The first synthesis and characterization of [9]-, [12]-, and [18]cycloparaphenylene was demonstrated utilizing a novel aromatization reaction. We refer to these fascinating structures as "carbon nanohoops" due to their structural similarity to carbon nanotubes. Additionally, we have utilized computational methods to understand the unique properties of these fully conjugated macrocycles

Bertozzi, Carolyn R.

Bhattacharjee, Joydeep

Jasti, Ramesh

Neaton, Jeffrey B.

English

PubMed

Neaton, Jeffrey B

Bertozzi, Carolyn R

eScholarship - University of California

UC BerkeleyUC Berkeley Previously Published WorksTitleSynthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop StructuresPermalinkhttps://escholarship.org/uc/item/8g01j6z4JournalJournal of the American Chemical Society, 130(52)ISSN0002-7863AuthorsJasti, RameshBhattacharjee, JoydeepNeaton, Jeffrey Bet al.Publication Date2008-12-31DOI10.1021/ja807126u Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaSynthesis, Characterization, and Theory of [9]-, [12]-, and[18]Cycloparaphenylene: Carbon Nanohoop StructuresRamesh Jasti,†,‡ Joydeep Bhattacharjee,‡ Jeffrey B. Neaton,‡ and Carolyn R. Bertozzi*,†,‡,§Departments of Chemistry and Molecular and Cell Biology, Howard Hughes Medical Institute, UniVersity ofCalifornia, and The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720Received September 8, 2008; E-mail: crb@berkeley.eduDue to their unique architectural features, the cycloparaphe-nylenes (1, Figure 1) have attracted attention from many vantagepoints.1 Their strained and distorted aromatic systems and radiallyoriented p orbitals (2) have intrigued synthetic chemists, theoreti-cians, supramolecular chemists, and materials scientists alike.Despite this widespread interest, the cycloparaphenylenes remainan unsolved synthetic challenge.2-4 Here we describe the firstsynthesis of [9]-, [12]-, and [18]cycloparaphenylene. We also utilizecomputational methods to understand the structural and opticalproperties of these new macrocycles. As these structures correspondto the shortest-possible segment of an armchair carbon nanotube,5we refer to them as “carbon nanohoops.”The heart of the synthetic challenge for the cycloparaphenyleneslies in the strain energy of the bent aromatic system.6 Our strategywas to build up strain sequentially during the synthesis, using a3,6-syn-dimethoxy-cyclohexa-1,4-diene moiety as a masked aro-matic ring in a macrocyclic intermediate (see structure 4, Scheme1). We reasoned that the cyclohexadiene unit would provide thecurvature and rigidity necessary for macrocyclization to afford amarginally strained intermediate. Subsequent aromatization wouldthen provide the driving force necessary to achieve the strainenergies of target compounds, calculated to be 47, 28, and 5 kcal/mol for [9]-, [12]-, and [18]cycloparaphenylenes, respectively (Videinfra).The synthesis of the macrocyclic precursors to cycloparaphenylenesis depicted in Scheme 1. Monolithiation of diiodobenzene andsubsequent addition of benzoquinone generated a syn diol,7 which wasthen alkylated with methyl iodide to produce diiodide 4 in an overallyield of 34%. In preparation for a tandem Suzuki coupling/macrocy-clization sequence, portions of diiodide 4 were converted to diboronate5 in a straightforward manner. Advantageously, both coupling partnerscould easily be prepared on a gram scale without the need for columnchromatography. Standard Suzuki reaction conditions afforded mac-rocycles 6, 7, and 8 in 22% combined yield (Scheme 1). Formationof macrocycle 6 was unexpected and likely involves homocouplingof diboronate 5 as the intermediate step.8 Although the yield ofmacrocyclization products was modest, the synthetic route providedrapid access to three different macrocycles, which were easily separatedby column chromatography. It is notable that the construction ofmacrocycle 8 involved formation of 18 carbon-carbon bonds in only4 steps.With the macrocyclic precursors in hand, we turned our attentionto the aromatization reaction. To date, no efficient reactions to convertthe cyclohexadiene moiety to a para-substituted aryl ring had beenreported in the literature.9 Cyclohexadiene compounds similar to ourshave been aromatized under acidic conditions, but only with concomi-tant alkyl shifts generating 2,4-dialkylanisole derivatives.10 We at-tempted to circumvent such rearrangements with the use of strongnucleophilic hydride sources11 but without success. Similarly, reducingconditions employing Stephen’s reagent12 or low-valent titanium13 ledto complex mixtures due to the Lewis acidic nature of the reagents.Lithium napthalenide, a reagent known to reduce benzylic ethersat low temperature,14 provided the eventual solution (Figure 2).Treatment of each macrocycle (6, 7, and 8) with lithium napthalenideat -78 °C afforded the corresponding cycloparaphenylenes (9, 10, and11) in moderate yields. Mechanistically, the reaction is envisioned toproceed via an initial one-electron transfer which generates stabilizedradical intermediate 12 and an equivalent of lithium methoxide. A† Department of Chemistry, University of California.‡ The Molecular Foundry, Lawrence Berkeley National Laboratory.§ Department of Molecular and Cell Biology, University of California, andHoward Hughes Medical Institute.Figure 1. Cycloparaphenylenes (1) and a schematic of their radiallyoriented p orbitals (2).Scheme 1. Preparation of Macrocyclic Precursorsaa Reagents and conditions: (a) (i) nBuLi, THF, -78 °C, (ii) benzo-quinone; (b) (i) NaH, THF, 0 °C, (ii) MeI, 0 °C to rt; (c) (i) nBuLi, THF,-78 °C, (ii) isopropyl pinacol borate (Bpin), -78 °C; (d) Pd(PPh3)4, Cs2CO3,toluene/methanol (10:1), 80 °C.Figure 2. Aromatization reaction and proposed mechanism.Published on Web 12/04/200810.1021/ja807126u CCC: $40.75  2008 American Chemical Society17646 9 J. AM. CHEM. SOC. 2008, 130, 17646–17647second electron transfer then produces the alkyl lithium 13 which canaromatize via loss of a second equivalent of lithium methoxide.Especially noteworthy is the formation of cycloparaphenylene 9, themost strained carbon nanohoop of the series, using these low-temperature conditions.With cycloparaphenylenes in hand for the first time, we analyzedtheir properties using a variety of methods.15 Interestingly, their UV/visible absorption spectra showed symmetric peaks with a maximumat ∼340 nm regardless of carbon nanohoop size (Figure 3a). Indeed,the smallest carbon nanohoop (9), which has the poorest geometryfor orbital overlap and fewest number of aryl rings, has a slightlysmaller optical absorption gap than that of the larger carbon nanohoops.This result stands in sharp contrast to the acyclic paraphenylenes inwhich a narrowing of the optical absorption gap occurs with extendedconjugation.16 The fluorescence spectra also provided some unexpectedresults. As carbon nanohoop size decreased, the Stokes shift increased.Interestingly, the smallest carbon nanohoop 9 shows an especially broadspectrum with a large Stokes shift of approximately 160 nm.We further investigated the structural and optical properties of thecarbon nanohoop molecules with computational methods based ondensity functional theory (DFT).15 Our calculations indicate the favoredgeometry for the even-membered nanohoops, [12]- and [18]cyclopar-aphenylene, is a staggered configuration in which the dihedral anglebetween two adjacent phenyl rings alternates between 33° and 34°,respectively. This arrangement minimizes steric repulsion betweenneighboring aryl rings. For [9]cycloparaphenylene, the situation isslightly more complex. Reduced symmetry due to an odd number ofphenyl rings results in a lowest energy conformation in which thedihedral angle varies between 18°, 30°, 31°, and 33° around the carbonnanohoop (Figure 3b).15 We found that chiral Mobius-strip likearrangements, where the dihedral angle is successively increased by∼35° around the ring, were higher in energy by at least 2 kcal/molper phenyl ring. The calculated strain energies of carbon nanohoops9, 10, and 11 were 47, 28, and 5 kcal/mol with diameters of 1.2, 1.7,and 2.4 nm, respectively.We also performed DFT calculations to understand the counterin-tuitive trends in the observed optical data. We estimated the averageoptical absorption energy gap, Eg, with the following approximateformula: Eg ) ionization potential (IP) - electron affinity (EA) -Ee-h, where Ee-h is the electron-hole interaction energy between thehighest-occupied and lowest-unoccupied DFT one-electron wavefunctions.15 As expected, we observed that IP - EA decreases as thenumber of phenyl rings increase, in both the cyclic and acyclic cases.Remarkably, however, we found that the magnitude of Ee-h growsmore dramatically with the decreasing number of phenyl rings forcarbon nanohoops than for their acyclic counterparts.17 This computedtrend in Ee-h more than compensates for the increase in IP - EA andresults in an overall decrease in Eg with nanohoop diameter, inagreement with the experimentally observed trends. The fluorescencedata were also rationalized with DFT methods. The large Stokes shiftobserved in these spectra is governed by the degree of structuralrelaxation in the optically excited state. In the smaller nanohoops,enhanced curvature leads to greater sp3 hybridization and increasinglyasymmetric p orbitals, with their smaller lobes oriented inside thenanohoops. This factor reduces steric interactions and facilitates a largerdecrease in dihedral angle in the excited state for the smaller nanohoop.Thus smaller hoops have the potential for larger structural relaxationand Stokes shifts. Indeed constrained DFT calculations confirmed theobserved trend that the smallest carbon nanohoop has the largestrelaxation in its excited state.15In conclusion, we have demonstrated the first synthesis andcharacterization of [9]-, [12]-, and [18]cycloparaphenylene, the fun-damental unit of an armchair carbon nanotube. It may be possible toprepare extended carbon nanotubes using similar solution phasechemistry without the need for high temperatures. Preparation of carbonnanotubes of a specific chirality5 utilizing these carbon nanohoopstructures as templating agents is an intriguing prospect.Acknowledgment. This work was performed at the MolecularFoundry. Experimental work was supported by the Office ofScience, Office of Basic Energy Sciences, of the U.S. Departmentof Energy under Contract No. DE-AC02-05CH11231. Theoreticalwork was supported by the National Science Foundation throughthe Network for Computational Nanotechnology, Grant EEC-0634750.Supporting Information Available: Procedures for the synthesisand characterization of all new compounds and details of the compu-tational analyses. This material is available free of charge via theInternet at http://pubs.acs.org.References(1) (a) Schröder, A.; Mekelburger, H.-B.; Vögtle, F. In Topics in CurrentChemistry; Springer: Berlin, 1994; Vol. 172, p 179. (b) Girreser, U.;Giuffrida, D.; Kohnke, F. H.; Mathias, J. P.; Philp, D.; Stoddart, J. F. PureAppl. Chem. 1993, 65, 119. (c) Scott, L. T. Angew. Chem., Int. Ed. 2003,42, 4133. (d) Tahara, K.; Tobe, Y. Chem. ReV. 2006, 106, 5274. (d) Kawase,T.; Kurata, H. Chem. ReV. 2006, 106, 5250.(2) To our knowledge, the earliest attempt to synthesize the cycloparaphenyleneswas reported in 1934: Parekh, V. C.; Guha, P. C. J. Indian Chem. Soc.1934, 11, 95.(3) Vögtle et al. suggested a variety of approaches that to date have not led tothe synthesis of cycloparaphenylenes: Friedrich, R.; Nieger, M.; Vögtle,F. Chem. Ber. 1993, 126, 1723.(4) Herges et al. have demonstrated the synthesis of the quinodimethane formof benzoannelated [4]cycloparaphenylene: Kammermeier, S.; Jones, P. G.;Herges, R. Angew. Chem., Int. Ed. 1996, 35, 2669.(5) For a brief overview of carbon nanotube structure, see: Dai, H. Acc. Chem.Res. 2002, 35, 1035.(6) Jagadeesh, M. N; Makur, A.; Chandrasekhar, J. J. Mol. Model. 2000, 6, 226.(7) The syn stereochemistry is favored due to electrostatic interactions in thetransition state: Alonso, F.; Yus, M. Tetrahedron 1991, 47, 7471.(8) (a) Song, Z. Z.; Wong, H. N. C. J. Org. Chem. 1994, 59, 33. (b) Yamaguchi,S.; Ohno, S.; Tamao, K. Synlett 1997, 1199.(9) The one relevant example in the literature produces a mixture of para-and meta-substituted aryl rings: Morrow, G. W.; Schwind, B. Synth.Commun. 1995, 25, 269.(10) Alonso, F.; Yus, M. Tetrahedron 1992, 48, 2709.(11) Highly reactive nucleophiles have been shown to outcompete alkyl shiftsin similar systems: (a) Alonso, F.; Yus, M. Tetrahedron 1991, 47, 313. (b)Alonso, F.; Yus, M. Tetrahedron 1991, 47, 9119.(12) Newman, M. S.; Kanakarajan, K. J. Org. Chem. 1980, 45, 2301.(13) Walborsky, H. M.; Wuest, H. H. J. Am. Chem. Soc. 1982, 104, 5807.(14) Liu, H.-J.; Yip, J.; Shia, K.-S. Tetrahedron Lett. 1997, 38, 2253.(15) For full details, see Supporting Information.(16) Nijegorodov, N. I.; Downey, W. S.; Danailov, M. B. Spectrochim. Acta,Part A 2000, 56, 783.(17) This tendency can be directly related to the difference between linear finite(acyclic) and closed curved (cyclic) geometries. In the cyclic system, theelectron and hole states are delocalized over the entire circumference ofthe molecule. In the acyclic system, electron and hole states are localizedaway from the edges, toward the middle of the molecule. The spatialdistribution of these optically active electronic states results in differentelectron-hole interaction energetics.JA807126UFigure 3. (a) Absorption (solid line) and fluorescence (dashed line) spectraof carbon nanohoops 9, 10, and 11. (b) Energy-minimized geometry of carbonnanohoop 9 by DFT calulations.J. AM. CHEM. SOC. 9 VOL. 130, NO. 52, 2008 17647C O M M U N I C A T I O N S

Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures

Acta, Part A

For a brief overview of carbon nanotube structure, see:

For full details, see Supporting Information.

have demonstrated the synthesis of the quinodimethane form of benzoannelated [4]cycloparaphenylene:

Highly reactive nucleophiles have been shown to outcompete alkyl shifts in similar systems: (a) Alonso,

In Topics in Current Chemistry;

suggested a variety of approaches that to date have not led to the synthesis of cycloparaphenylenes:

Synlett

The one relevant example in the literature produces a mixture of paraand meta-substituted aryl rings:

The syn stereochemistry is favored due to electrostatic interactions in the transition state: Alonso,

This tendency can be directly related to the difference between linear ﬁnite

To our knowledge, the earliest attempt to synthesize the cycloparaphenylenes was reported in

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2709987

Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California