Abstract:Electronic and steric factors governing syn-anti isomerism in the Diels-Alder reaction have been investigated. Pentachlorocyclopentadiene (1) with many dienophiles gives bicycloheptene Diels-Alder adducts in which the predominant isomer has the chlorine atom on the bridge anti to the double bond. The amount of this isomer, which is disfavored on steric grounds, is increased by the addition of Lewis acids, in particular aluminum chloride. A greater percentage of this unexpected isomer is formed when the catalys… Show more
“…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)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)) and computationally (17)(18)(19)(20)(21)(22)(23)(24)(25). These investigations revealed that a number of factors are important and various suggestions have been advanced as possible sources of the pattern observed.…”
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...
“…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)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)) and computationally (17)(18)(19)(20)(21)(22)(23)(24)(25). These investigations revealed that a number of factors are important and various suggestions have been advanced as possible sources of the pattern observed.…”
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...
“…The Diels-Alder reactions of α, β-unsaturated carbonyl dienophiles is well established to undergo rate acceleration with Lewis acids by LUMO lowering of the dienophile xvii,xviii,xix,xx and a similar activation is believed to operate for hydrogen bonding catalysts. xxi In order to limit the number of different interactions between the substrates and the hydrogen bonding catalyst, the monodentate substrate methyl vinyl ketone was selected along with a nonbonding diene, cyclopentadiene (eq 2).…”
A sensor to quickly and simply assess the relative reactivity of different hydrogen bonding catalysts has been developed. Specifically, blue shifts seen upon treatment of hydrogen bonding catalysts with the colorimetric compound, 7-methyl-2-phenylimidazo[1,2-a]pyrazin-3(7H)-one, correlate well to the Keq of binding to the sensor. The blue shifts also show a high degree of correlation with relative rates in Diels-Alder reactions of methyl vinyl ketone and cyclopentadiene employing the hydrogen bonding catalysts. The relevance of the sensor blue shifts to the LUMO lowering abilities of the hydrogen bonding catalysts is discussed.
“…An early example was reported by Woodward, who found that 5-acetoxycyclopentadiene 112 adds ethylene on its syn side . The same contrasteric prejudice has been observed with 1,2,3,4,5-pentachlorocyclopentadiene, but 5-iodocyclopentadiene undergoes Diels−Alder cycloaddition at its en face; evidently, if the 5-substituent is bulky enough, the reaction becomes subject to steric control. …”
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