Low temperature Kumada–Corriu cross-coupling of polychlorinated acene derivatives and a synthesis of sterically demanding acenes
Graphical abstract
Introduction
Linear acene derivatives such as substituted anthracenes and tetracenes are well known for their properties as organic semiconductors and fluorophores.1 5,6,11,12-Tetraphenyltetracene (rubrene), in particular, has exceptional charge carrier mobility values of up to 20 cm2/V s.2 Traditional approaches for the synthesis of linear acenes often involve rearrangements and condensations with low yields and difficult purifications, complicating the synthesis of any derivatives.3 More recent syntheses of substituted acenes via Grignard or organolithium additions into acene-quinones are quite effective, but require stoichiometric amounts of reducing agents to aromatize diol intermediates.4
An alternative to the above-mentioned methods are metal-catalyzed cross-couplings reactions, which are often reliable and powerful methods for forming carbon–carbon bonds.5 The instability or low availability of iodo- and bromoacenes, however, makes their use difficult in cross-coupling reactions despite success in highly sterically demanding cross-coupling reactions with polyiodo- or polybromobenzenes.6 On the other hand, chloroacenes are more readily available, but their use is complicated by difficulties in coupling sterically hindered aryl chlorides. The cross-coupling of aryl chloride electrophiles possessing two ortho substituents is notoriously difficult.7, 8 We are unaware of any cross-coupling reactions of linear acene-type aryl chlorides with peri-substitution, which adds an additional element of steric demand not often considered in cross coupling.
We now report a Kumada–Corriu coupling for hindered polychlorinated acenes containing two ortho substituents to produce linear acene derivatives. Optimized conditions allow the couplings to take place at room temperature, an improvement over higher temperature coupling conditions.7 In some cases, even peri-substituted polychlorinated acenes undergo effective cross coupling. Moreover, our conditions enable late-stage diversification of known polychlorinated acenes and the synthesis of new organic electronic materials.
Section snippets
Optimization of Kumada–Corriu cross-coupling
Our studies began with 5,6,11,12-tetrachlorotetracene 1a (Table 1),9 which possesses organic semiconducter properties and is easy to synthesize on a large scale. Each chlorinated site contains two ortho substituents. Suzuki coupling of tetrachlorotetracene 1a with Organ’s PEPPSI-IPr catalyst7a provided tetramethyltetracene 1b albeit in a modest yield (entries 1 and 2).10 By substituting the methyl boroxine, carbonate base, and mol sieves for all in one MeMgBr (3 equiv per chlorine) we were able
Conclusion
In conclusion, we have developed a Kumada–Corriu cross-coupling method for linear polychlorinated acenes containing two ortho substituents. Cross-coupling could often be achieved with low catalyst loadings at room temperature. These conditions were successful even with sterically crowded substrates and can be used to prepare ‘twisted acenes’20 such as octamethylnaphthalene.
Experimental section
General procedure for low-temperature Kumada–Corriu couplings: synthesis of 5,6,11,12-tetramethyltetracene (1b). In a 50-mL Schlenk flask, dried, and flushed with nitrogen, were added 5,6,11,12-tetrachlorotetracene4 (50 mg, 0.137 mmol), PEPPSI-IPr (11 mg, 0.0165 mmol) and 6 mL 1,4-dioxane (freshly distilled from CaH2). MeMgBr (3 M solution in diethyl ether, 0.6 mL, 1.65 mmol) was slowly added at room temperature. After the addition, white precipitate was formed and the solution was slightly warm. The
Acknowledgments
The NSF Materials Research Science and Engineering Centers program under DMR-0212302 provided primary financial support of this work. Partial support for facilities and supplies was provided from the 3M Foundation (Non-tenured faculty grant to C.J.D.). We thank Professor John E. Anthony (UKY) for discussions in the early stages of this work and Ms. Rosalind P. Douglas (UMN) for assistance in the preparation of this Letter.
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