Reactions of HDDA-Derived Benzynes with Perylenes: Rapid Construction of Polycyclic Aromatic Compounds

Article abstract of DOI:10.1021/acs.orglett.6b02878

Benzynes produced by the thermal cycloisomerization of tetrayne substrates [i.e., by the hexadehydro-Diels-Alder (HDDA) reaction] react with perylenes to produce novel naphthoperylene derivatives. Cyclic voltammetry and absorption and emission properties of these compounds are described. DFT studies shed additional light on the dearomatization that accompanies the reaction as well as some of the spectroscopic behavior.

Full text of DOI:10.1021/acs.orglett.6b02878

Letter
pubs.acs.org/OrgLett
Reactions of HDDA-Derived Benzynes with Perylenes: Rapid
Construction of Polycyclic Aromatic Compounds
Feng Xu, Xiao Xiao, and Thomas R. Hoye*
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
S
* Supporting Information
ABSTRACT: Benzynes produced by the thermal cycloisomerization of
tetrayne substrates [i.e., by the hexadehydro-Diels−Alder (HDDA)
reaction] react with perylenes to produce novel naphthoperylene
derivatives. Cyclic voltammetry and absorption and emission properties
of these compounds are described. DFT studies shed additional light on
the dearomatization that accompanies the reaction as well as some of the
spectroscopic behavior.
Scheme 1. (a) Synthetic Strategies Based on Perylene for
olycyclic aromatic compounds (PACs) have long been
Pimportant not only from fundamental theoretical and
structural perspectives but also because of their biological and
environmental impact, especially with respect to their
carcinogenicity.1 More recently, PACs have attracted consid-
erable attention because of their promising applications in
optoelectronic materials such as organic light-emitting diodes,
organic field-effect transistors, and organic photovoltaic cells.2
The development of facile and efficient strategies for creating
novel PACs remains of significance in the fields of both
chemistry and materials.3
the Synthesis of Some Polycyclic Aromatic Compounds
(PACs);5b (b) Possible Routes to PACs through HDDA
Cascade Reactions
Strategies for the synthesis of π-extended PACs based on
de novo construction of the aromatic cores often leads to
lengthy and/or inefficient routes. One way to circumvent this
limitation is by starting from a building block that already
contains multiple aromatic rings. Perylene (1) has been
employed in this role to good advantage.4 Examples
particularly relevant to the work reported here are shown in
Scheme 1a. o-Benzyne (2) adds in [4 + 2] fashion to 1 to
give naphtho[1,2,3,4-ghi]perylene (4) following thermal
extrusion of dihydrogen from the intermediate adduct 3.5,6
The bis(benzyne) precursor 5 was recently used to capture 1
under conditions where 6 precipitated from solution,
mitigating its further conversion to the second benzyne.
Subsequent treatment with fluoride ion, now in the presence
of a Pd(0) catalyst, led to the insoluble trimer 7, which was
characterized at atomic resolution by scanning probe
microscopy.7
The thermal cycloisomerization of a 1,3-diyne bearing a
tethered alkyne diynophile (see 8) leads to the formation of a
carbocyclic benzyne moiety (see 9),8 a process we have
termed the hexadehydro-Diels−Alder (HDDA) reaction
(Scheme 1b).9 Recently we have turned our attention to
exploring the application of the HDDA reaction to the
synthesis of compounds of potential utility in various
photonic or electronic materials. One avenue has been to
examine the reactions of 9 with 1. If these were to proceed by
way of the precedent in Scheme 1a, multiple substituents and
functionality could readily be incorporated into the naphtho-
[1,2,3,4-ghi]perylene products 10. We report the results of
some of our studies here.
Received: September 23, 2016
© XXXX American Chemical Society
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One of the first reactions we explored was that between
perylene (1) and the tetrayne precursor 8a (Scheme 2), which
Diels−Alder adduct between 9a and 1 (cf. Scheme 1a) to
rearomatize the perylene subunit. The ESI mass spectrum
confirmed that fact.
With this result in hand, we explored some aspects of the
substrate scope for this reaction (Scheme 2). Tetraynes 8a−c
demonstrate that arenes having both electron-poor and -rich
character are well-tolerated. The three-atom tether in the
tetrayne can include a sulfonamide (8d) or ether (8e)
functionality as the central linking group. The electron-
deficient perylene derivative containing ester groups at the 3,
4, 9, and 10 positions was also an effective benzyne trapping
agent, even when used in a near-stoichiometric quantity (1.2
equiv). It is noteworthy that each of the tetraynes 8a−e can
be prepared in just two steps from commercially available
alkyne precursors (alkyne bromination and Cadiot−Chodkie-
wicz cross-coupling; see the Supporting Information (SI)).
In the reaction giving 10g, a small amount (5%) of the
bisadducts 11-syn and 11-anti (Figure 2) were also formed.
Scheme 2. Synthesis of Polycyclic Aromatic Compounds
10a−g through Reactions of Tetraynes 8a−e with Perylene
(1)
a
In the reactions leading to 10f and 10g, 1.2 equiv of tetraethyl
3,4,9,10-perylenetetracarboxylate was used in place of 1.
contains a dipropargylated malonate ester linker. This HDDA
cascade, effected simply by heating a CHCl3 solution of 8a
and (only) 1.5 equiv of 1 at 85 °C for 10 h, provided an air-
stable yellow powder that could be purified by column
chromatography to remove excess perylene followed by
addition of methanol to a dichloromethane (DCM) solution
of this material to precipitate the pure product. Analysis of the
1D and 2D 1H NMR spectra allowed us to assign the
structure as 10a. Resonances corresponding to H5, H6, and
H10, each of which resides in a bay region of the
naphtho[1,2,3,4-ghi]perylene moiety, were observed at ca.
8.9 ppm (Figure 1). In contrast, the resonance for H1, the
fourth bay region proton, as well as that of H2 resided
significantly farther upfield, a reflection of the shielding
provided by the adjacent aryl group of the biaryl moiety. No
alkyl methine protons were observed, suggesting that two
hydrogen atoms had been lost from the presumed initial
Figure 2. Double adducts from benzyne addition to 10f.
These could be chromatographically separated from 10g but
coeluted as a 2:1 mixture that gave an orange powder upon
solvent removal. These arise from subsequent head-to-tail and
head-to-head additions of the benzyne from 8c to the product
10g, which indicates that dihydrogen was lost during the
course of the HDDA cascade reaction. Trace amounts of
analogous double adducts were observed in the crude NMR
spectra of most of the product mixtures leading to 10a−f, but
in those other instances solubility issues made isolation of
pure material challenging. Experiments using an excess of
tetrayne substrate over the amount of perylene (or perylene
tetraester) did not allow the efficient production of any of the
expected bisadducts. It is worth noting that perylene has been
converted by dienophiles into double-addition adducts in a
single operation in only a few instances and never in high
yield.10
DFT [SMD(chloroform)/B3LYP-D3BJ/6-311+G(d,p)//
B3LYP/6-31G(d)] calculations were performed on a model
system to further probe mechanistic aspects of these reactions.
We calculated the energy profile for the reaction between
benzyne (2) and perylene (1) (Figure 3). The Diels−Alder
1
Figure 1. H NMR spectrum of 10a (500 MHz, CDCl3).
B
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decomposition. All four of the PACs derived from the parent
perylene compound (i.e., 10a, 10b, 10c, and 10e) showed
very similar behavior (oxE1/2 = 0.68−0.71 eV and no reduction
half-wave). The oxidation potentials of the core structures
present in this subset of PACs, each an unsubstituted perylene
subunit, were insensitive to the electronic character of the
remote substituents R present on the phenyl rings.
In contrast, the PAC 10g gave rise to a CV in which both
oxidation (1.15 eV) and reduction (−1.50 and −1.75 eV)
half-waves were observable. The four carboethoxy groups
present on the pyrene-derived core in 10g lowered the
reduction potential, a feature that was also seen for 10f
(+0.93, −1.68, and −1.92 eV, respectively). In this pair, each
having an electron-deficient perylene core, the redox
potentials were responsive to the nature of the substituents
on the two phenyl ringsOMe for the more easily oxidized
and difficult to reduce 10f versus CO2Me for 10g.
Figure 3. Computed free energy changes for the reaction of benzyne
(2) with perylene (1) to give naphthoperylene 4 via extrusion of
dihydrogen from the primary Diels−Alder adduct 3.
Absorption and fluorescence spectra were also recorded for
each of 10a−c and 10e−g (see the SI). Again, representative
examples are shown here for 10c and 10g (Figure 5a). The
lowest-energy absorption, reflective of the HOMO−LUMO
gap, is red-shifted ca. 25 nm in the UV−vis spectrum of the
naphthoperylenetetraester 10g relative to 10c. The emission
spectra for both compounds mirror the absorption bands,12
and again the maximum is red-shifted ca. 25 nm in the
spectrum for 10g.
The HOMO and LUMO energies and molecular orbital
maps were computed for each of 10a−c and 10e−g
[(TD)DFT, B3LYP/6-31G(d)]. In two instances only, the
HOMO and LUMO were seen to be located in two
significantly different portions of the molecule (Figure 5c).
Namely, the HOMO in 10a resided principally on the
naphthoperylene core while the LUMO was largely localized
on the alkynyl p-carbomethoxyphenyl substituents. The
reverse was true for 10f, the naphthoperylenetetraester
bearing p-methoxyphenyl substituents. It might then be
expected that emission from the excited-state singlets for
each of these compounds would reflect the intramolecular
charge transfer character in each. Indeed (and uniquely among
all of the compounds 10), the emission spectra of both 10a
and 10f (Figure 5b) are noticeably broader and red-shifted in
comparison with those of their counterparts 10b−e (see, e.g.,
10c in panel a) and 10g (panel a), respectively. It is notable
that the redistribution of orbital density in both of these
molecules principally involved the arene substituent on the
alkyne (rather than the biaryl) since the former can (and does
in the DFT optimization) adopt coplanarity with the
naphthoperylene polycycle.
(DA) reaction is computed to have a free energy of activation
(cf. TS1) of ca. 11 kcal·mol−1. That step is highly exergonic
because, in addition to the usual driving force for π- to σ-
bond change accompanying a DA event, the high degree of
strain energy in benzyne more than compensates for the
lowering of the perylene aromatic resonance stabilization. We
also located a transition structure, TS2 (27.4 kcal·mol−1
uphill), that directly connects the DA adduct 3 and product
4. TS2 was computed to have a symmetrical structure,
suggesting a concerted loss of dihydrogen. In comparison with
the activation energy required for similar concerted
dehydrogenation of dihydronaphthalene derivatives (ca. 31
kcal·mol−1, computed)6 and dihydrobenzene (42.7 kcal·mol−1,
experimental),11 we suggest that the activation energy for the
conversion of 3 to 4 would be even lower for the 3c,13a-
dihydro adducts leading to products 10 because of the
additional π substituents present in 10. In other words, the
DA adduct of the HDDA reaction of benzyne and perylene
may require an activation energy even less than 27.4 kcal·
mol−1 for the release of dihydrogen, consistent with the fact
that we never detected an intermediate dihydro adduct in any
of the reactions reported here.
We measured the cyclic voltammograms (CVs) of dichloro-
methane (DCM) solutions of all of the adducts 10a−g (see
the SI) except for 10d, which showed only limited solubility
in all common solvents, including DCM. Two representative
CVs (those of 10c and 10g) are shown in Figure 4. The PAC
10c showed a reversible oxidation wave (at 0.69 eV), but no
reduction was observed before the onset of solvent
In conclusion, we have described a series of new polycyclic
aromatic compounds that can be readily accessed through
reaction between HDDA-generated benzynes 9 and perylene
(1) or its 3,4,9,10-tetraethyl ester derivative. The reaction is
accompanied by in situ aromatization of the initial Diels−
Alder adduct, presumably by spontaneous ejection of
dihydrogen. Novel bisadducts 11 were observed. CVs showed
an interesting lack of sensitivity to electronic substitution for
the derivatives of the parent perylene (i.e., 10a−e) but a
significant responsiveness to MeO versus MeO2C substituents
in the electron-deficient perylenes 10f versus 10g. Finally,
absorption and emission spectra revealed a behavior that was
consistent with the differences in the computed HOMO and
LUMO maps.
Figure 4. Cyclic voltammograms for 10c (blue) and 10g (orange).
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AUTHOR INFORMATION
Corresponding Author
■
*hoye@umn.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This investigation was supported by a grant awarded by the
U.S. Department of Health and Human Services (National
Institute of General Medical Sciences, GM65597, and the
National Cancer Institute, CA76497). Portions of this work
were performed with hardware and software resources
available through the University of Minnesota Supercomput-
ing Institute (MSI). Some of the NMR data were recorded
using instrumentation purchased with funds awarded through
the NIH Shared Instrumentation Grant Program
(S10OD011952). We thank Gereon Yee (University of
Minnesota) and Joseph J. Dalluge (University of Minnesota)
for help in collecting the cyclic voltammetry and HRMS (for
11) data, respectively.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02878.
Experimental procedures for the synthesis, spectro-
scopic characterization data, and 1H and 13C NMR
spectra for new compounds; DFT computational
methods; and CV and UV and fluorescence spectral
data for 10a−c and 10e−g (PDF)
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DOI: 10.1021/acs.orglett.6b02878
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