Conversion of 2-Iodobiaryls into 2,2′-Diiodobiaryls via Oxidation-Iodination Sequences: A Versatile Route to Ladder-Type Heterofluorenes

Article abstract of DOI:10.1002/anie.201503134

Even though 2,2′-diiodo- and 2,2′-dibromobiaryls represent accomplished precursors for heterofluorenes and other extended π-conjugated systems, their preparation still remains nontrivial when structural diversity of the biaryl backbone is required. Herein, we report a convenient method for the preparation of various 2,2′-diiodobiaryls from 2-iodobiaryls via cyclic diaryliodonium intermediates. An iodinative ring-opening of the diaryliodonium salts, mediated by a copper/diamine catalyst system, is able to afford the corresponding 2,2′-diiodobiaryls under mild conditions. The versatility of this two-step approach is demonstrated by the preparation of hitherto unexplored tetraiodoteraryls and their conversion into ladder-type π-conjugated systems. Biaryl iodination: 2-Iodobiaryls can be readily converted to 2,2′-diiodobiaryls through an initial oxidation to form cyclic diaryliodonium salts, followed by a copper/diamine-catalyzed iodinative ring-opening under mild conditions. The versatility of this two-step protocol is demonstrated by the preparation of hitherto unexplored tetraiodoteraryls and their conversion into ladder-type π-conjugated systems.

Full text of DOI:10.1002/anie.201503134

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503134
German Edition: DOI: 10.1002/ange.201503134
Iodination Reaction
Conversion of 2-Iodobiaryls into 2,2’-Diiodobiaryls via Oxidation-
Iodination Sequences: AVersatile Route to Ladder-Type
Heterofluorenes**
Bin Wu and Naohiko Yoshikai*
Abstract: Even though 2,2’-diiodo- and 2,2’-dibromobiaryls
represent accomplished precursors for heterofluorenes and
other extended p-conjugated systems, their preparation still
remains nontrivial when structural diversity of the biaryl
backbone is required. Herein, we report a convenient method
for the preparation of various 2,2’-diiodobiaryls from 2-
iodobiaryls via cyclic diaryliodonium intermediates. An iodi-
native ring-opening of the diaryliodonium salts, mediated by
a copper/diamine catalyst system, is able to afford the
corresponding 2,2’-diiodobiaryls under mild conditions. The
versatility of this two-step approach is demonstrated by the
preparation of hitherto unexplored tetraiodoteraryls and their
conversion into ladder-type p-conjugated systems.
H
eteroatom-bridged biaryls (heterofluorenes or dibenzo-
heteroles), containing Group 13–16 heteroatom elements,
have garnered significant attention as building blocks of p-
conjugated functional materials with optical and electronic
applications.[1–4] Over the years, a variety of preparative
methods for the synthesis of heterofluorenes have been
established, especially well for the naturally occurring nitro-
gen- and oxygen-containing analogues (carbazoles and diben-
zofurans).[5] But regardless, synthetic methods for the inclu-
sion of other bridging heteroatoms in heterofluorenes still
represent desirable research targets, as they provide a means
to modulate the electronic nature of the p-conjugated
systems.[6] The most frequently used precursors to such
heterofluorenes are 2,2’-dihalobiaryls (X = Br or I), which
are usually subjected to halogen–lithium exchange, followed
by trapping with boron,[1a] silicon,[7] phosphorus,[8] or other
heteroatom electrophiles (route A, Scheme 1a).[9,10] Recently,
Scheme 1. a) Common synthetic approaches to heterofluorenes
(routes A–C) and b) conversion of 2-iodobiaryls to 2,2’-diiodobiaryls
through oxidation and iodinative ring-opening.
bearing appropriate ortho-substituents (Si, P, S) may undergo
an intramolecular heteroatom–aryl bond formation, thus
affording dibenzosiloles,[14] dibenzophospholes,[15] or diben-
zothiophenes (route C, Scheme 1a).[16] Even though each
synthetic route has its own advantages, they also have
individual drawbacks. Route A, for example, allows the
conversion of common 2,2’-dihalobiaryl precursors into
a variety of heterofluorenes, but the synthesis of a structurally
diverse set of 2,2’-dihalobiaryls is laborious and nontrivial.[17]
Routes B and C are suitable for the synthesis of specific
heterofluorenes with various substituents, but are not a priori
divergent with respect to the heteroatom. Consequently,
a practical solution to enable diversity of both the central
heteroatom and the peripheral substituents would be attrac-
tive.[18]
Herein, we disclose an expansion to route A for the
synthesis of heterofluorenes enabled by the conversion of
readily available 2-iodobiaryls (1) into the corresponding 2,2’-
diiodobiaryls (3) in two steps through oxidation to cyclic
diaryliodonium triflates (2),[19] followed by a copper/diamine-
catalyzed iodinative ring-opening reaction (Scheme 1b).
Although conversion of 1 to 3 was previously achieved by
the thermal decomposition of cyclic diaryliodonium iodide at
À
approaches based on C H functionalizations have emerged.
À
À
For example, the Pd-catalyzed intramolecular C X/C H
coupling of silicon-,[11] phosphorus-,[12] or sulfur-tethered[13]
arenes allows the efficient preparation of the corresponding
heterofluorenes (route B, Scheme 1a). Alternatively, biaryls
[*] B. Wu, Prof. N. Yoshikai
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Singapore 637371 (Singapore)
E-mail: nyoshikai@ntu.edu.sg
[**] This work was supported by the Singapore National Research
Foundation (NRF-RF2009-05) and Nanyang Technological Univer-
sity. We would like to thank Dr. Yongxin Li and Dr. Rakesh Ganguly
(Nanyang Technological University) for assistance with the X-ray
diffraction analyses.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503134.
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200–2508C,[20] this classical method requires several steps
under harsh conditions, and has hence found limited appli-
cations. In contrast, the present approach allows the facile
preparation of a wide variety of 2,2’-diiodobiaryls, as well as
the generation of hitherto unexplored tetraiodoteraryl deriv-
atives, which may serve as versatile precursors for the
construction of ladder-type p-conjugated systems.[6b]
can be readily scaled up to 5 mmol (entry 11, Table 1). It is
worth noting that the use of [(nBu)4N]Br instead of
[(nBu)4N]I promoted the brominative ring-opening of 2a to
afford 2-iodo-2’-bromobiphenyl (53% yield; data not shown).
We next explored the scope of the iodinative ring-opening
reaction (Scheme 2). Various 2-iodobiaryls (1b–1q) were
The key to the present two-step preparation of 2,2’-
diiodobiaryls is the iodinative ring-opening of the cyclic
diaryliodonium salts. We envisioned that an oxidative addi-
tion of the aryl IIII bond to a CuI catalyst should generate a 2-
À
iodo-2’-cuprio(III)biaryl intermediate (A),[19a,21] which should
subsequently be intercepted by an iodide anion (B) to furnish
À
the desired 2,2’-diiodobiaryl through C I bond-forming
reductive elimination (Scheme 1b). On the basis of this
hypothesis, dibenzo[b,d]iodol-5-ium triflate (2a) was treated
with tetrabutylammonium iodide in dioxane at room temper-
ature in the presence of various copper catalysts (10 mol%;
Table 1). Using CuI in the absence of any additives afforded
Table 1: Optimization of the reaction conditions for the iodinative ring-
opening of cyclic diaryliodonium triflate 2a.[a]
Entry
Ligand
IÀ source
Yield [%][b]
1
2
3
4
5
6
7
8
none
L1
L2
L3
[(nBu)4N]I
[(nBu)4N]I
[(nBu)4N]I
[(nBu)4N]I
[(nBu)4N]I
[(nBu)4N]I
[(nBu)4N]I
NaI
10
95 (89)
74
64
83
57
66
82
83
L4
2,2’-bipyridyl
1,10-phenanthroline
L1
L1
L1
L1
9
KI
Scheme 2. Two-step conversion of 2-iodobiaryls to 2,2’-diiodobiaryls.
Reaction conditions: i) mCPBA (1.5 equiv), TfOH (3 equiv), CH2Cl2, RT,
1 h; ii) CuI (10 mol%), L1 (20 mol%), [(nBu)4N]I (2 equiv), dioxane,
RT, 24 h. The yield refers to that of the second step, and the overall
yield over two steps is shown in parentheses. [a] Reaction in DMSO at
608C. [b] Reaction in DMF at 608C. Tf =trifluoromethanesulfonyl,
10[c]
11[d]
[(nBu)4N]I
[(nBu)4N]I
20
(84)
[a] Scale: 0.2 mmol. [b] Determined by 1H NMR spectroscopy using
1,1,2,2-tetrachloroethane as an internal standard; yield of isolated
product in parentheses. [c] Reduced loadings of CuI (5 mol%) and L1
(10 mol%). [d] Scale: 5 mmol.
readily oxidized using a mixture of mCPBA (1.5 equiv) and
TfOH (3 equiv) in CH2Cl2 (room temperature, 1 h),[19,23] thus
affording the corresponding cyclic diaryliodonium triflates
(2b–2q). The subsequent iodinative ring-opening of 2b–2q
proceeded smoothly and generated the corresponding 2,2’-
diiodobiaryls (3b–3q) in moderate to good yields (58–92%),
resulting in respectable combined yields (40–77%) over both
steps. The solubility of some iodonium triflates (2l, 2m, 2o–
2q) was limited in dioxane, which required a modification of
the reaction conditions (DMSO or DMF, T= 608C) to
achieve the desired reaction. The method showed tolerance
toward electron-donating methoxy groups (3b, 3i, 3n) as well
as toward electron-withdrawing chloro (3e, 3o–3q), acetyl
(3 f), cyano (3g), trifluoromethyl (3h, 3n), and nitro (3q)
2,2’-diiodobiphenyl (3a) merely in low yield (10%; entry 1,
Table 1). However, using a combination of CuI and N,N’-
dimethylcyclohexane-1,2-diamine (L1, 20 mol%)[22] resulted
in the clean conversion of 2a to 3a in 95% yield as
determined by 1H NMR spectroscopy (89% yield of the
isolated product; entry 2, Table 1). Other 1,2-diamine ligands
(L2–L4, Table 1), 2,2’-bipyridine, and 1,10-phenanthroline
also promoted the iodinative ring-opening, albeit in some-
what lower yields (57–83%; entries 3–7, Table 1). As alter-
native sources of iodide, NaI and KI could be used instead of
[(nBu)4N]I (entries 8 and 9, Table 1). Even though reduced
loadings of CuI (5 mol%) and L1 (10 mol%) resulted in
significantly decreased yields (entry 10, Table 1), the reaction
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groups. Whereas iodination at the C3 position of thiophene
was successfully achieved (3m), the attempted iodination at
the C2 position of thiophene or the C3 position of indole
failed due to decomposition of the starting material in the first
step. Note that the regioselectivities of iodination in the
products 3i and 3k were expected from previous studies by
Wen et al.[19a]
We extended the present method to the conversion of
diiodoteraryls into the corresponding tetraiodoteraryls
(Scheme 3). The oxidation of 2’,5’-diiodo-p-terphenyl deriv-
Scheme 4. Conversion of tetraiodoteraryl 6b into ladder-type p-conju-
gated systems. a,b) Reaction conditions: i) CuI (20 mol%), sulfur
powder, K2CO3, DMF, 908C; ii) CuI (20 mol%), selenium powder,
K2CO3, NMP, 1208C; iii) 1) nBuLi, THF, À788C; 2) Et2GeCl2, À788C to
RT; iv) 1) nBuLi, THF, À788C; 2) PhPCl2, À788C to RT; 3) H2O2;
v) Pd(OAc)2 (20 mol%), IPr·HCl (10 mol%), P(tBu)3·HBF4 (10 mol%),
AgOAc, p-xylene, 1308C. c) ORTEP drawings of 11, 12, and syn-13
(thermal ellipsoids at 50% probability; hydrogen atoms and solvent
molecules were omitted for clarity).
Scheme 3. a) Conversion of 2’,5’-diiodo-p-terphenyl derivatives 4a–4d
to 2,2’,2’’,5’-tetraiodo-p-terphenyl derivatives 6a–6d, and b) conversion
of 2,5-bis(2-iodophenyl)thiophene 7 to 3,4-diiodo-2,5-bis(2-iodophe-
nyl)thiophene 9. Reaction conditions: i) mCPBA (3 equiv), TfOH
(6 equiv), CH2Cl2, RT, 2 h; ii) CuI (20 mol%), L1 (40 mol%), [(nBu)4N]I
(4 equiv), MeCN, 608C, 24 h. Inset: ORTEP drawing of 5b (thermal
ellipsoids set at 50% probability), whereby hydrogen atoms, counter-
anions, and solvent molecules were omitted for clarity.
and K2CO3 in DMF[25] resulted in a fourfold C S bond
À
atives 4a–4d proceeded cleanly to afford bis-iodonium
triflates 5a–5d in high yields (87–98%; Scheme 3a). The
ladder-type structure of these bis-iodonium triflates was
confirmed by X-ray crystallographic analysis of single crystals
of 5b (see inset in Scheme 3a).[24] A twofold iodinative ring-
opening of 5a–5d took place in acetonitrile at 608C, affording
the desired 2,2’,2’’,5’-tetraiodo-p-terphenyl derivatives 6a–6d
as light-sensitive solids in moderate to good yields (51–72%;
Scheme 3a). Furthermore, 2,5-bis(2-iodophenyl)thiophene
(7) was also amenable to a twofold oxidation/ring-opening,
which resulted in the formation of 3,4-diiodo-2,5-bis(2-
iodophenyl)thiophene (9) in moderate yield (44%; Sche-
me 3b).
The diiodobiaryl and tetraiodoteraryl derivatives synthe-
sized by the present method should be able to serve as
versatile starting materials for the synthesis of heterofluor-
enes and other extended p-conjugated systems. This idea was
validated by conversion of tetraiodoteraryls 6b and 9 into
various ladder-type p-conjugated systems (Scheme 4a,b).
Exposure of 6b to a catalytic amount of CuI, sulfur powder,
formation to afford benzo[1,2-b:4,5-b’]bis[b]benzothio-
phene[26] 10 in 86% yield. A similar Cu-catalyzed method
using selenium and (N-methyl-2-pyrrolidone) (NMP) instead
of sulfur and DMF[18a] allowed conversion of 6b and 9 into
benzo[1,2-b:4,5-b’]bis[b]benzoselenophene[26a]
(11)
and
dibenzo[d,d’]thieno[3,2-b;4,5-b’]diselenophene (15). These
results suggest great promise for a catalyst system based on
copper and elemental sulfur/selenium to prepare dibenzo-
thiophenes and dibenzoselenophenes directly from 2,2’-diio-
dobiaryls.[27] A twofold iodine–lithium exchange/electrophilic
trapping sequence proved effective for the preparation of
germanium- and phosphoryl-bridged p-teraryls 12 and
13,[15a,28] respectively. The latter product was obtained as
a separable mixture of syn- and anti-isomers. Besides the
heteroatom-bridged p-teraryls, the Pd-catalyzed reaction of
6b with 5-decyne, using Wuꢀs conditions,[29] resulted in
a twofold annulation to afford benzo[k]tetraphene derivative
14. Single-crystal X-ray diffraction analysis was performed for
all of these ladder-type products except anti-13, which
invariably demonstrated high levels of coplanarity of their
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[13] T. Wesch, A. Berthelot-BrØhier, F. R. Leroux, F. Colobert, Org.
skeletons in the solid state (see Scheme 4c for 11, 12, and syn-
13, and the Supporting Information for further details).[24]
In summary, we have developed a convenient two-step
method for the preparation of 2,2’-diiodobiaryls from readily
accessible 2-iodobiaryls through oxidative cyclization/Cu-
catalyzed iodinative ring-opening sequences. Even though
cyclic diaryliodonium intermediates have previously been
reported to serve as direct precursors to carbazoles and some
other bridged biaryl compounds,[19,30] the method presented
herein should substantially enhance their utility for the
synthesis of a wide range of heterofluorenes. Indeed, the
versatility of this method has been demonstrated by the
preparation of novel tetraiodoteraryls and their conversion
into various ladder-type p-conjugated systems. Furthermore,
the present method may also be useful for the diversification
of Kitaꢀs 2,2’-diiodobiaryl-based organocatalysts for oxidation
reactions.[31]
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Keywords: biaryls · copper · diaryliodonium salts ·
heterofluorenes · iodination
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Received: April 6, 2015
Published online: June 8, 2015
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