Design of Living Ring-Opening Alkyne Metathesis

Abstract

Despite the tremendous impact of alkene ring-opening metathesis polymerization (ROMP) on the design and the synthesis of polymers and functional materials, examples for the analogous alkyne ROMP remain scarce. This imbalance can be rationalized by the limited availability of commercially available catalysts and the lack of alkyne metathesis monomers. We present here the synthesis and a systematic study of the ROMP reactivity of an easily functionalized alkyne precursor derived from a ring-strained dibenzo[a,e][8]annulene. We find that both tungstenand molybdenumbased catalysts yield high-molecular-weight polymers with alternating alkane and alkyne linkages along the poly(orthophenylene) backbone. A systematic study of different phenols and alcohols as the activating ligands in the trialkoxymolybdenum(VI) alkylidyne elucidates the design rules to create living alkyne ROMP. Previously we found that the ROMP reaction of 5,6didehydrodibenzo[a,e]cyclooctatetraene proceeds with Schrock s tungsten catalyst. Unfortunately this monomer cannot be easily derivatized because of its poor thermal and photochemical stability. The poor stability also leads to high polydispersity and a non-living polymeric system. To overcome this limitation we substituted the olefin bridge in the 5,6-didehydrodibenzo[a,e]cyclooctatetraene with a saturated ethyl linker, 1a–c (Scheme 1). These strained alkynes (1a–c) can be conveniently synthesized on a multigram scale starting from the readily available 1,2-bis-(3-methoxyphenyl)ethanes 2a–c as depicted in Scheme 1 (synthetic details are in the Supporting Information). Reaction of 2a–c with tetrachlorocyclopropene and AlCl3 yields the intermediate aromatic dichlorocyclopropenes that were hydrolyzed in situ to give the cyclopropenones 3a–c. Cleavage of the methyl ethers and subsequent alkylation of the hydroxy groups in 4a–c with solubilizing alkyl chains provides 5a–c. Photochemical decarbonylation of the cyclopropenone produces the substituted [8]annulenes 1a–c in 77–89% yield. These monomers are far superior to other strained alkynes used in ROMP because they are indefinitely stable under ambient conditions and exposure to light. The structure of 1a–c is consistent with its previously reported derivatives. The ring strain stored in the compression of the triple bond angles from 1808 to 1558 (Figure 1) is Scheme 1. Reaction conditions: a) AlCl3, tetrachlorocyclopropene, CH2Cl2, 78 8C to 24 8C, 60–81%; b) BBr3, CH2Cl2, 78 8C, 99%; c) NaH, C12H25Br, DMF, 24 8C, 51–63%; d) THF/MeOH, hn, 24 8C, 77– 89%.