Synthesis of Geometrically Well-Defined Covalent Acene Dimers for

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Synthesis of Geometrically Well-Defined Covalent Acene Dimers for Mechanistic Exploration of Singlet Fission Thomas James Carey, Jamie L. Snyder, Ethan G. Miller, Tarek Sammakia, and Niels H. Damrauer J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b00602 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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The Journal of Organic Chemistry

Synthesis of Geometrically Well-Defined Covalent Acene Dimers for Mechanistic Exploration of Singlet Fission Thomas J. Carey, Jamie L. Snyder, Ethan G. Miller, Tarek Sammakia*, and Niels H. Damrauer* Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States *[email protected] *[email protected]

Abstract We report the first synthesis of norbornyl-bridged acene dimers (2 and 3) with well-defined and controlled spatial relationships between the acene chromophore subunits. We employ a modular 2-D strategy wherein the central module, common to all our compounds, is a norbornyl moiety. The acenes are attached to this module using the Diels-Alder reaction, which also forms one of the acene rings. Manipulation of the Diels-Alder adducts provides the desired geometrically defined bis-acenes. The modular nature of this synthesis affords flexibility and allows for the preparation of a variety of acene dimers, including functionalized tetracene dimers.

ACS Paragon Plus Environment


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Introduction Singlet fission (SF) is a non-radiative photophysical process by which a singlet excited state in certain materials or molecular systems converts to a singlet–coupled pair of triplets (1TT) and then ultimately to a set of uncorrelated triplets according to eq 1.1-4 (1) Because spin is conserved in the initial forward reaction (k1), the process can be rapid provided that it is not significantly endergonic. While these photophysics were discovered more than 45 years ago in molecular crystals of tetracene,1,5 recent interest has blossomed as researchers attempt to develop next-generation strategies for solar energy conversion; SF offers routes to process higher energy solar photons into pairs of electronic excitations rather than wasting excess energy as heat.4,6 While not an absolute requirement, it is generally the case that SF-active systems involve the spatial juxtaposition of multiple chromophore subunits (as is the case in a molecular crystal), each of which has a low-energy triplet and a singlet excited state that is approximately, or greater than, twice the energy of the triplet. These conditions are important by way of ensuring that the formation of 1TT is energetically accessible or favorable. Longer polyacenes such as tetracene and pentacene satisfy this condition and have played a major, although not exclusive, role in the growth of the field.3,4,7-32 For much of the development of SF, condensed phase systems including polycrystalline films,4,9,10,33 amorphous films,8 and nanoparticle/supramolecular chromophore aggregates25,34-38 have been exploited. While much less developed, there are good reasons to also consider molecular systems and in particular covalent dimers comprised of the same types of chromophores that have proven effective in the condensed-phase settings.39-50 On a practical


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level, SF-active dimers may be useful in dye-sensitized solar cells.4,6 More fundamentally, dimers – and the systematic variation of their structure and electronic properties using synthesis – offer a platform for exploring mechanistic details important for future development of effective SF systems. Our group has been particularly interested in understanding how to control interchromophore electronic couplings that are tied to the magnitude of the k1 rate constant in eq 1.12,51-53 In recent computational and theoretical studies, we have explored the role of interchromophore bridge structure,53 orbital symmetry,51-53 and intramolecular vibrations and their symmetries.52 The structural motif that is central to those studies underpins the synthetic exploration herein. More generally, known dimers for SF39-44 have significant conformational flexibility that can lead to uncertainty in the nature of the S1 reactant state as well as the electronic coupling between it and the product 1TT. We have been interested in identifying dimer platforms permitting systematic exploration of factors – electronic coupling, reorganization energy, and driving force – that fundamentally control SF rates. By way of controlling interchromophore couplings in dimers, we were drawn to work of Paddon-Row and coworkers who developed platforms for exploring donor-acceptor interactions relevant to both energy-transfer and electron-transfer using bicyclic bridges based on norbornyl fragments.54,55 The norbornyl alkyl spacers provided superb control over the spatial juxtaposition of electro-active moieties by providing two covalent attachments to the bridge per chromophore. This controls the geometric arrangement of the chromophores and their electronic interactions with high fidelity and limited conformational flexibility. In particular, we were inspired by a set of naphthalene dimers56,57 that exhibit an elegant manifestation of controlled interchromophore interactions (Figure 1). For the molecule with the largest bridge, BN3, a single absorption band is observed that is indicative of two approximately uncoupled chromophores where the transition is



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S3←S0. As the bridge size is decreased to BN2 and then to BN1, that band splits (so-called Davydov splitting) to an increasing degree, indicating interchromophore interactions that are tunable via distance. The work herein reports the first synthesis of tetracene analogues to BN1, which we call 2, and 3 (Figure 2), while using the previously synthesized 158 as a model that also provides the photophysical background for an un-coupled chromophore of this type.

Figure 1. a) Pictorial representation of Davydov splitting between two chromophores (C1 and C2) and b) naphthalene dimers studied by Paddon-Row and coworkers that exhibit such an interaction.

Figure 2. Dimeric targets and previously synthesized monomer. It is noted here that 2 and 3 are not, at the outset, expected to be optimal for SF and studies confirming this have recently been reported.45 Our previous theoretical and computational explorations52,53 suggest that orbital symmetry manifests in such a way that electronic coupling


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for SF goes to zero for C2v-symmetric acene dimers, even when interchromophore electronic coupling that manifests in absorption (i.e., Davydov splitting) is high. These dimer systems will allow us to test this prediction and to explore the role of vibronic coupling for enabling SF.12,52 Further, we expect the building blocks that are explored in this initial synthesis to be highly useful in developing design rules based on the control of electronic coupling in SF via symmetry properties.51 Results and Discussion As discussed in the introduction, we became interested in polyacene dimer systems related to those originally synthesized by Patney and Paddon-Row where inter-chromophore electronic coupling could be controlled using bicyclic alkyl spacers.56 The general synthetic strategy described by these workers involved the use of the Diels-Alder reaction wherein the polyacene arm moieties bear diene functionalities that react with norbornadiene. For example, a naphthalene dimer with a single bicyclic bridge (BN1) was synthesized from α,α,α´,α´tetrabromo-o-xylene (4) which was subjected to in situ reductive elimination of Br2 with NaI to form the corresponding dibromo-o-quinodimethane (5, Scheme 1). This species serves as the diene and reacts with 0.17 equiv norbornadiene as a dienophile to provide the bis-Diels-Alder adduct, which undergoes spontaneous loss of four equivalents of HBr to provide BN1. By using an excess of norbornadiene (10 equiv) the corresponding mono-annulated polyacene (N1) can be prepared by an analogous process.56



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Scheme 1. Patney and Paddon-Row synthesis of bridged naphthalene derivatives

Motivated by their successful use of dibromo-o-quinodimethane, we wished to explore a related strategy using homologues derived from tetrabromo-dimethylanthraquinone (6, Scheme 2). The quinone functionality could then be reduced to form the desired tetracene arms late in the synthesis. These routes were explored; however, difficulty in accessing the requisite tetrabromo starting materials, partly due to allergic reactions observed among laboratory workers (we note that benzyl halides are well known lachrymators), led us to abandon this approach. An alternate route to access o-quinodimethane 8 via the corresponding sultine (7) was then explored59 but abandoned due to reactivity issues; extrusion of SO2 to provide the requisite o-quinodimethane intermediate required excessively high temperatures leading to complex mixtures of products even when attempts were made to trap the reactive intermediate in situ.

Scheme 2. Failed approach towards 2


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With these preliminary results, we required an alternate route and devised a different DielsAlder strategy for obtaining 2 and 1 via quinones 9 and 12, respectively. In this new route, the role of diene is assigned to the bridge, and that of the dienophile to the eventual tetracene arm (Scheme 3). By this strategy, the synthesis of 2 requires tetraene 11,60 and anthraquinone 10,61 and the synthesis of 1 requires triene 13.62 All of these are known in the literature.

Scheme 3. Alternative retrosynthesis for 2 and 1

There are two main challenges involved with this synthetic strategy: accomplishing the Diels-Alder reaction and reduction of the quinone carbonyls to the polyacene. We chose to initially study these in route to the synthesis of the corresponding model monomer, 1. Our synthesis of 1 proceeds via quinone 12, and begins with a Diels Alder reaction between dienophile 1061 and triene 1362 (Scheme 4). Subjection of 10 and 13 to thermal Diels-Alder conditions (150 °C, toluene, sealed tube, 3 days) provided 14. This material was not amenable to purification by flash chromatography as partial oxidation to 12 was observed. Instead, the crude material was adsorbed on alumina and stirred for 2 days under an oxygen atmosphere63 to provide 12 in 76% crude yield. Compound 12 also proved difficult to purify due to poor solubility in nonpolar solvents, which led to co-elution with close running bands. As such, a small quantity was purified for characterization purposes and the crude was used in subsequent steps.



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Scheme 4. Synthesis of 12

We then turned our attention to the reduction of the quinone moiety to the corresponding hydrocarbon. For optimization purposes, commercially available 5-12-naphthacenequinone (15) was used as a model for this transformation. Literature precedent suggests that reduction of one of the carbonyls of the quinone can be accomplished via hydrogenation with Pd/C. This transformation was observed by Nuckolls and co-workers in 64% yield on 5,12naphthacenequinone derivative 21 which bears bis-o-methoxy substitution (Scheme 5).64 The product of this reduction is the keto-tautomer 22 rather than the aromatic phenol, a tautomeric preference that is known for larger polyacenes wherein the diminishing returns associated with aromatization65,66 are less significant than the stability of ketones relative to enols.67,68 In our case, mono-reduction of compound 15 would provide ketone 16, which could then be reduced to the corresponding alcohol and subjected to elimination to provide tetracene 17. Subjecting 15 to H2 and Pd/C (or other heterogeneous catalysts) did not provide reduction of the ketone; instead, reduction of the interior ring proximal to the quinone to provide compound 18 was observed. We found this surprising, especially in contrast to the comparatively electron-rich and, thus, less reactive carbonyls in Nuckolls’ system. Attempts to convert this to useful material by tautomerization to 19, or 20 via in situ mesylation of 19, were unsuccessful. We attributed this to


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the known instability of diol 19 towards disproportionation to provide 15 and other products as described by Fieser.69

Scheme 5. Initial attempts at reduction sequence on the model system

The use of metal hydride reducing agents to convert 15 to 17 was then studied (Scheme 6). Interestingly, subjecting 15 to LAH at reflux in toluene provided the fully reduced product (17), albeit in modest yield (30%). This transformation required reaction workup under nitrogen; otherwise, partial conversion to oxidation products - including the starting quinone (15), the corresponding diol (23), and other unidentified species - was observed. Subsequently in our studies, we found that subjecting 15 to NaBH4 provided the corresponding diol (23) as a 2:1 mixture of diastereomers (95%). Treatment of this product with HCl and SnCl2 provides 17 in 93% yield and overall yield of 90% for the two steps.70-72 A similar procedure was used in the synthesis of 1. Treatment of 12 with NaBH4 provided diol 24 as a 1:1.4:1 ratio of 3 diastereomers in 88% yield. Diol 24, however, is sensitive to oxidation to the quinone; as such, a one pot procedure was used wherein quinone 12 was treated with NaBH4 in MeOH/CHCl3 and then directly treated with SnCl2. This procedure provides 1 in 71% yield over two steps.



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Scheme 6. Forward synthesis of 1

We then turned our attention to the synthesis of 2 via bisquinone 9. We imagined subjecting tetraene 1160 to a 2-directional strategy that consists of two consecutive Diels-Alder reactions in one pot using dienophile 10 in order to install both arms in a single operation to provide 28 (Scheme 7). Subjecting 11 to 10 (1 equiv) in toluene at 70 °C provided the mono-Diels-Alder product, 25 as the keto tautomer,67,68 presumably derived from endo-addition,60 as a single isomer to the limits of NMR detection. We then studied more forcing conditions (refluxing toluene and 5 equiv 10) but were unable to obtain the bis-Diels-Alder product, even in the presence of a Lewis acid. Such reluctance to undergo consecutive Diels-Alder reaction is known in related systems.73 Compound 25 undergoes oxidation during attempted purification, as do related compounds in this series. As such, we were unable to study a subsequent Diels-Alder reaction on purified 25. Instead, we simply added basic alumina to the crude reaction product without removal of solvent, and rapidly stirred the slurry in air and obtained 27 in 60% yield from compound 11. Interestingly, we found that oxidation of 25 to 27 can occur at higher temperatures prior to exposure to air or alumina, and we speculate that starting 10 can act as an


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oxidant under these reaction conditions. Evidence for this is the presence of 26 in the reaction mixture wherein the isolated alkene has been reduced.

Scheme 7. Forward synthesis of 9

We hypothesize that the steric demands of 25 hinder endo-approach of the dienophile and thereby preclude subsequent Diels-Alder reactivity. In support of this hypothesis, geometry optimized DFT calculations of 25 indicate that the newly added arm adopts a conformation blocking endo approach to the diene (Figure 3).



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Figure 3. Geometry optimized DFT calculation of 25 showing steric blocking of subsequent Diels-Alder reactions (see S.I. for computational details).

The synthesis then continued with 27, which we note is linear and cannot adopt a bent conformation similar to that of 25 (Scheme 7). Compound 27 was treated with 3 equiv 10 under solvent-free conditions (the solid reactants were first taken up in chloroform, then the solvent removed under reduced pressure to provide a homogeneous paste) and heated to 220 °C under nitrogen. This provided 29, which was oxidized in the presence of Al2O3 and air to form bisquinone 9 in 65% yield from 27. The overall yield from 11 to 9 is 39% over 4 steps after purification by flash chromatography. The reduction of 9 to 2 was then studied and we made use of the NaBH4 / SnCl2 procedure described above. Subjecting bisquinone 9 to NaBH4 provided a complex diastereomeric mixture of tetraols (30) in 75% yield (Scheme 8). Subsequent reduction to the bisacene dimer 2 was not as facile as the model system (17), and an unproductive side reaction to provide ketonic products was partially observed. We speculate that this reaction proceeds via the cation (31) derived from protonation and ionization of one of the alcohols of the tetraol (Scheme 9). This intermediate can either undergo reduction with SnCl2, or loss of a proton to provide the phenol (32) which preferentially exists as the keto-tautomer as described above.


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Scheme 8. Final synthesis of 2

Several aspects of the reaction were varied in order to minimize this elimination pathway, including increasing the stoichiometry of SnCl2, increasing the concentration of the reaction, and varying the solvent. We found that in certain solvents, such as acetonitrile or chloroform/methanol, HCl is unnecessary; presumably the Lewis acidity of SnCl2 enables the loss of hydroxide and generation of the carbocation as shown in Scheme 9. The use of a large excess of SnCl2 (~50 equiv) also favors reduction over elimination, leading to the desired polyacene product. These conditions allow for the conversion of tetraol 30 to the desired polyacene dimer 2 in 91% crude yield (Scheme 8).

Scheme 9. Possible mechanism for reduction vs elimination with only the central ring shown for clarity

Attempts to purify 2 proved challenging. Polyacenes can suffer from partial oxidation on silica as well as aggregation, which provides insoluble material that does not elute. Multiple crystallization methods were attempted, but none were successful. A workable solution was found in the differential solubility of 2 relative to its impurities; 2 is less soluble in chloroform



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than its oxidation products, such as 9. As a result, pure samples can be obtained by subjecting crude material to several chloroform extractions. While this provides material of sufficient purity for spectroscopic studies, it reduces the amount of pure 2 obtained. The reduction in the yield of 2 by this procedure varies and can be 30% or more depending on the scale and the run. Photophysical studies, including a UV-Visible absorption spectrum, of 2 have been reported45 and have shown Davydov splitting of the S3←S0 transition in the UV region from ~260–320 nm as discussed in the Introduction and as expected for 2 (versus 1). The modularity of our synthesis allows us to prepare systems with improved physical properties. For example, in order to circumvent the aforementioned difficulties tied to purification, solubility, and study of 2, an analogue, 3, bearing triisopropylsilyl (TIPS) acetylene substituents was targeted for synthesis (Scheme 10). Such substitution is known to increase the stability of acene systems towards oxidation while at the same time increasing solubility in organic media.71,74 To accomplish this, 9 was subjected to alkynylation with the Grignard reagent derived from TIPS-acetylene to provide the corresponding tetraol as a mixture of diastereomers (33). This material was not isolated, but instead treated directly with SnCl2/HCl to the tetraalkyne substituted polyacene 3 in 81% yield. As in the case of the parent system, the UV-Visible absorption spectrum of this material also shows Davydov splitting in the region from ~275-350 nm (see Figure S1 in supporting ionformation).


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Scheme 10. Final synthesis of 3

Conclusion Acene dimers 2 and 3 were synthesized via a modular Diels-Alder approach. The syntheses proceed via bisquinone intermediate 9 that can be subjected to either hydride addition or alkynylation followed by SnCl2 reduction to provide the aromatic product. The TIPS-alkyne derivative was synthesized for its superior stability and solubility as compared to the parent hydrocarbon. Our route has allowed us to prepare tetracene compounds, and this route can be applied to the synthesis of other geometrically defined acene derivatives. For example, we have studied the extension of this chemistry to the pentacene series, and while we have evidence that this route is viable for the synthesis of the pentacene homologue of 3, we have encountered issues of solubility, purification, and reactivity, that must be addressed before we can prepare analytically pure material for photophysical studies. Finally, as a general platform, acene dimers of this nature provide a framework for testing hypotheses about the role of orbital symmetry in determining state coupling for SF and work is currently underway to synthesize variants51 bearing Cs and C2 point group symmetry by related routes.



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Experimental

General Information All reactions were run under a nitrogen atmosphere unless otherwise specified. Tetrahydrofuran was distilled from sodium benzophenone ketyl, and toluene from calcium hydride prior to use. Hexanes was distilled prior to use to remove non-volatile impurities, and acetonitrile was HPLC grade and stored over 4Å mol sieves. Unless otherwise noted, all reagents were obtained from commercial suppliers and used without further purification. Compounds 10,61 and 11,60 were prepared according to literature procedures. Compound 1362 was prepared by a modification of a literature procedure described below. 1H NMR spectroscopy was performed at 500 MHz or at 300 MHz in CDCl3 using residual CHCl3 as an internal standard (7.26 ppm).

13

C NMR

spectroscopy was performed at 75 MHz in CDCl3 using the center line of solvent (77.16 ppm) as an internal standard. High resolution mass spectra (HRMS) were recorded using electrospray ionization (ESI q-TOF). HRMS was calculated on the molecular ion [M]+, the protonated [M + H]+, or the lithiated [M + Li]+ species. Molecular ions [M]+ were observed on polyacene compounds 1, 2, and 3, and while this is unusual with ESI ionization, it is not without precedent75,76 and has been observed in related polyacenes.41,77

5,6-dimethylidene-2-norbornene (13) A 350 mL glass sealed-tube reactor was flame-dried, and 5,6-di(chloromethyl)-2-norbornene62 (3.0 g. 15.7 mmol, 1 equiv) was added by syringe followed by potassium isopropoxide (77 mL of a 0.91 M solution in isopropanol; 70.1 mmol, 4.5 equiv). The flask was sealed then heated to 120 °C behind a plexiglass blast-shield. The reaction was monitored via TLC (20:1 hexanes / EtOAc;


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KMnO4 visualization). Upon disappearance of starting material (~48 h), the reaction was cooled to room temperature and distilled pentane was added followed by deionized water. The layers were separated and the organic layer was washed five times with water to remove residual alcohol. The organic layer was dried over MgSO4, filtered, and concentrated by rotary evaporation without heating to provide 13 as a light yellow oil (1.37 g, 11.6 mmol, 74%).62 1

H NMR (500 MHz, Chloroform-d) δ 6.22 (t, J = 1.8 Hz, 2H), 5.19 (s, 2H), 4.98 (s, 2H), 3.33

(app. pent, J = 1.7 Hz, 2H), 1.79 (A of ABX2, J = 8.2, 1.6 Hz, 1H), 1.60 (B of ABX2, br, J = 8.2, 1.5 Hz, 1H);

13

C NMR (100 MHz, Chloroform-d) δ 149.1, 136.8, 101.6, 51.7, 50.9; HRMS

(ESI/Q-TOF) m/z: [M + H]+ Calcd for C9H10H 119.0861; Found 119.0872. Hexacyclo[18.2.1.02,19.04,17.06,15.08,13]tricosa-2(19),3,6,8,10,12,14,17,21-nonene-5,16-dione (12) Compounds 13 (0.20 g, 1.69 mmol, 1.25 equiv), 1061 (0.282 g, 1.36 mmol, 1 equiv), and toluene (7 mL) were added to a 150 mL glass sealed-tube reactor and the vessel was sealed and heated to 150 °C behind a blast-shield. The reaction was monitored via TLC (20:1 hexanes / EtOAc; KMnO4 visualization) for the disappearance of starting material (~48 h) upon which the solution was cooled and concentrated at reduced pressure to provide a brown solid. This material was not purified but was instead subjected to oxidative aromatization by the addition of basic alumina (~16 g). This suspension was stirred for 10 minutes, then the solvent was removed by rotary evaporation to provide a homogeneous orange powder. This adsorbent was stirred dry under an atmosphere of O2 for 2 days. Aliquots were removed and the reaction was monitored by 1HNMR (an aliquot was flushed with 1:1 CHCl3/EtOAc and concentrated). Upon disappearance of starting material, the powder was washed thoroughly with EtOAc to provide a yellow solid upon concentration at reduced pressure (333 mg, 1.03 mmol, 76% yield). Purification of the bulk



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sample was difficult due to poor solubility. As such a small aliquot was purified for the purposes of characterization, and the crude product was taken on to the next step. 1

H NMR (400 MHz, Chloroform-d) δ 8.81 (s, 2H), 8.19 (s, 2H), 8.10-8.08 (m, 2H), 7.69-7.67

(m, 2H), 6.83 (t, J = 1.9 Hz, 2H), 4.11 (app. pent, J = 1.6 Hz, 2H), 2.46 (A of ABX2, J = 7.7, 1.5 Hz, 1H), 2.33 (B of ABX2, br, J = 7.7, 1.5 Hz, 1H); 13C NMR (100 MHz, Chloroform-d) δ 183.5, 159.3, 142.7, 135.2, 132.9, 130.2, 130.0, 129.39, 129.36, 119.7, 69.3, 50.7; HRMS (ESI/Q-TOF) m/z: [M + H]+ Calcd for C23H14O2H 323.1072; Found 323.1061. Hexacyclo[18.2.1.02,19.04,17.06,15.08,13]tricosa-2(19),3,5,7,9,11,13,15,17,21-decene (1) To a flame-dried 50 mL RBF was added 12 (55 mg, 0.171 mmol, 1 equiv), in CHCl3 (2 mL) and MeOH (2 mL). The solution was cooled in an ice bath, and NaBH4 (13.5 mg, 0.355 mmol, 2.0 equiv) was added in a single solid portion and the reaction was warmed to room temperature. The reaction was monitored by TLC (CHCl3 eluent, UV visualization) for the disappearance of the starting material (~1 h), upon which stannous chloride (324 mg, 1.71 mmol, 10 equiv) was added. This solution turned a light-yellow hue, then bright orange over the course of 10 minutes. The flask was wrapped in aluminum and the solution was stirred overnight under N2 after which toluene (10 mL) and water (10 ml) were added. The heterogeneous mixture was filtered through a plug of Celite®, washed with toluene, and the biphasic mixture was separated. The organic layer was washed with water (3 times) dried over MgSO4, filtered then concentrated to a yellow solid. Purification by flash chromatography (1:1 hexanes / chloroform) provided 1 (35 mg, 0.122 mmol, 71%) as a yellow powder. 1

H NMR (500 MHz, Chloroform-d) δ 8.58 (s, 2H), 8.43 (s, 2H), 7.99-7.97 (m, 2H), 7.66 (s, 2H),

7.39-7.37 (m, 2H), 6.68 (s, 2H), 3.98 (s, br, 2H), 2.35 (A of AB, br, J = 7.8 Hz, 1H), 2.21 (B of


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AB, br, J = 7.8 Hz, 1H); HRMS (ESI/Q-TOF) m/z: [M]+ Calcd for C23H16 292.1252; Found 292.1249. 21,22-Dimethylenehexacyclo[18.2.1.02,19.04,17.06,15.08,13]tricosa-2(19),3,6,8,10,12,14,17octaene-5,16-dione (27) Compounds 1160 (208 mg, 1.44 mmol, 1.0 equiv) and 1061 (301 mg, 1.44 mmol, 1.0 equiv) were dissolved in toluene (40 mL), and stirred at 70 °C (sand bath). The reaction was monitored by TLC (hexanes eluent, UV visualization) and subjected to workup when 11 was no longer visible (~48 h). Basic alumina (~3 g) was then added to the mixture and the solution stirred vigorously while exposed to air. The progress of the reaction was followed by NMR and the reaction was subjected to workup when the initial Diels-Alder adduct was no longer visible (~48 h). The solution was then filtered and the alumina was rinsed with chloroform (~400 mL). The filtrate was concentrated under reduced pressure and purified by flash chromatography on silica gel (packed with hexanes then eluted with 7:1 hexanes/EtOAc) to provide compound 27 as a yellow powder (302 mg, 0.867 mmol, 60% over two steps). 1

H NMR (300 MHz, CDCl3) δ 8.82 (s, 2H), 8.23 (s, 2H), 8.11-8.06 (m, 2H), 7.71-7.66 (m, 2H),

5.29 (s, 2H), 5.19 (s, 2H), 4.06 (X of ABX2, J = 1.5 Hz, 2H), 2.20 (A of ABX2, J = 9, 1.5 Hz, 1H), 2.07 (B of ABX2, J = 9, 1.5 Hz, 1H);

13

C NMR (75 MHz, CDCl3) δ 183.3, 153.3, 146.6,

135.3, 134.1, 130.2, 130.0, 129.51, 129.48, 119.8, 104.3, 52.8, 51.4; HRMS (ESI/Q-TOF) m/z: [M + Li]+ Calcd for C25H16O2Li 355.1310; Found 355.1309. Decacyclo[18.18.1.02,19.04,17.06,15.08,13.021,38.023,36.025,34.027,32]nonatriaconta2(19),3,6,8,10,12,14,17,21(38),22,25(34),26,28,30,32,36-hexadecaene-5,16,24,35-tetrone (9) Compounds 27 (201 mg, 0.577 mmol, 1.0 equiv) and 1061 (361 mg, 1.73 mmol, 3.0 equiv) were dissolved in chloroform (50 mL). The chloroform was then removed under reduced pressure to



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provide a red-orange paste that was heated under nitrogen at 220 °C (sand bath). The reaction was monitored by NMR and subjected to workup when 27 was no longer visible (~24 h). The solids were dissolved in chloroform (40 mL), and basic alumina (~3 g) was added to the mixture. The solution was then stirred vigorously while exposed to air, and the progress of the reaction was monitored by NMR. The reaction was subjected to workup when the initial Diels-Alder adduct was no longer visible (~24 h). The mixture was then filtered and the alumina rinsed with chloroform (~400 mL). The filtrate was concentrated under reduced pressure and purified by flash chromatography on silica gel (packed with hexanes then eluted with CHCl3) to provide 9 as a brown powder (207 mg, 0.375 mmol, 65%). 1


4.76 (s, 2H), 2.79 (s, 2H);

13

C NMR (75 MHz, CDCl3) δ 183.0, 155.5, 135.2, 133.8, 130.3,

129.7, 129.63, 129.58, 120.9, 51.7; HRMS (ESI/Q-TOF) m/z: [M + Li]+ Calcd for C39H20O4Li 559.1522; Found 559.1551. Decacyclo[18.18.1.02,19.04,17.06,15.08,13.021,38.023,36.025,34.027,32]nonatriaconta2(19),3,6,8,10,12,14,17,21(38),22,25(34),26,28,30,32,36-hexadecaene-5,16,24,35-tetrol (30) Compound 9 (24 mg, 0.043 mmol, 1.0 equiv) was dissolved in methanol (3 mL) and chloroform (0.4 mL) and cooled to 0 °C (ice water bath). NaBH4 (176 mg, 4.65 mmol, 107 equiv) was slowly added in three portions (brief exposure to air) over 2 h. The reaction was then allowed to slowly come to room temperature. The reaction was monitored by TLC (CHCl3 eluent, UV visualization) and subjected to workup when 9 was no longer visible (~24 h). The reaction was concentrated under reduced pressure, re-dissolved in water, and the insoluble product was filtered. The solids were allowed to dry overnight to yield 30 as a light brown powder (18 mg,


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0.032 mmol, 74%). This compound was isolated as an inseparable mixture of up to 7 diasteromers and provided a complex 1H NMR spectrum that was not amenable to tabulation. HRMS (ESI/Q-TOF) m/z: [M + Li]+ Calcd for C39H28O4Li 567.2148; Found 567.2145. Decacyclo[18.18.1.02,19.04,17.06,15.08,13.021,38.023,36.025,34.027,32]nonatriaconta2(19),3,5,7,9,11,13,15,17,21(38),22,24,26,28,30,32,34,36-octadecaene (2) Compounds 30 (15 mg, 0.027 mmol, 1 equiv) was dissolved in acetonitrile (5.4 mL) via sonication and stannous chloride (267 mg, 1.41 mmol, 52 equiv) was added. The reaction was monitored by TLC (CHCl3 eluent, UV visualization) and subjected to workup when 30 was no longer visible (~30 minutes). The reaction was concentrated under reduced pressure, re-dissolved in toluene via sonication, filtered to remove the insoluble metal salts, and the filtrate concentrated under reduced pressure to provide crude 2 (12 mg, 0.024 mmol, 91%). This was rinsed with chloroform (1 mL) to provide 2 as a yellow powder of sufficient purity for spectroscopic studies and 1H NMR. Because this compound is poorly soluble in common organic solvents, obtaining a

13

C NMR was not practical. In addition, using

119

Sn NMR on a model

system, we confirmed by comparison with solutions of known concentration that tin compounds are efficiently removed by the workup procedure. We used a model system that is available in larger scale so as to have better sensitivity in order to detect traces of tin. 1

H NMR (500 MHz, CDCl3) δ 8.57 (s, 4H), 8.48 (s, 4H), 7.97-7.95 (m, 4H), 7.84 (s, 4H), 7.37-

7.35 (m, 4H), 4.62 (s, 2H), 2.61 (s, 2H); HRMS (ESI/Q-TOF) m/z: [M]+ Calcd for C39H24 492.1878; Found 492.1881. 5,16,24,35-tetra(2-(tri-isopropyl-silyl)-ethynyl)decacyclo[18.18.1.02,19.04,17.06,15.08,13.021,38.023,36.025,34.027,32]nonatriaconta2(19),3,5,7,9,11,13,15,17,21(38),22,24,26,28,30,32,34,36-octadecaene (3)



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To an oven-dried round bottom flask under N2 was added isopropyl magnesium chloride (0.45 mL, 2.0 M in THF, 0.9 mmol, 25 equiv) followed by (triisopropylsilyl)acetylene (0.10 mL, 0.45 mmol, 12.5 equiv) and anhydrous toluene (1 mL). This was slowly heated to 110 oC for 15 minutes. In a separate flask, compound 9 (20 mg, 0.036 mmol, 1.0 equiv) was dissolved via sonication in anhydrous toluene (2 mL) and added to the reaction dropwise via cannula while at reflux. The reaction was maintained at reflux overnight, and subjected to workup after 24 hours, at which point 9 was no longer visible by TLC (CHCl3 eluent, UV visualization). The reaction was removed from the heat source, allowed to cool for 5 minutes and SnCl2 (2 mL of saturated SnCl2 in a 1.0 M HCl solution) was slowly added dropwise while the solution was still warm. The reaction was allowed to stir for 1 h, then filtered through a plug of silica (CH2Cl2 eluent). The combined organics were washed with water (2 x 10 mL), dried over Na2SO4, and concentrated under reduced pressure. The crude material was purified by flash chromatography on silica gel (packed with hexanes then eluted with hexanes then 20:1 hexanes/ EtOAc) to provide 3 as a red paste (35.6 mg, 0.029 mmol, 81%). 1


4.65 (s, 2H), 2.68 (s, 2H), 1.39-1.33 (m, 84H);

13

C NMR (75 MHz, CDCl3) δ 146.4, 132.7,

132.0, 130.2, 128.6, 126.1, 125.9, 118.8, 118.3, 50.8, 19.1, 18.6, 11.8; HRMS (ESI/Q-TOF) m/z: [M]+ Calcd for C83H104Si4 1212.7215; Found 1212.7214.

Acknowledgments We thank Dr. Ryan Michael for experimental assistance and Dr. Akin Akdag for useful discussions. We acknowledge support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Science, U.S. Department of Energy through grant


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DE-FG02-07ER15890. This work utilized the Hopper supercomputer of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors would like to acknowledge the University of Colorado Boulder Central Analytical Laboratory Mass Spectrometry Core Facility (partially funded by NIH S10 RR026641) for mass spectrometry analysis. Supporting Information Proton and carbon NMR spectra for compounds 1, 2, 3, 9, 12, 13, and 27, UV-vis spectrum of 3, and computational data for DFT geometry optimization of 25.

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Synthesis of Geometrically Well-Defined Covalent Acene Dimers for

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