No 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 catalyst (A1C13, TiCl4, BF3) can form a complex with the dienophile (e.g., maleic anhydride, methyl acrylate, acrylonitrile). The catalytic activity parallels the Lewis acid strength. These results lend further support to the hypothesis that the primary factor governing syn-anti isomerism in systems of this type is an attractive interaction due to dipole-dipole, dipole-induced dipole, and London dispersion forces between the diene and the dienophile (the polarity of which is increased on complexing with the Lewis acid). Lewis acids also increase the endo-exo ratio of the products and cause a very slight acceleration in the rate of this kinetically controlled Diels-Alder reaction.

Electronic and steric factors governing syn-anti isomerism in the Diels-Alder reaction have been investigated employing 1,2,3,4,5-pentachlorocyclopentadiene as the plane asymmetric dienophile. The predominance of ti«?/-7-chloro isomers in many cases has been ascribed to forces arising from dipole-dipole, dipole-induced dipole, and London-dispersion interactions, the predominance of endo over exo isomers to secondary orbital interactions and the lack of anti-exo isomers to steric hinderance in the transition state.heptatriene9 and cyclooctatetraene10 gives 1 and 2, respectively. On the other hand 5-acetoxy-1,3 -cyclopen-1 (syn) 2 (syn)3 (anti)tadiene with ethylene gives 311 in which the dienophile(1) Presented in part at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1-5, 1968, and the Halocarbon Conference held at the Eastern Research Laboratory of the Dow Chemical Co., Wayland, Mass., May 5-8, 1968.(2) Taken from the thesis of Mrs. Hsu submitted to Mount Holyoke College in partial fulfillment of the requirements for the Ph.D. degree.

Die Diels‐Alder‐Reaktion des Cyclopentadiens (I) mit Fumarsäuredimethylester führt nur zum Bicyclohepten (II) (76,5% Ausbeute).

Das Cyclopentadien (I) reagiert mit Maleinsäureanhydrid (II) zu den anti/syn‐Isomeren (III) und (IV) und mit den Dienophilen (V) zu den anti‐endo‐Addukten (VI), den syn‐endo‐Addukten (VII) und den syn‐exo‐Addukten (VIII).