SYNTHETIC AND COMPUTATIONAL STUDIES OF SILAMETACYCLOPHANES - MACROCYCLIC CAGE COMPOUNDS

Multiple Pd(0)-catalyzed coupling reactions of 9-borabicyclo[3.3.1]nonane (9-BBN) adducts of allylsilanes and bromobenzenes are used to prepare several unusual silametacyclophanes. Reaction of the 9-BBN adduct of dimethyldiallylsilane and 1,3-dibromobenzene gives 4-dimethyl-4-sila[7]metacyclophane in 32% yield, while reaction of the 9-BBN adduct of methyltriallylsilane and 1,3,5-tribromobenzene leads to 4-methyl-4-sila[3(4,10)][7]metacyclophane in 4% yield. NMR data are presented to help characterize these compounds. Computational studies of the cyclophanes HE[-Y-(CH2)(n)-]3C6H3 for E = C, Si, Y = CH2, O, and n = 2, 3 reveal various important factors in stabilizing compounds with H at E inside the cage. These include that the macrocycle be large enough to accommodate the EH bond and that the EH bond be polarized so that its H atom is positive when directed toward the pi cloud of the benzene ring. Even the small n = 2 macrocycles are only moderately strained at both E and the benzene base.