Polyaromatic ribbons from oligo-alkynes via selective radical cascade: Stitching aromatic rings with polyacetylene bridges

Article abstract of DOI:10.1021/ja3023626

Selective radical generation in conjugated oligomeric o-aryleneethynylenes initiates an intramolecular cascade which involves five fast radical cyclizations followed by aromatization via a 1,5-H shift with a >93% yield per step. This radical cascade trans

Full text of DOI:10.1021/ja3023626

Article
pubs.acs.org/JACS
Polyaromatic Ribbons from Oligo-Alkynes via Selective Radical
Cascade: Stitching Aromatic Rings with Polyacetylene Bridges
Philip M. Byers and Igor V. Alabugin*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States
S
* Supporting Information
ABSTRACT: Selective radical generation in conjugated
oligomeric o-aryleneethynylenes initiates an intramolecular
cascade which involves five fast radical cyclizations followed by
aromatization via a 1,5-H shift with a >93% yield per step. This
radical cascade transformation opens a new avenue for the
systematic and controlled preparation of functionalized
graphene nanoribbons where, potentially, each of the
peripheral aromatic rings can be different.
Scheme 1. B3LYP Calculated Barriers for Radical Cascade
Transformation of Tris-o-aryleneethynylenes via Selective
Intermolecular Activation
1. INTRODUCTION
Interest in polycyclic aromatic hydrocarbons (PAHs) was
refueled by their structural relation to graphene, a form of
carbon whose mechanical and electronic properties1,2 are
distinctly different from those other carbon nanostructures.3
Because structural variations (e.g., size, shape, curvature,4 edge
geometry,5 and composition6) affect electronic properties
strongly, progress in this field depends on the development
of rationally designed “bottom-up”7 approaches toward chemi-
cally and structurally uniform graphene substructures. While a
number of elegant approaches8 to the design of symmetric
graphene nanoribbons have been reported, efficient and flexible
approaches to nonsymmetrically substituted graphene nano-
ribbon structures are scarce.9−11 These approaches become
even more valuable considering recent reports where relatively
large polycyclic aromatic hydrocarbons were used to create
even larger carbon nanostructures using surface-assisted
coupling12 and template-initiated growth.13
Alkyne functionality is an attractive entry point for the
preparation of carbon-rich materials14 due to its high carbon
content (atom economy), the possibility of modular assembly
via reliable cross-coupling chemistry, and the propensity of
alkynes to participate in well-choreographed cascade trans-
formations leading to the rapid construction of polycyclic
frameworks.15 These advantages are illustrated by the recently
reported radical cascade transforming triynes (1,2-bis(2-
arylethynyl)phenyl ethynes) into an extended polyaromatic
system (benzo[a]indeno[2,1-c]fluorene) in >70%.16 The
efficiency of this cascade hinges on the chemoselective
intermolecular attack of tributyl tin (Bu3Sn) radical at the
central alkyne of the triyne system (Scheme 1). This step
initiates an efficient cascade, where each of the subsequent steps
(5-exo-dig, 6-exo-dig cyclizations, attack at the aromatic ring,
and 1,5-shift-induced rearomatization) has a low barrier
following a favorable energy landscape to progressively more
and more stable intermediates.
Mechanistic studies, together with DFT computational
analysis, revealed the reasons for this efficiency and suggested
that this cascade can be expanded to the preparation of larger
molecules of different sizes and architectures. In particular, we
were encouraged by the lower calculated barriers for the key 5-
exo-dig and 6-exo-dig cyclization steps17 (∼3 and ∼6 kcal/mol,
respectively) relative to the barrier for the final ring formation
via radical attack at an aromatic system (12 kcal/mol, Scheme
1). According to these values, the final cyclization step is ∼100
000 times slower than the 6-exo-dig closure on the pendant
alkyne. The energetics suggest that if the starting material had
Received: September 9, 2011
Published: May 23, 2012
© 2012 American Chemical Society
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additional triple bonds capable of extending the cascade,
preparation of larger carbon nanostructures would be feasible.
However, when we applied intermolecular initiation to the
longer polyynes, the selectivity decreased dramatically and
complex mixtures were formed.18 For this cascade to go to
completion, the radical cascade must be initiated via selective
attack at the central alkyne. Attack at any other alkyne would
require an unfavorable cascade that involves a relatively
inefficient19 5-endo (or 4-exo) closure (Scheme 2).
Sonogashira coupling of the terminal enediyne 1f with 1-
iodonaphthalene (Scheme 3).
Scheme 3. Synthesis of Enediyne Compounds for Model
Radical Cascade Reactions
Scheme 2. Proposed Extension of the Alkyne Radical
Cascade towards Longer Graphene Ribbons and Importance
of Selective Radical Attack at the Central Alkyne
Our initial efforts using triethylsilane and tris(trimethylsilyl)-
silane (Et3SiH and TTMSS) led to the products of the desired
cascade transformation. However, the reactions were sluggish
and the yields were low (21−25%). A dramatic increase in
efficiency was observed when the Bu3SnH/AIBN system was
used for the chemoselective cascade initiation. We also
attempted anionic 6-exo-dig cyclization but found that
phenoxide elimination dominates, leading to compounds 5,
which can be converted into heterocycle 4, via an anionic 5-
endo-dig closure (Scheme 4).
Scheme 4. Optimization of Cascade Conditions
2. RESULTS AND DISCUSSION
2.1. Enediyne Synthesis and Radical Cascades. The
similarity of electronic and steric properties of alkynes in the
larger polyalkyne oligomers precludes selective intermolecular
radical attack at the central alkyne. We describe here a strategy
that targets the central alkyne by intramolecular radical attack.
A suitable initiator (the “weak link”) is built-in directly at the
central ring to ensure that the cascade begins at the right part of
the molecule. Selective radical formation in the presence of
multiple triple bonds initiates the complete “zipping” of the
oligo alkynes into a fully aromatic ribbon.
Since radical cascades are often compatible with a variety of
functional groups,15 we have concentrated on radical initiation
of the cascade. Because finding suitable conditions for the
chemoselective alkyl radical generation in the presence of
multiple alkynes is challenging, we turned to model compounds
with two triple bonds (enediynes).
We chose the first intramolecular step in the radical cascade
to 6-exo-dig closures to avoid the unfavorable arrangement of
two fused five-membered rings.17c The requisite library of
enediynes was synthesized through the Sonogashira reaction of
1-(2-bromoethoxy)-2,3-diiodobenzene with a number of
substituted alkynes 1a−1e. Bis-1-naphthyl enediyne 1g was
prepared through a slightly different procedure based on
Selective radical attack at the alkyl halide in the presence of
two alkynes is remarkable considering that alkynes are also
reactive under these conditions.20 Structures of the cascade
cyclization products were elucidated by heteronuclear single
quantum coherence (HSQC), heteronuclear multiple bond
correlation (HMBC) and nuclear Overhauser effect (NOE)
NMR experiments. The 2D experiments confirmed that
products originate from the 6-exo-dig, 5-exo-dig cascade.
Results for the cyclization of the p-methoxy substituted
enediyne are discussed in more detail.
No gHMBC correlations for the vinyl C−H and the C3−H
at the trisubstituted benzene ring were observed (Scheme 5).
The lack of correlation implies that a 5-exo-dig cyclization took
place in the second step giving an exocyclic double bond. The
product of an endo-dig cyclization would be expected to display
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not be expected due to the additional bond separating these
atoms. Finally, protons in the CH2 groups remain magnetically
equivalent, indicating the absence of radical closure at the
terminal aryl group (in contrast to the cascade transformation
of tetraynes discussed in a subsequent section).
Scheme 5. HSQC and HMBC Experiments for the
Cyclization of p-OMe-Substituted Enediyne 2d
a
2.2. Computational Analysis of Enediyne Cascade. As
expected from general trends in alkyne cyclizations,16,21 both
intramolecular radical attacks at the triple bonds proceed via an
exoclosure leading to the selective formation of substituted 1-
benzylidene-2-phenyl-3,4-dihydro-1H-5-oxa-acenaphthylenes 2.
Literature examples of two consecutive ring closures in bis-
TMS-acetylenes22 are scarce. The 87% yield for the cyclization
of bis-TMS-enediyne 2e is a particularly convincing illustration
for the efficiency of this radical cascade.
We have explored the cyclization potential energy surfaces
for all of the enediynes computationally (Scheme 6). Several
Scheme 6. Yields and Potential Energy Surfaces for the
Cascade Cyclization of Substituted Enediynes (B3LYP/6-
31+G(d,p), kcal/mol)
a
Top HSQC NMR highlights select direct C−H couplings. The inset
shows all H and C chemical shifts for enediyne 2d. Bottom HMBC
NMR shows selected C/H through-bond correlations (colored
arrows).
this gHMBC correlation. All protons of the OMe-substituted
rings were identified by their multiplicity and long-range
couplings between the two deshielded C(−OMe) carbons and
the respective aromatic meta C−H bonds (H at 7.53 couples to
the carbon at 159.7 ppm and H at 7.34 ppm couples to the
carbon at 158.6 ppm). One of the two meta C−H groups (7.53,
130.3 ppm) also displays a HMBC correlation with the vinyl
C−H (7.19, 134.1 ppm), which identifies the terminal anisole
moiety. The gCOSY spectrum shows hydrogen correlations in
the methoxy-substituted benzene rings and also correlation
between the CH2’s of the −OCH2CH2− moiety.
In the tricyclic core, the quaternary carbon (150.8 ppm) ipso
to the oxygen of the −OCH2CH2− moiety shows HMBC
couplings with both the aromatic meta-C−H (6.92 and 126.5
ppm) and the O−CH2 protons (4.30 and 67.2 ppm). Both the
ortho proton at 7.25 ppm and the β-CH2 group (2.96, 25.3
ppm) correlate with the quaternary aromatic carbon (114.2
ppm) at the junction of the three cycles (see SI section for the
full list of HMBC correlations), confirming that an initial
cyclization followed a 6-exo-dig rather than the 7-endo-dig
path. In the latter case, correlation with the β-protons would
observations are noteworthy. The activation barrier for the first
(6-exo) step is ∼5 kcal/mol higher than the barrier for the
second (5-exo) ring closure. Although both cyclization barriers
are relatively insensitive to the nature of substituents in the
aromatic ring (0.3−0.6 kcal/mol variations), the sterically
demanding TMS group deactivates the alkyne moiety and
increases the activation barriers for both cyclizations by ∼2
kcal/mol. Because this additional barrier does not lead to a
lower experimental yield for the cascade transformation of the
TMS-substituted enediyne. (In fact, the cascade product yield is
the highest for this substrate!) We conclude that reaction yields
reflect the relative selectivity of C−Hal bond activation vs
attack at a triple bond rather than relative cyclizations rates for
the different substituents. Importantly, both cyclization steps
are highly exothermic and irreversible and the cascade moves
down the potential energy slope with each newly formed C−C
bond.
2.3. Radical Cascades of Tetraynes. Encouraged by these
results, we investigated the cascade cyclization of tetra-alkynes
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11a−c. The fully “zipped” cascade product (Scheme 7) formed
as expected. Although the yield was low for X = Br (26−35%),
Me, see SI for the details). The absence of triple bond carbon
resonances confirms that cascade proceeded fully utilizing all
alkynes in the process. The presence of diastereotopic CH2
groups confirms the formation of a chirality center in the
molecule. Connectivity in the OCH2CH2CH-moiety is
supported by the 2D correlations in the gHSQC spectrum, in
which the two hydrogens at 4.97 and 4.69 ppm are bound to
the 65.2 ppm carbon, the two hydrogens at 3.57 and 1.98 ppm
to the 30.8 ppm carbon, and the methine proton at 2.03 ppm to
the 21.1 ppm carbon. For the bottom part of the ribbon, 2D
correlations readily identify all carbons in the free-standing and
the fused tolyl groups. The CH singlet at 5.28 ppm on the
bottom part of the ribbon displays gHMBC correlations with
carbons 135.8 in the ipso position of the free tolyl and 129.7
ppm carbon of the isolated CH on the fused tolyl ring. The
5.28 ppm singlet also confirms that the final cyclization and
subsequent aromatization took place. Full cascade is also
consistent with the presence of only one set of characteristic
doublets of a para substituted aromatic rings. Interior aromatic
carbons can be identified by comparing gHMBC with gHSQC.
The presence of gHMBC cross-peaks which are not present in
the gHSQC spectrum, indicates the interior quaternary carbons
in the aromatic region. The ortho aromatic C−H in the central
ribbon can be identified due to their significant deshielding
(9.08, 128.5 and 8.50, 124.4 on the side with the fused tolyl ring
and 9.02, 128.9 and 7.91, 123.4 on the other side). By
identifying these outlying CH bonds along the perimeter of the
structure, one can map the connections of the interior carbons
(Figure 1).
Scheme 7. Synthesis of o-Aryleneethynylene Tetramers and
the Proposed Mechanism of Cascade Transformation
The proposed cascade mechanism is presented in (Scheme
7). Once selective activation of the “weak link” in the presence
of four alkynes is accomplished, a cascade of four sequential exo-
dig alkyne cyclizations occurs. Only after all four alkynes are
consumed by the radical cascade does the vinyl radical attack
the terminal aryl ring. This attack disrupts aromaticity in this
ring and is only observed when three or more alkynes are
present in the starting material (e.g., this step does not occur in
the enediyne cyclizations). We suggest that, for these longer
oligomers, this loss of aromaticity is partially compensated by
aromatization in one of the previously formed rings (i.e., ring 3,
Scheme 7). The subsequent 1,5 H-shift is also favorable (ΔG ≈
−34 kcal/mol) because it is assisted by rearomatization (rings 4
and E).24 With each following step, this molecular system
continues thermodynamic descent, resulting in an overall
transformation that is ∼160 kcal/mol exergonic (Scheme 8)!
The final H-abstraction step has low stereoselectivity due to
the near planarity of the sterically unencumbered radical center.
The fully “zipped” ribbon has good solubility in common
organic solvents due to the presence of nonplanar parts in the
structure. In particular, the polycyclic core is distorted due to
the steric C−H interactions between o-hydrogens in the
peripheral aryl groups.
the bromide can be quantitatively exchanged to form the iodo
tetraalkynes 12a−c, which transform to the fully closed
nanoribbons 13a−c in higher yields (52−55% overall,23
∼93% per step). This increase again indicates that the reaction
yields reflect the chemoselectivity of the first step (intermo-
lecular activation) rather than the efficiency of the cyclizations
(intramolecular propagation). Mass-spectrometry of side
products isolated in minor amounts suggests that addition of
Bu3Sn moiety to the triple bonds is indeed still a problem.
2D HSQC and HMBC NMR analysis fully confirmed the
expected polycyclic skeleton of the two diastereomers of the
product (Figure 1 presents the results for this analysis for R =
3. CONCLUSIONS
To a large extent, the highly favorable thermodynamics of the
reaction cascade stems from the high energy content of alkyne
functional group. This feature has been harnessed before for the
preparation of graphene ribbons (Scheme 9).
For example, Swager and co-workers utilized electrophile-
induced alkyne cyclizations to anneal additional cycles onto a
polyphenylene backbone.25 Among the many approaches
Figure 1. (a) HMBC NMR correlations used to establish the structure
of cascade product 13a and (b) H and C chemical shifts for cascade
product 13a.
developed by Mullen and co-workers for the preparation of
̈
polyaromatic scaffolds, the Diels−Alder polymerization of bis-
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Scheme 8. Relative Energies and Structures of Intermediates for the Cascade Radical Transformation of Tetraynes (B3LYP/6-
a
31+G(d,p), kcal/mol)
a
HOMO and LUMO for the final product 13b are given on top.
of functionalized graphene nanoribbons where, potentially,
each of the peripheral aromatic rings can be unique. Future
work includes expansion of this radical cascade toward the
preparation of longer and wider nanoribbons with a variety of
functional groups, development of new “weak links” which
avoid pentagons in the ribbon, and optimization of conditions
for the further aromatization at the peripheral aromatic rings.
Scheme 9. Conceptual Approaches for the Use of Alkyne
Functionality in Preparation of Graphene Ribbons
ASSOCIATED CONTENT
* Supporting Information
■
S
Full synthetic procedures, 1H and 13C NMR spectra, and
HRMS of all new compounds, gCOSY for all enediyne cascade
products, gHMBC and gHMQC data for compound 2d and 13,
DFT geometries and energies for all reactants, transition states,
intermediates, and products for the cascade cyclizations of
enediynes. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
alabugin@chem.fsu.edu
■
cyclopentadienones with bis-alkynes incorporates alkyne
carbons in the backbone.26
Notes
The authors declare no competing financial interest.
Expansion of radical cascades described in our work to longer
oligoalkynes would potentially lead to the conceptually different
approach toward the preparation of graphene ribbons. In this
approach, the central “polyacetylene” backbone is fully
assembled from the alkyne groups of the starting material via
radical polymerization.
In summary, we have found that selective radical generation
from alkyl halides is possible in conjugated oligomeric o-
aryleneethynylenes. Subsequent intramolecular transformation
proceeds with a yield of >93% per step as a cascade of five fast
radical cyclizations followed by aromatization via a 1,5-H shift.
This radical cascade may open a new avenue for the preparation
ACKNOWLEDGMENTS
Support by National Science Foundation (CHE-0848686/
1152491) is gratefully acknowledged.
■
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