Thermal Electrocyclic Ring Opening Research Articles

Modifying Woodward–Hoffmann Stereoselectivity Under Vibrational Strong Coupling

AbstractVibrational strong coupling (VSC) has recently been shown to change the rate and chemoselectivity of ground‐state chemical reactions via the formation of light–matter hybrid polaritonic states. However, the observation that vibrational‐mode symmetry has a large influence on charge‐transfer reactions under VSC suggests that symmetry considerations could be used to control other types of chemical selectivity through VSC. Here, we show that VSC influences the stereoselectivity of the thermal electrocyclic ring opening of a cyclobutene derivative, a reaction which follows the Woodward–Hoffmann rules. The direction of the change in stereoselectivity depends on the vibrational mode that is coupled, as do changes in rate and reaction thermodynamics. These results on pericyclic reactions confirm that symmetry plays a key role in chemistry under VSC.

Modifying Woodward-Hoffmann Stereoselectivity Under Vibrational Strong Coupling.

Vibrational strong coupling (VSC) has recently been shown to change the rate and chemoselectivity of ground‐state chemical reactions via the formation of light–matter hybrid polaritonic states. However, the observation that vibrational‐mode symmetry has a large influence on charge‐transfer reactions under VSC suggests that symmetry considerations could be used to control other types of chemical selectivity through VSC. Here, we show that VSC influences the stereoselectivity of the thermal electrocyclic ring opening of a cyclobutene derivative, a reaction which follows the Woodward–Hoffmann rules. The direction of the change in stereoselectivity depends on the vibrational mode that is coupled, as do changes in rate and reaction thermodynamics. These results on pericyclic reactions confirm that symmetry plays a key role in chemistry under VSC.

Altered torquoselectivity of fluorine in the iron-tricarbonyl-mediated thermal ring opening of 3-fluorocyclobutene: a density-functional exploration

There are eight possible pathways for the iron-tricarbonyl-assisted thermal electrocyclic ring opening of fluorocyclobutene due to variations in the orientation of the binding of Fe(CO)3 relative to the fluorine substituent (R1 or R2), stereoselectivity (conrotatory or disrotatory, i.e., C or D), and the torquoselectivity of fluorine (inward-facing or outward-facing, i.e., in or out). A density functional study revealed that the energetically favored pathway is R1 Din. Not only is the D mode favored energetically, but the in configuration was observed to be the lowest-lying mode of R1, despite the general tendency of fluorine substituents to prefer an out configuration. Data on the activation hardness and aromaticity indices such as BAC and HOMA lead to the same conclusion. However, the R2 mode surprisingly shows no particular preference for either the Cout or the Dout pathway (i.e., the R2 mode shows less stereoselectivity than R1). This behavior occurs due to the influences of both the fluorine substituent and metal coordination. Also, the geminal bond orbitals σ(C-F) and σ*(C-F) appear to participate in ring opening, given the excellent correlation of ∆BE with the activation barrier of the transition state.

Rh(I)-catalyzed benzo/[7+1] cycloaddition of cyclopropyl-benzocyclobutenes and CO by merging thermal and metal-catalyzed C-C bond cleavages.

A Rh-catalyzed benzo/[7+1] cycloaddition of cyclopropyl-benzocyclobutenes (CP-BCBs) and CO to benzocyclooctenones has been developed. In this reaction, CP-BCB acts as a benzo/7-C synthon and the reaction involves two C-C bond cleavages: a thermal electrocyclic ring-opening of the four-membered ring in CP-BCB and a Rh-catalyzed C-C cleavage of the cyclopropane ring.

1,3‐Dipolar Cycloaddition of Azomethine Ylides to Aldehydes: Synthesis of anti α‐Amino‐β‐Hydroxy Esters

AbstractA 1,3‐dipolar cycloaddition reaction of azomethine ylides to aldehydes is described. The azomethine ylides, generated by thermal electrocyclic ring opening of aziridines, adds to aldehydes in good yields with moderate to good selectivities to furnish oxazolidines. The oxazolidines were subsequently hydrolyzed to the corresponding amino alcohols, giving the anti diastereomer as the major product.

Stereoselective Olefination of Unfunctionalized Ketones via Ynolates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective olefination of unfunctionalized ketones via ynolates.

Ynolates react with ketones at room temperature to afford alpha,beta,beta-trisubstituted acrylates (tetrasubstituted olefins) with 2:1-8:1 geometrical selectivities. This can be regarded as a new olefination reaction of ketones giving tetrasubstituted olefins in good yield, even in the case of sterically hindered substrates. The reaction mechanism involves cycloaddition of ynolates with a carbonyl group and subsequent thermal electrocyclic ring-opening of the resulting beta-lactone enolates. The stereoselectivity is determined in the ring-opening, which is regulated by torquoselectivity. In this paper, we describe the scope and limitations of olefination of ketones via ynolates and discuss the stereocontrol mechanism.

Demoniac Intervention in the Thermal Electrocyclic Ring Opening of Cyclobutenes: Fe(CO)3 Complexation of Pericyclic Transition Structures

Can metal complexation of organic π-systems alter the stereoselectivity of pericyclic reactions? We address this question for the Fe(CO)3-templated electrocyclic ring opening of cyclobutene using hybrid HF-DFT calculations. These calculations show that the preferred pericyclic pathway in the presence of Fe(CO)3 corresponds to the pathway that is orbital-symmetry-forbidden in its absence, validating predictions made previously based on extended Hückel calculations. The three-dimensional aromaticity of the preferred metal-complexed transition state is revealed by NICS calculations.

Demoniac Intervention in the Thermal Electrocyclic Ring Opening of Cyclobutenes: Fe(CO)3 Complexation of Pericyclic Transition Structures

Demoniac Intervention in the Thermal Electrocyclic Ring Opening of Cyclobutenes: Fe(CO)3 Complexation of Pericyclic Transition Structures

Synthesis of cis-Disubstituted Cyclobutenyl Nucleoside Analogues

cis-Disubstituted cyclobutene nucleosides analogues were prepared by a linear synthesis starting from cis-cyclobutene dicarboxylic anhydride. This strategy involved mild reaction conditions with intent to restrict the thermal electrocyclic ring opening into (Z,E)-dienes.

スクアリン酸を合成素子とする多官能型炭素および複素環化合物の高選択的合成

New method for derivatization of squaric acid was developed by means of Lewis acid-catalyzed reaction of the acid family with unsaturated organosilanes to give 4-substituted 4-hydroxy (or, 4- ethoxy) -2-cyclobutenones. Thermal electrocyclic ring opening of 4-allylated and 4-acylmethylated products was followed by intramolecular [2 + 2] cycloaddition and cyclization of minor stereoisomer of enol ketene to give bicyclo [3.2.0] heptenones and tetronates, respectively. Further, new synthetic potential as a C4-synthon was found in ring transformation of 4-hydroxycyclobutenones involving radical- and cation-mediated ring opening and reclosure processes, which afforded highly substituted 2- (5H) furanones and 4-cyclopentene-1, 3-diones. These reactions included interesting mechanistic features such as 5-endo cyclization of acyl radical, ionic rearrangement of hypoiodite and 1, 2-silylmigration of alkynylsilane. In particular, ab initio calculation for 5-endo cyclization of 5-oxa-2, 4-pentadienoyl radical suggested unusual non-radical cyclization mechanism. The synthetic utility of the above ring transformation strategies was demonstrated in natural product synthesis of (E) -basidalin and (Z) -multicolanate and construction of a triquinane skeleton.

The preparation and electrocyclic ring-opening of cyclobutenes: Stereocontrolled approaches to substituted conjugated dienes and trienes

Thermal electrocyclic ring-opening of 4-alkyl-2-cyclobutene-1-carbaldehydes occurs at low temperature to give (2 Z,4 E)-alkadienals exclusively, and the process is exploited in transforming cis-3-cyclobutene-1,2-dimethanol 1 into a variety of naturally occurring 1,3,5-alkatrienes and 2,4-decadienoates. Desymmetrisation of 1 with Pseudomonas fluorescens lipase gives access to both enantiomers of 3-oxabicyclo[3.2.0]hept-6-en-2-one 4, for use in stereocontrolled routes to 6-oxygenated (2 Z,4 E)-alkadienals.

ChemInform Abstract: Thermal Electrocyclic Ring‐Opening of Cyclobutenes (I), (IV): Substituents with Complementary Conrotatory Preferences Induce High Stereoselectivity.

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.

Thermal electrocyclic ring-opening of cyclobutenes: stereoselective routes to functionalised conjugated ( Z,E)- and ( E,E)-2,4-dienals

The cyclobutenecarbaldehyde 12 undergoes thermal electrocyclic ring-opening at low temperature, producing the (2 Z,4 E)-hexadienal 13 exclusively. By contrast, unsymmetrical derivatives of cis-3-cyclobutene-1,2-dicarboxylic acid undergo ring-opening at 80–110°C with low levels of stereoselectivity, which vary according to the balance of the electronic and, to a lesser extent, the steric nature of the substituents located on the rehybridising carbon atoms.

36-Dimethoxybenzocyclobutenone: a reagent for quinone synthesis

3,6-Dimethoxybenzocyclobutenone 4 is prepared in four efficient steps from 2,5-dimethoxybenzoic acid 8. The derived benzocyclobutenol 13 undergoes electrocyclic ring opening at 110–115°C to give the hydroxy- o-quinone dimethide 21, which reacts with dienophiles to give 5,8-dimethoxy-1,2,3,4-tetrahydro-1-naphthol derivadves stereoselecdvely. Since the ketone 4 can be functionalised at C-5 using electrophiles and at C-2 via hoimolytic bromination, the ring opening and cycloaddition sequence offers a flexible route to linear fused hydroquinone and quinone derivatives. In model studies, the benzocyclobutenol derivative 48 underwent thermal electrocyclic ring opening and intramolecular cycloaddition to give 49, while the analogous reaction with 52 failed due to adverse steric effects during the cycloaddition step. In photochemical experiments, attempts to generate the silyi ether 57 by in situ silylation of the dienol 55 and to prepare the benzocyclobutenol 62 via irradiation of the o-phthalaldehyde monoacetal 60 were unsuccessful.

Ringerweiterung bei der Reaktion eines 1,3‐Cyclohexandions mit Diphenylcyclopropenon

Ring Expansion during the Reaction of a 1,3‐Cyclohexanedione with DiphenylcyclopropenoneThe reaction of 5,5‐dimethyl‐1,3‐cyclohexanedione (1) in form of its Na‐salt with diphenylcyclopropenone (2) in DMF yielded the bicyclic triketone 3 (58 %), the structure of which was deduced as an enolizeable bicyclo[5.2.0]nonane‐β‐diketone from spectral data and from the following reactions: hydrolysis or methanolysis of 3 cleaved the β‐dicarbonyl moiety, thereby opening the 4‐membered ring to yield the keto acid 9 or its methyl ester 10. Methylation of 3 afforded the two enol ethers 4 and 5. The ether 5 readily underwent a thermal electrocyclic ring opening to the monocyclic enol ether 8, whereas the ether 4 was thermally stable. The same electrocylic ring opening (in boiling benzene) converted 3 (probably via 3b) to the monocyclic triketone 7, which was also the hydrolysis product of the ring‐opened enol ether 8. By heating 3 in DMF/H2O, a partial (56 %) conversion to the lactone 6 took place. The tricyclic intermediate 11 was found useful to rationalize the ring expansion during the formation of 3 from 1 and 2 as well as the corresponding ring contraction during the conversion of 3 into 6.