Synthesis of Linear and Cyclic Discrete Oligomers with Defined Sequences via Efficient Anionic Coupling Reaction

We hereby report a new strategy to prepare sequence-defined discrete oligomers with uniform molecular weight through carbanionic addition and coupling reactions using specific monomers possessing 1,1-diphenylethylene (DPE) and tert-butyldimethylsilyloxy (TBS) moieties. The strategy allows the molecular weight growth by iterative attaching of the sec-butyllithium (s-BuLi) activated monomer or dimer unit to the halomethyl end-group, released from deprotection of TBS, of the proceeding oligomer starting from a dichloromethyl core. A series of linear discrete oligomers with alternating and ABCD/ACBD sequences are synthesized. These linear sequence-defined oligomers are subsequently modified with double vinyl groups and cyclized through ring-closing metathesis (RCM) reaction into corresponding sequence-defined macrocycles. The molecular structures of the linear and cyclic oligomers are analyzed by using size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The features of the present approach are the presynthesized monomers or dimers with specific functionalities, the efficient carbanionic reactions, as well as the easy separation of the products due to the increasing difference between the molecular weights of the monomer/dimer and the products. More interestingly, sequence and topology effects were observed on Förster resonance energy transfer (FRET) measurements using naphthalene (donor) and pyrene (acceptor) functionalized linear and cyclic ABCD/ACBD oligomers having identical molecular weights and compositions for the two sequences.