Cationic cyclocodimerization. 1. Novel synthesis of [3.3]paracyclophane skeleton from 1,3-bis(p-vinylphenyl)propane

Abstract

The trapping of the cationic species, which was generated by the protonation of 1,3-bis(p-vinylphenyl)propane , was tried by using a number of olefins as nucleophiles. Among the olefins, styrene, p-methylstyrene, p-chlorostyrene, m-(trifluoromethyl)styrene, 2-phenylpropene, 1,1 -diphenylethylene, indene, 1,3-cyclohexadiene, trans-1,3-pentadiene, and St-C3-St itself were able to trap the cation to give cisand iraní-l-methyl-3-substituted[3.3]paracyclophanes (cis/trans ratio, 1.0-7.4) in 8-46% yields. 2-Phenylpropene gave the highest yield. Other a,w-bis(p-vinylphenyl)alkanes (St-C"-St, = 4-6) also afforded cis-and trans-1 -methyl-3-styryl [3 ,n]paracyclophanes ( = 4-6) in 43-61% yields in this cationic cyclocodimerization with styrene. From the consideration of the reaction mechanism, the propriety of olefins as a comonomer was clarified by using their ionization potentials and total delocalization energies, which were calculated for the face-to-face oriented benzyl cation-olefin pairs. Styrene derivatives and dienes of which ionization potentials lie in the range of 8.1 to 8.6 eV are concluded to be suitable for the comonomers.The methyl group of trans-1 -methyl-3-X-substituted[3.3]paracyclophane (X = methyl, styryl, a-methylstyryl, or indenyl) showed temperature-dependent NMR resonance, which indicates conformational inversion at the disubstituted linkage. The line-shape analyses showed that the relative bulkiness of the substituents to the methyl group increased in the order of styryl < a-methylstyryl « indenyl. Their activation free energies at 273 K were almost equal (11.5-11.9 kcal/mol), because of the compensative contribution of AH* and AS*. Especially the indenyl group substituted cyclophane showed remarkably small AH* and largely negative AS*. These temperature-dependent NMR spectra also gave definite evidence for the cis/trans structural assignments of isomers.Cationic cyclization, in which a carbocation intramolecularly reacts with unsaturation(s) to form a ring(s), is a general and useful method for the common-ring formation. Johnson's wellknown biomimetic synthesis1 of steroid skeletons from polyenes is one of the excellent and sophisticated applications of the method. However, carbocations are so labile in most organic solvents that the cationic C-C bond formations are often accompanied by several side reactions. Therefore, they are seldom used to make macrocyclic systems from open-chain starting materials that have