[4723-74-4] · C5Cl4O · Tetrachlorocyclopentadienone · (MW 217.85)
(highly reactive diene for Diels-Alder reactions)
Physical Data: unstable as the monomer; matrix-isolated IR spectrum has been recorded.1
Form Supplied in: must be prepared in situ.
Preparative Methods: conveniently generated in the presence of dienophiles, starting from hexachlorocyclopentadiene. Treatment of this material with concentrated Sulfuric Acid at 80 °C affords in good yield the pentachlorocyclopentenone,2 which is readily dehydrohalogenated with either sodium acetate in MeCN or Sodium Iodide in 1,2-dimethoxyethane (eq 1).3
The utility of tetrachlorocyclopentadienone as a reactive diene has received very little attention. The reagent has only rarely been employed as a trap for dienophiles. Dietsche reported the first Diels-Alder reactions of the reagent with norbornadiene and cyclopentadiene in yields of 80% and 60%, respectively (eqs 2 and 3).3 The norbornadiene cycloadduct undergoes thermal extrusion of carbon monoxide and subsequent retro-Diels-Alder reaction to give the 1,2,3,4-tetrachlorobenzene (eq 4). Maleic anhydride, acrylonitrile, and less reactive dienophiles such as cyclopentene, cyclohexene, norbornene, and phenylacetylene all fail to react with the reagent; dimerization presumably occurs preferentially.
Fray reported the Diels-Alder reaction of the reagent with acenaphthylene to afford the endo and exo cycloadducts in a ratio of 4.2:1 (eq 5).4 Reinvestigation of the reaction with cyclopentadiene gave as the major products the endo- and exo- adducts in a ratio of about 170:1, along with the Cope-rearranged derivative (eq 6).
Because of the propensity to dimerize, the reagent is more commonly employed as its dimethyl acetal.2
Keith R. Buszek
Kansas State University, Manhattan, KS, USA