Medium-sized cyclophanes. Part XVIII. Iodine-induced cycloisomerization of methyl-, ethyl-, and isopropyl-[2.2]metacyclophanes. π-Basicity and additional strain energy terms as controlling factors

Abstract

Methyl-, ethyl-, and isopropyl-[2.2]metacyclophanes (4)–(14), in which up to eight alkyl groups are present, have been prepared. The relative rates of iodine-induced cycloisomerization were determined by competitive experiments and gave the order of reactivity as (12) > (9)≃(10) > (11) > (5) > (8) > (4) > (3). The presence of alkyl groups adjacent to the bridging methylene enhanced the rate. For the methyl series, the rate increased with inreasing number of methyl groups except for (13) and (14) which were recovered unchanged. Though the ethyl compound (10) had reactivity similar with that of the methyl derivative (9), the isopropyl compound (11) had decreased reactivity in accord with lower equilibrium constant for π-complexation. The results were ascribed to two factors: (a)π-basicity of the substrate as determined by the tetracyanoethylene complexes and (b) additional strain imposed by the alkyl groups adjacent to the methylene bridge to the ten-membered ring.