Selective Vicinal Diiodination of Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1002/ejoc.202000954

Vicinally diiodinated polycyclic aromatic hydrocarbons (I₂-PAHs) are accessible from the corresponding diborylated B₂-PAHs through boron/iodine exchange. The B₂-PAHs have been prepared via twofold electrophilic borylation reactions templated by a vicinally disilylated benzene. Our protocol is applicable to fluorenes, acenes, annulated acenes, oligoaryls, and even [5]helicene. Using B₂-naphthalene as the example, we have shown that the reaction scope can, in principle, be expanded to include the synthesis of vicinally dibrominated and dihydroxylated PAHs. The usefulness of the building blocks provided by our method in the field of optoelectronic materials was demonstrated by the successful conversion of I₂-fluoranthene to the analogous doubly alkynylated fluoranthene emitter.

Full text of DOI:10.1002/ejoc.202000954

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Selective Vicinal Diiodination of Polycyclic Aromatic
Hydrocarbons
Authors: Tanja Kaehler, Alexandra John, Tao Jin, Michael Bolte, Hans-
Wolfram Lerner, and Matthias Wagner
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000954
Link to VoR: https://doi.org/10.1002/ejoc.202000954
01/2020

10.1002/ejoc.202000954
European Journal of Organic Chemistry
COMMUNICATION
Selective Vicinal Diiodination of Polycyclic Aromatic Hydrocarbons
Tanja Kaehler+, Alexandra John+, Tao Jin, Michael Bolte, Hans-Wolfram Lerner and Matthias Wagner*
[*]
M. Sc. T. Kaehler, Dr. A. John, M. Sc. T. Jin, Dr. M. Bolte, Dr. H.-W. Lerner, Prof. Dr. M. Wagner
Institut für Anorganische Chemie, Goethe-Universität Frankfurt
Max-von-Laue-Strasse 7, 60438 Frankfurt (Main) (Germany)
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Vicinally diiodinated polycyclic aromatic hydrocarbons
(I2-PAHs) are accessible from the corresponding diborylated B2-PAHs
through boron/iodine exchange. The B2-PAHs have been prepared
via twofold electrophilic borylation reactions templated by a vicinally
disilylated benzene. Our protocol is applicable to fluorenes, acenes,
annulated acenes, oligoaryls, and even [5]helicene. Using
B2-naphthalene as the example, we have shown that the reaction
scope can, in principle, be expanded to include the synthesis of
vicinally dibrominated and dihydroxylated PAHs. The usefulness of
the building blocks provided by our method in the field of
optoelectronic materials was demonstrated by the successful
conversion of I2-fluoranthene to the analogous doubly alkynylated
fluoranthene emitter.
underscores the considerable demand for 1Br in the chemical
community.[11]
This demand results from the numerous applications of
2,3-dibromonaphthalene as a synthetic building block across
fields as varied as pharmaceuticals[12] and organic optoelectronic
materials: (1) Introduction of new (cooperating) substituents in the
naphthalene scaffold, such as Lewis basic ‒OR/SR groups or
electropositive ‒SiMe3/SnMe3 moieties.[6,13,14] (2) Consecutive
C‒C- or C‒N-coupling reactions to immediately generate
oligonaphthylenes or π-expanded (hetero)arenes.[9,10,13,15–17]
(3) Mono- or dialkynylations with subsequent cyclization reactions
to furnish naphthopentalenes as well as naphthannulated
(hetero)aromatics.[8,18,19] (4) Br2 elimination to liberate
2,3-naphthalyne,[11,13,20] followed by [4+2]-cycloadditions to
prepare polycyclic aromatic hydrocarbons (PAHs) and
triptycenes.[7,11,21–24] It is to be emphasized at this point that
2,3-dibromonaphthalene not only provides rare access to
2,3-naphthalyne (via Br2 elimination), but also to 3-bromo-1,2-
naphthalyne (via HBr elimination). The respective trapping
products still contain one Br atom at the naphthalene fragment,
which can be employed for further transformations or renders the
product itself a potential aryne precursor.[21,22]
Given the remarkably diverse uses even of the moderately large
naphthalene system 1Br, it is desirable to be able to employ also
larger vicinally dibrominated PAHs. In 2019, two optimized
protocols for the synthesis of 2,3-dibromoanthracene (2Br;
Scheme 1, middle) have been disclosed: Bettinger’s five-step
approach starts from readily available phthalide to generate an
isobenzofuran intermediate and is therefore reminiscent of the
benzyne route (3) to 1Br (overall yield of 2Br: 18%).[25] The
synthesis put forward by Bunz and co-workers involves a
The importance of aryl halides as building blocks in organic
synthesis can hardly be overestimated. Particularly valuable are
vicinally dihalogenated derivatives, which offer the possibility of
annulating an additional ring onto the pre-existing arene scaffold.
While simple ortho-dihalogenated benzenes are in large supply,
already the next higher 2,3-dihalonaphthalenes require tedious
syntheses. As an example, three different access routes to
2,3-dibromonaphthalene (1Br) have been elaborated and
optimized, but each of them still has its shortcomings in terms of
atom economy, reaction times and temperatures, or yields
(Scheme 1, top):[1] (1) Naphthalene is protected on one of its rings
through
Diels-Alder
reactions
with
2
equiv
of
hexachlorocyclopentadiene (HCCPD). Subsequent bromination
occurs selectively in the b positions of the diadduct A, and a
thermally induced retro-cycloaddition reaction finally liberates the
target compound together with the environmentally malignant
HCCPD (overall yield of 1Br: 20%).[2–4] (2) ortho-Phthalaldehyde is
converted to 2,3-dibromonaphthalene through the assembly of a
second benzene ring by following a sequence of Corey-Fuchs,
HBr elimination (cf. B), and Bergman cyclization reactions (overall
yield: 30%).[5] (3) Monolithiation of 1,2,4,5-tetrabromobenzene in
the presence of excess furan gives the corresponding
Vollhardt
cyclization
of
bis(propargyl)benzene
with
bis(trimethylsilyl)acetylene, followed by bromodesilylation and
oxidation (yield over 6 steps from 1,2-dimethylbenzene: 38%).[26]
The general application fields of 2Br are very similar to those
outlined above for its naphthalene congener 1Br.[15,16,18,23,27]
Our group now reports the first versatile strategy that is general
enough to prepare a large variety of vicinally dihalogenated PAHs
by applying always the same reaction sequence (Scheme 1,
bottom). We are exploiting our previous observation that a broad
palette of PAHs undergoes vicinal electrophilic diborylation upon
treatment with 4,5-dichloro-1,2-bis(trimethylsilyl)benzene and
BBr3 (Scheme 1, bottom).[28] The resulting B2-PAHs 1B–8B can
afterwards be converted to the aimed-for ortho-I2-PAH products
1I‒8I via straightforward boron/iodine exchange (‘H.C. Brown
chemistry’; Figure 1).[29]
dibromoepoxynaphthalene
C via a benzyne intermediate.
Reduction of C using Zn/TiCl4 furnishes the desired 1Br (overall
yield: 62%[6]).[6–10] In our hands, the benzyne route (3) turned out
to be the most efficient one, if 2,3-dibromonaphthalene is to be
synthesized from scratch. With environmental concerns set aside,
the main bottleneck of route (1) is the synthesis of the protected
naphthalene A, because it forms only at high temperatures and in
equilibrium with its constituents.[2,3] A has meanwhile been
commercialized so that this problem is remedied, which
1
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10.1002/ejoc.202000954
European Journal of Organic Chemistry
COMMUNICATION
(1) 3-step synthesis
Cl6
Cl6
Br
Br
+
Cl6
(a)
(c)
(b)
Cl6
Cl6
A
(2) 3-step synthesis
Br
Br
Br
Br
O
Br
Br
Br
Br
(f)
(d)
(e)
O
1Br
B
overall yield
20–62%
(3) 2-step synthesis
Br
Br
Br
Br
Br
Br
O
O
+
(h)
(g)
C
(4)
5-step synthesis
Br
Br
Br
Br
O
O
EtO
O
EtO
O
Br
Br
O
O
(i)
(j)
(k)
(l)
(m)
BF4
Br
(5) 6-step synthesis
SiMe3
SiMe3
SiMe3
Br
2Br
Me3Si
Me3Si
Br
Br
SiMe3
(q)
MgBr
(o)
Br
Br
overall yield
18–38%
(n)
(p)
(r)
(s)
SiMe3
3-step synthesis
R
B
this work:
boron / iodine
exchange;
Cl
Cl
SiMe3
SiMe3
Cl
Cl
I
Wagner 2017:
vicinal electrophilic
borylation
+
π
π
π
(t)
(u)
B
R
I
novel I2-PAHs
1B–8B
1I–8I
Scheme 1. Syntheses of 2,3-dibromonaphthalene 1Br, 2,3-dibromonanthracene 2Br, and the ortho-diiodinated PAHs presented herein. Reagents and conditions for
1Br and 2Br. (a) 160 °C, 120 h. (b) Br2, Fe, tetrachloroethane, reflux, 3 h. (c) Continuous high-vacuum distillation, 220 °C. (d) CBr4, PPh3, CH2Cl2, 0 °C, 2 h. (e)
KOtBu, H2O, THF, –78 °C, 5 min. (f) γ-Terpinene, o-dichlorobenzene, 180 °C, 2 h. (g) nBuLi, toluene, –25 °C, 3 h to rt, 12 h. (h) Zn/TiCl4, THF, reflux, 18 h. (i)
[Et3O]BF4, 1,2-dichloroethane, rt, 72 h. (j) LiAlH4, EtOH, THF, rt, 2 h. (k) MeLi, cat. iPr2NH, Et2O, rt. 3 h. (l) MeLi, Et2O, reflux, 17 h. (m) Zn/TiCl4, THF, 0°C to reflux,
17 h. (n) NBS, CH3CN, irradiation by a spotlight (500 W), 2 h. (o) CuI, THF, reflux, 14 h. (p) AgNO3, NaCN, H2O, EtOH, rt, 30 min.(q) [CoCp(CO)2], irradiation by a
spotlight (500 W), 5 h. (r) NBS, CH3CN, rt, 16 h. (s) DDQ, toluene, reflux, 4 h. Reagents and conditions for 1B–8B (R = Br, OH, or Mes) and 1I–8I (for 1Br and 2Br. (t)
1) BBr3, n-hexane, sealed ampoule, 120 °C, 2.5 d (R = Br). For R = Mes: 2) MesMgBr in THF, toluene, 0 °C to rt, 17 h. For R = OH: 2) H2O, toluene, 0 °C to rt, 17
h. (u) I2, K2CO3, CH3CN, 80 °C, 17 h. Best results were achieved with R = Mes; the byproducts 2-iodomesitylene and 1,2-dichloro-4,5-diiodobenzene can be recycled.
The regioselectivity of the dihalogenation process is governed by
three factors: (1) The ortho-disilylated benzene precursor leads to
a corresponding ortho-diborylated intermediate, which imprints its
substitution pattern onto the PAH in the process of assembling a
strain-free, six-membered B2C4 ring. (2) For steric reasons, the
preferred sites of attack on each PAH have H atoms in
neighboring positions. (3) No substituent scrambling occurs
during boron/halogen exchange. These factors, together with the
nodal structure of the PAH’s HOMO,[28] provide a set of reliable
selection rules to predict which types of vicinally dihalogenated
PAHs are accessible by our method. As a proof-of-principle, the
precursor borane 1BOH has been transformed both to 2,3-dibromo-
and 2,3-diiodonaphthaline (see below and the Supporting
Information). In all other cases, only the vicinally diiodinated PAHs
have been prepared, because C‒I bonds tend to be more reactive
than C‒Br bonds in subsequent halogen/metal exchanges,[30]
Pd-catalyzed cross-couplings,[31] and radical reactions.[32] As an
additional advantage, the higher molecular mass of the aryl
iodides reduces weighing errors when working with small
quantities. Compared to electrophilic aromatic chlorination or
bromination, iodination reactions are less efficiently catalyzed by
conventional Lewis acids (an exception being CuCl2)[33] and
2
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10.1002/ejoc.202000954
European Journal of Organic Chemistry
COMMUNICATION
I
I
I
1,2-dichloro-4,5-diiodobenzene and 2-iodomesitylene in yields of
>90% and sufficient purity to be recycled for the synthesis of 4,5-
I
dichloro-1,2-bis(trimethylsilyl)benzene
and
MesMgI.
Our
1I
93%
2I
86%
diiodination protocol is thus considerably more atom economic
than it seems at first glance.
The 1H NMR spectra of 1I–8I show one (or two, depending on the
symmetry) characteristic singlets assignable to the protons
occupying the positions adjacent to the iodination sites. Cross
peaks with one (or two) 13C resonances in the typical shift range
of iodinated aryl-C atoms (102.6–105.9 ppm) have been detected
in all cases in the HMBC experiments. The remaining 1H/13C
signal patterns of 1I–8I are similar to those of the respective
pristine PAHs.
I
I
I
I
I
I
3I
94%
5I
83%
4I
25%
X-ray crystal structure determinations were carried out for all
products except the known compounds 1I and 2I (see Figure 2 and
the Supporting Information).[26,38,39] Key structural features will
exemplarily be discussed for the diiodo[5]helicene 8I and
compared to those of its diborylated precursor 8BMes. Compound
8I crystallizes with four crystallographically independent
molecules in the asymmetric unit (8IA–8ID). The individual I–C
bond lengths of 8IA–8ID are the same within the experimental error
margins (av. 2.10 Å). The intramolecular I×××I distances vary only
slightly between 3.674(4) and 3.692(4) Å and are thus 0.6 Å
smaller than the double van der Waals radius of iodine
(4.30 Å[40,41]). A somewhat crowded situation within the I–C–C–I
fragment is also indicated by the associated torsion angles of
–7(4)° to 13(5)° and the moderately expanded internal I–C–C
bond angle of av. 123.1°. We take the shortest distance between
the best plane through the heteroatom-substituted benzene ring
and the centroid of the C–C bond at the opposite end of the
respective [5]helicene as the helix pitch.
I
I
I
I
I
I
6I
34%
7I
85%
8I
87%
Figure 1. Ortho-diiodinated PAHs 1I–8I obtained via the synthesis protocol
shown in Scheme 1 (bottom). Yields are given relative to the respective B2-PAH
precursor.
therefore often require the addition of strong oxidants,[34] which
limits the functional group tolerance. Given that the boron/halogen
exchange is equally facile for X = Cl, Br, and I, the detour via our
double electrophilic borylation reaction appears particularly
justified for synthesizing the otherwise harder-to-get iodo
derivatives.[35]
The transformations of the B2-PAHs 1BMes–8BMes (R = Mes) to the
ortho-I2-PAHs 1I–8I were achieved by heating a mixture of the
respective aryl borane, excess I2, and K2CO3 in CH3CN to 80 °C
for 17 h.[36] The corresponding aryl iodides were isolated in yields
of 25 – 94% after (flash) column chromatography. The following
points are noteworthy: (1) In most cases, a much shorter reaction
time should be sufficient. We opted for ‘17 h’ with the sole reason
to guarantee full conversion especially in those cases were more
valuable PAHs are to be diiodinated.[37] The current protocol can
conveniently be carried out overnight and by working in simple
septum-capped microwave vials. (2) Due to the weak
electrophilicity of I2, overiodination as a possible consequence of
longer reaction times was never an issue. In contrast, the
corresponding boron/bromine exchange on 1BOH (R = OH) tends
to furnish the desired 2,3-dibromonaphthalene contaminated with
triply brominated side products (see the Supporting Information
for more details). (3) We initially assumed that it would be most
practical to quench the primary bromoborane products 1BBr–8BBr
(R = Br) with H2O and perform the boron/halogen exchange on
the borinic acids 1BOH–8BOH. However, already the borinic acid
2BOH was so poorly soluble that the crude lump always enclosed
some unreacted anthracene, which was impossible to separate
from the 2,3-diiodoanthracene target product by chromatographic
workup. The problem was solved through the introduction of
solubilizing mesityl substituents to generate 1BMes–8BMes. The
steric demand of the Mes groups had no negative impact on the
subsequent iodination step and a stock solution of the Grignard
reagent required for their introduction is stable enough to be
stored for months under an inert atmosphere. (4) In addition to the
aimed-for ortho-I2-PAHs, chromatographic workup also furnishes
Figure 2. Molecular structures of 4I, 6I, 8BMes, and 8IA in the solid state (B: bright
green, Cl: olive-green, I: magenta).[42] Displacement ellipsoids are drawn at the
50 % probability level: H atoms, except those of the CH2 group of 6I, are omitted
for clarity. Selected I–C bond lengths [Å]: 4I: 2.104, 6I: 2.093, 8IA: 2.09.
3
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10.1002/ejoc.202000954
European Journal of Organic Chemistry
COMMUNICATION
a)
during distillation) or orally consumed. (a) K. M. Abdo, C. A. Montgomery,
W. M. Kluwe, D. R. Farnell, J. D. Prejean, J. Appl. Toxicol. 1984, 4, 75–
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6144–6150.
I
I
(i)
[3]
[4]
[5]
7I
7CCPh
92%
T. K. Wood, W. E. Piers, B. A. Keay, M. Parvez, Chem. Eur. J. 2010, 16,
12199–12206.
b)
OH
B
M. Białkowska, W. Chaładaj, I. Deperasińska, A. Drzewiecka-Antonik, A.
E. Koziol, A. Makarewicz, B. Kozankiewicz, RSC Adv. 2017, 7, 2780–
2788.
Cl
Cl
HO
HO
(ii)
B
[6]
[7]
[8]
[9]
S. Kirschner, J.-M. Mewes, M. Bolte, H.-W. Lerner, A. Dreuw, M. Wagner,
Chem. Eur. J. 2017, 23, 5104–5116.
1BOH
1OH
OH
H. Hart, A. Bashir-Hashemi, J. Luo, M. A. Meador, Tetrahedron 1986, 42,
1641–1654.
Scheme 2. Syntheses of 7CCPh and 1OH. Reagents and conditions. (i) 3.5 equiv
phenylacetylene, 16 mol% CuI, 6 mol% Pd(PPh3)2Cl2, THF/HN(iPr)2 (4:1), 90 °C,
5 h. (ii) 4 equiv m-CPBA, H2O/EtOH (1:2), 0 °C to rt, 6 h, quantitative conversion.
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12166; Angew. Chem. 2015, 127, 12331–12334.
For 8IA–8ID, values of 2.86, 3.06, 3.15, and 3.15 Å have been
determined, which attest to a certain sensitivity of this parameter
to even subtle packing effects. The average value of 3.06 Å,
however, is almost identical to that of the pristine [5]helicene (2.90
Å) and also the B2-substituted 8BMes (2.96 Å; own structure
determinations, see the Supporting Information). We therefore
conclude that possible steric interactions with the neighboring
bulky mesityl substituent do not increase the helix pitch.
As a representative follow-up reaction, we conducted the double
Sonogashira alkynylation of 7I, which proceeded with yields of
92% to afford 7CCPh (Scheme 2a), a versatile precursor for further
annulations (see the related examples mentioned above).
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dihydroxylated PAHs: upon treatment with m-chloroperbenzoic
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acid
(m-CPBA),
1BOH
is
readily
transformed
to
2,3-dihydroxynaphthalene 1OH (Scheme 2b).[43] Compounds of
this kind have been used by Bunz and co-workers for the
preparation of N-heteroacenes.[15]
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Based on a boron/iodine exchange protocol, we devise a
universal method for the vicinal diiodination of PAHs. A set of
selection rules to predict the specific iodination sites is provided.
Remarkably, these sites differ from the positions that are
commonly attacked by halogens, i.e., the 1-positions of
naphthalenes, the 9,10-positions of anthracenes, the localized
double bonds of phenanthrene substructures, and the benzylic
hydrogen atoms of fluorenes.[44] Except I2-naphthalene (1I) and
I2-anthracene (2I), none of the I2-PAHs presented herein have so
far been accessible. Owing to this lack of alternatives, the
moderate yields obtained in the borylation steps seem an
affordable price to pay for obtaining synthetic building blocks as
useful as 1I–8I.
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Keywords: halogenation • iodine • polycyclic aromatic
hydrocarbons • synthetic methods
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Entry for the Table of Contents
Ortho-diiodinated PAHs
The magic I ‒ versatile and so far elusive vicinally diiodinated PAHs have been prepared via a sequence of electrophilic borylation
and boron/iodine exchange reactions.
Institute and/or researcher Twitter usernames: Tanja Kaehler: @KaeTanja
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