Selective dimerization by a radical chain reaction in γ-irradiated crystals of 1,3-bis(3-acetoxyprop-1-ynyl)-4-methoxy-6-methylbenzene

Abstract

A single dimer is formed by a radical chain reaction upon γ-irradiation of 1,3-bis(3-acetoxyprop-1-ynyl)-4-methoxy-6-methylbenzene (1) crystals, sealed in a glass ampoule under vacuum at room temperature. The dimerization is initiated by the addition of the prop-2-ynyl-type radical attached at the 1-position of the benzene ring to the β-carbon of the triple bond attached at the 3-position of the benzene ring. The resulting vinyl radical undergoes H-abstraction from the prop-2-ynyl methylene of the 3-acetoxyprop-1-ynyl group attached at the 1-position resulting in a chain reaction. The dimerization proceeds after the γ-irradiation is stopped. The kinetic chain length of the dimerization may be more than 2 × 10³. Such an exclusive formation of a dimeric product by the γ-irradiation was observed neither for benzene solutions of 1 nor for crystals of other disubstituted bis(3-acetoxyprop-1-ynyl)benzenes. The crystal structure of 1 is responsible for the selective dimerization by the chain reaction. A mechanism for the chain reaction is proposed on the basis of the crystal structure.