrometry (FAB-MS), matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and differential scanning calorimetry (DSC). The existence of the cyclic oligomers was proven by all of the above methods. In addition, it was found that small amounts of linear species were formed, especially in the TEGT system.

Ring-Chain Equilibrium between an [18]Cyclacene Derivative and a Ladder Oligomer

The synthesis of the AB-type Diels-Alder (DA) monomer 12 is reported whose self-condensation leads to the formation of the linear ladder oligomer 17 and its cyclic congener 18. 18 is the largest [n]cyclacene derivative known (n = 18) and can be obtained by thermal treatment of 17 in yields of up to 45% utilizing the reversibility of the DA cyclization involved. 17 can be considered a “storage form” for 18.

Thermal degradation of poly- l-lactide—studies on kinetics, modelling and melt stabilisation

The kinetics of changes in molecular weight during thermal treatment of crude and purified poly- l-lactide (PLA) were studied in sealed ampouls in nitrogen atmosphere in the temperature range of 180–230 °C. The changes in molecular weight were modelled by using a two-parameter model, consisting of a statistical degradation step and a recombination step. From the results, the catalytic effect of tin-(II) on degradation and recombination reaction can be seen, as well as the effect of tin concentration on the existing ring-chain equilibrium. The thermal stabilisation of Poly- l-lactide during processing in the melt is possible by the addition of tropolone.

Cyclic polyesters: 5. Cyclics prepared by poly(decamethylene terephthalate) ring-chain reactions

Cyclic oligomers of poly(decamethylene terephthalate) (PDT) were prepared successfully and cleanly from a ring-chain equilibrium reaction in dilute solution of chlorobenzene. A variety of catalysts were investigated for this purpose. The reactions were monitored up to equilibrium and the molar cyclization equilibrium constants K x deduced. Satisfactory yields of cyclics [O(CH 2) 10OCO C 6H 4CO] x were obtained, together with a higher molar mass linear polymer. Mass spectroscopic investigation showed that no monomeric ring was formed in the PDT reaction. The individual cyclics with x = 2–14 corresponding to 36–252 skeletal bonds were completely resolved using PL-gel mixed-E gel permeation chromatographic columns. The lower mass cyclics ( x = 2–5) were cleanly extracted and then characterized. The yield of cyclics in ring-chain equilibrates corresponding to a solvent/polymer dilution ratio of 30 1 , 60 1 and 100 1 were ca 11%, ca 30% and ca 50%, respectively. The K x values for PDT cyclics were used to compare the statistical conformations of the corresponding PDT chains in chlorobenzene with an aliphatic polyester, poly(decamethylene adipate), which was studied previously.

Synthesis and properties of unique mesoionic 1,3-thiazolium-4-olates

N-bridged 1,3-thiazolium-4-olates were synthesized by reaction of 3-substituted 3-aminothioacrylanilides with bromoacetic acid ethyl ester in refluxing xylene. Their structural aspects were investigated by means of mass, NMR, and absorption spectroscopy. They display an unusual ring-chain tautomeric equilibrium, which is governed by the nature of the solvents and thepH value.

Synthesis and polymerization of bis(hexamethylene carbonate) and bis(2,2,3,3,4,4,5,5‐octafluorohexamethylene carbonate)

AbstractBis(hexamethylene carbonate) (HC) and bis(2,2,3,3,4,4,5,5‐octafluorohexamethylene carbonate) (FHC) were synthesized starting from the corresponding diols and diphenyl carbonate in a two‐step reaction: (i) first polycondensation occurs, (ii) then ring‐closing depolymerization leads to HC and FHC, respectively. The polymerization of HC is performed with sec‐butyllithium in toluene solution, and the corresponding polymer is obtained in high yield. Polymerization of FHC is perfomed either in tetrahydrofuran solution with dibutylmagnesium as initiator or in the melt with dibutyltin dimethoxide as initiator. Poly(HC) is a semicrystalline polymer with a melting point of 54.0°C and a glass transition temperature of −51.3°C. Poly(FHC) was obtained either as a semicrystalline material with a melting point of 40.8°C or an amorphous material with a glass transition temperature of −39.8°C. Copolymers of HC and FHC with a diad ratio of 19:60:21 were obtained by melt polymerization with dibutyltin dimethoxide as initiator. The ringchain equilibrium established during polymerization was studied for HC in toluene solution and for FHC in the melt. The concentration of the cyclic oligomers at equilibrium was determined, and for poly(HC) the characteristic ratio was calculated to be 10.0 ± 1.0.

Ring-opening polymerization of macrocyclic aryl ether ketone oligomers containing the 1,2-dibenzoylbenzene moiety

Facile ring-opening polymerization of cyclic aryl ether oligomers containing the 1,2-dibenzoylbenzene moiety to form high molecular weight linear polymers in the presence of a nucleophilic initiator is described. The polymerization can be initiated in the melt in the presence of a nucleophilic initiator such as potassium carbonate, cesium fluoride, and alkali phenoxides. Various alkali phenoxides were investigated as potential nucleophilic initiators. The polymerization reaction rate in the melt increases in the order of K+ > Na+ > Cs+, and in the order of −OPhPhO− > PhO− > PhOPhO− > PhPhO−. However, the polymerization in an aprotic dipolar solvent is faster in the presence of cesium phenoxide than in the presence of potassium phenoxide. Polymerization of the cyclic oligomers in solution demonstrates that the ring-opening polymerization proceeds via a chain-growth mechanism and involves a transetherification reaction between linear and cyclic aryl ether oligomers. The ring-chain equilibrium is much more favorable towards linear polymers. Since little or no ring strain exists in the cyclic system, the transetherification reactions are indiscriminate with regards to cyclic or linear chains and the interchain equilibration is also a facile process during polymerization. This intermolecular transetherification has been demonstrated by using low molecular weight aryl ethers to control the molecular weight of the polymer formed via ring-opening polymerization. © 1996 John Wiley & Sons, Inc.

Mass spectrometric behaviour of simple oxazolidines under electron impact and chemical ionization

AbstractThe 70 eV electron impact mass spectra of 33 differently substituted oxazolidines were studied to determine the effect of substituents and the existence of the ring—chain tautomeric equilibrium on the decomposition of the molecular ions. Most of the fragmentations can be rationalized to start as the cleavage initiated by the radical site at nitrogen. Isomeric compounds showed different spectra and were easily differentiated. The position of the ring—chain equilibrium could be located only roughly. The chemical ionization mass spectra of the compounds were also recorded, with ammonia, isobutane, acetone or methane as reagent gas. Methane was the only reagent gas that promoted extensive fragmentation of the protonated molecules. However, no information about the position of ring‐chain tautomerism was obtained under these conditions. Analogously to other related five‐membered heterocycles, the oxazolidines reacted under acetone chemical ionization conditions to afford [M + CH3CO]+ adduct ions. These adducts were stable, however, and unlike those of 1,3‐dioxolanes and 1,3‐oxathiolanes, they did not decompose and form stable oxonium ions.