5-substituted Cyclopentadienes Research Articles

Hyperconjugative Aromaticity and Antiaromaticity Control the Reactivities and π-Facial Stereoselectivities of 5-Substituted Cyclopentadiene Diels-Alder Cycloadditions.

The reactivities and π-facial stereoselectivities of Diels-Alder reactions of 5-substituted cyclopentadienes were studied using density functional theory. Burnell and co-workers previously showed that the π-facial selectivities result from the energies required to distort the reactants into the transition state geometries. We have discovered the origins of these distortions. C5-X σ-donors predistort the cyclopentadiene into an envelope conformation that maximizes the stabilizing hyperconjugative interaction between the C5-X σ-bond and the diene π-system. This envelope conformation geometrically resembles the anti transition state. To minimize the destabilizing effect of negative hyperconjugation, C5-X σ-acceptors predistort in the opposite direction toward an envelope geometry that resembles the syn transition state. We now show how hyperconjugative effects of the C5-X substituent influence the stereoselectivities and have developed a unified model rationalizing the stereoselectivities and reactivities of 5-substituted cyclopentadiene Diels-Alder reactions.

Effect of Ring Size and Migratory Groups on [1,n] Suprafacial Shift Reactions. Confirmation of Aromatic and Antiaromatic Transition-State Character by Ring-Current Analysis.

Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes, 3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studied computationally at the MP2/6-31+G* level for structures and energetics and with the ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The hydrogen shifts in cyclopentadienes have a diatropic ring current indicating aromatic, cyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-fluorocyclopentadiene which requires much more energy and has a paratropic ring current in the TS pointing to antiaromatic, cyclopentadienyl cation character. [1,3] hydrogen shifts in cyclopropenes are very difficult, passing through transition states that have an extended C-C bond. For 3-fluorocyclopropene, the [1,3] fluorine shift is much easier than the hydrogen shift. For 7-fluorocycloheptatriene, the [1,7] hydrogen shift is predicted but requires very high energy and has a paratropic ring current and antiaromatic character. The [1,7] suprafacial fluorine shift is relatively easy, having a TS with cycloheptatrienyl cation character. Patterns of currents, and the reversal for H and F migration, are rationalized by orbital analysis based on the ipsocentric method. Calculated charges and structural features for reactants and transition states support these conclusions.

Schleyer hyperconjugative aromaticity and Diels–Alder reactivity of 5‐substituted cyclopentadienes

Schleyer's discovery of hyperconjugative aromaticity and antiaromaticity in 5-substituted cyclopentadienes further expanded our understanding of the pervasive influence of aromaticity. Acceptors induce antiaromatic character by Schleyer's negative hyperconjugative aromaticity, and donors have the opposite effect. We computationally explored the Diels-Alder reactivity of 5-substituted cyclopentadienes with ethylene and maleic anhydride. The predicted billionfold difference in the computed gas phase rate constants at room temperature for the Diels-Alder reactions of 5-substituted cyclopentadienes with ethylene or maleic anhydride results from differences in the transition state distortion energies, which are directly related to the hyperconjugative aromaticity of these molecules.

An ab initio study of 1,5-suprafacial shifts in 5-substituted 1,3-cyclopentadienes

A computational study of the degenerate 1,5-suprafacial migration of the 5-substituted cyclopentadienes is presented. The substituents considered are the Group 14–17 elements of the second through fifth periods, saturated by hydrogens as needed to fulfill normal valence requirements. The geometry of most transition states are remarkably similar in the carbon framework. The activation barrier to migration was shown to correlate well with a dimensionless "stretching" parameter, especially at the MP2 level.Key words: 5-substituted 1,3-cyclopentadiene, 1,2-haptotropic shift, 1,5-suprafacial shift, ab initio, transition states.

An ab initio study of conformations and IR spectra of 5-substituted 1,3-cyclopentadienes

A computational study of 5-substituted cyclopentadienes is presented. The substituents considered are the group 14–17 elements of the second through fifth periods, saturated by hydrogens as needed to fulfill normal valence requirements. The conformational characteristics are examined and rationalized using bond–antibond interactions and steric arguments. Trends in vibrational frequencies are discussed and compared with experiment where possible.Key words:cyclopentadiene, ab initio, spectra.

On the influence of spiroconjugation and electron-withdrawing effects on the Frontier orbital energies of 5-substituted cyclopentadienes

Ab initio calculations reveal that the influence of spiroconjugation on the HOMO of 5-substituted cyclopentadienes is weak relative to electron withdrawing effects and that both the HOMO and LUMO energy levels can be lower than in cyclopentadiene contrary to the predictions of qualitative perturbation theory. In accord with these theoretical results, the reaction of 5,5-dicyanocyclopentadiene with cyclopentadiene yields only heterodimers.

Ｄｉｅｌｓ‐Ａｌｄｅｒ反応におけるπ面選択性と発現機構

Controlling of the π-facial stereoselectivity in Diels-Alder reactions has been one of the most fundamental and attractive subjects of theoretical and experimental studies.Increasing attention has been focused on the selectivity in the reaction of 5-substituted cyclopentadienes, the simplest diene having unsymmetrical π-plane. The, π-facial selectivity is highly dependent on the substituents 1) The diene having 5-alkyl or 5-trimethylsilyl group reacted with dienophiles with anti-π-facial selectivity; 2) 5-Hydroxy, 5-acetoxy, or 5-fluoro moiety leads to the opposite stereochemical results. The former case is simply attributable to the steric repulsion between the substituents and dienophiles. However, the origin of the syn selectivity in the latter case has been the subject of intensive studies.The present paper describes a short review on the theories developed for the prediction and explanation of the π-facial selectivity in the Diels-Alder reactions of these simple dienes.

π-Facial Selectivity in the Diels–Alder Reaction of 5-Substituted 1,3-Cyclopentadienes

Abstract Theoretical and experimental studies are presented about π-facial selectivities in Diels–Alder reactions of 5-substituted cyclopentadienes (1). The HOMO and the NHOMO of 1 were readily predicted from the orbital mixing rule to be distorted to favor the syn- and anti-attack, respectively. The frontier orbital is dependent on the n-orbital energy (εn) of the substituent relative to the π-HOMO energy (επ) of the diene. For επ > εn, the syn π-facial selectivity is predicted since the HOMO contains the diene π-HOMO as the main component. For επ < εn, the anti π-facial selectivity is predicted since the π-HOMO most contributes to the NHOMO. For ε π εn, the loss of π-facial selectivity is predicted since the HOMO and the NHOMO both contribute to the reaction. The qualitative theory was examined by ab initio molecular orbital calculation on 1 (X = NH2, PH2, AsH2, OH, SH, SeH, F, Cl, and Br) and PM3 calculation of the activation energies on Diels-Alder reactions of 1 (X = NH2, PH2, AsH2, SbH2, OH, SH, SeH, TeH, F, Cl, Br, and I) with maleic anhydride. The observed selectivities of chalcogen-substituted cyclopentadienes (X = SPh and SePh) were in agreement with the theoretical prediction.

Computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes. Evidence that n–π orbital mixing in the HOMO is not the source of the syn π-facial selectivity in Diels–Alder reactions

A series of C5 monomethyl (1a–1f) and pentamethylcyclopentadienes (2a–2f) bearing stereogenic C5 heteroatom substituents (-NH2, -OH, -OCH3, and -SCH3) have been studied computationally (abinitio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds 1d–1f and 2d–2f, the conformations 3 and 4 of Cs symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. On the other hand, the twisted-anti conformers 5a and 5b are the most stable geometries of 1c and 2c. Analysis of the computed MO eigenvalues (orbital energies), MO eigenvectors (orbital coefficients), and the pe spectra of cyclopentadienes 2a–2f established that n–π orbital mixing is not important in the HOMO's of 1c–1d or 2c–2d. That the ionization energy of the HOMO is found to be virtually independent of the substituent at C5 in the series 2a–2f, provides support for the computational results. Because 2c–2d undergo Diels–Alder reactions selectively, syn to the heteroatom substituent, n–π orbital mixing in the HOMO cannot be the source of the π-facial selectivity observed for these compounds.

Cyclic unsaturated compounds Communication 51. direction of lactonization of bicyclo[2.2.1]-2-heptene-5,6-dicarboxylic acids

1. A study was made of the NMR spectra of some methyl-substituted lacto esters that are derivatives of the acids that are formed in the spontaneous lactonization of the correspondingbicyclo[2.2.1]-2-heptene-5.6-dicarboxylic acids. 2. In the mentioned acids with a monosubstituted double bond the closure of the γ-lactone ring is accomplished at the C2-center (containing the substituent). In the acids with a disubstituted double bond the substituent at C3 in the formed lactones assumes an exo-configuration. 3. Additional proof was obtained for the endo-configuration of the anhydride ring in the adducts of substituted cyclopentadienes with maleic anhydride, and in the case of the 5-substituted cyclopentadienes, for the syn-configuration of the substituent at the C7-center (at the bridge) in the adducts that predominate in the corresponding mixtures of the stereoisomers.

Alkylation of Thallous Cyclopentadienide with Tropylium Ion Synthesis of 11,12-Dihydrosesquifulvalene

Abstract The alkylation of thallous cyclopentadienide (1) has been shown to give 5-substituted cyclopentadienes without the complications due to prototropic shifts which accompany alkylation of alkali metal cyclopentadienides.1 In seeking a route to the interesting (CH)12 hydrocarbon2 11,12-di-hydrosesquifulvalene (3), it appeared likely that alkylation of 1 with tropylium fluoroborate (2) would afford a ready means of access to this system.