Convenient methods for preparing π-conjugated linkers as building blocks for modular chemistry

Convenient methods for preparing π-conjugated linkers as building blocks for modular chemistry Jiří Kulhánek, Filip Bureš* and Miroslav Ludwig Full Research Paper Address: Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, nám Čs. legií 565, Pardubice, 532 10, Czech Republic Email: Filip Bureš* - Open Access Beilstein Journal of Organic Chemistry 2009, 5, No. 11. doi:10.3762/bjoc.5.11 Received: 12 February 2009 Accepted: 07 April 2009 Published: 14 April 2009 Associate Editor: P. Skabara * Corresponding author Keywords: boronic acid; donor/acceptor; linker; Sonogashira reaction; property tuning; push-pull; Suzuki–Miyaura reaction © 2009 Kulhánek et al; licensee Beilstein-Institut. License and terms: see end of document. Abstract Simple, straightforward and optimized procedures for preparing extended π-conjugated linkers are described. Either unsubstituted or 4-donor substituted π-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl π-conjugated backbone are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building blocks in Suzuki–Miyaura or Sonogashira coupling reactions. Introduction Development of new organic compounds with improved and advanced properties is one of the most important goals of modern material chemistry. Organic chemists steadily attempt to design and synthesize novel and well-defined organic pushpull systems with prospective applications as chromophores for nonlinear optics (NLO) [1-5], dyes [6], electronic and photonic devices [7,8], organic light-emitting diodes (OLED) [9] or functional polymers [10-13]. A typical push-pull chromophore consists of a polar A-π-D system with a planar π-system endcapped by a strong electron donor (D) and a strong electron acceptor (A). The π-conjugated system ensuring charge-transfer (CT) between the donor (D = NR2, OR groups etc.) and the acceptor (A = NO 2 , CN groups etc.) is most commonly comprised of double and triple bonds, aromatic and heteroaro- matic rings as well as their combinations [14-19]. A typical synthetic approach to CT chromophores involves either a stepwise formation of the target molecule [19,20] or a separate preparation of the donor as well as the acceptor moieties and their final combination [21,22]. It is already well known that the HOMO/LUMO gap and polarizability of the push-pull systems with the given donors and acceptors can be finely tailored by the extension or shortening of the π-conjugated path between the donor and acceptor [19,21,23-25]. Thus, the latter modular synthetic approach seems to be more suitable for the property tuning described above. The final combination, C–C bond formation, of the donor and acceptor chromophore moieties is usually accomplished by cross-coupling reactions, in particular by the Suzuki–Miyaura [26,27] or the Sonogashira [28] reacPage 1 of 5 (page number not for citation purposes) Beilstein Journal of Organic Chemistry 2009, 5, No. 11. tions. Consequently, the availability of the suitably substituted π-conjugated linkers of various lengths bearing boronic ester functionality or terminal acetylene is crucial for such a synthetic approach. Hence, we report here a convenient synthesis as well as characterization of either unsubstituted (R = H) or donor substituted (R = NMe2, OMe) π-conjugated linkers designed for the Suzuki–Miyaura and Sonogashira cross-couplings with a systematically varied and enlarged π-conjugated path (Figure 1). Whereas the simplest linkers such as 4-substituted phenylboronic pinacol esters 1a–c [29] and ethynylbenzenes 2a–c [30] are well known and also commercially available, the proposed dioxaborolanes 3–6 feature styryl (series 3), biphenylyl (series 4), (E)-phenylethenylphenyl (series 5), and phenylethynylphenyl (series 6) linkers and H (series a), OMe (series b), and NMe 2 (series c) groups as the substituent R, respectively (Figure 1, Table 1). The terminal acetylenes 7c–9c possess only the strongest NMe2 donor and have an identical backbone to the one described above (Figure 1, Table 1). Results and Discussion Synthesis of boronic pinacol esters 3–6 Whilst two styryl dioxaborolanes 3a–b are known [31,32] and commercially available, the N,N-dimethylamino substituted derivative 3c needed to be synthesized. In order to achieve pure (E)-3c, at first, a hydroboration of the commercially available terminal acetylene 2c with catecholborane was examined. Despite all attempts to optimize the reaction conditions, 3c could not be prepared this way and was not even detected in the crude reaction mixture. Thus the above hydroboration reported by Perner and co-workers [33] proved to be infeasible. However, in light of the report by Itami and Yoshida [34], we attempted the Mizoroki–Heck C–H arylation of 4-bromo-N,Ndimethylaniline with an equimolar amount of vinylboronate pinacol ester leading to the desired 3c in 73% yield (Scheme 1, Method A). 4-Substituted 4′-bromobiphenyl intermediates necessary for the preparation of 4a–c were synthesized by the Suzuki–Miyaura cross-coupling of 1-bromo-4-iodobenzene with the corresponding boronic acids/esters 1a–c in the yields of 82, 84, and Table 1: Optimized synthetic procedures and yields for the preparation of 3–9. Entry Product (R) Method Yield (%) 1 2 3 4 5 6 7 8 9 10 11 12 3c (NMe2) 4b (OMe) 4c (NMe2) 5a (H) 5b (OMe) 5c (NMe2) 6a (H) 6b (OMe) 6c (NMe2) 10/7c 11/8c 12/9c A B B C C C D D D E E E 73a 81a 83a 76b 69a 82a 78a 72a 91a 98/92c 97/91c 99/89c aYield of the final coupling step. bHorner–Wadsworth–Emmons reaction. cYield of the Sonogashira cross-coupling (10–12) and final TMS-group removal to the terminal acetylenes 7c–9c. Figure 1: Basic and newly proposed π-conjugated linkers designed for the Suzuki–Miyaura and Sonogashira cross-coupling. Page 2 of 5 (page number not for citation purposes) Beilstein Journal of Organic Chemistry 2009, 5, No. 11. Scheme 1: Convenient synthetic methods leading to π-linkers 3–6. 91%, respectively. A routine procedure involving a lithiation and reaction with triisopropyl borate followed by esterification with pinacol afforded dioxaborolanes 4b–c in the yields of 81 and 83% (Scheme 1, Method B). The biphenyl-4-boronic acid corresponding to 4a was also commercially available while 4b was reported as a side product [35] without full characterization. Whereas 5a was easily accessible as a pure (E)-product from the pinacol ester of 4-formylphenylboronic acid and diethyl benzylphosphonate through the Horner–Wadsworth–Emmons reaction in 76% yield [36], methoxy and N,N-dimethylamino substituted (E)-4-bromostilbenes were synthesized from the corresponding benzaldehydes and 4-bromobenzyl(triphenyl)phosphonium bromide [37] by the Wittig reaction [37,38] in 37 and 54% yields, respectively. In contrast to the Horner–Wadsworth–Emmons reaction, this procedure afforded both (E)- and (Z)-stilbenes that we (...truncated)