π-Facial diastereoselectivity in Diels-Alder reactions of 2,5-dimethylthiophene oxide

Naperstkow, Arvin M; Macaulay, John B.; Newlands, M. J.; Fallis, Alex G.

doi:10.1016/s0040-4039(01)93451-3

Cited by 65 publications

(33 citation statements)

References 28 publications

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“…But there is no marked interaction of u framework (orbitals) with the .rr system of the dieneophile 1. interactions; in HOMO-3 there appears to be an interaction between the lone pair on oxygen and the .rr system of the dieneophile in accord with Anh's proposal (16). The point here is that no MO or combination of MOs corresponds to the array of orbitals written to describe hyperconjugative interactions in terms of the Cieplak effect (29). Six frontier MOs of the anti transition structure are given in Fig.…”

Section: Analysis Of Frontier Molec~~lar Orbitalsmentioning

confidence: 99%

“…This latter interpretation proposed by Cieplak for reactions of cyclohexanones is based on the concept of transition state stabilisation by a donation from an adjacent a bond into the vacant a * orbital associated with the incipient bond (26,27). This explanation was used to account for the syn approach of 2,3-dimethylbutadiene to C5-fluoroadamantane-2-thione (28) and rationalize the preference observed for addition anti to the antiperiplanar a bond that is the better donor in the reactions of dienophiles with C5-substituted pentamethylcyclopentadienes (29,30) and 2,5-dimethylthiophene oxide (15). Clearly, it is desirable to continue research in this area to gain a complete understanding of the factors which affect the m-facial selectivity so that the stereochemistry of D-A reactions of C5-substituted cyclopentadienes can be predicted with certainty and the adducts utilized for the total synthesis of complex molecules.…”

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An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities

Ma¹

1994

Can. J. Chem.

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NICK HENRY WERSTIUK and JIANGONG MA. Can. J. Chem. 72,2493 (1 994).The Diels-Alder reactions of maleic anhydride (1) with a group of C5-substituted cyclopentadienes and 2,5-dimethylthiophene oxide (5) have been studied with the semi-empirical method AM1. We find that the facial selectivities and relative reactivities found experimentally for six dienes (2n-2f) are accurately reproduced at this level of theory. The results of the calculational study provide no concrete support for the proposal that a-a* hyperconjugation in the transition state -an interaction between a a bond at C5 of the cyclopentadiene and the incipient a bonds anti to it (the Cieplak effect) -is the major source of the syn facial selectivity observed for 2a-2d. In addition, the relative reactivities and facial selectivities are predicted for a number of cyclopentadienes which will be synthesized and studied experimentally.NICK HENRY WERSTIUK et JIANGONG MA. Can. J. Chem. 72,2493Chem. 72, (1994. Faisant appel 2 la methode semi-empirique AM I, on a CtudiC les rkactions de Diels-Alder de l'anhydride maleique (I) avec un certain nombre de cyclopentadiknes substituCs en position 5 et avec l'oxyde du 2,5-dimCthylthiophkne (5). On a trouvC que, h ce niveau de la thCorie, il est possible de reproduire fidklement les sClectivitCs faciales et les rCactivitCs relatives observCes expCrimentalement pour les six diknes (2n-2f). Les rCsultats de I'Ctude par calculs ne fournit aucun support concret pour la proposition qui a CtC faite selon laquelle de l'hyperconjugaison a** dans I'Ctat de transition -une interaction entre une liaison a au niveau C5 du cyclopentadibne et les liaisons a qui se forment anti par rapport h elles (effet Cieplak) -serait la cause principale de la sClectivitC faciale syn observCe avec les composCs 2n-2d. De plus, on a prCdit les rCactivitCs relatives et les sClectivitCs faciales d'un certain nombre de cyclopentadiknes qui seront synthCtisCs et CtudiCs expCrimentalement.[Traduit par la redaction] Introduction The synthetic utility of the Diels-Alder reaction is well established although the ability to control the facial selectivity which would render the reaction enantioselective with asvmmetric planar 1,3-dienes and potentially generate five or more chiral centres in one operation, is limited in the absence of chiral auxiliaries. In the reaction of C5-substituted cyclopentadienes where n-facial diastereoselectivity has been observed in the absence of chiral auxiliaires, the origin of the selectivity is still imperfectly understood even though a wide range of substrates have been studied experimentally (1-16) and computationally (17-25). These investigations revealed that a number of factors are important and various suggestions have been advanced as possible sources of the pattern observed. These include steric effects, complexation between diene and dienophile, secondary orbital interactions, polarizability, electrostatic interactions, orbital mixing, and transition state hyperconjugation. This latter interpretation proposed by Ci...

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Section: Analysis Of Frontier Molec~~lar Orbitalsmentioning

confidence: 99%

mentioning

confidence: 99%

An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities

Ma¹

1994

Can. J. Chem.

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“…co-workers extended these ideas to Diels-Alder reactions of 2,5-dimethylthiophenes oxides (21) and C-5 substituted cyclopentadienes (22) to rationalize the preference observed for cycloaddition anti to the antiperiplanar a bond that is the better donor. The synthetic utility of the Diels-Alder reaction is well established (23).…”

Section: Introductionmentioning

confidence: 99%

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

Ma¹,

Macaulay²,

Fallis³

1992

Can. J. Chem.

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A series of C5 monomethyl (la-If) and pentamethylcyclopentadienes (2a-2f) bearing stereogenic C5 heteroatom substituents (-NH,, -OH, -OCH3, and -SCH3) have been studied computationally (ab initio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds Id-lf and 2d-2f, the conformations 3 and 4 of C, 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 56 are the most stable geometries of l c 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-n orbital mixing is not important in the HOMO'S of lc-ld 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-n orbital mixing in the HOMO cannot be the source of the n-facial selectivity observed for these compounds. . de rnonomCthy1 (la-lf) et de pentamCthylcyclopentadienes (2a-2f) portant des substituants hCtCroatomiques stCrCoginiques en C5 (-NH,, -OH, -OCH3 et -SCH3). Dans les cas des composds Id-lf et 2d-2f, les calculs ont permis d'Ctablir que les conformations 3 et 4 de symttrie C,, dans lesquelles les paires libres sont anti par rapport au cycle cyclopentadibne, possbdent les gtomCtries les plus stables. Par ailleurs, les conformbres anti dCformCs, 5a et 56, correspondent aux gComCtries les plus stables des composes l c et 2c. Une analyse des valeurs propres calculCes pour les OM (Cnergies des orbitales), des vecteurs propres des OM (coefficients des orbitales) et des spectres pe des cyclopentadibnes 2a-2f a permis d'ttablir qu'un melange d'orbitales n-n n'est pas important dans les OM hautes occupCes des produits lc-ld ou 2c-2d. Le fait que 1'Cnergie d'ionisation de I'OM haute occupCe soit virtuellement independante de la nature du substituant en C5, dans la sene 2a-2f, confirme les rCsultats des calculs. Compte tenu du fait que les composCs 2c-2d subissent des reactions de Diels-Alder ~Clectivement syn par rapport au substituant hCtCroatomique, on en conclut que le melange d'orbitals n-n dans I'OM haute occupee ne peut pas &tre la source de la sClectivitC T-faciale observee pour ces composCs.[Traduit par la redaction]

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“…Torssell 1 and Fallis and co-workers 2 have shown that oxidative cycloaddition occurs on treating alkyl-substituted thiophenes with m-chloroperbenzoic acid (m-CPBA) in the presence of an electron-poor dienophile. We have studied the reaction in greater detail and found it valuable to synthesize novel alkyl-substituted 7-thiabicyclo[2.2.1]heptene 7-oxides in connection with crown ether systems.…”

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confidence: 99%

“…A sulfone bridge in these molecules is much less stable than the corresponding sulfoxy bridge. Although it is believed that SO 2 itself is extruded from the molecules, the gas phase over the reaction solution has not yet been analysed for SO 2 . For most systems tried, a satisfactory transformation could be achieved.…”

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confidence: 99%

Preparation and Electrooxidative SO-Extrusion of Halogenated 7-Thiabicyclo[2.2.1]heptene 7-Oxides†

Thiemann¹,

Melo²,

Neves³

et al. 1998

J. Chem. Res.

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Halogenated thiophenes have been cycloadded oxidatively to maleimides to give halogenated thiabicyclo[2.2.1]heptene S-oxides which have been subjected to an electrochemical extrusion of SO to give various halogeno-substituted phthalimides. Torssell1 and Fallis and co-workers 2 have shown that oxidative cycloaddition occurs on treating alkyl-substituted thiophenes with m-chloroperbenzoic acid (m-CPBA) in the presence of an electron-poor dienophile. We have studied the reaction in greater detail and found it valuable to synthesize novel alkyl-substituted 7-thiabicyclo[2.2.1]heptene 7-oxides in connection with crown ether systems. 3 Recently, it was shown that the yield for the oxidative cycloaddition of alkyl-substituted thiophenes, proceeding via intermediate thiophene S-monoxides, could be greatly enhanced by adding BF 3 Á Et 2 O to m-CPBA.4 While the uncatalysed reaction can be run at 0 8 8C to ambient temperature, the catalysed reaction is run typically at À78 8 8C. The greater stability of thiophene S-oxides towards further oxidation under catalysed conditions was demonstrated. 4,5 The further oxidation of thiophene S-monoxides to the S,S-dioxides is a main side reaction in the oxidative cycloaddition.Oxidation of thiophenes at sulfur proceeds readily when the thiophenes are substituted with electron donors such as alkyl groups. Electron-withdrawing or more ambidentate substituents hinder the oxidation. For this reason the oxidative cycloaddition of halogenated thiophenes does not proceed well at room temperature. Nevertheless, at more elevated temperatures, 7-thiabicyclo[2.2.1]heptene S-oxides from readily in the reaction of 2,5-chlorinated or brominated thiophenes with such idenophiles as maleimides or maleic anhydride (Scheme 1). The products are not very soluble in dichloromethane and partly precipitate during the reaction. Further precipitation can be induced by addition of diethyl ether. Apart from diligent washing with diethyl ether, the products need no additional puri®cation for a further transformation, although for analytical purposes column chromatography and recrystallization have been performed.It has been found that the addition of BF 3 Á Et 2 O has no bene®cial eect on the reaction, if the thiophene is halogenated at position 2 and/or 5. In the case of 3,4-dibromo-2,5-dimethylthiophene 1d, however, an acceptable yield of the cycloadduct was obtained when the reaction was run at À20 8 8C in presence of BF 3 Á Et 2 O. In the cycloaddition ®ve stereocentres are created. Nevertheless, only one isomer is isolated. Although no crystal structural analysis has been performed, it is thought to be the endo-product with the lone-electron pair of the sulfur directed to the newly formed ole®nic bond. This is evidenced by comparison to compounds formed in cycloadditions with donor-substituted compounds. On the other hand, 3 may be viewed as precursors for corresponding arenes. Photochemical 6 and oxidative extrusion 3 of SO in 3 has been found to proceed at much lower temperatures than the purely thermal e...

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π-Facial diastereoselectivity in Diels-Alder reactions of 2,5-dimethylthiophene oxide

Cited by 65 publications

References 28 publications

An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities

An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities

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

Preparation and Electrooxidative SO-Extrusion of Halogenated 7-Thiabicyclo[2.2.1]heptene 7-Oxides†

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