OAc AcO O O O O O O O O O O X X n n 1 2 X = Cl 7 X = NH 2 8 X = Br 3 5 Me CO 2 Me CO 2 Me 4 n OAc AcO X X OAc Br Br OAc OAc OAc O – Na + O – Na + CO 2 Me CO 2 Me 6 X = Br 11 X = H 9 n n 10 12 Very dark green Synthesis of poly(anthra-9,10-quinone-2,6-diyl) Gerald Power, Philip Hodge,*† Ian D. Clarke, Michael A. Rabjohns and Ian Goodbody Chemistry Department, University of Manchester, Oxford Road, Manchester, UK M13 9PL The synthesis, via a precursor polymer, of poly(anthra- 9,10-quinone-2,6-diyl) 1 is described and some of its properties are reported. Poly(anthra-9,10-quinone-2,6-diyl) 1 is of interest for several reasons. For example, it is expected to have novel redox properties and these might lead to applications of the polymer in electrochromic displays and/or as charge injection layers in electroluminescent displays. A previous attempt 1,2 to synthesise polymer 1 directly from 2,6-dichloroanthraquinone 2, by nickel(0)-mediated couplings, led only to very small oligomers because of the extremely low solubilities of compounds of this general type in all common organic solvents. Even the dimer 3 has very low solubilities. 3 We now report a synthesis of polymer 1, via the soluble precursor polymer 4, which produces a product with a number average molecular weight corresponding to an average degree of polymerisation, DP, of 13. Quinone-containing polymers have been described be- fore, 1,2,4 but most are electron-transfer polymers. 4 The latter are usually crosslinked polymers with quinone-containing pendant groups. 5 Very few quinone-containing polymers that have been described actually have the quinone moiety in the main chain and conjugated with the p-electron systems of the neighbouring units. The work most relevant to the present is the synthesis of poly(2-methylanthra-9,10-quinone-1,4-diyl) 5 and several very closely related polymers by nickel(0)-mediated couplings of dichloroquinones. 1,2 For steric reasons, the neighbouring an- thraquinone moieties in these polymers are essentially orthogo- nal to each other. The basic strategy of the present synthesis was to prepare Diels–Alder adduct 6, carry out a nickel(0)-mediated coupling to give the soluble precursor polymer 4, then, after characteri- sation of the latter, convert it into polymer 1. Thus, commercial 2,6-diaminoanthraquinone 7 was converted into 2,6-dibro- moanthraquinone 8, mp 282–283 °C (lit., 6 289–290 °C) (60% yield) using a Sandmeyer reaction. 7,8 Reductive acetylation of quinone 8 by treatment with zinc dust, Ac 2 O and NaOAc at reflux temperature gave the anthracene 9, mp 310 °C (de- comp.)‡ (81% yield), and treatment of this with an excess of maleic anhydride in xylene at reflux temperature for 18 h gave the Diels–Alder adduct. The latter was treated with MeOH at reflux temperature to give the half esters. These reacted smoothly with ethereal CH 2 N 2 to give compound 6, mp 195–200 °C (decomp.)‡, (35% overall yield of recrystallised product from compound 9). Treatment of compound 6 in DMA at 80 °C with 1.2 equiv. of Ni 0 [cod] 2 in the presence of cod (1.0 equiv.) and 2,2A-bipyridyl (1.2 equiv.) 2,8–11 gave precursor polymer 4‡ in 82% yield. The polymer was highly soluble in many organic solvents including CHCl 3 and THF. The infrared and 1 H NMR spectra of the product were consistent with structure 4. By gel permeation chromatography polymer 4 had, relative to polysty- rene standards, a number average molecular weight M n , of 5690, a weight average molecular weight M w , of 8980 and a peak molecular weight M p , of 6830. The M n value corresponds to a DP of 13. Heating polymer 4 to 240 °C did not bring about a clean retro- Diels–Alder reaction to give polymer 10 even though analogous reactions in syntheses of related polymers proceed cleanly. 8,12 However, model studies showed that the dimethyl maleate- 9,10-diacetoxyanthracene Diels–Alder adduct 11‡ reacts read- ily with 5 equiv. of NaOEt and EtOH in NMP at 20 °C under nitrogen to give a deep red solution of the disodium salt of 9,10-dihydroxyanthracene, and that exposure of this to air rapidly gives anthra-9,10-quinone in essentially 100% yield. We suggest that the key steps in this overall conversion are, successively, (i) a retro-aldol reaction, (ii) a retro-Michael reaction and (iii) an esterolysis (see Scheme 1). Treatment of polymer 4 with NaOEt under similar conditions to those used above gave a very dark green solution which we attribute to the formation of polymer 12. Passage of air through the green solution precipitated polymer 1, as a pale brown solid, in 95% overall yield from the precursor polymer 4. Chem. Commun., 1998 873 Published on 01 January 1998. Downloaded on 28/10/2014 18:39:54. View Article Online / Journal Homepage / Table of Contents for this issue