The question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels-Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these [4+2]-cycloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives—one-step reaction versus two-step reaction—considering all experimental and theoretical criteria.

Mechanistic Aspects of Diels‐Alder Reactions: A Critical Survey

Synthetic and mechanistic aspects of 1,3-diene photooxidation

Kinetische Daten fur die Umsetzung substituierter 1,3-Diene mit Tetracyanethylen (TCNE) und anderen Dienophilen werden unter Zugrundelegung des FMO-Modells interpretiert. Wahrend qualitativ die Erwartungen erfullt sind, stellt man ein Versagen quantitativer Korrelationen fest. Dies zeigt, das bei Diels-Alder-Reaktionen neben den HOMO-LUMO-Abstanden weitere Faktoren eine bedeutsame Rolle spielen, z. B. der 1,4-Abstand im Dien, die Gleichgewichtslage cisoide ⇌ transoide Konformation des Diens und andere. Die kinetischen Daten stehen nicht im Widerspruch mit der Vorstellung eines konzertierten Ablaufs der Cycloadditionen.



Reactivity of Substituted 1,3-Butadienes in Diels-Alder-Reactions



Kinetic data for the reaction of substituted 1,3-dienes with tetracyanoethylene (TCNE) and other dienophiles are interpreted in terms of the FMO-model. While the expectations are fulfilled qualitatively, the kinetic data cannot be correlated quantitatively. This shows that in Diels-Alder reactions besides HOMO-LUMO separation other factors play an important role, for instance the 1,4-distance in the diene and the conformational equilibrium cisoid ⇌ transoid of the diene. The kinetic data are in accord with a one-step mechanism of the cycloaddition reaction.

Reaktivität substituierter 1,3‐Butadiene in Diels‐Alder‐Reaktionen

Internal and external factors influencing the Diels–Alder reaction

Note: Univ lausanne,inst chim organ,ch-1005 lausanne,switzerland. Reference LGSA-ARTICLE-1982-005doi:10.1002/hlca.19820650321 Record created on 2005-11-09, modified on 2017-05-12

Face Selectivity of the Diels‐Alder Additions of Exocyclic Dienes Grafted onto 7‐Oxabicyclo[2.2.1]heptanes

Die Reaktivitat von Cyclopentadien und cis-fixierten Di-exo-methylenverbindungen in Diels-Alder-Reaktionen wird im Rahmen des Grenzorbitalmodells untersucht. Es wird gezeigt, das neben der Energiedifferenz der dominanten Grenzorbitalwechselwirkung auch der Dien-1,4-Abstand eine wichtige Funktion besitzt. Je kurzer dieser ist, umso reaktiver ist das Dien. Dies zeigt die Kinetik der Umsetzung von Tetracyanethylen und Maleinsaureanhydrid mit Cyclopentadien und den Di-exo-methylenverbindungen 2–6, deren 1,4-Abstand auf Grund von Kraftfeldrechnungen bekannt ist. Zwischen 1,4-Abstand des Diens und log k2 der Cycloaddition besteht ein linearer Zusammenhang, der sich theoretisch begrunden last.



Significance of Diene-1,4-Distance on the Reactivity in Diels-Alder Reactions



The reactivity of cyclopentadiene and di-exo-methylene compounds in Diels-Alder reactions is investigated using the frontier molecular orbital method. It is shown that besides the HOMO-LUMO energy difference the diene 1,4-distance plays an important part for the rate of reaction. The shorter this distance is the higher becomes the reactivity. Experimentally this is shown by the rates of reaction of tetracyanoethylene and maleic anhydride with cyclopentadiene and di-exo-methylene compounds 2–6. The 1,4-distance of the latter is known from force field calculations. 1,4-Distance and log k2 exhibit a linear relationship which is supported by theoretical considerations.

Der Einfluß des Dien‐1,4‐Abstandes auf die Reaktivität bei Diels‐Alder‐Reaktionen

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Der Einfluß des Dien‐1,4‐Abstandes auf die Reaktivität bei Diels‐Alder‐Reaktionen