Donor- and/or Acceptor-Substituted Expanded Radialenes: Theory, Synthesis, and Properties

Abstract

ABSTRACT:The synthesis of donor-(D) and/or acceptor (A)-expanded [4]radialenes has been developed on the basis of readily available dibromoolefin (7), tetraethynylethene (10 and 20), and vinyl triflate (12) building blocks. The successful formation of D/A radialenes relies especially on (1) effective use of a series alkynyl protecting groups, (2) Sonogashira crosscoupling reactions, and (3) the development of ring closing reactions to form the desired macrocyclic products. The expanded [4]radialene products have been investigated by spectroscopic (UV−vis absorption and emission) and quantum chemical computational methods (density functional theory and time dependent DFT). The combined use of theory and experiment provides a basis to evaluate the extent of D/A interactions via the cross-conjugated radialene framework as well as an interpretation of the origin of D/A interactions at an orbital level. ■ INTRODUCTIONConjugated carbon-rich macrocycles are intriguing targets of study for both theoretical and experimental chemists because of their often symmetrical shape and aesthetically appealing structures. They are, however, useful molecules as well, with potential as the organic component for electronic, optical, and nonlinear optical applications.1−8 A specific subcategory of conjugated macrocycles are the [n]radialenes, which are cyclic, carbon-rich molecules with a general formula C 2n H 2n that contain n ring atoms and n exocyclic double bonds (1, Figure 1). "Expanded radialenes" are derivatives of radialenes that originate by formal insertion of an unsaturated spacer between each pair of exomethylene fragments of a radialene, giving rise to macrocycles such as 2 and 3 (Figure 1). 8−12 Work with expanded radialenes was pioneered by Diederich and co-workers 13−16 via the introduction of diacetylene moieties into the radialene framework to give derivatives with the general structure 3 17,18 as well as structurally related radiannulenes.19−27 More recently, expanded radialenes 2 composed of repeating enyne units have been realized and studied. 28−30The two-dimensionally (2D) conjugated structure of expanded [n]radialenes is rather special because it combines a number of linearly and cross-conjugated pathways placed on a nonbenzoid carbon framework.31−39 One key question concerning the properties of expanded [n]radialenes has been the role played by cross-conjugation 9,40,41 to the overall electronic makeup of these unique molecules. The influence of D−A or D/A interactions 42 via cross conjugation in acyclic systems has been explored by a number of groups. 9,10,43−46 To date, however, only a few examples of donor-or acceptor-expanded radialenes have been reported. 18,28 Finally, donor−acceptor-expanded radialenes remain unknown, and there have been no attempts to document cross-conjugated interactions in these derivatives using theory.Our group has recently reported a modular approach for the synthesis of perphenylated expanded [n]radialenes 29 and radiaannulenes. 22,29 This approach has been especially useful ...

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…Bicyclic radiaannulenew as obtained by using tetrabromoethenei nstead of 1,1-dibromo-2,2-diphenylethene( Figure 71 d). [356] Bisradialene 236 a was not obtainedo wing to its highly strained structure, whereas less-strained 236 b was isolatedi n1 8% yield. X-ray crystallographic analysis revealed that 236 b has an almostp lanar structure and contains ah ighly strained alkyne next to two exo-methylene moieties (CCDC:6 52688).…”

“…The strained planar structure of 233 b was determined by using X‐ray crystallographic analysis (CCDC: 652685) . The introduction of a larger substituent on an exo ‐methylene moiety caused the puckering of π‐expanded [4]radialene 235 (38.5°) like cyclobutane rings, which was observed by X‐ray crystallographic analysis (CCDC: 952887; Figure c) . Although a PM3 calculation suggests that the bond angle at the sp carbon of 233 a is smaller than 160°, 233 a was isolated in 32 % yield through the Sonogashira–Hagihara coupling.…”

“…This synthetic method could be applied to the preparation of cyclopentamer (45 % yield), but is not applicable to cyclohexamer (<5 %) owing to the competitive polymerization. Bicyclic radiaannulene was obtained by using tetrabromoethene instead of 1,1‐dibromo‐2,2‐diphenylethene (Figure d) . Bisradialene 236 a was not obtained owing to its highly strained structure, whereas less‐strained 236 b was isolated in 18 % yield.…”

“…[346,354] The introduction of al arger substituent on an exo-methylene moiety caused the puckering of p-expanded [4]radialene 235 (38.58)l ike cyclobutaner ings, which was observed by X-ray crystallographic analysis( CCDC: 952887; Figure 71 c). [355,356] Although aP M3 calculation suggests that the bond angle at the sp carbon of 233 a is smaller than 1608, 233 a was isolated in 32 %y ield through the Sonogashira-Hagihara coupling. By introducing phenylethynyl groups,the crystal structure of p-expanded [3]radialene derivative 234 could be analyzed (CCDC:9 42887; Figure 71 b).…”