Synthese von Cyclobutan durch Dehalogenierung von 1,4-Dihalogenbutanen mit Alkalimetalldämpfen. Zur Relation von radikalischen und metallorganischen Reaktionskanälen

Synthesis of Cyclobutane by Dehalogenation of 1,4-Dihalogenbutane in Alkali-Metall-Bathway. The Relation of Radical and Metallorganic Reaction-channels. Gasphase dehalogenation of 1,4-dichloro-, 1-chlor-4-bromo, 1,4-dibrombutanes and 1,4-dibrompentane by Na/K vapors produces cyclobutane resp. methylcyclobutane with selectivities of resp. 55.5, 46.7, 32.0 and 17 moles/100 moles. Side products of the reaction are ethylene and hydrogen, yields of which increase with increasing temperature and rising degree of substitution of chlorine by bromine. A mechanism is discussed in which the initiation step is the generation of halobutyl radicals of the type H₂Ċ (CH₂)₂ CH₂X (X = Cl, Br) which are reacting further in two ways – by formation of tetramethylenediradical, splitting off the second halogen atom, or by the formation of the metallorganic compound 1-kalium-4-halogenbutane, which further cyclisize to cyclobutane splitting off KX. Using some methods of theoretical chemistry such as quantumchemical calculations based on ab initio (STO-3G) and semiempirical (AM1) structures, on the relative energies of conformeres of halogenbutylradicals and on models of the metallorganic intermediates, but also by conformation calculations of the content of rotation isomers of the intermediate formed, the correlation between the two reaction channels in dependence of the type of halogen was estimated. The calculations showed, that only about 20% of cyclobutane are generated by the radical pathway. The high predominance of the metallorganic channel in the case of 1,4-dichlorbutaneis caused by the high lifetime of chlorobutyl radicals in comparison with his bromoanalogues.