339531-21-4Relevant articles and documents
Insulated molecular wires: Dendritic encapsulation of poly(triacetylene) oligomers, attempted dendritic stabilization of novel poly(pentaacetylene) oligomers, and an organometallic approach to dendritic rods
Schenning, Albertus P. H. J.,Arndt, Jan-Dirk,Ito, Masato,Stoddart, Alison,Schreiber, Martin,Siemsen, Peter,Martin, Rainer E.,Boudon, Corinne,Gisselbrecht, Jean-Paul,Gross, Maurice,Gramlich, Volker,Diederich, Francois
, p. 296 - 334 (2007/10/03)
Multinanometer-long end-capped poly(triacetylene) (PTA) and poly(pentaacetylene) (PPA) oligomers with dendritic side chains were synthesized as insulated molecular wires. PTA Oligomers with laterally appended Frechet-type dendrons of first to third generation were prepared by attaching the dendrons (8, 13, and 17, respectively. Scheme 1) to (E)-enediyne 18 by a Mitsunobu reaction and subsequent Glaser-Hay oligomerization under end-capping conditions (Scheme 2). Whereas first-generation oligomers up to the pentamer were isolated (1a-e), for reasons of steric overcrowding, only oligomers up to the trimer (2a-c) were formed at the second-generation level, and only the end-capped monomer and dimer (3a.b) were isolated at the third-generation level. By repetitive sequences of hydrosilylation (with the Karstedt catalyst), followed by allylation or vinylation, a series of carbosilane dendrons were also prepared (Schemes 3 and 4). Attachment of the second-generation wedge 40 to (E)-enediyne 18, followed by deprotection and subsequent end-capping Hay oligomerization, provided PTA oligomers 4a-d with lateral carbosilane dendrons (Scheme 5). UV/VIS Studies (Figs. 5-10) demonstrated that the insulating dendritic layers did not alter the electronic characteristics of the PTA backbone, even at the higher-generation levels. Despite distortion from planarity due to the bulky dendritic wedges, no loss of π-electron conjugation along the PTA backbone was detected. A surprising (E)-(Z) isomerization of the diethynylethene (DEE) core in the third generation derivative 3a was observed, possibly photosensitized by the bulky Frechet-type dendritic wedge. Electrochemical investigations by steady-state voltammetry and cyclic voltammetry showed that the first reduction potential of the PTA oligomer with Frechet-type dendrons is shifted to more negative values as the dendritic coverage increases. With compounds 5a-c, the first oligomers with a poly(pentaacetylene) backbone were obtained by oxidative Hay oligomerization under end-capping conditions (Scheme 6). The synthesis of dendritic PPA oligomers by oxidative coupling of (E)-enetetrayne 60 under end-capping conditions provided oligomers 61a-d, which were formed as mixtures of stereoisomers due to unexpected thermal (E)-(Z) isomerization (Scheme 8). In another novel approach towards dendritic encapsulation of molecular wires with a Pt-bridged tetraethynylethene (TEE) oligomeric backbone, the trans-dichloroplatinum(II) complex trans-67 with dendritic phosphane ligands (Fig. 14) was coupled under Hagihara conditions to mono-deprotected 69 under formation of the extended monomer 65 (Scheme 12). Again, an unexpected thermal (E)-(Z) isomerization, possibly induced by steric strain between TEE moieties and dendritic phosphane ligands in the unstable complex, led to the isolation of 65 as an isomeric mixture only.
