Extended anthracenes and their use as dienes in Diels ‐ Alder reactions

Hasnaa Sadeq, Thies Thiemann*, John P. Graham, Yosef Al Jasem, Bernhard Bugenhagen, Nathir al Rawashdeh, Mazen al Sulaibi 1 Department of Chemistry, Faculty of Science, UAE University, PO Box 15551, Al Ain, Abu Dhabi, United Arab Emirates 2 Department of Chemical Engineering, Faculty of Engineering, UAE University, Al Ain, United Arab Emirates 3 Institute of Inorganic Chemistry, University of Hamburg, Martin‐Luther‐King Platz 6, D‐20146 Hamburg, Germany 4 Department of Applied Chemical Sciences, Jordan University of Science & Technology, P.O. Box 3030, Irbid‐22110, Jordan

Anthracenes have interesting electroptical properties. 9,10-Disubstituted anthracenes have been forwarded as blue light emitting molecules in organic light emitting diodes. 1 Also, anthracenes as subunits in polymers have elicited interest such as as material in the emitting layer of OLEDs. 2 Additionally, anthracenes, and especially 9,10-disubstituted derivatives 1, have been used as sensors. 3 The authors have interest to use 3-(anthran-9-yl)acrylates as building blocks for anthracene-containing polymers as well as for dimeric anthracenes of type 3 connected with a cyclobutyl unit as potential sensors that can be connected to surfaces by manipulation of the ester functions in 3. Polymerisation was envisaged to be possible by photochemically initiated radicals 4 and dimer formation was envisaged to be proceeding photochemically. Now, the photochemistry of anthracenes themselves has been investigated in great detail, 5,6 and it has been found that the outcome of the reaction depends on the substituents. Anthranylacrylates (9) have not been studied photochemically, to our knowledge. So, apart from the acrylate unit in the anthracenes 2, the anthracenes themselves are known to undergo photochemical [4 + 4]-cycloadditions under certain conditions, 7 and thus competing reactions can be foreseen. Initially, the anthranylacrylates were grown in a crystal with the aim of photolysing in the crystal. The structures of representative anthranylacrylates in the crystal have been measured by us, with an X-ray crystallographic determination of ethyl (E)-3-(anthran-9-yl)propenoate ethyl and of (E)-3-[9bromoanthran-10-yl]propenoate. 8 In the crystal of ethyl (E)-3-[9-bromoanthran-10-yl]propenoate ( Figure 3), the closest distance between double bonds of two molecules, which are molecules in one strand, is 5.321 (3) Å. 8b Although this distance is smaller than the smallest distance found in the nonbrominated parent compound (5.549 (3) Å) ( Figure 2), 8a it is still larger than in many of those found for aryl-enes that undergo [2 + 2]-photodimerization readily. 9,10 The anthranyl units themselves, while aligned parallel to each other in one strand, are off-set to each other and are much further apart (5.321 (3) Å for C1-C1 and C8-C8) than in anthracenes (less than 4.2 Å) that have been reported to undergo [4 + 4]-photodimerization in the crystal. 11,12 This makes a photodimerisation in the crystal highly unlikely. Also, initial results from photoreactions of the anthranylacrylates in solutions have found the photochemistry of the anthranylacrylates to be complex.   For this reason, it was decided to attempt to "protect" the anthranyl unit first by cycloaddition reaction, subsequently carry out the photochemical reaction, and thereafter liberate the anthranyl unit by retro-Diels-Alder reaction. In the present communication, the authors detail the preparation of a variety of anthranylacrylates and aroylethenylanthracenes and discuss their cycloaddition behavior versus a number of common deactivated enes. The photochemical behavior of the products and their subsequent manipulation will be communicated later.

i. Preparation of anthranylacrylates and aroylethenylanthracenes
Commercially available 9-anthranylcarbaldehyde (5) should lend itself to Wittig olefination with conjugated phosphoranes such as alkoxycarbonylmethylidenetriphenylphosphoranes 6 or substituted aroylmethylidenetriphenylphosphorane 7. However, first reaction of 5 with formylmethylidenetriphenylphosphorane showed that 9-anthranylcarbaldehyde (5) does not react with ease, where here the temperature needed for any reaction to occur resulted in decomposition of the phosphorane, even when the reaction was carried out under nitrogen. Also, the Wittig reaction of 9-anthranylcarbaldehyde (5) with fluorophenylcarbonylmethylidenetriphenylphosphorane (8-F), run in refluxing xylene at 125 o C, was not yet complete after 24h. It is believed that it is the steric hindrance exerted by the two ancillary benzo groups of the anthracene that leads to sluggish reactions. In former times, the authors have studied Wittig olefinations in solventless and under solvent-reduced systems. [13][14][15] When 5 is reacted with ethoxycarbonylmethylidenetriphenylphosphorane (6) in very small amounts of CHCl 3 at 110 o C, the reaction to the Wittig-product is complete after 90 min (Scheme 3). In later experiments 5 and 6 were reacted solventless, where a magnetically stirred mixture of 5 and 6 was heated to 130 o C. At 130 o C, the mixture produces a melt, and this liquid undergoes Wittig olefination rapidly. No inert atmosphere is used when running the experiment. While such reaction conditions may be seen as a contribution to green chemistry, the easiest way of separating the product from the phosphorane, used in slight excess, and the triphenylphosphine oxide is by column chromatography with benzene as eluent. Also, the aroylmethylidenetriphenylphosphoranes 7 react with 5 under solventless conditions at 140 o C, albeit in reaction times of 3-4h (Scheme 3). Again, these reactions are run in air. 2a was subjected to bromination (Br 2 , CHCl 3 , rt). Little addition to the acrylate system was observed, although a small amount (15%) of (Z)-3-[10-bromoanthran-9-yl]-2-bromopropenoate (9b) was formed. Bromination took place at the central ring of the anthracene to give ethyl (E)-3-[9-bromoanthran-10yl]propenoate (9a, 75%) (Scheme 4). Reaction of 9-anthranylcarbaldehyde (5) with bromine under analogous conditions (Br 2 , CHCl 3 ) does not funish the corresponding bromoanthranylcarbaldehyde but 9,10-dibromoanthracene (10), formally a decarbonylative bromination (Scheme 4). Ethyl (E)-3-[9bromoanthran-10-yl]propenoate (9a) lends itself to arylation through Suzuki cross-coupling reactions at C9. These were easily performed with 4-methoxy-, 3-nitro-, and 3-chlorophenylboronic acids 11 and phenylboronic acid itself [11, X=H, Pd(PPh 3 ) 2 Cl 2 , aq. Na 2 CO 3 , dioxane] to furnish the pi-extended systems 12 (Scheme 5). For further use (not shown) such as anchoring to a surface, 12 could be hydrolyzed to the respective acrylic acids.  ii. Diels-Alder reactions of the pi-extended anthracenes

ii.a. Preparation of the dienophiles
Maleic anhydride (14) and diethyl fumarate were acquired commercially. N-Arylmaleimides 16 were produced analogous to known literature procedures. 16  ii.b.
[4+2]-Cycloaddition reaction of 9-anthranylcarbaldehyde, alkyl anthranylacrylates, and aroylethenylanthracenes [4+2]-Cycloaddition with substituted anthracenes have been studied. 17 As it was not certain whether the vinyl substituent on C9 of the anthracene would have a detrimental effect or would participate in the cycloaddition reaction of the anthracene, at first it was attempted to carry out the Diels-Alder reaction of the anthracene at the stage of the 9-anthranylcarbaldehyde (5), followed by a Wittig reaction of the carbaldehyde containing cycloadduct 17 (Scheme 7). 9-Anthranylcarbaldehyde (5) has been submitted to [4+2]-cycloaddition reactions previously, 18 although not to cycloaddition reactions with maleimides 16. In our studies, 9-anthranylcarbaldehyde was subjected to solventless Diels Alder reactions at 110 -115 o C with a number of maleimides. After 13h, the reaction mixtures were cooled and diluted with ether/hexane, whereupon the cycloadducts crystallized. The reactions gave appreciable yields (78%quant.), and in no instance could a concomitant decarbonylation be observed. Solventless reactions have been used previously in the cycloaddition of anthracenes with liquid mono-activated alkenes such as with alkyl acrylates or propiolates, 17,18 but not with maleimides. The subsequent step of the Wittig reaction of the cycloadduct carbaldehyde was found to be more difficult, most likely due to the steric crowding in the transition state. Thus, in refluxing CHCl 3 , 17-Br with 6 produced the Wittig product 18-Br in only 31% yield after 90 min (Scheme 8). Neither additional reaction time nor a second addition of the phosphorane to the reaction mixture gave satisfactory results. Also, the separation of the product from the starting material proved not to be easy. Therefore, it was decided to carry out the Wittig-olefination with the 9-anthranylcarbaldehyde first and subsequently subject the acrylates (2a: alkyl = ethyl; 2b: alkyl = methyl) thus obtained to Diels Alder type reactions. This worked well, where diene and dienophile were heated, again under solventless conditions for the Diels Alder reaction to proceed. The mixture provided a melt at the reaction temperature of 120 o C, which started to solidify as the reaction proceeded. Although the meltingsolidification behavior indicates that the reaction may be completed after 15-20 min., this type of reaction was run for 3-4 h.    . Spatial distribution and energies of HOMO -LUMO for representative anthranylacrylates and aroyl and acetylethenylanthracenes. It can be seen that the HOMOs and the LUMOs, respectively, for the compounds are similar in energy and spatial distribution. All calculations were performed using the Gaussian 09W C01 program. 19 Geometry optimizations for the each molecule were performed without symmetry constraints in the gas-phase using the B3LYP 20,21 hybrid functional and 6-31G(d) basis set. Optimized structures were confirmed to be energy minima through vibrational frequency calculations Orbital energies and isosurfaces were calculated using the same functional and basis set.
Anthranylacrylates and aroylethenylanthracenes seem to react equally well, the major difference in the reaction being associated with the physical property of the melting point of the starting materials rather than with significantly different electronic characteristics of the starting materials. This can also be viewed from the computational data of HOMO-LUMO of selected vinylanthracenes, where no significant differences in HOMO -LUMO data is discernable, when comparing the compounds (Figure 4).
Lastly, the reaction of benzoquinone 25 with 9-formylanthracene 5 leads to the primary cycloadduct 26 which is oxidatively dehydrogenated to 27 (Scheme 13). The dehydrogenation can be accelerated significantly by photoirradiation of 26 adsorbed to silica gel. For 26, with an easily enolizable structure lastly leading to quinone system, this may not be surprising. Momentarily, studies are underway to see whether other cycloadducts show a similar behavior as a further possible reaction pathway in the photoirradiation of the cycloadducts.

General
The phosphoranes were prepared by reaction of the phenacyl bromides and of alkyl bromoacetates with triphenylphosphine and subsequent dehydrobromination of the phosphonium bromides with aq. Na 2 CO 3 , analogous to ref. 22. For the general preparation of 2a, see ref. 8a.

Conclusions
Anthranylacrylates 2 and phenylacylethenylanthracenes 8 can be easily produced by solventless Wittig olefination of anthranylcarbaldehyde 5. Anthranylacrylates 2 can be easily brominated in the central aromatic ring system to bromoanthranylacrylates 9a, which can be transformed by Suzuki-Miyaura cross-coupling reaction to the pi-extended anthranylacrylates 12. 2 (and 5) lend themselves to solventless cycloaddition reactions with maleimides 16. Cycloaddition reactions can performed with the aroylethenylanthracenes 8, albeit in refluxing xylene. Cycloaddition reactions with the extended anthranylacrylates 12 are possible, also, but the molecules are less reactive.