Electrocyclic Ring Closure Reactions Research Articles

DMSO-assisted environmentally benign synthesis of benzo[c]-chromeno[4,3,2-gh]phenanthridines by remote oxidative hetero cross-coupling cyclization and aromatization reaction.

An unprecedented metal-free and catalyst-free synthesis of benzo[c]chromeno[4,3,2-gh]phenanthridine derivatives, a class of 1,6-diheterophenalenoid heterocycle, is reported for the first time. The oxidative cross-coupling reaction for the remote cyclization is achieved through the in situ generated o-quinone methide intermediate followed by an electrocyclic ring closure reaction. The aromatization of the cyclohexane ring is achieved by sequential H shift, hydroxylation, and elimination reaction. DMSO-assisted concomitant cyclization and aromatization reactions are also disclosed for the first time.

Designing Force Probes Based on Reversible 6π-Electrocyclizations in Polyenes Using Quantum Chemical Calculations.

The conjugated π-system in polyenes can be interrupted by electrocyclic ring-closure reactions. In this work, this 6π-electrocylization is shown by means of density functional calculations to be reversible by the application of an external mechanical pulling force at the terminal ends of the interrupted polyene chain. The test systems were constrained in a fused ring system, thus locking the orientation of three π-bonds and generally promoting 6π-electrocyclic ring-closure reactions. For several systems, the forward reaction is exergonic and the corresponding reaction barrier is comparable to those reported in the literature. The reverse reaction is triggered by an external pulling force of 2 nN (nano-Newton) or less and also becomes exergonic in all investigated polyenes under these force conditions. Moreover, it proceeds via a low reaction barrier when a pulling force of 2 nN is active, indicating that the mechanical force is an efficient stimulus for triggering ring-opening reactions. Analysis of the strain energy induced by this mechanical force confirms an optimal activation of the corresponding C-C σ-bond that breaks upon ring opening when the pulling positions are located on the polyene chain.

Construction of Complex Bisether-Bridged Medium-Sized Cyclic Compounds from o-(1-(Acyloxy)propargyl)benzaldehydes under Base and Acid Catalysis.

We report herein our serendipitous discovery of the rapid and straightforward accesses to unprecedented diverse complex molecular structures from readily available starting materials. Catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene under mild conditions, o-(1-(acyloxy)propargyl)benzaldehydes 1 underwent efficient and selective dimerization reactions to produce novel complex bisether-bridged tricyclic products 3 and 4. The reactions proceeded most probably through dimerizations between 3-methylene-3H-isochromene intermediate and its zwitterionic resonance structures which were generated from a concerted 6-π electrocyclic ring closure reaction from the initially formed (2-formylphenyl)allene intermediates derived directly from o-(1-(acyloxy)propargyl)benzaldehydes. Treatment of the resulting product simply with NaOEt in ethanol and aqueous HCl, respectively, enabled further development of complex molecular diversities.

Synthesis of 1,2,3,4,5‐Penta(methoxycarbonyl)cyclopentadienides through Electrocyclic Ring Closure and Ring Contraction Reactions

The mechanism for the reaction of dimethyl malonate with dimethyl acetylenedicarboxylate in the presence of pyridine and acetic acid to form isomeric octa(methoxycarbonyl)cycloheptadienes was established. The formation of intermediary N‐[di(methoxycarbonyl)vinyl]pyridinium acetate was proven by means of NMR spectroscopy. On the basis of the established mechanism, dimethyl dibromosuccinate was proposed to be used instead of dimethyl acetylenedicarboxylate. A novel method for the synthesis of penta(methoxycarbonyl)cyclopentadienylsodium through the reduction of hepta(methoxycarbonyl)cycloheptatriene with sodium borohydride, electrocyclic ring contraction and retro‐[2+2]‐cycloaddition was developed.

Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene.

The thermal electrocyclic ring-closure reaction of vinylheptafulvene (VHF) to form dihydroazulene (DHA) is elucidated herein by using DFT and 1 H NMR spectroscopy. Two different transition states were found computationally; one corresponds to a disrotatory pathway, which is allowed according to the Woodward-Hoffmann selection rules, whereas the other corresponds to a conrotatory pathway. The conrotatory pathway is found to be zwitterionic in the transition state, whereas the disrotatory transition state varies in zwitterionic character depending on solvent and substituents in the molecular framework. The conrotatory and disrotatory transition states are found to have similar energy and their relative stability varies with solvent polarity and functionalization at the C1 position. To support these findings, we chemically ring-opened diastereomerically pure 1-(benzothiazol-2-yl)-DHA to give the VHF form, then subsequently thermally reconverted the VHF to DHA in a range of solvents with various polarities. We found that, depending on solvent polarity, different ratios of anti- and syn-diastereoisomers of DHA were formed in a systematic manner, which supports the existence of two distinct thermal ring-closure pathways for VHF.

A Study of Electrocyclic Reactions in a Molecular Junction: Mechanistic and Energetic Requirements for Switching in the Coulomb Blockade Regime.

Molecular photoswitches incorporated in molecular junctions yield the possibility of light-controlled switching of conductance due to the electronic difference of the photoisomers. Another isomerization mechanism, dark photoswitching, promoted by a voltage stimulus rather than by light, can be operative in the Coulomb blockade regime for a specific charge state of the molecule. Here we elucidate theoretically the mechanistic and thermodynamic restrictions for this dark photoswitching for donor-acceptor substituted 4n and 4n+2 π-electron open-chain oligoenes (1,3-butadiene and 1,3,5-hexatriene) by considering the molecular energies and orbitals of the molecules placed in a junction. For an electrocyclic ring closure reaction to occur for these compounds, we put forward two requirements: a) the closed stereoisomer (cis or trans form) must be of lower energy than the open form, and b) the reaction pathway must be in accordance to the orbital symmetry rules expressed by the Woodward-Hoffmann rules (when the electrodes do not significantly alter the molecular orbital appearances). We find these two requirements to be valid for the dianion of (1E,3Z,5E)-hexa-1,3,5-triene-1,6-diamine, and the Coulomb blockade diamonds were therefore modeled for this compound to elucidate how a dark photoswitching event would manifest itself in the stability plot. From this modeling of conductance as a function of gate and bias potentials, we predict a collapse in Coulomb diamond size, that is, a decrease in the height of the island of zero conductance.

Thermodynamics of an Electrocyclic Ring-Closure Reaction on Au(111)

We have computationally studied the effects of temperature on the reaction pathway of an electrocyclic ring-closure reaction on the Au(111) surface, particularly focusing on thermodynamic aspects of the reaction. The electrocyclic ring closure is accompanied by a series of dehydrogenation steps, and while it is found that temperature, in terms of vibrational entropy and enthalpy, has a reducing effect on most energy barriers, it does not alter the qualitative appreciation of the reaction kinetics. However, it is found that the way the abstracted hydrogen atoms are treated is crucial for the thermodynamics of the reaction. The overall reaction is highly endothermic but becomes thermodynamically favorable due to the entropy gain of the hydrogen byproducts, which desorb associatively from the surface as H2. The study provides new outlooks for the theoretical treatment of reactions related to on-surface synthesis, anticipated to be instructive for future studies.

Formation of Thermally Robust Frustrated Lewis Pairs by Electrocyclic Ring Closure Reactions

AbstractThe phosphorus/boron‐substituted hexatriene systems 6 undergo thermally induced electrocyclic ring closure to yield the cyclohexadiene‐derived P/B frustrated Lewis pairs (FLPs) 7. Subsequent TEMPO oxidation gives the phenylene‐bridged FLPs 8. Both systems activate dihydrogen and the thermally robust FLPs undergo carbon–carbon coupling reactions at a mesityl group upon treatment with dimethyl acetylenedicarboxylate at elevated temperatures.

Formation of Thermally Robust Frustrated Lewis Pairs by Electrocyclic Ring Closure Reactions.

The phosphorus/boron-substituted hexatriene systems 6 undergo thermally induced electrocyclic ring closure to yield the cyclohexadiene-derived P/B frustrated Lewis pairs (FLPs) 7. Subsequent TEMPO oxidation gives the phenylene-bridged FLPs 8. Both systems activate dihydrogen and the thermally robust FLPs undergo carbon-carbon coupling reactions at a mesityl group upon treatment with dimethyl acetylenedicarboxylate at elevated temperatures.

ChemInform Abstract: The Thriving Chemistry of Ketenimines

AbstractReview: ca. 100 refs.

Synthesis, X-ray Structure and Photo Chemistry of 2-(Methyl(1-Phenylvinyl)Carbamoyl)Phenyl Acetate

2-(Methyl(1-phenylvinyl)carbamoyl)phenyl acetate (SM), empirical formula C18H17NO3, crystallizes in the triclinic space group, P−1, with unit cell parameters a = 9.10870(10) A, b = 10.56940(10) A, c = 16.1340(2) A, α = 78.0590(10)°, β = 89.5080(10)°, γ = 86.6830(10)°, Z = 4. The 2-(methyl(1-phenylvinyl)carbamoyl)phenyl acetate (SM) molecule, upon irradiation of UV-light below 3,000 A, releases acetate group to give six membered cyclic lactam 1 as the major product. The quantum yield for the lactam 1 was found to be 0.053. An alternate conrotatory 6-pi electron photochemically allowed electrocyclic ring closure reaction also occurs upon irradiation to give a six-membered cyclic zwitterionic intermediate which retains acetate as leaving group. A [1,5]-H shift would then give the isomerized product 2. The quantum yield for the lactam 2 was found to be 0.026. A minor oxidation product lactam 3 that retains the leaving group also formed, likely from loss of a proton and tautomerization of enol. The loss of one hydrogen molecule is the driving force for revival of aromaticity in the minor product 3. The quantum yield for the lactam 3 was found to be 0.012. The low value of total quantum yield (0.091) for photocyclization of SM can be explained as the photoproducts absorb light in competition with the photoreactant. In this study, Synthesis, X-ray structure and photochemistry of 2-(methyl(1-phenylvinyl)carbamoyl)phenyl acetate (SM) is reported .

Synthesis of Pyrroles through a 4π-Electrocyclic Ring-Closure Reaction of 1-Azapentadienyl Cations

1-Azapenta-1,4-diene-3-ols 4a-m are easily accessible from 1-azapenta-1,4-dien-3-ones 3a-i and organolithium compounds. Treatment of the compounds 4a-m with strong acid (triflic acid) generates 1-azapentadienyl cations in situ upon protonation at the hydroxyl oxygen atom and subsequent water elimination. The intermediate cations undergo facile 4π-electrocyclization under ambient condition to give diversely substituted pyrroles 6a-m in moderate to good yield. The product pyrrole 6k could be characterized by X-ray diffraction. Quantum chemical calculations were performed to elucidate the mechanism of this reaction with respect to starting compounds, transition states, and products. They support the proposed mechanism of a 4π-conrotatory Möbius-type electrocyclic ring-closure reaction.

The thriving chemistry of ketenimines

Ketenimines are an important class of reactive species and useful synthetic intermediates. During the last two decades several practical and versatile approaches to ketenimines have been developed, leading to exhaustive investigations on ketenimine chemistry and the discovery of a variety of highly efficient reactions. Five types of reactions for ketenimines have been reported, including nucleophilic additions (ketenimines can be used as both electrophiles and nucleophiles), radical additions, cycloaddition reactions, electrocyclic ring closure reactions, and σ rearrangements. Furthermore, numerous complex organic compounds, particularly the biologically interesting heterocycles, have been constructed using these methodologies. The review of these accomplishments is presented here.

X-Ray Crystal Structure and Photo Chemistry of N-Methyl-N-(1-phenylvinyl)benzamide

N-methyl-N-(1-phenylvinyl)benzamide (I) has empirical formula C16H15NO, crystallizes in the monoclinic space group, P21/n, with unit cell parameters a = 8.9101(2) A ˚ , b = 15.1416(4) A ˚ , c = 9.7737(2) A ˚ , a = 90, b = 109.3320(10), c = 90, Z = 4. Interestingly, the Compound (I) undergoes photocyclization upon irradiation of light to give two cyclic lactams (A and B) below 3100 A ˚ . Compound (I) can be cyclized at either of two equivalent ortho positions. The mechanism for formation of lactam A may involve electrocyclic ring closure reaction upon irradiation of light to give six-membered cyclic zwitterionic intermediate. The ring proton rearranges via (1, 5)-H shift to give isomerized product A. The minor photoproduct B is supposed to be formed via photocyclization process fol- lowed by a loss of proton and the tautomerized product undergoes oxidation for the revival of aromaticity.

Competing Thermal Electrocyclic Ring-Closure Reactions of (2Z)-Hexa-2,4,5-trienals and Their Schiff Bases. Structural, Kinetic, and Computational Studies

The electrocyclic ring closure reactions of (2Z)-hexa-2,4,5-trienals (vinylallenals) to alkylidene-2H-pyrans and of the corresponding Schiff base derivatives to alkylidenepyridines can be concurrent. Rates of vinylallenal cyclization and imine formation upon addition of n-butylamine determine the extent of the competition. The activation energies for the electrocyclization of a series of 6-substituted-(2Z)-4-tert-butyl-3-methylhexa-2,4,5-trienals 2 and trienimines 4 depend on the steric interactions between the substituents at C6 and the tert-butyl group at C4. Mixtures of alkylidene-2H-pyrans 3 and alkylidenepyridines 5 are obtained with bulky groups at C6, whereas only the latter is obtained with a C6-t-Bu and only the former with substituents of moderate size at C6. The reaction rates were indirectly derived from the empirical observations using a global optimization study based on differential evolution. The cyclizations are torquoselective, and the kinetically favored (E)-alkylidene heterocycles evolve by electrocyclic ring opening/ring closure toward the thermodynamic Z isomers upon extended reaction times. Density functional theory (DFT) calculations of the electrocyclizations helped in their characterization as monorotatory processes with pseudopericyclic features and made it possible to rationalize the reactivity trends and the torquoselectivity.

DFT Study of the Mechanisms of In Water Au(I)-Catalyzed Tandem [3,3]-Rearrangement/Nazarov Reaction/[1,2]-Hydrogen Shift of Enynyl Acetates:  A Proton-Transport Catalysis Strategy in the Water-Catalyzed [1,2]-Hydrogen Shift

A computational study with the Becke3LYP density functional was carried out to elucidate the mechanisms of Au(I)-catalyzed reactions of enynyl acetates involving tandem [3,3]-rearrangement, Nazarov reaction, and [1,2]-hydrogen shift. Calculations indicate that the [3,3]-rearrangement is a two-step process with activation free energies below 10 kcal/mol for both steps. The following Nazarov-type 4pi electrocyclic ring-closure reaction of a Au-containing dienyl cation is also easy with an activation free energy of 3.2 kcal/mol in CH2Cl2. The final step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a computed activation free energy of 20.2 kcal/mol) when dry CH2Cl2 is used as the solvent. When this tandem reaction was conducted in wet CH2Cl2, the [1,2]-hydride shift step in dry solution turned to a very efficient water-catalyzed [1,2]-hydrogen shift mechanism with an activation free energy of 16.4 kcal/mol. Because of this, the tandem reaction of enynyl acetates was found to be faster in wet CH2Cl2 as compared to the reaction in dry CH2Cl2. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule is involved in catalysis or a proton-relay stabilizer when a water cluster is involved in catalysis. Water is found to act as a proton shuttle in the proton-transport catalysis strategy. Theoretical discovery of the role of the acetoxy group in the water-catalyzed [1,2]-hydrogen shift process suggests that a transition metal-catalyzed reaction involving a similar hydrogen shift step can be accelerated in water or on water with only a marginal effect, unless a proton-accepting group such as an acetoxy group, which can form a hydrogen bond network with water, is present around this reaction's active site.

Novel photosensitive polyurethanes based on photochromic dithienylethene monomers: synthesis, characterization and photophysical properties of a new film-building material for photonic applications

Abstract The synthesis of a novel photochromic dithienylethene-based polyurethane and its characterization by optical spectroscopy and scanning tunneling microscopy are presented. This linear polyurethane builds up perfect free-standing films suited for photonic applications. Moreover it may be extruded to long mechanically stable fibers. The photochromic reactions of the polyurethane, as being the electrocyclic ring-closure and ring-opening reactions of the dithienylethene unit in the polymer backbone, occur in a completely reversible and highly efficient fashion.

Quantum-Chemical Treatment of Cyclization Reactions: XXIV. Superposition-Additivity Approach

The superposition-additivity approach which is characterized by a very simple calculation scheme provides a sufficiently reliable description of electronic structure, physicochemical parameters, and chemical behavior (specifically, in electrocyclic ring closure reactions) of large organic molecules with conjugated double bonds on the basis of the calculation or experimental data for less extended analogs.

Theoretical Investigation of Electrocyclic Ring Closure Reactions in Bis(aryloxy)bis(η2-iminoacyl)zirconium and Isoelectronic Complexes

A theoretical investigation of the ligand coupling reactions in the isoelectronic series (HO)2Zr(η2-CHO)2, 9, (HO)2Zr(η2-CHNH)2, 10, and (HO)2W(η2-HCCH)2, 11, has been carried out at the hybrid density functional level of theory. The structures of 9−11 and their experimentally observed analogues are isolobal to 20-valence-electron species of the form Cp2M(η2-L)2 which have previously been shown to have a Cp2M(η2-L)(η1-L) structure. Our calculations show that the bent geometry of the (HO)2M fragment greatly reduces metal−oxygen π-overlap in two of the four metal−oxygen π-interactions, which allows the (HO)2M metal fragment to bind two ligands in an η2-fashion without exceeding coordinative saturation at the metal. The mechanism of formyl, iminoformyl, and acetylene coupling in 9−11 follows a path in which the coordinated ligands undertake a scissors-like motion that pulls the carbon atoms away from the metal and toward one another. In agreement with experimental observations, formyl coupling in 9 occurs more rapidly (ΔE⧧ = 14.0 kcal/mol) than iminoformyl coupling in 10 (ΔE⧧ = 19.4 kcal/mol), which, in turn, occurs more rapidly than acetylene coupling in 11. This latter reaction was found to be formally symmetry forbidden. We attribute this relative order of reactivity to the steadily decreasing electronegativity of the η2-bound ligands that is manifested in two ways in the electronic structures of 9−11 as they undergo ligand coupling. Overall the conversions of 9 to enediolate 12, 10 to enediamide 13, and 11 to metallacyclopentatriene 14 are strongly exothermic by 58.1, 45.4, and 21.6 kcal/mol, respectively.

ChemInform Abstract: Synthesis of the Naphthoic Acid Component of Kedarcidin Chromophore by Routes Employing Photochemical and Thermal Electrocyclic Ring Closure Reactions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.