Solid Phase Synthesis of M-Phenylene Ethynylene Oligomer Heterosequences

Abstract

The synthesis of m-phenylene ethynylene (mPE) oligomers is currently a tedious process. To address this, two methods for the solid-phase synthesis of mPE oligomers (including heterosequences) have been developed. The first strategy employs a silyl-acetylene linker attached to ArgoGel(TM) solid support, and uses palladium-catalyzed cross-coupling reactions to form the desired mPE backbone in a monomer-by-monomer fashion. In order to rapidly access hexameric oligomers in a period of one to two days, efforts were taken to limit the need for deprotection steps and ensure cross-coupling conditions were complete within two hours. This led to the ability to produce mPE heterosequence hexamers with aryl bromide and terminal acetylene endgroups in good yields (typically 55-75%), in twelve hours. To increase the diversity of mPE oligomer endgroups available from solid-phase methods, a new strategy based on a triazene linker and Merrifield resin was also investigated. In this approach the mPE backbone was constructed in the same fashion as used in the silyl-acetylene linker approach. However, due to differences in resin swelling, some modification of reaction conditions were required. These new conditions allowed mPE heterosequences with aryl iodide and TMS-acetylene endgroups to be produced in 45-65% yields in forty-eight hours. Both solid-phase methods were able to produce oligomers of up to nine or ten repeat units. A method for obtaining longer-length oligomers was, however, necessary. To achieve this, a method of on-resin fragment coupling was investigated. This strategy employed previously established synthetic conditions to couple oligomer fragments onto a solid support through a silyl-acetylene linker. The on-resin mPE fragments were then subjected to a sequence of deprotection and coupling steps in order to access the desired length mPE oligomers in low, but acceptable yields. Aggregation of resin-bound oligomers may be attributed to the lack of conversion observed for the room temperature on-resin fragment coupling. Disruption of on-resin aggregates was achieved by elevating the temperature of coupling reactions.