Cyclopent-2-enone, photodimerisation

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

The cyclopent-2-enone required for the photodimerisation is prepared by the hydrolysis and oxidation of 3-chlorocyclopentene, which is obtained by the low temperature addition of hydrogen chloride to cyclopentadiene. The latter is obtained by heating dicyclopentadiene. This depolymerisation is an example of a reverse (or retro) Diels-Alder cycloaddition reaction the diene readily reforms the dicyclopentadiene on standing at room temperature. 

Photodimerisation of cyclopent-2-enone. cis,tra.ns,cis-3,3,8-Tetramethoxy-tricyclo[5.3.0.02 6]decane and c s,. ra.ns,c s-tricyclo[5.3.0,02 6]decane-3,10-dione. Place a solution of 49.9 g (0.61 mol) of cyclopent-2-enone in 800 ml of methanol (distilled from solid potassium hydroxide) in a photochemical reactor vessel of 1-litre capacity equipped with a 100-W medium pressure mercury arc lamp surrounded by a Pyrex cooling jacket (Section 2.17.5, p. Ill), flush the magnetically stirred solution with nitrogen for 10 minutes and then irradiate under nitrogen overnight. Filter the white crystalline solid which separates and continue irradiating until no further crystalline material separates from the methanol solution. About 21.5 g of material is obtained after approximately 40 hours irradiation. Crystallisation from methanol gives cis,trans,cis-3,3,8,8-tetramethoxytricyclo[5.3.0.02,6]decane as white plates, m.p. 173-174 °C. The yield is 19.3 g (30%). 

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