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Gold(I) Mediated Rearrangement of [7]-Helicene to Give a Benzo[cd]pyrenium Cation Embedded in a Chiral Framework†
Cite this: DOI: 10.1039/x0xx00000x
Raphael J. F. Berger,a Matthew J. Fuchter,*b Ingo Krossing,c Henry S. Rzepa,b Julia Schaeferc and Harald Schererc
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
The facile gold-mediated skeletal rearrangement of [7]helicene to a cationic polyaromatic hydrocarbon is described. We report in depth studies on the structure and aromaticity of this novel stable cation and propose a mechanism for its formation. Extended polyaromatic hydrocarbons (PAHs) have initially been studied intensely in astrophysics,1 but are now gaining significant attention as organic semiconductors.2 In this context, PAHs are often used as models to study graphene,3 and the linear acenes, pentacene in particular, have been shown to give exceptional charge mobilities in organic transistors.2,4 This is, in part, due to their conjugated framework and periodic (co-planar) alignment in the solid state. We have recently reported the use of a structurally unique class of PAHs, the helicenes,5 in organic electronic devices. By employing enantiomerically pure derivatives of these helically chiral PAHs, we were able fabricate organic devices that emit6 and detect7 circularly polarised (CP) light, with the handedness of the CP light controlled by the handedness of the helicene PAH employed. Despite growing interest in this broad class of compounds, extended and functionalized PAHs represent challenging synthetic targets.8,9 6
6H-benzo[cd]pyrene (1) X AgX15
THIS WORK
X Ag
CH2Cl 2
AuCl, AgX CH 2Cl 2, rt 3 85%
2
X = [Al{OC(CF 3) 3}4]
Scheme 1
This journal is © The Royal Society of Chemistry 2012
6H-Benzo[cd]pyrene (1), synthesised as a mixture of isomers in 1965,10a recently reinvestigated in terms of its aromaticity,10b and also dubbed “olympicene”,11 is an interesting example of a PAH (Scheme 1). It is known to be able to readily form a cationic, radicaloid or anionic species with partial charges or spindensity mostly localized at the six-position.8,10 While C6 is formally not involved in the aromaticity of the neutral 6H-benzo[cd]pyrene (1), the benzo[cd]pyrenium cation is expected to function as a 18 electron Hückel aromatic system, with electron delocalisation through the formally vacant p-orbital. Herein we report a unique example of a facile gold-mediated skeletal rearrangement of [7]helicene (2) to give a stable C30H17 cation 3 (Scheme 1). We give details of the cation’s synthesis, structure, aromaticity, and propose a mechanism for its formation. Since, the isolated product has an aromatic benzo[cd]pyrenium (see 3, Scheme 1, highlighted in blue) PAH embedded in a chiral framework it may have future potential in the burgeoning field of chiral organic electronic devices.6,7,12-14 Previously, we have reported the ability of [7]-helicene (2) to act as a “molecular tweezer” for a silver(I) cation.15 Treatment of the helicene with a silver salt bearing the weakly coordinating counterion [Al{OC(CF3)3}4]– resulted in the silver-helicene adduct (Scheme 1). Salts containing such weakly coordinating counterions are highly soluble in non-polar solvents16 and therefore perfectly suited to studies conducted in apolar media. By analogy to this previous work, Ag[Al{OC(CF3)3}4] and [7]-helicene (2) were dissolved in CH2Cl2, and AuCl was added, in an attempt to form a gold(I) helicene complex. The solution was stirred for 48 h at room temperature. The colour of the solution turned from colourless to green. Since precipitation of AgCl was not observed, the reaction was sonicated for a further 50 h, during which time a grayish precipitate was observed. Filtration of the mixture, followed by cooling to –20 °C gave black crystals of a novel rearrangement product, [C30H17]+[Al{OC(CF3)3}4]– (3) (Scheme 1), which was isolated in excellent yield (85%). The fact this dramatic skeletal rearrangement occurs in excellent yield under mild conditions is
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remarkable, particularly since helicenes are often noted for their high stability.17 Indeed, whilst skeletal rearrangements have been reported for certain helicenes, these usually occur only at very high temperatures (>120 oC) and the products are only isolated in low yield.18,19 We encountered no instability issues with cation 3 when handled under an inert atmosphere. The product 3 crystallised in the triclinic space group P-1 (Figure 1), with one cation, one anion and one solvent molecule in the asymmetric unit. All of the carbons of the five ortho-condensed six-carbon rings and C122 are in plane (plane 1) within to 10 pm (av. 5 pm), with the exception of C101 and C102 (17 and 13 pm deviation from the plane). A second plane is constructed from the isolated benzene unit containing C124 and C17 (plane 2). The angle between these two planes is ~107°. The distances between the sp2 carbon atoms in the ring system are in the range of 132.7-150.1 pm. The bridging CH2-group (C123) projects out from main plane by ca. 119°. There are 40 weak F-H contacts ranging from 245 to 335 pm and 16 F...C contacts between 301 and 338 pm between the cation and the counterion. The rather weak contacts between anion and cation are in agreement with the CM5 charges (see Figure S6, ESI),20 which indicates delocalization of the positive charge over the planar conjugated system. In the extended solid-state, two cationic units are aggregated through π-stacking in such a way that there is an inversion center between them (see Figure 1, lower image). The distance between the two associated cationic planes is 335.9 pm. These are associated with neighbouring molecules by contacts between the isolated benzene rings. The shortest distance between these rings is 332.4 pm, the distance between the centroids of the isolated rings is much longer (464.7 pm). The cationic units are arranged in long strands and the anions are located in the gaps between the strands (see Figure S5, ESI).
Figure 1. Top left: Front view of [C30H17]+ cation 3; Top right: Side view of carbocation 3. The counterion and the co-crystallized solvent molecule were omitted for clarity. Selected bond lengths [pm] and angles [°] in 3: C122C123 157.6(8), C123-C100 151.2(7), C122-C124 154.2(7), C100-C17 155.1(7), C122-C120146.6(8), C100-C105 159.4(8); C120-C122-C123 111.3(8), C105-C100-C123 108.3(5), C122-C123-C100 100.0(4); Bottom: Extended solid state structure of 3. 1
H and 13C NMR spectra were recorded for crystalline product 3, following its dissolution in CD2Cl2. Experimental assignments were made on the basis of HMBC and HSQC spectra for the [C30H17]+ cation (see ESI). The signals of the [Al{OC(CF3)3}4]– anion are found at the expected shifts in the 19F and 27Al spectra.16 The 1H shifts for hydrogen atoms bonded to planar cationic ring system are in the region 8.58-9.38, and thus significantly deshielded, presumably by the induced ring current of this aromatic system. The 13C shifts of this ring are also significantly deshielded. Further prominent downfield shifts are observed for a number of carbon atoms in the 13C NMR spectrum of 3 (see Table
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C3CC46986G S2, ESI. Atom numbers depicted in Figure 1). Pronounced downfield shifts include C105 (δ = 167.7), C120 (δ = 154.9), C101 (δ = 149.6), C107 (δ = 148.7), C118 (δ = 144.8 ) and C103 (δ = 146.6). Consideration of the mesomeric forms for cation 3 (Figure S1, ESI) is consistent with these carbons having positive character. 13 C NMR spectra at the B3LYP/6Predicted 311++G(d,p)/SCRF=dcm level qualitatively reproduce the experimentally obtained spectra (Chart S1, ESI). The perturbative influence of the very large counterion (not included in the calculation) may account for the largest errors in measured vs simulated shift being associated with the carbon atoms with positive character. Previously, we have reported an analogous rearrangement of [7]-helicene, albeit in low yield and under harsh conditions (120 o C, strong Lewis acid), where we did not isolate a cationic intermediate, but instead a regioisomeric mixture of ketone products (see Figure S2, ESI).19 Formally, these products would arise from attack of water onto C118 or C107 of 3, followed by aerobic oxidation of the product. The regioselectivity of this attack is consistent with the NMR data; C118 and C107 have significant cation character, whilst not being sterically shielded by the bridge system. We also computed the UV-vis spectrum of cation 3 (see ESI) which can be compared that the experimental spectra of [7]helicene (2) and previously isolated ketone product derived from this cation (vide supra).19 The calculated spectrum is qualitatively similar to our reported spectrum of the cation derived ketone product, and predicts significant absorbance across the almost entire visible range. This is consistent with the observed dark green colour of 3 in dilute solution (or black in the solid state).
Figure 2. Magnetically induced current density vectors (purple) in 3 when a magnetic field perpendicular to the main molecular plane of 1 T field strength is applied. Shown are current vectors about 0.53 Å above the molecular plane, resulting mainly from the contribution from π orbitals. Integrated currents (dark green) flowing through a plane symbolized by half-lines (dark green) are shown as well as the experimentally observed 13C-NMR shifts (blue values).
The magnetically induced ring currents of the mono-cation 3 were further studied computationally in order to examine the aromaticity of this unusual scaffold. Figure 2 depicts the induced ring currents when a 1 Tesla magnetic field is applied perpendicular to the main plain of the molecule. The current plane displayed lies 0.53Å above the molecular plane. The total circulating current integrates to a 25 nAT–1 diamagnetic contribution and –15 nAT–1 paramagnetic contribution. Therefore overall the current is diamagnetic and
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amounts to 10 nAT–1; approximately the same as that observed for benzene and hence indicates the presence of a magnetically induced ring current of typical size for an aromatic molecule. There is a strong sidebranch of the total current flowing along C120–C121– C105–C104. The sidebranch has a diatropic contribution of 15 nAT– 1 and paratropic contribution of –8 nAT–1 and in part can explain the observed chemical shifts in the 13C NMR. The alternating strongly deshielded and moderately deshielded 13C signals along the circular chain bearing the strongest branch of the induced currents is apparently a consequence of the paramagnetic terms in the shielding tensors and cannot be explained with the ring currents since these are covered by the diamagnetic terms alone. Since an embedded benzo[cd]pyrenium cation is one description of the molecular framework of this scaffold, which should function as a 18 electron Hückel aromatic system (vide supra), we were curious whether the corresponding anion system (see ESI) yielded from a formal addition of two electrons to the mono-cation 3 would still be aromatic. The total circulating current of the anion integrates to a 17 nAT–1 diamagnetic contribution and –14 nAT–1 paramagnetic contribution. Therefore overall the current is diamagnetic and amounts to 3 nAT–1, which is considerably less than in the cation (in agreement with Hückel’s rule). Since the paramagnetic contribution does not overly change in comparison with the cation, it should not be considered anti-aromatic. Previously the contribution of the six-position charge in benzo[cd]pyrene (1) to the aromaticity of this PAH has been underappreciated. Our results concerning the topology and strengths of the magnetically induced ring currents in 3 fully agree with the recent theoretical results from Ramos-Berdullas et al10b on the benzo[cd]pyrenium cation. Moreover the distribution of partial charges, which can be explained in terms of Kekulé structures (see ESI), is in good agreement with our findings on cation 3.
ring A (Scheme 2), as observed for silver(I),15 which lowers the barrier for cycloaddition. Hydride abstraction from intermediate 5 by the gold cation would then give the cationic intermediate 5. The presumed gold hydride species would rapidly decompose to elemental gold and hydrogen gas.25 Thus, this reaction represents a highly unusual oxidation of a hydrocarbon by gold(I). A proton transfer event, converting cation 5 into cation 7, would then occur, mediated by the weakly coordinating counterion, [Al{OC(CF3)3}4].26 The calculated pKa values of cation 5, H[Al{OC(CF3)3}4], and cation 7 in CH2Cl2 are 21.7, 27.5, and 39.1 respectively (BP86/SV(P), COSMO solvation model for CH2Cl2). Therefore this proton transfer is in agreement with the calculated relative acidities of the species involved. Finally, a likely facile [1-2] shift would result in product 3. The relative free energies (ωB97XD/6311G(d,p)) for the cationic species 5, 7 and 3 are 0.0, –23.8 and –44.5 kcal mol–1. Thus the thermodynamics of this scheme are entirely viable. In conclusion, we have discovered a facile and high yielding gold-mediated rearrangement of [7]-helicene into a stable C30H17 cation, which can be described as an aromatic benzo[cd]pyrenium PAH embedded in a chiral framework. Not only does this represent a highly unusual gold-mediated oxidation of a hydrocarbon, but results in a chiral PAH that we believe should be of use in the flourishing field of chiral organic electronic devices.6,7,12-14
[Al(OR) 4]
G F [4+2]
C
B
H
H
D
A 2
Au[Al(OR)4]
H
E
Au+ mediated 4
AuH
5
Au + H 2 - H[Al(OR) 4]
G [Al(OR) 4] E
H
[Al(OR) 4]
1,2-shift
Au A [ (l O R ) 4
+ H[Al(OR) 4]
D
A C
B
7
6
3
Scheme 2 Our current mechanistic hypothesis is shown in Scheme 2. Intramolecular Diels–Alder cycloaddition of rings A and F of [7]helicene (2) would result in intermediate 4. Whilst an intramolecular Diels–Alder reaction was originally considered as a plausible mechanism for helicene racemization,21,22 this has since been dismissed.23 Indeed, at the rwb97xd/6-311g(d,p) level, the computed free energy of activation for the first Diels Alder step of [7]-helicene (2) is 42.2 kcal mol–1, a barrier that would require a high temperature to proceed at a reasonable rate. It is likely however that the gold cation promotes this unfavorable Diels-Alder reaction, especially since the intermediate formed (4) subsequently undergoes an irreversible oxidation step. Such involvement of the gold cation is implied by the mild conditions under which this rearrangement takes place (vide supra). We suggest this occurs through transient complexation of the gold cation to the π-system24 of the terminal
This journal is © The Royal Society of Chemistry 2012
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Notes and references
Chemistry of Materials, Hellbrunnerstr. 34, Paris Lodron-Universität, A5020 Salzburg, Austria b Department of Chemistry, Imperial College London, London SW7 2AZ, UK. Tel: +442075945815; E-mail : [email protected] c Institute for Inorganic and Analytical Chemistry, Freiburger Materialforschungszentrum FMF and Freiburg Institute for Advanced Studies (FRIAS), Albert–Ludwigs–Universität Freiburg, Albertstr. 21, 79104 Freiburg, Germany. † We thank the Leverhulme Trust (grant F/07058/BG) for funding this work. We also thank Dilraj K. Judge and Benjamin Wardzinski for [7]helicene synthesis support. IK thanks the ERC for the UniChem Advanced Grant for funding. Electronic Supplementary Information (ESI) available: Supplementary Figures, experimental procedures, spectroscopic and crystallographic data. See DOI: 10.1039/c000000x/ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24. 25 26
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a
A. G. G. M. Tielens, Annu. Rev. Astron. Astrophys., 2008, 46, 289. C. Wang, H. Dong, W. Hu, Y. Liu and D. Zhu, Chem. Rev. 2012, 112, 2208. A. Manjavacas, F. Marchesin, S. Thongrattanasiri, P. Koval, P. Nordlander, D. Sánchez-Portal and F. J. G. de Abajo, ACS Nano, 2013, 7, 3635. M. Watanabe, K. –Y. Chen, Y. J. Chang and T. J. Chow, Acc. Chem. Res. 2013, 46, 1606. Y. Shen and C. F. Chen, Chem. Rev. 2012, 112, 1463. Y. Yang, R. Correa da Costa, A. J. Campbell and M. J. Fuchter, Adv. Mater. 2013, 25, 2624. Y. Yang, R. Correa da Costa, M. J. Fuchter and A. J. Campbell, Nat. Photon. 2013, 7, 634. R. G. Harvey, Org. Prep. Proced. Int. 1997, 29, 243. Aromatic Ring Assemblies, Polycyclic Aromatic Hydrocarbons, and Conjugated Polyenes, ed. J. Siegel and Y. Tobe, Thieme, Germany, 2009, vol. 45b. a) D. H. Reid and W. Bonthrone, J. Chem. Soc., 1965, 5920; b) N. Ramos-Berdullas, S. Radenković, P. Bultnick, M. Mandado, J. Chem. Phys. A, 2013, 117, 4779. A. J. S. Valentine and D. A. Mazziotti, J. Phys. Chem. A., 2013, in press. DOI: 10.1021/jp312384b Y. H. Geng, A. Trajkovska, S. W. Culligan, J. J. Ou, H. M. Chen, D. Katsis and S. H. Chen, J. Am. Chem. Soc., 2003, 125, 14032 and references therein. J. Gilot, R. Abbel, G. Lakhwani, E. W. Meijer, A. P. H. J. Schenning and S. C. J. Meskers, Adv. Mater., 2010, 22, E131. L. Torsi1, G. M. Farinola, F. Marinelli, M. C. Tanese, O. H. Omar, L. Valli, F. Babudri, F. Palmisano, P. G. Zambonin and F. Naso, Nat. Mat. 2008, 7, 412. M. J. Fuchter, J. Schaefer, D. K. Judge, B. Wardzinski, M. Weimar, and I. Krossing, Dalton Trans. 2012, 41, 8238. (a) I. Krossing and A. Reisinger, Coord. Chem. Rev., 2006, 250, 2721; (b) I. Krossing and I. Raabe, Angew. Chem., Int. Ed., 2004, 43, 2066–2090; (c) I. Krossing, Chem.–Eur. J., 2001, 7, 490. US Pat., US6686065B2, 2004. J. H. Borkent, P. H. F. M. Rouwette and W. H. Laarhoven, Tetrahedron, 1978, 34, 2569. M. J. Fuchter, M. Weimar, X. Yang, D. K. Judge and A. J. P. White, Tetrahedron Lett. 2012, 53,1108. V. Marenich, S. V. Jerome, C. J. Cramer and D. G. Truhlar, J. Chem. Theor. Comput. 2012, 8, 527. R. H. Martin, Angew. Chem., Int. Ed. Engl. 1974, 13, 649. R. H. Martin and M. J. Marchant, Tetrahedron, 1974, 30, 347. R. H. Janke, G. Haufe, E. –U. Würthwein and J. H. Borkent, J. Am. Chem. Soc. 1996, 118, 6031. E. Herrero-Gómez, C. Nieto-Oberhuber, S. López, J. Benet-Buchholz and A. M. Echavarren, Angew. Chem. Int. Ed. 2006, 45, 5455. X. F. Wang and L. Andrews, Angew. Chem. Int. Ed. 2003, 42, 5201. A. Kraft, J. Beck, G. Steinfeld, H. Scherer, D. Himmel and I. Krossing, Organometallics 2012, 31, 7485.
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: R. J.F. Berger, M. Fuchter, I. Krossing, H. Rzepa, J. Schaefer and H. Scherer, Chem. Commun., 2013, DOI: 10.1039/C3CC46986G.
Volume 46 | Number 32 | 2010
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Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication.
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COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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Gold(I) Mediated Rearrangement of [7]-Helicene to Give a Benzo[cd]pyrenium Cation Embedded in a Chiral Framework†
Cite this: DOI: 10.1039/x0xx00000x
Raphael J. F. Berger,a Matthew J. Fuchter,*b Ingo Krossing,c Henry S. Rzepa,b Julia Schaeferc and Harald Schererc
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
The facile gold-mediated skeletal rearrangement of [7]helicene to a cationic polyaromatic hydrocarbon is described. We report in depth studies on the structure and aromaticity of this novel stable cation and propose a mechanism for its formation. Extended polyaromatic hydrocarbons (PAHs) have initially been studied intensely in astrophysics,1 but are now gaining significant attention as organic semiconductors.2 In this context, PAHs are often used as models to study graphene,3 and the linear acenes, pentacene in particular, have been shown to give exceptional charge mobilities in organic transistors.2,4 This is, in part, due to their conjugated framework and periodic (co-planar) alignment in the solid state. We have recently reported the use of a structurally unique class of PAHs, the helicenes,5 in organic electronic devices. By employing enantiomerically pure derivatives of these helically chiral PAHs, we were able fabricate organic devices that emit6 and detect7 circularly polarised (CP) light, with the handedness of the CP light controlled by the handedness of the helicene PAH employed. Despite growing interest in this broad class of compounds, extended and functionalized PAHs represent challenging synthetic targets.8,9 6
6H-benzo[cd]pyrene (1) X AgX15
THIS WORK
X Ag
CH2Cl 2
AuCl, AgX CH 2Cl 2, rt 3 85%
2
X = [Al{OC(CF 3) 3}4]
Scheme 1
This journal is © The Royal Society of Chemistry 2012
6H-Benzo[cd]pyrene (1), synthesised as a mixture of isomers in 1965,10a recently reinvestigated in terms of its aromaticity,10b and also dubbed “olympicene”,11 is an interesting example of a PAH (Scheme 1). It is known to be able to readily form a cationic, radicaloid or anionic species with partial charges or spindensity mostly localized at the six-position.8,10 While C6 is formally not involved in the aromaticity of the neutral 6H-benzo[cd]pyrene (1), the benzo[cd]pyrenium cation is expected to function as a 18 electron Hückel aromatic system, with electron delocalisation through the formally vacant p-orbital. Herein we report a unique example of a facile gold-mediated skeletal rearrangement of [7]helicene (2) to give a stable C30H17 cation 3 (Scheme 1). We give details of the cation’s synthesis, structure, aromaticity, and propose a mechanism for its formation. Since, the isolated product has an aromatic benzo[cd]pyrenium (see 3, Scheme 1, highlighted in blue) PAH embedded in a chiral framework it may have future potential in the burgeoning field of chiral organic electronic devices.6,7,12-14 Previously, we have reported the ability of [7]-helicene (2) to act as a “molecular tweezer” for a silver(I) cation.15 Treatment of the helicene with a silver salt bearing the weakly coordinating counterion [Al{OC(CF3)3}4]– resulted in the silver-helicene adduct (Scheme 1). Salts containing such weakly coordinating counterions are highly soluble in non-polar solvents16 and therefore perfectly suited to studies conducted in apolar media. By analogy to this previous work, Ag[Al{OC(CF3)3}4] and [7]-helicene (2) were dissolved in CH2Cl2, and AuCl was added, in an attempt to form a gold(I) helicene complex. The solution was stirred for 48 h at room temperature. The colour of the solution turned from colourless to green. Since precipitation of AgCl was not observed, the reaction was sonicated for a further 50 h, during which time a grayish precipitate was observed. Filtration of the mixture, followed by cooling to –20 °C gave black crystals of a novel rearrangement product, [C30H17]+[Al{OC(CF3)3}4]– (3) (Scheme 1), which was isolated in excellent yield (85%). The fact this dramatic skeletal rearrangement occurs in excellent yield under mild conditions is
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remarkable, particularly since helicenes are often noted for their high stability.17 Indeed, whilst skeletal rearrangements have been reported for certain helicenes, these usually occur only at very high temperatures (>120 oC) and the products are only isolated in low yield.18,19 We encountered no instability issues with cation 3 when handled under an inert atmosphere. The product 3 crystallised in the triclinic space group P-1 (Figure 1), with one cation, one anion and one solvent molecule in the asymmetric unit. All of the carbons of the five ortho-condensed six-carbon rings and C122 are in plane (plane 1) within to 10 pm (av. 5 pm), with the exception of C101 and C102 (17 and 13 pm deviation from the plane). A second plane is constructed from the isolated benzene unit containing C124 and C17 (plane 2). The angle between these two planes is ~107°. The distances between the sp2 carbon atoms in the ring system are in the range of 132.7-150.1 pm. The bridging CH2-group (C123) projects out from main plane by ca. 119°. There are 40 weak F-H contacts ranging from 245 to 335 pm and 16 F...C contacts between 301 and 338 pm between the cation and the counterion. The rather weak contacts between anion and cation are in agreement with the CM5 charges (see Figure S6, ESI),20 which indicates delocalization of the positive charge over the planar conjugated system. In the extended solid-state, two cationic units are aggregated through π-stacking in such a way that there is an inversion center between them (see Figure 1, lower image). The distance between the two associated cationic planes is 335.9 pm. These are associated with neighbouring molecules by contacts between the isolated benzene rings. The shortest distance between these rings is 332.4 pm, the distance between the centroids of the isolated rings is much longer (464.7 pm). The cationic units are arranged in long strands and the anions are located in the gaps between the strands (see Figure S5, ESI).
Figure 1. Top left: Front view of [C30H17]+ cation 3; Top right: Side view of carbocation 3. The counterion and the co-crystallized solvent molecule were omitted for clarity. Selected bond lengths [pm] and angles [°] in 3: C122C123 157.6(8), C123-C100 151.2(7), C122-C124 154.2(7), C100-C17 155.1(7), C122-C120146.6(8), C100-C105 159.4(8); C120-C122-C123 111.3(8), C105-C100-C123 108.3(5), C122-C123-C100 100.0(4); Bottom: Extended solid state structure of 3. 1
H and 13C NMR spectra were recorded for crystalline product 3, following its dissolution in CD2Cl2. Experimental assignments were made on the basis of HMBC and HSQC spectra for the [C30H17]+ cation (see ESI). The signals of the [Al{OC(CF3)3}4]– anion are found at the expected shifts in the 19F and 27Al spectra.16 The 1H shifts for hydrogen atoms bonded to planar cationic ring system are in the region 8.58-9.38, and thus significantly deshielded, presumably by the induced ring current of this aromatic system. The 13C shifts of this ring are also significantly deshielded. Further prominent downfield shifts are observed for a number of carbon atoms in the 13C NMR spectrum of 3 (see Table
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C3CC46986G S2, ESI. Atom numbers depicted in Figure 1). Pronounced downfield shifts include C105 (δ = 167.7), C120 (δ = 154.9), C101 (δ = 149.6), C107 (δ = 148.7), C118 (δ = 144.8 ) and C103 (δ = 146.6). Consideration of the mesomeric forms for cation 3 (Figure S1, ESI) is consistent with these carbons having positive character. 13 C NMR spectra at the B3LYP/6Predicted 311++G(d,p)/SCRF=dcm level qualitatively reproduce the experimentally obtained spectra (Chart S1, ESI). The perturbative influence of the very large counterion (not included in the calculation) may account for the largest errors in measured vs simulated shift being associated with the carbon atoms with positive character. Previously, we have reported an analogous rearrangement of [7]-helicene, albeit in low yield and under harsh conditions (120 o C, strong Lewis acid), where we did not isolate a cationic intermediate, but instead a regioisomeric mixture of ketone products (see Figure S2, ESI).19 Formally, these products would arise from attack of water onto C118 or C107 of 3, followed by aerobic oxidation of the product. The regioselectivity of this attack is consistent with the NMR data; C118 and C107 have significant cation character, whilst not being sterically shielded by the bridge system. We also computed the UV-vis spectrum of cation 3 (see ESI) which can be compared that the experimental spectra of [7]helicene (2) and previously isolated ketone product derived from this cation (vide supra).19 The calculated spectrum is qualitatively similar to our reported spectrum of the cation derived ketone product, and predicts significant absorbance across the almost entire visible range. This is consistent with the observed dark green colour of 3 in dilute solution (or black in the solid state).
Figure 2. Magnetically induced current density vectors (purple) in 3 when a magnetic field perpendicular to the main molecular plane of 1 T field strength is applied. Shown are current vectors about 0.53 Å above the molecular plane, resulting mainly from the contribution from π orbitals. Integrated currents (dark green) flowing through a plane symbolized by half-lines (dark green) are shown as well as the experimentally observed 13C-NMR shifts (blue values).
The magnetically induced ring currents of the mono-cation 3 were further studied computationally in order to examine the aromaticity of this unusual scaffold. Figure 2 depicts the induced ring currents when a 1 Tesla magnetic field is applied perpendicular to the main plain of the molecule. The current plane displayed lies 0.53Å above the molecular plane. The total circulating current integrates to a 25 nAT–1 diamagnetic contribution and –15 nAT–1 paramagnetic contribution. Therefore overall the current is diamagnetic and
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amounts to 10 nAT–1; approximately the same as that observed for benzene and hence indicates the presence of a magnetically induced ring current of typical size for an aromatic molecule. There is a strong sidebranch of the total current flowing along C120–C121– C105–C104. The sidebranch has a diatropic contribution of 15 nAT– 1 and paratropic contribution of –8 nAT–1 and in part can explain the observed chemical shifts in the 13C NMR. The alternating strongly deshielded and moderately deshielded 13C signals along the circular chain bearing the strongest branch of the induced currents is apparently a consequence of the paramagnetic terms in the shielding tensors and cannot be explained with the ring currents since these are covered by the diamagnetic terms alone. Since an embedded benzo[cd]pyrenium cation is one description of the molecular framework of this scaffold, which should function as a 18 electron Hückel aromatic system (vide supra), we were curious whether the corresponding anion system (see ESI) yielded from a formal addition of two electrons to the mono-cation 3 would still be aromatic. The total circulating current of the anion integrates to a 17 nAT–1 diamagnetic contribution and –14 nAT–1 paramagnetic contribution. Therefore overall the current is diamagnetic and amounts to 3 nAT–1, which is considerably less than in the cation (in agreement with Hückel’s rule). Since the paramagnetic contribution does not overly change in comparison with the cation, it should not be considered anti-aromatic. Previously the contribution of the six-position charge in benzo[cd]pyrene (1) to the aromaticity of this PAH has been underappreciated. Our results concerning the topology and strengths of the magnetically induced ring currents in 3 fully agree with the recent theoretical results from Ramos-Berdullas et al10b on the benzo[cd]pyrenium cation. Moreover the distribution of partial charges, which can be explained in terms of Kekulé structures (see ESI), is in good agreement with our findings on cation 3.
ring A (Scheme 2), as observed for silver(I),15 which lowers the barrier for cycloaddition. Hydride abstraction from intermediate 5 by the gold cation would then give the cationic intermediate 5. The presumed gold hydride species would rapidly decompose to elemental gold and hydrogen gas.25 Thus, this reaction represents a highly unusual oxidation of a hydrocarbon by gold(I). A proton transfer event, converting cation 5 into cation 7, would then occur, mediated by the weakly coordinating counterion, [Al{OC(CF3)3}4].26 The calculated pKa values of cation 5, H[Al{OC(CF3)3}4], and cation 7 in CH2Cl2 are 21.7, 27.5, and 39.1 respectively (BP86/SV(P), COSMO solvation model for CH2Cl2). Therefore this proton transfer is in agreement with the calculated relative acidities of the species involved. Finally, a likely facile [1-2] shift would result in product 3. The relative free energies (ωB97XD/6311G(d,p)) for the cationic species 5, 7 and 3 are 0.0, –23.8 and –44.5 kcal mol–1. Thus the thermodynamics of this scheme are entirely viable. In conclusion, we have discovered a facile and high yielding gold-mediated rearrangement of [7]-helicene into a stable C30H17 cation, which can be described as an aromatic benzo[cd]pyrenium PAH embedded in a chiral framework. Not only does this represent a highly unusual gold-mediated oxidation of a hydrocarbon, but results in a chiral PAH that we believe should be of use in the flourishing field of chiral organic electronic devices.6,7,12-14
[Al(OR) 4]
G F [4+2]
C
B
H
H
D
A 2
Au[Al(OR)4]
H
E
Au+ mediated 4
AuH
5
Au + H 2 - H[Al(OR) 4]
G [Al(OR) 4] E
H
[Al(OR) 4]
1,2-shift
Au A [ (l O R ) 4
+ H[Al(OR) 4]
D
A C
B
7
6
3
Scheme 2 Our current mechanistic hypothesis is shown in Scheme 2. Intramolecular Diels–Alder cycloaddition of rings A and F of [7]helicene (2) would result in intermediate 4. Whilst an intramolecular Diels–Alder reaction was originally considered as a plausible mechanism for helicene racemization,21,22 this has since been dismissed.23 Indeed, at the rwb97xd/6-311g(d,p) level, the computed free energy of activation for the first Diels Alder step of [7]-helicene (2) is 42.2 kcal mol–1, a barrier that would require a high temperature to proceed at a reasonable rate. It is likely however that the gold cation promotes this unfavorable Diels-Alder reaction, especially since the intermediate formed (4) subsequently undergoes an irreversible oxidation step. Such involvement of the gold cation is implied by the mild conditions under which this rearrangement takes place (vide supra). We suggest this occurs through transient complexation of the gold cation to the π-system24 of the terminal
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Notes and references
Chemistry of Materials, Hellbrunnerstr. 34, Paris Lodron-Universität, A5020 Salzburg, Austria b Department of Chemistry, Imperial College London, London SW7 2AZ, UK. Tel: +442075945815; E-mail : [email protected] c Institute for Inorganic and Analytical Chemistry, Freiburger Materialforschungszentrum FMF and Freiburg Institute for Advanced Studies (FRIAS), Albert–Ludwigs–Universität Freiburg, Albertstr. 21, 79104 Freiburg, Germany. † We thank the Leverhulme Trust (grant F/07058/BG) for funding this work. We also thank Dilraj K. Judge and Benjamin Wardzinski for [7]helicene synthesis support. IK thanks the ERC for the UniChem Advanced Grant for funding. Electronic Supplementary Information (ESI) available: Supplementary Figures, experimental procedures, spectroscopic and crystallographic data. See DOI: 10.1039/c000000x/ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24. 25 26
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a
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