Thermal Ring-opening Reaction Research Articles

Thermodynamic control of the electrocyclic ring opening of cyclobutenes: C=X substituents at C-3 mask the kinetic torquoselectivity.

The thermal ring-opening reactions of 4-phenyl-1,3,3-triethoxycarbonylcyclobutene and 4-methyl-1,3,3-triethoxycarbonylcyclobutene yield dienes that result from an unexpected selectivity for "inward" rotation of the phenyl and methyl groups. With 1-ethoxycarbonyl-4-phenylcyclobutene, "outward" rotation of the phenyl group occurs exclusively. Density functional theory was used to investigate the role of the 3,3-geminal diester groups and the origin of torquoselectivity in these electrocyclic reactions. The rules of torquoselectivity still hold, with a calculated 6-8 kcal/mol preference for outward rotation of the methyl and phenyl substituents. However, cyclization of the "out" dienes to pyran intermediates allows for isomerization and thermodynamic control of stereoselectivity.

Thermal ring-opening reaction of N-polynitromethyl tetrazoles: facile generation of nitrilimines and their reactivity

Thermal ring-opening reaction of N-(R-dinitromethyl)-5-nitrotetrazoles (R=NO 2, F, Cl) yielded highly reactive N-(R-dinitromethyl)-nitrilimine intermediates under mild conditions. These species underwent facile and regiospecific reaction with nitriles and acetylenes to afford the corresponding nitrotriazoles and nitropyrazoles in 50–90% yields. Treatment of N-(chlorodinitromethyl)-5-nitrotetrazole with the excess of azide led to a unique compound with the molecular formula C 2N 14. Based on the analytical data, it was assigned a structure of diazidomethylenecarbonohydrazonic diazide.

Synthesis and photochromic properties of thiophenophan-1-enes containing a polyether bridge

[2.n]Thiophenophan-1-ene derivatives containing a polyether bridge have been synthesized by the coupling reaction of bis(iodomethyl)dithienylethene with polyethylene glycols under high dilution conditions. All prepared [2.n]thiophenophan-1-enes were photochromic, as photocyclization of the open forms took place by UV irradiation to give a yellow solution of the closed form and their ring opening reactions occurred upon visible irradiation. There were significant differences in the quantum yields for the photocyclization reactions and the photo ring-opening reactions due to the difference in their structures since their electric properties are similar. The kinetic parameters of the thermal ring opening reaction of the closed forms of thiophenophan-1-enes were estimated and they were also dependent on their bridges.

Effect of Amino-Modified Silica Nanoparticles on the Corrosion Protection Properties of Epoxy Resin-Silica Hybrid Materials

In this paper, a series of organic-inorganic hybrid materials consisting of epoxy resin frameworks and dispersed nanoparticles of amino-modified silica (AMS) were successfully prepared. First of all, the AMS nanoparticles were synthesized by carrying out the conventional acid-catalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) in the presence of (3-aminopropyl)-trimethoxysilane (APTES) molecules. The as-prepared AMS nanoparticles were then characterized by FTIR, 13C-NMR and 29Si-NMR spectroscopy. Subsequently, a series of hybrid materials were prepared by performing in-situ thermal ring-opening polymerization reactions of epoxy resin in the presence of as-prepared AMS nanoparticles and raw silica (RS) particles. The as-prepared epoxy-silica hybrid materials with AMS nanoparticles were found to show better dispersion capability than that of RS particles existed in hybrid materials based on the morphological observation of transmission electron microscopy (TEM). The hybrid materials containing AMS nanoparticles in the form of coating on cold-rolled steel (CRS) were found to be much superior in corrosion protection over those of hybrid materials with RS particles when tested by a series of electrochemical measurements of potentiodynamic and impedance spectroscopy in 5 wt% aqueous NaCI electrolyte. The increase of corrosion protection effect of hybrid coatings may have probably resulted from the enhancement of the adhesion strength of the hybrid coatings on CRS coupons, which may be attributed to the formation of Fe-O-Si covalent bond at the interface of coating/CRS system based on the FTIR-RAS (reflection absorption spectroscopy) studies. The better dispersion capability of AMS nanoparticles in hybrid materials were found to lead more effectively enhanced molecular barrier property, mechanical strength, surface hydrophobicity and optical clarity as compared to that of RS particles, in the form of coating and membrane, based on the measurements of molecular permeability analysis, dynamic mechanical analysis, contact angle measurements and ultraviolet-visible transmission spectra, respectively.

Studies on the Thermal Ring-Opening Reactions of cis-3,4-Bis(organosilyl)cyclobutenes

Three cis-3,4-bis(organosilyl)cyclobutenes were synthesized, and their thermal ring-opening reactions were studied. The ring-opening reaction of cis-3,4-bis(trimethylsilyl)cyclobutene proceeded remarkably faster than that of cis-3,4-dimethylcyclobutene. The significant rate acceleration was explained by assuming (i) stabilization of the transition state by electron delocalization from the cyclobutene HOMO to the Si-CH3 sigma* orbital, (ii) destabilization of the ground state by intramolecular interaction between the C-Si sigma orbitals and the pi orbital of cyclobutene, and (iii) through-space steric repulsion of the two bulky trimethylsilyl groups in a cis arrangement. The ring-opening reaction of unsymmetrical cis-3,4-bis(arylsilyl)cyclobutenes having electronically different arylsilyl groups was also examined. The inward preference increased in the order, p-CH3OC6H4-Si, C6H5-Si, p-CF3C6H4-Si, supporting the interpretation of the origin of the inward preference of silyl substituents on the basis of a stabilizing interaction between the cyclobutene HOMO and the Si-C sigma* orbital at the transition state.

Torque control by metal-orbital interactions

Abstract The silyl substituent of 3-silylcyclobutene prefers inward rotation rather than outward rotation during a thermal ring-opening reaction, giving the Z-isomer predominantly. This intriguing behavior was explained by assuming electron-accepting interactions between the low-lying σ*-orbital of the silicon-carbon linkage and the highest occupied molecular orbital (HOMO) of the opening cyclobutene system, which are possible only in the inward transition state. On the basis of this finding, a novel method for the stereoselective synthesis of functionalized 1,3-butadiene derivatives from cyclobutenones was developed. Boryl substituents exhibit even stronger preference for inward rotation than silyl substituents as a result of electron delocalization from the cyclobutene HOMO into the vacant p-orbital of boron at the inward transition state.

Contrasteric Stereochemical Dictation of the Cyclobutene Ring-Opening Reaction by a Vacant Boron p Orbital

Cyclobutene, having a pinacolatoboryl group at the 3-position, was prepared by the reaction of trimethyl borate with a cyclobutenyl anion, which was generated by reductive lithiation of 3-(phenylselenyl)cyclobutene. Its thermal ring-opening reaction provided (Z)-1-borylbuta-1,3-diene selectively despite the arising steric congestion. The contrasteric behavior was accounted for by assuming an electron-accepting interaction of the vacant boron p orbital with the distorted breaking sigma orbital in the inward transition state.

Synthesis and thermal ring opening of trans-3,4-disilylcyclobutene.

The electrocyclic ring opening of substituted cyclobutenes is a classical textbook example of concerted pericyclic reactions that obey the Woodward–Hoffmann rules. Substituents located at the 3and 4-positions can move either toward the breaking bond (inward) or away from it (outward) during the thermal ring-opening reaction, provided that their movements are conrotatory. For example, (E)-penta-1,3-diene and the corresponding Z isomer can both arise from 3-methylcyclobutene without violating the Woodward–Hoffmann rules. However, experimentally, the E isomer is formed exclusively. The outward preference is intuitively reasonable, because significant steric congestion develops when the methyl substituent rotates inward during the ring-opening reaction. The inward transition state was calculated to be energetically higher than the outward transition state by 5.3 kcalmol 1 (MP2/631G(d)//3-21G) in the case of 3-methylcyclobutene. On the other hand, theoretical studies predicted that an electron-accepting substituent would prefer to rotate inward. The delocalization of electron density from the highest occupied molecular orbital (HOMO) of the opening cyclobutene skeleton to the electron-accepting substituent stabilizes the inward transition state. This prediction has been verified experimentally with carbonyl-substituted cyclobutenes. We recently discovered the remarkable effects of a silyl substituent located at the 3-position. 7] Although silicon is more electropositive than carbon, silyl substituents accelerate the reaction and promote inward rather than outward rotation, in spite of steric considerations. Stabilization of the inward transition state was explained by assuming that the energetically low-lying antibonding s* orbital of the silicon– carbon bond accepts electron density from the HOMO. 9] In the case of trans-3,4-disubstituted cyclobutenes, the disadvantage of the inward transition state in which both substituents rotate in the sterically unfavorable direction would be enormous. For example, (2E,4E)-hexadiene (2) is exclusively formed during the ring-opening of trans-3,4dimethylcyclobutene (1) (Scheme 1). The calculated energy difference of the inward and outward transition states amounts to 13.0 kcalmol 1 (RHF/3-21G). Organic chemistry textbooks present this reaction as a typical example of conrotatory ring opening. Previous studies on the effects of silyl substituents prompted us to examine the ring-opening reaction of trans3,4-bis(trimethylsilyl)cyclobutene (9 ; Scheme 2), in which the two methyl substituents of 1 are replaced with two significantly bulkier trimethylsilyl substituents. The steric congestion arising from the inward rotation of two bulkier substituents may become insurmountable. Thus, we consider this a true test of the power of electronic stabilization. The synthesis of trans-3,4-bis(trimethylsilyl)cyclobutene (9) is shown in Scheme 2. Cyclobutenedione 3, prepared according to a literature procedure, was reduced to the cis

Theoretical and experimental studies on the thermal ring-opening reaction of cyclobutene having a stannyl substituent at the 3-position.

The ring-opening reaction of 3-(trimethylstannyl)cyclobutene gave a mixture of the (Z)- and (E)-1,3-dienes, whereas that of 3-tert-butylcyclobutene exclusively afforded the (E)-1,3-diene due to the steric influences. The contrasting rotational behaviors suggest that there is some effect operating with the 3-stannylcyclobutene to stabilize the inward transition state, counteracting the steric influences. This contrasteric effect is ascribed to negative hyperconjugation. The stannyl group in the inward transition state accommodates electron density from the breaking sigma orbital of the cyclobutene into its low-lying tin-carbon sigma orbital. Theoretical studies supported this interpretation.

Origins of inward torquoselectivity by silyl groups and other sigma-acceptors in electrocyclic reactions of cyclobutenes.

Recent reports of inward torquoselectivities in thermal electrocyclic ring-opening reactions of 3-silylcyclobutenes have revealed that saturated silyl substituents, just like the extensively studied pi-acceptors, can exert contrasteric effects. The origins of torquoselectivity for substituents lacking pi orbitals have been explained using B3LYP density functional calculations. Orbital interactions involving the substituent vacant orbitals and the occupied orbitals associated with the breaking bonds are found to control these contrasteric torquoselectivities, with minor contributions from electrostatic effects. Reaction energetics and transition states for electrocyclic ring-opening reactions of 3-silyl, fluorosilyl, difluorosilyl, trifluorosilyl, methylsilyl, methyl, fluoromethyl, difluoromethyl, trifluoromethyl, ammonio, phosphonio, formyl, and borohydrido cyclobutenes are reported to complement previous extensive studies of unsaturated substituents. Inward stereoselectivities are predicted for various silyl and phosphonium substituents, along with potent pi-acceptors studied earlier. Cope and Diels-Alder reactions involving silyl substituents are also computed.

Theoretical quantum chemical study of spironaphthoxazines and their merocyanines: Thermal ring-opening reaction and geometric isomerization

We calculate potential energy curves of isomerization in the ground state of spironaphthoxazines (SNO) and their merocyanines (EZ, ZZ, EE and ZE) by ab initio methods. We find that in the curves cleavage of the spiro-bond gives firstly cisoid-ZZ, followed by the two paths to the ZZ isomer by rotation of the central C–N single bond or to the ZE isomer by inversion at the nitrogen atom. There is an interconversion path between the resulting ZZ and ZE, through cisoid-ZE. In addition, ZZ and ZE have paths to EZ and EE, respectively, by rotation of the CC bond. We also discuss the photochromic process in terms of the behavior of the specific merocyanines involved in the process.

A Silyl Substituent Can Dictate a Concerted Electrocyclic Pathway: Inward Torquoselectivity in the Ring Opening of 3-Silyl-1-cyclobutene

A preference for inward over outward rotation is found for the thermal conrotatory ring-opening reaction of 1-cyclobutene 1, to produce 1,3-butadiene 2, when a silyl substituent is located at the 3-position. This preference exists in spite of the bulkiness of the substituent.

Novel Graft Stabilizers for the Free Radical Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide

This paper reports the free radical polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO2) using novel graft copolymer stabilizers. The stabilizers are formed by the thermal ring-opening reaction of poly(methyl vinyl ether-alt-maleic anhydride) with 1H,1H,2H,2H-perfluorohexan-1-ol (1) or 1H,1H,2H,2H-perfluorooctan-1-ol (2). These stabilizers are effective at low concentrations (0.1−2 wt % with respect to MMA) for the dispersion polymerization of MMA leading to high yields of poly(methyl methacrylate) (PMMA) with reasonable molecular weights, narrow molecular weight distribution, and narrow particle size distribution. The increase of graft chain length by only two carbon units has a substantial effect on the stabilizers' activity.

Photooxidation of 1-Alkenes in Zeolites: A Study of the Factors that Influence Product Selectivity and Formation

Photochemical oxidation of hydrocarbons with molecular oxygen is potentially an environmentally benign method for the selective oxidation of hydrocarbons. In this study, in situ FT-IR spectroscopy and ex situ NMR spectroscopy were used to investigate the factors that influence product formation and selectivity in the room-temperature photooxidation of 1-alkenes in zeolites. Upon irradiation with broadband visible light, propylene, 1-butene, and 1-pentene loaded in BaY were photooxidized with molecular oxygen. As discussed in the literature, initial excitation of alkene and molecular complexes results in the selective formation of unsaturated aldehydes and ketones, proposed to occur through a hydroperoxide intermediate. In addition, epoxide and alcohol products are formed when the hydroperoxide intermediate reacts with an unreacted parent alkene molecule. Here it is shown that saturated aldehydes and ketones are formed as well through both a thermal ring-opening reaction of the epoxide in BaY and a second photochemical oxidation route involving a dioxetane intermediate. The yield of saturated aldehydes and ketones increased with decreasing wavelength, increasing temperature, and at a given temperature and wavelength, increasing chain length. Photooxidation of propylene in BaX, BaZSM-5, and BaBeta zeolites was also investigated. Photooxidation in BaX is very similar to that of BaY. In zeolites, BaZSM-5 and BaBeta, propylene polymerized upon adsorption. The polymer, polypropylene, also undergoes photooxidation with molecular oxygen to form an oxygenated polymer product. The results of this study show that product formation and selectivity in the photooxidation of 1-alkenes in zeolites depends on several factors. These factors include thermal reactions of the reactant and photoproduct molecules in the zeolite at ambient temperatures. Several reactions of 1-alkenes in cation-exchanged zeolites contribute to the loss of selectivity; they include: epoxide ring opening, double-bond migration, and alkene polymerization. Some of these reactions are proposed to occur at Brønsted acid sites that are present in various amounts in cation-exchanged zeolites.

New hyperbranched poly(ether amide)s via nucleophilic ring opening of 2-oxazoline-containing monomers

An AB 2 monomer containing one oxazoline and two phenolic units was synthesized. The polymerization of the monomer 2-(3,5-dihydroxyphenyl)-1,3-oxazoline in N-methylcaprolactam resulted in a well-defined and fully soluble, high molar mass, hyperbranched polymer with etheramide structure. The hydrolysis of some oxazoline groups as side reaction limits the achieved molar mass when the polymerization is carried out in tetramethylene sulfone or in bulk. The polymers were characterized by one (1D) and two (2D) dimensional 1 H and 13 C NMR spectroscopy which allowed to determine the degree of branching to be 50% as expected from statistics. DSC and TGA measurements revealed a glass transition temperature of 176°C and a decomposition onset of 330°C. The thermal ring-opening reaction was studied in situ by DSC measurements.

Control of the thermal reaction of a photochromic spirobenzopyran by the enzyme-like activity of albumins

The thermal ring-opening reaction of a negative photochronic spirobenzopyran was investigated in the presence and absence of albumins. In the presence of the proteins, the formation of the merocyanine form from the spiro form is enhanced markedly by two orders of magnitude. The results for five different albumins indicate the enzyme-like activity of the proteins. The influence of pH, temperature and various ligands was examined in detail for bovine serum albumin (BSA) and human serum albumin (HSA). In particular, it was deduced that warfarin shows reversible purely competitive inhibition for BSA with an inhibitor constant K 1 of 1.6×10 −3M. The results suggest that the catalytic centre for the reaction with the spirobenzopyran is different for BSA and HSA. Albumin can control the velocity of the thermal reaction of the photochromic system.

SOLVENT EXTRACTION OF BORON WITH l,2-DIHYDROXY-4-OXADODECANE (DHD) IN n-AMYL ALCOHOL

ABSTRACT In the work presented here, l,2-dihidroxy-4-oxadodecane (DHD) was synthesized and used for the solvent extraction of boron. DHD was prepared from glycidol and octanol by thermal epoxide ring opening reaction without using acid or base catalyst. The structure of this new extracting reagent was identified according to its spectroscopic data i.e. 13C NMR, 1H NMR and elemental analysis results. The fixed standard solution of boron (as borax) of 10−4 M was extracted with different volume ratios of DHD and n-amyl alcohol (AA) at different equilibrium pH values. The best extraction result was obtained at high pH values with a mixture of equal volumes of DHD and n-amyl alcohol. The different equilibrium pH values were plotted against the extraction yields (%R) and the pH0·5 value was determined by using this graph. The extraction of boron from aqueous solutions with equal volumes of DHD and various solvents were studied and it was found that petroluem ether, n-amyl alcohol and diisopropyl ether are convinient solvents for boron extraction. Stripping of boron loaded organic phase was achieved with H2SO4 aqueous solutions. The effect of stripping as a function of H2SO4 cocentration was also studied

Cyclopropyl building blocks for organic synthesis. Part 22. Facile synthesis of stable analogs of 2-oxocyclobutanecarboxylates: 2-[(diphenylmethylene)amino]cyclobutenecarboxylates, derivatives and reactions

An efficient two-step synthesis of 2-[(diphenylmethylene)amino] cyclobutenecarboxylate (4a) and some analogous derivatives from 2-chloro-2-cyclopropylideneacetates 2, 17, 22, and 25 and nonenolizable ketimines, especially diphenylmethyleneamine (DPMA-H), is described. A likely mechanism for the formation of 4a from the primary Michael adduct 3 of DPMA-H to 2 and its substituted analogues is presented. The unique neighboring group effect of the DPMA moiety to allow formation of an azaspiropentane intermediate and its regioselective rearrangement to cyclobutenamine derivatives is discussed and further exemplified by an extremely facile SET α-chlorination. Compound 4a and derivatives undergo a thermal ring-opening reaction to the corresponding butadienes with subsequent formation of 1,3-disubstituted 3,4-dihydroisoquinolines 39. Further transformations of 4a and some derivatives include transesterification, hydrolysis to methyl 2-oxocyclobutanecarboxylates, and addition of N-phenyltriazolinedione

Cycloaddition reactions of cyclophanes

Abstract

ポリフルオロアルキル基を有する第四級アンモニウム塩の反応 ＩＩ １‐（ポリフルオロアルキル）‐４‐アザ‐１‐アゾニアビシクロ〔２．２．２〕オクタン塩の熱開環反応

ポリフルオロアルキル基を有する第四級アンモニウム塩の反応 ＩＩ １‐（ポリフルオロアルキル）‐４‐アザ‐１‐アゾニアビシクロ〔２．２．２〕オクタン塩の熱開環反応