Cationic Ring-Opening Oligomerization of the Two Stereoisomers of 4-Bromo-6,8-dioxabicyclo[3.2.1]octan-7-one

Abstract

ABSTRACT:Ring-opening oligomerization of the two stereoisomers of a bicyclic lactone 4-bromo-6,8-dioxabicyclo(3.2.l ]octan-7-one was investigated in dichloromethane and chloroform at -40 and 0°C using antimony pentachloride, phosphorus pentafluoride, and trifluoromethanesulfonic acid as initiators. The axial isomer (Sa) showed a tendency to cyclodimerize, particularly at higher temperature, whereas the equatorial isomer (Se) was much less reactive and it afforded only a small amount of cyclic oligomers. Reductive debromination using tri-n-butylstannane converted the cyclic dimer (6) of Sa to the cyclic dimer (2) of 6,8-dioxabicyclo[3.2. l]octan-7-one (1), indicating that 6 was composed of a pair of the enantiomeric monomeric units. The specific formation of the cyclic dimer 6 from the axial isomer Sa was interpreted in terms of the participation of bromonium ions in the propagation.KEY WORDS Ring-Opening Oligomerization / Bicyclic Lactone / Stereoisomer / 4-Bromo-6,8-dioxabicyclo(3.2. l]octan-7-one / Cyclic Dimer/ Bromonium Ion / Neighboring Group Participation / Ring-opening polymerization of cyclic compounds is often accompanied by the formation of cyclic oligomers of different ring sizes, and in some cases, cyclic oligomers are produced as major products. 1 -3 Previously, we found specific formation of 10-, 20-, and 25-membered macrocyclic oligoesters (cyclic dimer (2), tetramer (3), and pentamer (4)) in the cationic ring-opening polymerization of a bicyclic lactone, 6,8-dioxabicyclo[3.2.1 ]octan-7-one (1). 4 -7 From one of the antipodes of 1, only the cyclic tetramer and pentamer were produced. These oligomers were exclusively or highly selectively produced under proper selection of reaction conditions, particularly solvent and temperature. Interestingly, cyclic dimer 2 is a meso compound consisting of a pair of the enantiomeric monomeric units, while cyclic tetramer 3 and cyclic pentamer 4 derived from the racemic monomer 1 are racemic mixtures, that is, they are equimolar mixtures of the corresponding cyclic oligomers of the optically active monomers, (lR)" or (1S)" (n = 4, 5).

Cationic Ring-Opening Co- and Terpolymerizations of Lactic Acid-Derived 1,3-Dioxolan-4-ones with Oxiranes and Vinyl Ethers: Nonhomopolymerizable Monomer for Degradable Co- and Terpolymers

“…17 6,8-Dioxabicyclo[3.2.1]octan-7-one, which has a DOLO moiety in its bicyclic structure, was also reported to undergo cationic polymerization. 18 In this study, we examined cationic homo-, co-, and terpolymerizations of DOLOs. The homopolymerizations of DOLOs did not proceed probably due to both the low ring strain in DOLOs and the inefficient generation of the hemiacetal ester structure in a polymer chain.…”