Synthetic Attempts towards Polymers with Pentafulvene Structural Units

Abstract

Synthetic attempts towards fully conjugated polymers 9 with pentafulvene-diyl structural units are described. Cationic polymerization of pentafulvenes la (R = X = Me) and lb (R = X = MeS) nearly quantitatively gives polymers 8a and Sb with typical M , and M , values of 38800 and 53750, respectively, for 8a, and 12000 and 35900, respectively, for 8b. Key step of the conversion 8a + 9a (Scheme 6 ) is a quantitative bromination Sa -+ 32a, the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23. Best results in view of two-fold the HBr elimination 32a + 9a are obtained with Et,N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 (Scheme 4). Here again, bromination 21 + 23 is quantitative, while two-fold HBr elimination 23 -+ 22 with Et,N proceeds in 51 YO yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29. Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3) is not successful: For pentafulvene lb (R = X = MeS), the main reaction is Diels-Alder-type dimerization l b -+ 1% (Scheme 2), even under anionic conditions. 1. Introduction. -Pentafulvenes ( = 5-methylidenecyclopenta-1,3-dienes) 1 are cyclic cross-conjugated molecules with a five-membered ring whose electronic properties are strongly varying with exocyclic substituents [&6] influencing the extent of 71-delocalization as well as the HOMO-LUMO gap. If these substituent effects would be operative in polymers containing pentafulvene-diyl structural units as well, then polymers with considerably varying electronic properties could in principle be available. In fact, semiempirical calculations [7] predict that some types of long-chain polyenes with fulvene-diyl or fulvalene-diyl structural units should be characterized by a relatively small energy difference between the HOMO and the LUMO. Therefore, they are very attractive in view of polymers with unique electrical and optical properties. Furthermore, polymers of this type or their dihydro precursors (see Scheme 1 ) should be of interest in view of the synthesis of 'nonclassical non-alternant polymers ' [8].Some synthetic pathways to polymers with pentafulvene-and pentafulvalene-(5-(cyclopenta-2,4-dien-l-ylidene)cyclopenta-1,3-diene)-derived structural units are summarized in Scheme 1. Considering the fact that pentafulvenes 1 are easily attacked by nucleophiles at C(6) to give substituted cyclopentadienides and, after protonation, substituted cyclopentadienes (for examples, see [9-1 l]), anionic polymerization of pentafulvenes 1 to 2 followed by HX elimination (+ 3; Route A ) should be an attractive procedure. On