Carbon networks based on dehydrobenzoannulenes. 3. Synthesis of graphyne substructures

Article abstract of DOI:10.1021/ol005623w

(equation presented) This Letter describes the synthesis of the first macrobicyclic subunits of the hypothetical all-carbon network graphyne. Key to synthetic success is an intramolecular Sonogashira cross-coupling sequence.

Full text of DOI:10.1021/ol005623w

ORGANIC
LETTERS
2000
Vol. 2, No. 7
969-972
Carbon Networks Based on
Dehydrobenzoannulenes. 3. Synthesis of
Graphyne Substructures1
Joshua M. Kehoe, James H. Kiley, Jamieson J. English, Charles A. Johnson,
Ryan C. Petersen, and Michael M. Haley*
Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403-1253
haley@oregon.uoregon.edu.
Received February 4, 2000
ABSTRACT
This Letter describes the synthesis of the first macrobicyclic subunits of the hypothetical all-carbon network graphyne. Key to synthetic
success is an intramolecular Sonogashira cross-coupling sequence.
The area of highly conjugated, all-carbon and carbon-rich
macromolecules continues to be of intense interest.2 The
properties of these novel systems are of great relevance in
the search for organic conductors, electrochromic display
materials, liquid crystals, synthetic ferromagnets, and non-
linear optical substances.3 Recent theoretical studies predict
stable, low-energy phases of carbon consisting of stacked
planar carbon layers equally occupied by sp and sp2 states.4
Replacement of one-third of the carbon-carbon bonds of
graphite with ethyne units results in the formation of one
such network composed entirely of phenyl rings and triple
bonds, dubbed graphyne (1). In addition to predictions of
strong nonlinear optical behavior, graphyne is expected to
be a large band gap semiconductor (Eg ) 1.2 eV).4a Alkali
metal charge-transfer complexes of 1 are predicted to be
metallic.4 Network 1 should be the most stable of the sp/sp2
hybrid allotropes with a calculated heat of formation
(∆Hf(g,C)) of 14.2 kcal per mol of C, which is comparable
to the experimentally determined values for C60 and C70
(10.16 and 9.65 kcal per mol of C, respectively).5 Despite
this thermodynamic instability, the crystalline fullerenes are
kinetically stable molecules and require high temperature and
pressure to force their conversion to the more stable forms
of graphite and diamond; hence, graphyne might exhibit
analogous kinetic stability and thus be resistant to graphitiza-
tion.
(1) For part 2 of this series, see: Wan, W. B.; Brand, S. C.; Pak, J. J.;
Haley, M. M. Chem. Eur. J. In press.
(2) (a) Topics in Current Chemistry (Carbon-Rich Compounds I); de
Meijere, A., Ed.; Springer: Berlin, 1998; Vol. 196. (b) Topics in Current
Chemistry (Carbon-Rich Compounds II); de Meijere, A., Ed.; Springer:
Berlin, 1999; Vol. 201.
(3) (a) Conjugated Polymers and Related Materials: The Interconnection
of Chemical and Electronic Structure; Salaneck, W. R., Lundstro¨m, I.,
Ranby, B., Eds.; Oxford University Press: Oxford, 1993. (b) Photonic and
Optoelectronic Polymers; Jenekhe, S. A., Wynne, K. J., Eds.; American
Chemical Society: Washington, DC, 1995. (c) Nonlinear Optics of Organic
Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press: Boca
Raton, FL, 1997. (d) Electronic Materials: The Oligomer Approach; Mu¨llen,
K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998.
Synthetic accessibility has been the primary deterrent to
graphyne-related research. Controlled oligotrimerization of
(5) (a) Beckhaus, H.-D.; Ru¨ckhardt, C.; Kao, M.; Diederich, F.; Foote,
C. S. Angew. Chem., Int. Ed. Engl. 1992, 31, 63-64. (b) Beckhaus, H.-D.;
Verevkin, S.; Ru¨ckhardt, C.; Diederich, F.; Thilgen, C.; ter Meer, H.-U.;
Mohn, H.; Mu¨ller, W. Angew. Chem., Int. Ed. Engl. 1994, 33, 996-998.
(4) (a) Baughman, R. H.; Eckhardt, H.; Kerte´sz, M. J. J. Chem. Phys.
1987, 87, 6687-6699. (b) Narita, N.; Nagagi, S.; Suzuki, S.; Nakao, K.
Phys. ReV. B 1998, 58, 11009-11014.
10.1021/ol005623w CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/11/2000

cyclo[12]carbon (2) could conceivably produce 1; however,
the detection of 2 outside a mass spectrometer has proven
elusive.6 Hexaethynylbenzene (3) can be viewed as an
acetylenic scaffold off of which graphyne mimics can be
built.7 Unfortunately, 3 proved to be highly sensitive to both
heat and oxygen, a common problem found in perethynylated
π-systems.
cyclotrimerization of o-ethynyliodobenzene provides annu-
lene 4 in 47% yield along with an 8% yield of cyclotetramer
and traces of cyclohexamer.12 Although the cyclooligomer-
ization strategy has furnished numerous other annulenic
systems,9 the concomitant isolation of tetramer and hexamer
along with 4 suggests that use of intermolecular methods to
prepare more complex structures such as 5 and/or 6 will be
problematic at best.
To minimize the formation of side products, we devised
an intramolecular approach for the assembly of DBAs 4-6.
The preparation of 4, illustrated in Scheme 1, allowed us to
Scheme 1a
An alternative method is to prepare macrocyclic segments
as network mimics. Dehydrobenzoannulenes (DBAs) are a
class of molecules that are well suited for this purpose.8 In
addition to being substructures of synthetic carbon allotropes,
DBAs have garnered tremendous interest in recent years as
ligands for organometallic chemistry, as hosts for binding
guest molecules, as probes for investigating weak induced
ring currents, and as precursors to fullerenes, “bucky” tubes,
“bucky” onions, and other carbon-rich materials.9 More
importantly, the annulenic compounds tend to be more stable
to heat, light, and oxygen than other ethynylated π-systems.
With this alternative strategy in mind, we report herein the
preparation of DBAs 5 and 6, the first macrobicyclic subunits
of network 1, and a new synthesis of 4.10
a Legend: (a) i. NaNO2, HCl, H2O, ii. Et2NH, K2CO3, H2O; (b)
Me3SiCtCH, PdCl2(PPh3)2, CuI, Et3N; (c) for 9, K2CO3, MeOH,
THF; (d) for 10, MeI, 120 °C; (e) 9 + 10, PdCl2(PPh3)2, CuI, Et3N;
(f) 9, PdCl2(PPh3)2, CuI, Et3N; (g) Pd(dba)2, PPh3, CuI, Et3N.
test and fine-tune our synthetic route. Sequential Sonogashira
cross-coupling reactions,13 manipulation of iodines masked
(6) Tobe, Y.; Matsumoto, H.; Naemura, K.; Achiba, Y.; Wakabayashi,
T. Angew. Chem., Int. Ed. Engl. 1996, 35, 1800-1802.
(7) Diercks, R.; Armstrong, J.; Boese, R.; Vollhardt, K. P. C. Angew.
Chem., Int. Ed. Engl. 1986, 25, 268-269.
(8) Haley, M. M. Synlett 1998, 557-565.
(9) (a) Haley, M. M.; Pak, J. J.; Brand, S. C., in ref 2b, pp 81-130. (b)
Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 107-119.
(10) For the synthesis of a hexaethynylated version of 4, an “advanced”
monocyclic subunit of 1, see: Eickmeier, C.; Junga, H.; Matzger, A. J.;
Scherhag, F.; Shim, M.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl.
1997, 36, 2103-2108.
(11) (a) Campbell, I. D.; Eglinton, G.; Henderson, W.; Raphael, R. A.
J. Chem. Soc., Chem. Commun. 1966, 87-89. (b) Staab, H. A.; Graf, F.
Chem. Ber. 1970, 103, 1107-1118.
Triyne 4, the smallest subunit, has been synthesized in a
variety of ways9 over the 30+ years since its initial
preparation.11 Nevertheless, an improved version of the
original approach remains the most direct route. Cu-mediated
(12) Youngs, W. J.; Tessier, C. A.; Bradshaw, J. D. Chem. ReV. 1999,
99, 3153-3180 and references therein.
(13) (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley: New York,
1995. (b) Modern Cross-Coupling Reactions; Stang, P. J., Diederich, F.,
Eds.; VCH: Weinheim, 1997.
970
Org. Lett., Vol. 2, No. 7, 2000

as dialkyltriazenes,14 and deprotection of silylated alkynes15
represent most of the chemistry involved in DBA construc-
tion. Diazotization of commercially available o-iodoaniline
(7) and trapping with diethylamine followed by cross-
coupling with (trimethylsilyl)acetylene provided triazene 8
in 82% yield. Transformation of this key intermediate into
9 or 1016 could be accomplished by either protiodesilylation
with K2CO3 in MeOH or triazene decomposition with MeI
at 120 °C, respectively, in essentially quantitative yield.
Cross-coupling 9 with 10 afforded triazene 11. A second
iteration of triazene decomposition and cross-coupling gave
triyne 12. Triazene decomposition, protiodesilylation, and
intramolecular alkynylation with Pd(dba)2 under high dilution
conditions furnished 4 as the sole product in 69% yield for
the three steps. More importantly, larger cyclooligomeric
macrocycles were not detected, facilitating product isolation
and purification.
Anticipating that solubility problems would likely be
encountered during the synthesis of other graphyne subunits,
solubilizing tert-butyl substituents were incorporated for the
preparation of bis-macrocycle 6 (Scheme 3). Iodination of
Scheme 3a
The intramolecular cyclization strategy is indispensable
for the construction of more complex substructures of the
graphyne network, such as bis-macrocycle 5 (Scheme 2),
Scheme 2a
a Legend: (a) BnEt3N+ICl2-, CaCO3, CH2Cl2, MeOH; (b) i.
NaNO2, HCl, MeCN, H2O, ii. Et2NH, K2CO3, H2O; (c) Me3SiCtCH,
i
PdCl2(PPh3)2, CuI, Et3N; (d) MeI, 120 °C; (e) Pr3SiCtCH,
PdCl2(PPh3)2, CuI, Et3N; (f) K2CO3, MeOH, THF; (g) N,N-diethyl-
o-iodophenyltriazene, PdCl2(PPh3)2, CuI, Et3N; (h) Bu4N+F-, THF,
EtOH; (i) 15, PdCl2(PPh3)2, CuI, Et3N; (j) Pd(dba)2, PPh3, CuI,
Et3N.
a Legend: (a) K2CO3, MeOH, THF; (b) 1,5-dibromo-2,4-
diiodobenzene, PdCl2(PPh3)2, CuI, Et3N; (c) Me3SiCtCH, PdCl2-
(PPh3)2, CuI, Et3N, (d) MeI, 120 °C; (e) Pd(dba)2, PPh3, CuI, Et3N.
commercially available 4-tert-butylaniline with the mild
reagent (BnNEt3)+ICl2- 18 provided 2,6-diiodo-4-tert-butyl-
aniline in high yield. This in turn was transformed into iodo
intermediate 15 and subsequently triazene 16 using chemistry
established in Schemes 1 and 2. Protiodesilylation and cross-
coupling with a second equivalent of 15 furnished pentayne
17. Cyclization as before gave the freely soluble, diamond-
shaped subunit 6 in 0.6% overall yield for 13 steps.
Attempts to obtain X-ray quality crystals of the bright
yellow compounds have been unsuccessful; nevertheless, the
spectral properties of the molecules agree fully with the
proposed structures. The IR spectra of 4-6 display very weak
ArCtCAr stretches centered around 2200 cm-1, character-
which are otherwise impossible to synthesize by intermo-
lecular routes. Using the same general sequence of reactions,
hexayne precursor 13 can be prepared efficiently from diyne
11. Triazene decomposition, protiodesilylation, and 2-fold
intramolecular cyclization gave, after vacuum sublimation,
a yellow solid that proved to be sparingly soluble in common
organic solvents. Although expected for much larger gra-
phyne mimics, the severity of the solubility problem was
surprising at this stage. It is probable that the low isolated
yield of the cyclization step (<15%) is due mainly to this
complication.17
(14) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991, 32,
2465-2466.
(17) Repetition of the synthetic sequence with solubilizing substituents
on the arenes has failed so far to furnish derivatives of 5. Full details will
be disclosed upon completion of this study.
(18) Kajigaeshi, S.; Kakinami, T.; Yamasaki, H.; Fujisaki, S.; Okamoto,
T. Bull. Chem. Soc. Jpn. 1988, 61, 600-602.
(15) Brandsma, L. PreparatiVe Acetylenic Chemistry; Elsevier: Am-
sterdam, 1988; pp 118-119.
(16) Nicolaou, K. C.; Dai, W.-M.; Hong, Y. P.; Tsay, S.-C.; Baldridge,
K. K.; Siegel, J. S. J. Am. Chem. Soc. 1993, 115, 7944-7953.
Org. Lett., Vol. 2, No. 7, 2000
971

istic of the highly symmetrical structures. The AA′BB′
multiplets in the H NMR spectra of DBAs 4 and 6 are
however, the presence of the two tert-butyl moieties in 6
results in sequential loss of the six methyl groups.
1
virtually superimposable (∆δ < 0.02), with the notable
exception of the four-proton singlet (δ 7.16) overlapping with
the BB′ signal in 6. Similar to that found in 4, the upfield
chemical shifts of the singlet and AA′BB′ multiplets of 6
suggest the presence of a weak paratropic ring current in
the 12-membered annulenic skeleton.19 The poor solubility
of 5 precluded acquisition of meaningful NMR data.
The UV-vis spectra of 4-6 display the characteristic
three-peak pattern of the diphenylacetylene chromophore (ca.
280, 290, and 300 nm). However, the last of these absorptions
in the macrocycles is about 2.5 times more intense than the
first two bands and 10 times more intense than the
comparable absorption in diphenylacetylene. This strong
band is attributed to the 0 f 0 transition of an arene π-π*
excitation11b and is typical for the hexadehydrotribenzo[12]-
annulene π-system. The UV-vis spectra of DBAs 5 and 6
do differ from that of 4 in two ways: (1) the presence of an
additional moderately strong absorption around 325 nm and
(2) extension of the weak, lower energy bands by an
additional 50-60 nm toward longer wavelengths. This
translates to end absorption points of 5 and 6 greater than
460 nm.
Unlike macrocycle 4, which melts at 210 °C, DBAs 5 and
6 exhibit no melting transition prior to decomposition. DSC
analyses of all three compounds show the onset of irrevers-
ible, exothermic reactions around 300-350 °C. The exo-
therms are extremely broad (w1/2 ≈ 30-40 °C) which suggest
that random polymerization of the materials is occurring.
Attempts to examine the DSC thermoproducts have been
severely hampered due to complete insolubility of the shiny,
black materials in common organic solvents. Therefore, the
structures of these thermoproducts as well as the nature of
thermal transformation remain uncertain.
In summary, we have synthesized dehydrobenzoannulenes
5 and 6, the most complete substructures of the graphyne
all-carbon network (1) prepared to date, via an intramolecular
cyclization strategy. In addition, we have developed an
alternative preparation of subunit 4. We are currently
optimizing the syntheses in order to obtain reliable correlation
studies relating macrocycle size with physical properties.
These results will be the subject of a future report.
Acknowledgment. We thank the National Science Foun-
dation (CHE-9704171), The Camille and Henry Dreyfus
Foundation (Teacher-Scholar Award 1998-2003 to M.M.H.),
and the University of Oregon for financial support.
Typical of polycyclic aromatic hydrocarbons, the molec-
ular ion peaks in the mass spectra of DBAs 5 and 6 (m/e
522 and 534, respectively) are also the base peaks. The mass
spectra also show strong M + 1 and M + 2 peaks, reflecting
the high carbon content of the bis-macrocycles. Analogous
to 4, compound 5 exhibits essentially no fragmentation;
Supporting Information Available: General experimen-
tal procedures and selected spectral data for compounds 4-6,
8-13, and 15-17. This material is available free of charge
via the Internet at http://pubs.acs.org.
(19) For a discussion of induced ring currents in dehydrobenzoannulenes,
see: (a) Wan, W. B.; Kimball, D. B.; Haley, M. M. Tetrahedron Lett. 1998,
39, 6795-6798. (b) Matzger, A. J.; Vollhardt, K. P. C. Tetrahedron Lett.
1998, 39, 6791-6794.
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