Synthesis of Oligoenes that Contain up to 15 Double Bonds from 1,6-Heptadiynes

This paper reports the synthesis of polyene oligomers (“oligoenes”) that contain up to 15 double bonds that are identical to the “all five-membered ring” species formed through cyclopolymerization of diisopropyldipropargylmalonate. The oligoenes contain an isopropylidene unit at each end. The isolated oligoenes range from the “dimer” (a pentaene, (E)-di-1,2-[1-(2-methyl-propenyl)-4,4-di-iso-propyl-carboxy-cyclopent-1-enyl]-ethene (3b2)) to the “heptamer” (3b7, a pentadecaene). Oligoenes 3b2, 3b3, 3b4, 3b5, and 3b7 were prepared through Wittig-like reactions between aldehydes and the appropriate monometallic Mo alkylidene or bimetallic Mo bisalkylidene species whose alkylidene is derived from an identical five-membered ring monomeric unit. Compounds 3b2, 3b4, and 3b6 were prepared through McMurry coupling reactions of aldehydes. A representative aldehyde (the “monomeric” aldehyde) is diisopropyl-3-formyl-4-(2-methylprop-1-enyl)cyclopent-3-ene-1,1-dicarboxylate (2b), McMurry coupling of which yields 3b2. A heptaene that contains a six-membered ring in the central unit also was prepared in a Wittig-like reaction involving a bimetallic Mo alkylidene; this species is a model for oligoenes that contain both six-membered and five-membered rings. X-ray structures of two bimetallic species that are employed in the synthesis of the oligoenes are reported.