Carbon networks based on benzocyclynes. 6. Synthesis of graphyne substructures via directed alkyne metathesis

Article abstract of DOI:10.1021/ol7014253

Intramolecular ring closing alkyne metathesis afforded the graphyne biscyclyne (3) in high macrocyclization yield and good overall yield. This methodology also furnished the tris[12]cyclyne 4, which contains the longest linear diphenylacetylene conjugatio

Full text of DOI:10.1021/ol7014253

ORGANIC
LETTERS
2007
Vol. 9, No. 19
3725-3728
Carbon Networks Based on
Benzocyclynes. 6. Synthesis of
Graphyne Substructures via Directed
Alkyne Metathesis§
Charles A. Johnson II,† Yunyi Lu,‡ and Michael M. Haley*,†
Department of Chemistry and Materials Science Institute, UniVersity of Oregon,
Eugene, Oregon 97403-1253, and Department of Chemistry, UniVersity of Illinois at
Urbana-Champaign, Urbana, Illinois 61801
haley@uoregon.edu
Received June 18, 2007
ABSTRACT
Intramolecular ring closing alkyne metathesis afforded the graphyne biscyclyne (3) in high macrocyclization yield and good overall yield. This
methodology also furnished the tris[12]cyclyne 4, which contains the longest linear diphenylacetylene conjugation pathway for any graphyne
substructure based on the tribenzo[12]cyclyne core.
Graphyne (1), a theoretical carbon allotrope composed of
sp- and sp2-hybridized carbon atoms, was first postulated
20 years ago1 and continues to elicit interest from both a
fundamental2 and applied standpoint.3 The highly unsaturated
network, calculated as the most stable carbon allotrope
containing alkyne units, is predicted to display a variety of
desirable materials properties,1,2b-d such as high-temperature
stability, a band gap (1.2 eV) lower than that of polyacet-
ylene, and the ability to intercalate Li+ and Na+ ions without
interlayer distortion. Despite these predicted advantageous
properties, a suitable synthetic route for the preparation of 1
has not been elucidated.
§ For Part 5, see ref 4b.
† University of Oregon.
‡ University of Illinois at Urbana-Champaign.
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graphdiyne4 and polyphenylene.5 However, the chemical
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© 2007 American Chemical Society
Published on Web 08/18/2007

literature contains only three different graphyne substructures
composed of fused tribenzo[12]cyclyne (2) subunits. The
short conjugation pathways of the reported substructures,
none of which are greater than two diphenylacetylene units
in linear length (n ) 2; e.g., 3, Figure 1), are not sufficient
substructure topologies. The length of our previous intra-
molecular approach3a for 3d as well as the dependence on
probability of intermolecular routes3c,d make these techniques
impractical for preparing 4 and 5. The synthetic challenge
associated with higher degrees of symmetry and construction
of the monoyne linkage of subunit 2 appeared straightfor-
wardly addressed with alkyne metathesis, which in recent
years has seen an extensive increase in catalyst development7-9
and use in macrocyclization.3c,9,10 Alkyne metathesis dis-
played an unprecedented efficiency for o-benzocyclyne
synthesis,3c,9c as shown by Vollhardt’s preparation of 3d in
6% yield via 6-fold intermolecular metathesis of 1,2-
dipropynylbenzene and 1,2,4,5-tetrapropynylbenzene. The
three-step route afforded an identical overall yield (4.4%)
to our first isolation of 3d by an 11-step convergent
pathway.3a We hypothesized that an intramolecular route
combining the efficiency of alkyne metathesis with pre-
organized propynyl groups would provide superior access
to graphyne substructures. Production of graphyne biscyclyne
3 via this new route and comparison to previous syntheses3a,c,d
was conducted to evaluate this assertion. Alkyl groups were
incorporated into the substructure’s peripheral arene rings
to combat expected solubility problems.
Synthesis of tetrabutylbiscyclyne 3a began with diyne 6a,
available in five steps (55% yield) from 4-butylaniline (see
the Supporting Information). Desilylation of 6a with base
followed by 4-fold Pd-catalyzed cross-coupling with 1,2,4,5-
tetraiodobenzene11 gave penultimate octayne 7a in 73% yield
(Scheme 1). Treatment of 7a with Schrock’s W-alkylidyne
catalyst7 at 80 °C afforded cycle 3a, with 50 mol % catalyst
loading and 3.3 mM reaction concentration providing the
highest yield (46%). Three components were recovered from
each experimental trial, all readily identifiable by fluorescent
emission color: biscyclyne 3a (green), oligomer (turquoise),
and starting material (6a, blue). Reaction monitoring by TLC
showed that product distribution did not change after 3 h,
suggesting that the catalyst had possibly become inactive.
This result seems to confirm reports that 2-butyne polymer-
izes in the presence of Mo(VI) and W(VI) complexes and
can deactivate the alkylidyne by a ring expansion polymer-
ization mechanism.9a,e
Figure 1. Target graphyne substructures 3-5.
to extrapolate electronic properties of the bulk network. Work
by our laboratory4 and others2a,3f,6 has shown that linear
phenylacetylene conjugation length dominates electronic
properties over the extent of conjugation for isomeric
substructures. We therefore sought to extend the length of
the linear conjugation pathway at least 2-fold beyond the
current limit. Substructures 4 (n ) 3) and 5 (n ) 4) were
proposed to accomplish this investigation (Figure 1).
In addition to isolating both 4 and 5, we sought to develop
a general method for construction of larger graphyne
The poor solubility of 3a prompted us to investigate
alternate solubilizing groups and substitution patterns. Both
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3726
Org. Lett., Vol. 9, No. 19, 2007

cocatalyst and successful acquisition of a 13C NMR spectrum,
the latter a notable deficiency of both tetrasubstituted 3a and
3b.
Scheme 1
The efficiency of our new intramolecular metathesis route,
as evidenced by a minimum 3-fold increase in overall and
cyclization yield when compared with previous reported
analogues 3d,e (Table 1), prompted us to next target 4 and
Table 1. Comparison of Route Efficiency for Biscyclyne 3
cyclization
yield (%)
overall
yield (%)
cycle
steps
ref
3d
3d
3e
3a
3ba
3c
11
3
8
8
8
<15
6
4
4
<1
19
21
17
6
7b
8
this work
this work
this work
1
46
87
91
7
a Isolated as 9:1 mixture with silanol-POSS.
5. Construction of larger substructures required an additional
key intermediate (8), available in five steps (48%) from 6b
(see the Supporting Information). Polyyne 9 was obtained
by 2-fold cross-coupling of desilylated 8 with 1,2-didecyl-
4,5-diiodobenzene (Scheme 2). Alkyne metathesis of 9
lengthening of the alkyl groups from butyl to tetradecyl (3b)
as well as incorporation of eight alkyl substituents into the
peripheral arene rings (3c) were attempted. The Mo-amido
catalytic system optimized by Moore9 was also considered
to overcome the incomplete reaction of starting material.
Scheme 2
Reaction of 7b, prepared analogously to 7a starting from
4-tetradecylaniline, with the Mo-alkylidyne and 4-nitrophe-
nol, using the literature method for intermolecular metath-
esis,9 provided no greater than 10% product formation with
the remainder of 7b unreacted. Increased catalyst loading
(30 mol %) and temperature (75 °C) afforded an improved
yield (∼30%) but still incomplete conversion of 7b to either
macrocycle or oligomer. Replacement of the phenol with
silanol-POSS, a ligand found to preferentially limit polym-
erization of 2-butyne via steric bulk,9d provided complete
consumption of 7b as well as excellent yield for 3b (87%)
under highly concentrated conditions (32 mM) and short
reaction time (30 min). Disappointingly, cycle 3b proved
equally poorly soluble and complete removal of the POSS
cocatalyst was also problematic.
Production of octasubstituted 3c began with arene 6c,
available in two steps from 1,2-didecyl-4,5-diiodobenzene12
(see the Supporting Information). Higher dilution (1.7 mM)
and increased reaction time (14 h) were used to produce 3c
in high yield, the result of enhanced macrocycle solubility
and propensity for further metathesis scrambling under the
concentrated conditions favoring 3b. The good solubility of
3c in organic solvents permitted removal of the POSS
afforded triscyclyne 4 in 19-31%, with the Mo-amido
catalyst and POSS ligand providing the higher yield. As with
3c, cycle 4 proved highly soluble in chlorinated organic
solvents, a factor attributed to the 1,2-didecylarene substitu-
tion.
To target substructure 5, sequential cross-coupling of
desilylated intermediates 6b and 8 to 1,4-dibromo-2,5-
(12) Zhou, Q.; Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59,
1294-1301.
Org. Lett., Vol. 9, No. 19, 2007
3727

diiodobenzene13 was required and furnished precursor 11 in
59% yield (Scheme 3). Attempts to produce 5 via 4-fold
Scheme 3
Figure 2. Absorption and emission spectra of 2, 3a-c, and 4.
metathesis of 11 with Schrock’s catalyst or the Mo-
alkylidyne with the 4-nitrophenol ligand afforded oligomeric
material or return of starting material, respectively. Use of
the Mo-amido catalyst with the POSS ligand resulted in
complete consumption of starting material and production
of a single discrete component, which spectroscopy identified
as an intermediate with three of the four [12]cyclyne subunits
cyclized. The poor solubility of this intermediate as well as
a kinetic energy barrier elucidated from modeling studies
are likely factors which prevented further conversion to
tetrakis[12]cyclyne target 5.
Macrocyclization of 3a-c and 4 results in an upfield shift
of the aromatic proton resonances (∆δ ≈ 0.2-0.6 ppm),
reflecting the paratropic nature of the [12]cyclyne subunit.14
The largest ∆δ values (-0.47, -0.58) are associated with
the internal arene rings and agree with calculations2a indicat-
ing decreased aromaticity from fusion with multiple paratropic-
[12]cyclyne cores.
Analysis of the absorption spectra of macrocycles 2,15 3a-
c, and 4 shows increased molar absorptivity and bathochro-
mic shifts in both λcutoff and λmax upon successive fusion of
[12]cyclyne subunits (Figure 2). Substructures 3 and 4 retain
the characteristic vibronic spectral pattern of 2, indicative
of a highly rigid and planar macrocycle.16 HOMO-LUMO
bandgaps calculated for 3a-c (∼2.73 eV) and 4 (2.43 eV)
from the lowest energy absorption λmax coincide with
computed values reported by Tobe et al.2a Cyclynes 3 and 4
exhibit large Stokes shifts (89-104 nm) as well as batho-
chromic λem and increased fluorescent quantum yield (3a-
c: ΦF ) 0.19-0.21; 4: ΦF ) 0.27) upon extension of the
linear conjugation pathway. Cycle 3c exhibits slightly lower
energy absorption and emission values than 3a,b due to
incorporation of the four additional electron-donating alkyl
substituents.
In conclusion, biscyclyne 3 was obtained in high overall
yield via an intramolecular metathesis cyclization route.
Substructure 4, which contains the longest linear diphenyl-
acetylene conjugation pathway and highest quantum yield
for graphyne substructures based on the [12]cyclyne subunit,
was also prepared. The assertion of linear phenylacetylene
conjugation pathway dominance was confirmed by com-
parison of electronic properties of 4 (n ) 3; absorption λcutoff
) 525 nm, ΦF ) 0.27, λem ) 515 nm) with an isomeric
trefoil topology (n ) 2; λcutoff ≈ 500 nm, ΦF ) 0.03, λem
≈
500 nm) prepared by the Tobe group.3f Attempts to further
elongate this chromophore linearly (i.e., 5) were unsuccessful.
Future work will include investigation of precipitation-driven
metathesis, sequential macrocyclization of [12]cyclyne sub-
units to further limit intermolecular oligomerization, and
incorporation of additional alkyl groups.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0414175 and -0718242) for support of this
research. C.A.J. acknowledges the University of Oregon for
a Doctoral Research Fellowship. We thank Prof. J. S. Moore
for his technical advice.
(13) Hart, H.; Harada, K.; Du, C. J. F. J. Org. Chem 1985, 50, 3104-
3110.
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2437.
(15) Staab, H. A.; Graf, F. Tetrahedron Lett. 1966, 751-757.
(16) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London,
UK, 1970. (b) Anstead, G.; Katzenellenbogen, J. A. J. Phys. Chem. 1988,
92, 6249-6258.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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