Synthesis, Properties, and Two-Dimensional Adsorption Characteristics of [6]Hexahelicene-7-carboxylic acid

Article abstract of DOI:10.1002/chem.201603058

A convergent synthesis of racemic [6]hexahelicene-7-carboxylic acid by cross-coupling of a bicyclic and a tricyclic component is described. A metal-catalyzed ring-closure is also a fundamental component of the synthetic approach. Scanning tunneling microscopy (STM) measurements of the racemate self-assembled on Au(111) at liquid–solid interface revealed the formation of ordered racemic 2D crystals.

Full text of DOI:10.1002/chem.201603058

DOI: 10.1002/chem.201603058
Full Paper
&
Cross-Coupling
Synthesis, Properties, and Two-Dimensional Adsorption
Characteristics of [6]Hexahelicene-7-carboxylic acid
Maarten W. van der Meijden,[a] Tatyana Balandina,[b] Oleksandr Ivasenko,[b]
Steven De Feyter,[b] Klaus Wurst,[c] and Richard M. Kellogg*[a]
Abstract: A convergent synthesis of racemic [6]hexaheli-
cene-7-carboxylic acid by cross-coupling of a bicyclic and
a tricyclic component is described. A metal-catalyzed ring-
closure is also a fundamental component of the synthetic
approach. Scanning tunneling microscopy (STM) measure-
ments of the racemate self-assembled on Au(111) at liquid–
solid interface revealed the formation of ordered racemic 2D
crystals.
Introduction
and which might be separated into enantiomers by preferen-
tial crystallization. To make further use of the unique properties
of the helicenes, it would be useful to prepare a helicene that
contains a functional group, which allows further derivatization
or immobilization.
Helicenes offer synthetic challenges rewarded by obtainment
of molecules with optical properties that invite application.[1]
For example, helicenes are currently being studied for use as
semiconductors[2] or as chiral sensors. Their possible use as mo-
lecular nanowires on an insulating surface is under investiga-
tion.[3] Interest has also been shown in the two-dimensional
crystallization of helicenes.[4] Multigram syntheses of helicenes
remain a challenge, although in recent years synthetic method-
ologies to prepare larger amounts have been described.[5] We
are particularly interested in the basic hexahelicene skeleton
provided with a substituent, which can be used as a handle
that allows further manipulation. Our interest in helicenes also
stems from ongoing investigation on conglomerates and dera-
cemization.[6,7] Since it is known from the literature that several
helicenes, including tetra- to nonahelicene, as well as some
heterohelicenes,[8–10] crystallize as conglomerates—enantio-
morphic crystals in non-centrosymmetric space groups—this
could offer perspective for new methods for resolution of con-
Results and Discussion
In view of the widespread synthetic activity in the preparation
of new types of helicenes, it is remarkable that relatively less
appears to have been done with the “grandfather of heli-
cenes”, namely hexahelicene,[11–13] to provide it with substitu-
ents that would allow further manipulation, particularly on sur-
faces. The synthesis of heptahelicene 2-carboxylic acid has
been described recently by means of cross-coupling followed
by [2+2+2] cyclization.[14] A straightforward convergent syn-
thesis of 5-aminohexahelicene has recently been described by
some of our group.[15] We wished also to develop a strategy
for placement of a carboxyl group in a central position on the
helicene periphery using synthetic knowledge already gained
from the cross-coupling approach
glomerates.[6,7] This hope was fueled by a recent publication
[5]
´
from the group of Teply, who screened helicenes that contain
a pyridine ring to find a salt that crystallizes as a conglomerate
we had developed.[15] However, de-
spite much effort (see the Support-
ing Information)
a synthesis of
[a] Dr. M. W. van der Meijden, Prof. Dr. R. M. Kellogg
Syncom BV
[6]hexahelicene-7-carboxylic acid
1 (Figure 1) based on the route of
ref. [15] failed.
Kadijk 3, 9747 AT Groningen (The Netherlands)
E-mail: r.m.kellogg@syncom.nl
[b] Dr. T. Balandina, Dr. O. Ivasenko, Prof. Dr. S. De Feyter
KU Leuven
The failed synthesis was based
on the concept of obtainment of
the substituent as carboxylic acid
with no need to adjust oxidation
levels. On the other hand, a methyl
group could be suitable provided
it could be oxidized at the end of
Department of Chemistry
Division of Molecular Imaging and Photonics
Celestijnenlaan 200F, 3001 Heverlee (Belgium)
Figure 1. Target [6]hexaheli-
cene-7-carboxylic acid 1.
[c] Dr. K. Wurst
University of Innsbruck
Institute of General, Inorganic and Theoretical Chemistry
Innrain 80–82 6020 Innsbruck (Austria)
the synthesis to the desired carboxylic acid. An approach to
key intermediate 8 required for synthesis by means of a modi-
fied cross-coupling approach is illustrated in Scheme 1.
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201603058.
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Scheme 2. Synthesis of bromoaldehyde 26.
DCM with a catalytic amount of dimethylformamide (DMF) led
to no reaction; on use of higher boiling 1,2-dichloroethane
(DCE) conversion took place. Using this procedure, the tribro-
moacetyl ester 5 was obtained without significant problems in
87–100% yield. The copper-catalyzed ring closure[22,23] of 5
using complex 6 gave somewhat lower yield than in the case
of the chloride, which was also tested, due to dehalogena-
tion.[24] The bromo-lithium exchange with 7 proceeded cleanly
to provide boronic acid 8 in 52–82% yield.
Scheme 1. Synthesis of boronic acid 8.
Reaction of 1-naphthol 2 with crotyl chloride gave crotyl
ether 3. Reaction with crotyl bromide gave slightly higher
yields, but also led to small amounts of unidentified impurities,
which in our hands could not be removed, even in later steps.
There were problems with regioselectivity of the Claisen reac-
tion with 3; in addition to the desired 4a in initial reactions
the linear alkene 4b was also obtained, possibly due to a com-
petitive cationic mechanism in the rearrangement.[16]
Bromoaldehyde 10 had to be prepared as the coupling
partner for 8 (Scheme 2).
Treatment of 1-tetralone 9 with PBr3 and DMF in DCM gave
10 in 56–60% yield.[25] The quality of 9 was of great impor-
tance. When a batch prepared in China was used, only 20–
30% yield was obtained, whereas a batch from Acros gave the
mentioned higher yield. Suzuki coupling of boronic acid 8 and
bromoaldehyde 10 gave 11 in reasonable yields. The
optimized conditions for the Suzuki reaction and further
conversions are depicted in Scheme 3.
Attempts to circumvent this problem by use of SnCl4 in
dichloromethane (DCM),[17] BCl3 in DCM,[18] microwave (MW)
heating (neat),[19] trifluoroacetic acid (TFA),[16] and Eu(FOD)3 in
[20]
CHCl3 were unrewarding. However, a paper was found in
which the use of N,O-bis(trimethylsilyl)acetamide is described
to prevent the abnormal Claisen rearrangement.[21] This ap-
proach worked beautifully and indeed only the desired prod-
uct 4a was obtained in 83–93% yield upon hydrolysis of the
TMS-protected phenol.
Dehydrogenation of 11 with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) was at first troublesome. When condi-
tions analogous to a similar conversion[15] were used (reflux in
benzene) the reaction took several weeks to go to full conver-
sion. When 1,2-dichlorobenzene (DCB) was used at 1308C (ex-
ternally), the reaction was finished within 2 h but unidentified
side products were formed. On use of refluxing toluene
(1108C), the reaction proceeded to completion overnight and
was somewhat cleaner than in DCB, although side products
were seen. However, we were pleased to find that if chloro-
benzene was used as solvent and the temperature held at
For preparation of 7, Cu-catalyzed ring closure of ester 5
was envisaged on the basis of reports of examples of such re-
markable transformations.[22,23] Tribromoacetylchloride, neces-
sary for tribromoacetylation of phenol 4a, is not commercially
available and must be prepared from reaction of tribromoace-
tic acid with thionyl chloride with a catalytic amount of DMF.
The material is not stable and must be used without isolation.
Reaction of tribromoacetic acid under reflux with SOCl2 in
Scheme 3. Final steps for preparation of helicene 1.
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1208C, the reaction was finished in 2 h without formation of
side products.
H2O2/K2CO3 in MeOH,[41] CrO3 in H2SO4,[42] and even SeO2 in
Dowtherm at 2608C (externally) in an open vessel. To our
amazement, no conversion was observed with any of these re-
agents. We have no reasonable explanation for the astonishing
intransigence of 16 to further oxidation.
Fully aromatized 12 can then be converted without prob-
lems to the desired alkyne 14 as long as during the work up
the mixture is washed with aqueous NaOH. If this step is omit-
ted, in the next step the dimethyl acetal of 12 instead of 14 is
Purification of the reaction mixture was difficult. At first, suit-
able column chromatographic conditions could not be found
and preparative HPLC was tested. This was attended by serious
solubility problems and the recovery was very low. The acid in
the crude mixture was converted to the pure benzyl ester with
Cs2CO3 and benzyl bromide in CH3CN at reflux. However, re-
moval of the benzyl group using a H-Cubeꢀ at 508C and
100 bar of H2 gave incomplete conversion and finally decom-
position. More or less acceptable purification by column chro-
matography over SiO2 was eventually possible, using 20% ace-
tone in toluene. This gives mainly mix fractions and some pure
fractions. The pure fractions were used for STM analysis. At-
tempt to purify the material by crystallization, or to separate
the enantiomers, failed.
[14]
isolated. Subsequent ring closure of 14 with PtCl2 led to 7-
methyl[6]helicene 15 in 63–100% yield.
The crystal structure of 15, as established by X-ray crystallo-
graphic analysis, is depicted in Figure 2. The space group is
P21/c and therefore 15 is a racemic compound rather than
a conglomerate.
Self-assembly at the liquid-solid interface
DFT (B97d/6-31G(d)) modeling of carboxylic hydrogen bonding
in vacuo (Figure 3Eand Figure S1 in the Supporting Informa-
tion), the strongest intermolecular interaction anticipated for 1,
indicated a very small (<0.7 kcalmolÀ1) preference for the for-
mation of enantiopure (MM and PP) dimers. Self-assembly at
Figure 2. X-ray structure of 7-methylhexahelicene 15.
Oxidation to the final product 1 was unexpectedly difficult.
[26,27]
Many different reagents were tested: (nBu)4NMnO4,
CrO3/
H2SO4/HOAc,[28] KMnO4/pyridine/water,[2,30] KMnO4 in butanone
or acetone,[31] MnO4/dicyclohexyl-18-crown-6 in benzene,[32]
HBr/O2/hv,[33] RuCl3·3H2O/NaIO4 in CCl4/CH3CN/water,[34] DDQ
(oxidation to the aldehyde),[35] and ceric ammonium nitrate
(CAN) in MeOH (oxidation to the aldehyde).[36] The oxidation of
several methylbenzophenanthrenes has been described using
Na2Cr2O7·2H2O in water at 2508C.[37] We did not have access to
suitable pressure vessels needed for such conditions. External
high temperature oxidation using KMnO4 or Na2Cr2O7·2H2O in
Dowtherm at 2508C was tested but no conversion was ob-
served. The use of (nBu)4NMnO4 in pyridine led to some con-
version (ca. 10%). Decomposition of the salt appears to be
more rapid than oxidation. Coating the permanganate com-
pound on Alox can improve the stability of tetraalkylammoni-
um permanganates.[38] This approach was also examined but
conversions remained low. The only approach that worked rea-
sonably well was the use of SeO2 in Dowtherm A at 2608C (ex-
ternally) in an open system while blowing air over the mixture
to remove the water formed during the reaction. A mixture of
aldehyde 16 and carboxylic acid 1 was obtained. At lower tem-
perature, only the aldehyde was obtained. If the temperature
was raised, a substantial amount of decomposition products
was obtained. Attempts to improve the conversion by addition
of K2CO3 in pyridine or Dowtherm[39] did not help. Attempts
were made to oxidize 15 to 1 with NaBO3·nH2O in HOAc,[40]
Figure 3. A) Amorphous assembly of 1 (1.1ꢁ10À4 m in TCB) on Au(111) sur-
face. The STM image was acquired immediately after the solution deposi-
tion. Imaging parameters: Is =100 pA, Vs =À400 mV. Scale bar=10 nm.
B) Ordered self-assembly of 1 (1.1ꢁ10À4 m in TCB) formed one hour after the
solution deposition. Imaging parameters: Is =84 pA, Vs =À410 mV. Scale
bar=10 nm. C),D) The histograms of the nearest-neighbor distances be-
tween the bright blobs in (A) and (B), respectively. E) models illustrating the
shortest and the longest separation between the uppermost benzene rings
of supramolecularly interacting helicene molecules.
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solid surfaces is known to enhance chiral discrimination of
enantiomers; there are even a few examples of homochiral as-
semblies formed by chiral molecules that did not undergo
enantiospecific crystallization.[43,44] That is why we chose to in-
vestigate the self-assembly of this racemic helicene on surfaces
using scanning tunneling microscopy (STM).
Drop-casting 1,2,4-trichlorobenzene (TCB) solutions of 1 on
highly oriented pyrolytic graphite (HOPG) did not yield any
self-assembly; only empty graphite surface was observed in
STM. Rationalizing that this is due to a very weak affinity of the
helicene to graphite, we turned our attention to stronger-bind-
ing Au(111) surfaces. Drop-casting a 1.1ꢁ10À4 m solution of
1 in TCB onto Au(111) immediately resulted in helicene adsorp-
tion, albeit with formation of disordered monolayers
(Figure 3A). With time (ꢀ1 h) however, the helicenes slowly re-
organized into an ordered pattern of co-aligned zig-zag rows
(Figure 3B). The pattern is composed of unstructured blobs of
different brightness.
The brightest blobs arranged into zig-zag rows are much
smaller (ꢁ0.6 nm in cross-section) than the molecular dimen-
sions of the helicene, which has a circular footprint of
ꢀ1.2 nm in diameter. This is understandable: electron tunnel-
ing through the molecular orbitals of the helicene spiral is
uneven, with (presumably) the uppermost annulated ring
being the brightest in STM.[45,46] The shortest distance
(ꢀ0.6 nm) between the uppermost parts of the interacting
helicene molecules will be in close-packed helicene dimers
bound by weak van der Waals interactions, whereas the lon-
gest (ꢀ1.9 nm) is anticipated for hydrogen-bonded dimers
(Figure 3E). Indeed, the nearest-neighbor separation analysis
confirmed that there are no contacts below 0.6 nm (Figure 3C).
Interestingly, there is a large peak around 2 nm in the histo-
gram of the ordered phase (Figure 3D), suggesting a possibility
of hydrogen bonding.
Figure 4. A) High resolution STM image of the self-assembled monolayer of
1 (1.1ꢁ10À4 m in TCB) on Au(111). Unit cell: a=1.3Æ0.2 nm, b=3.7Æ0.2 nm,
g=71Æ58. Blue and red circles map STM contrast features corresponding to
the footprint of 1. The direction of decline of STM brightness is marked with
blue- and red-colored spots and was used to build the tentative model.
Imaging conditions: Iset =84 pA, Vset =À410 mV, scale bar is 5 nm. B) The ten-
tative molecular model of the self-assembled monolayer of 1. The unit cell is
indicated in yellow. Molecules were color-coded in the same way as in (B).
C) STM image of a self-assembled pattern of 1 (1.1ꢁ10À4 m solution in TCB)
showing chiral domains. The size of the image is 50ꢁ30 nm. Imaging condi-
tions: Iset =76 pA, Vset =À427 mV.
rivative of 14. As explained in the Supporting Information, it is
clear that syntheses proceed better if electron-withdrawing
groups on the helicene skeleton are avoided. It is disappoint-
ing that the final oxidation of 14 to 1 is so difficult.
Racemic 1 undergoes self-assembly at the liquid/solid inter-
face on Au(111). Chiral recognition results in the formation of
an ordered racemic compound formed by both MM and PP
homochiral H-bonded dimers.
As has been shown previously, such STM contrast is typical
for helicenes and can be used to assign molecular handedness
directly.[47] Mapping out the features in a high-resolution STM
image (Figure 4A) with 1.2 nm circles (helicene dimensions),
while maintaining the periodicity and symmetry of the pattern
is relatively straightforward and led to a tentative molecular
model shown in Figure 4B. The model contains alternating
rows of homochiral (MM and PP) hydrogen-bonded helicene
dimers, which means that the ordered self-assembly of 1 is
a racemic compound. Such alternating packing of homochiral
dimers results in easily recognizable M-P zig-zag row motif,
well-known stable packing of racemic unsubstituted heli-
cenes.[47–49] Interestingly, despite the racemic composition, the
2D unit cell is asymmetric and mirror-image domains are
present on the Au(111) surface (Figure 4C).
Experimental Section
All NMR spectra were measured on a Varian VNMRS 300 MHz or
a Varian MercuryPlus 300 MHz apparatus. Either CDCl3 or [D6]DMSO
were used as solvents as indicated. STM imaging was carried out
at room temperature (20–248C) using a PicoSPM (Agilent) system
operating in constant-current mode with the tip immersed in the
supernatant liquid. STM tips were prepared by mechanical cutting
of Pt/Ir wire (80/20%, diameter 0.25 mm). As substrates, Au(111)
films on mica (Georg Albert PVD Company) or highly oriented
pyrolytic graphite (HOPG, grade ZYB, Advanced Ceramics Inc.,
Cleveland, USA) was used. Immediately prior to their use, Au(111)
substrates were annealed in an oven. A fresh HOPG surface was
prepared by removing the upper graphene layers with Scotch
tape. 1,2,4-trichlorobenzene (Sigma–Aldrich Co., 99%) was used as
the solvent. For each measurement, an 8 mL drop of the desired so-
Conclusions
The synthesis of 7-methyl[6]hexahelicene 15 proceeds very
well using the cross-coupling approach described here. The Pt
catalyzed conversion of 14 to 15 is particularly clean; one may
well expect the hexahelicene itself could be prepared on
a large scale quite readily by ring closure of the desmethyl de-
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lution was applied directly to the freshly prepared substrate. STM
imaging was carried out in the constant-current mode, and started
immediately after drop casting. The images were processed using
the WSxM software developed by Nanotec.[50] Supplementary DFT
calculations of H-bonding helicene dimers was performed using
Gaussian G09 software[51] at the B97D/6–31G(d) level of theory.
36.1, 27.9. 20.0 ppm. [M+NH4+] (API/ES) calculated: 491.88; found:
491.71.
4-Bromo-1-methylphenanthrene (7): Tribromoacetyl ester
5
(121.8 g contains some solvent, max. 432 mmol) in 1,2-dichloro-
benzene (DCB, 300 mL) was bubbled through with N2 for 10 min.
Copper catalyst 6 (5.7 g, 11.7 mmol, 2.7%, prepared according to
literature procedures[52,53]) was added and the mixture was heated
at reflux for 40 min. After NMR analysis revealed complete conver-
sion, DCB was removed by bulb-to-bulb distillation using a mem-
brane pump. The resulting mixture was coated on SiO2 (400 mL)
and filtered over a plug of SiO2 (1 L, heptane) to give 7 (0.7 g,
52%) as an orange solid, which contains debrominated product
(ca. 23%). This material is easily removed after the next step. For
analysis a sample was purified by column chromatography over
SiO2 (heptane).
1H NMR (CDCl3): d=9.96 (m, 1H), 7.89 (m, 3H), 7.79 (d, 1H), 7.61–
7.66 (m, 2H), 7.25 (d, 1H), 2.71 ppm (s, 3H). 13C NMR (CDCl3): d=
134.9, 134.2, 133.9, 133.2, 130.3, 128.9, 128.5, 128.4, 128.2, 127.4,
127.2, 125.3, 123.0, 117.3, 20.5 ppm. [M] (GC/MS) calculated:
270.00; found: 270.0.
(E)-1-(But-2-en-1-yloxy)naphthalene (3): Naphthol
2
(200 g,
1.4 mol), crotyl chloride (predominantly trans, 149 mL, 1.53 mol,
1.1 equiv) and K2CO3 (211 g, 1.53 mol, 1.1 equiv) were heated for
3 days at 808C (ext.) in acetone (5 L), divided over two flasks. After
NMR analysis revealed complete conversion, the recombined mix-
ture was filtered and concentrated and the residue was filtered
over SiO2 (heptane) to give ether 3 (204 g, 73%) as a yellow oil,
which was used without further purification in the next step.
1H NMR (CDCl3): d=8.30 (dd, 1H), 7.79 (dd, 1H), 7.33–7.51 (m, 4H)
6.83 (d, 1H), 5.80–5.99 (m, 2H), 4.78 (cis, d<1H), 4.65 (trans, d,
2H), 1.80 ppm (m, 3H). 13C NMR (CDCl3): d=154.9, 135.0, 130.2,
128.7, 127.8, 126.7, 126.6, 126.2, 125.5, 122.6, 120.6, 105.4, 69.2,
64.5 (small, cis), 18.3 ppm. [M] (GC/MS) calculated: 198.10; found:
198.1
(1-Methylphenanthren-4-yl)boronic acid (8): To bromide 7 (60.7 g,
224 mmol) in a flame dried flask in dry tetrahydrofuran (THF, 1 L)
was added BuLi (2.5m in hexanes, 103 mL, 257 mmol, 1.15 equiv).
After stirring for 1 h at À788C, NMR analysis of a sample revealed
complete conversion and B(OCH3)3 (51.7 mL, 470 mmol, 2.1 equiv)
was added in one portion. The resulting mixture was stirred for
30 min at À788C and 1.5 h at room temperature and was subse-
quently poured into 10% HCl (1.6 L). The mixture was extracted
with (C2H5)2O (2.5 L) and the organic layer was washed with water
(2 L), dried over Na2SO4, and concentrated. The residue was tritu-
rated from heptanes to give 8 (43.6 g, 82%) as an off-white solid.
1H NMR (CDCl3): d=8.48–8.51 (m, 1H), 7.88, (d, 1H), 7.91 (m, 1H),
7.80 (d, 1H), 7.70 (d, 1H), 7.61 (m, 2H). 7.44 (d, 1H), 4.72 (bs, 2H),
2.76 ppm (s, 3H). 13C NMR (CDCl3): d=136.7, 132.9, 132.7, 131.5,
131.3, 131.1, 128.8, 127.5, 127.2, 126.9, 126.8, 126.6, 126.3, 126.2,
123.3, 20.4 ppm. [M+Na] (API/ES) calculated: 259.09; found:
259.12.
2-(But-3-en-2-yl)naphthalen-1-ol (4a): Crotyl ether
3 (204 g,
1.0 mol) and N,O-bis(trimethylsilyl)acetamide (255 mL, 1.0 mol,
1 equiv) were heated at reflux for 30 min. After NMR analysis re-
vealed complete conversion, 3n HCl (2 L) was added and the re-
sulting mixture was extracted twice with (C2H5)2O (2 L, 1 L). The
combined organic layers were dried over Na2SO4 and concentrated.
The residue was stirred for 30 min with a mixture of CH3OH (1.75 L)
and 3n HCl (1.25 L) after which (C2H5)2O (5 L) was added. The
layers were separated and the organic layer was washed with
brine (1 L), dried over Na2SO4 and concentrated. To the residue was
added water (1 L) and the resulting mixture was extracted with
heptane (2 L, 1 L) and the combined organic layers were washed
with water (2ꢁ1 L), dried over Na2SO4 and concentrated to give 4a
(171.2 g, 84%) as an orange oil, which was used without further
purification in the next step.
1H NMR (CDCl3): d=8.18 (m, 1H), 7.78 (m, 1H), 7.45 (m, 3H), 7.27
(d, 1H), 6.14–6.25 (m, 1H), 5.73 (s, 1H), 5.31 (m, 2H), 3.78 (m, 1H),
1.51 ppm (d, 3H). 13C NMR (CDCl3): d=149.3, 142.5, 133.8, 127.7,
126.4, 126.1, 125.6, 125.4, 123.1, 121.8, 120.7, 115.3, 38.9, 18.8 ppm.
[M] (GC/MS) calculated: 198.10; found: 198.2.
1-Bromo-3,4-dihydronaphthalene-2-carbaldehyde (10): To dry
DMF (104 mL, 1.34 mol, 3 equiv) in dry DCM (1.8 L) was added
dropwise PBr3 (106 mL, 1.12 mol, 2.5 equiv) at 08C. a-Tetralone 9
(60 mL, 447 mmol) was added and the mixture was allowed to
warm to room temperature overnight. The mixture was heated at
reflux for 3 h, then poured onto ice (600 mL), giving a violent reac-
tion. The mixture was made basic with saturated aqueous NaHCO3.
The mixture was extracted with (C2H5)2O (3ꢁ800 mL) and the com-
bined organic layers were washed with water (400 L), dried over
Na2SO4, and concentrated to give 76.3 g crude 10 as a brown oil.
The crude material was filtered over approximately 500 mL SiO2
(toluene) to give 10 (74.6 g, 70%) as an orange oil. The obtained
material is NMR pure, but some minor, dark impurities can be re-
moved by column chromatography over SiO2 using ethyl acetate:-
heptanes 1:9 as eluting agent, giving an orange oil, which crystalli-
zes upon standing.
If 3 was allowed to rearrange in the absence of N,O-bis(trimethylsi-
lyl)acetamide significant amounts (10–20%) of 4b were formed.
This was not readily separable from 4a.
2-(But-3-en-2-yl)naphthalen-1-yl 2,2,2-tribromoacetate (5): Tri-
bromoacetic acid (296.7 g, 1 mol) and SOCl2 (145 mL, 2 mol,
2 equiv) and a few drops dimethyl formamide (DMF) were heated
overnight at reflux in DCE (2 L). The mixture was evaporated and
stripped with toluene to give tribromoacetylchloride (311 g, 99%)
as a dark oil, which was used as such.
To 4a (171.2 g, 864 mmol) in DCM (2 L) was added at 08C pyridine
(80 mL, 987 mmol, 1.14 equiv) and then tribromoacetyl chloride
(311 g, 987 mmol, 1.14 equiv) dropwise. After stirring at 08C for
1 h, NMR analysis revealed complete conversion and the mixture
was washed with 1m citric acid (2ꢁ1.5 L), dried over Na2SO4, and
concentrated to give ester 5 (425.5 g, quant., contains some
CH2Cl2) as a red oil, which was used without purification in the
next step.
1H NMR (CDCl3): d=7.98 (d, 1H), 7.79–7.88 (m, 2H), 7.48–7.59 (m,
2H), 7.42 (d, 1H), 6.01–6.12 (m, 1H), 5.12–5.18 (m, 2H), 3.96–4.00
(m, 1H), 1.43 ppm (d, 3H). 13C NMR (CDCl3): d=160.9, 141.4, 134.1,
133.6, 129.3, 128.5, 128.3, 127.8, 127.5, 126.5, 125.4, 120.9, 114.8,
1H NMR (CDCl3): d=10.26, (s, 1H), 7.90 (m, 1H), 7.35 (m, 2H), 7.19
(m, 1H), 2.84 (t, 2H), 2.62 ppm (m, 2H). 13C NMR (CDCl3): d=193.4,
139.3, 134.8, 133.3, 131.6, 129.0, 127.8, 127.4, 27.4, 23.1 ppm.
[M+1] (API/ES) calculated: 236.98; found: 236.99.
1-(1-Methylphenanthren-4-yl)-3,4-dihydronaphthalene-2-carbal-
dehyde (11): A solution of bromide 10 (16.7 g, 71 mmol, 1 equiv)
in toluene (400 mL) and water (1 mL) was bubbled through with
N2 for 10 min. K3PO4 (30 g, 141 mmol, 2 equiv), boronic acid 8
(16.6 g, 71 mmol), Pd(O2CCH3)2 (792 mg, 3.5 mmol, 5%) and S-Phos
Chem. Eur. J. 2016, 22, 1 – 8
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(3.62 g, 8.8 mmol, 12.5%) were added and the mixture was heated
at reflux overnight. After NMR-analysis revealed complete conver-
sion, water (1 L) was added and the mixture was extracted with
DCM (1 L, 500 mL, 300 mL). The combined organic layers were
dried over Na2SO4 and concentrated. The residue was purified by
column chromatography (ethyl acetate:heptanes 1:9) to give 11
(13.4 g, 54%) as a yellow solid.
1H NMR (CDCl3): d=9.49 (s, 1H), 8.61 (d, 1H), 8.07 (d, 1H), 7.85 (m,
2H), 7.43–7.53 (m, 2H), 7.21–7.34 (m, 3H), 6.91 (t, 1H), 6.74 (d, 1H),
3.08–3.17 (m, 2H), 2.76–2.93 (m, 2H), 2.86 ppm (s, 3H). 13C NMR
(CDCl3): d=193.5, 158.2, 139.0, 136.2, 134.9, 133.3, 133.2, 132.4,
131.5, 131.0, 130.6, 130.3, 128.9, 128.2, 127.6, 127.3, 127.2, 126.6,
126.0, 123.3, 27.7, 20.8, 20.6 ppm. [M+1] (API/ES) calculated:
349.15; found: 349.2.
brown solid. Crystallization from acetic acid gives 15 as yellow
crystals, suitable for X-ray structure determination.
1H NMR (CDCl3): d=8.14 (d, 1H), 7.97 (d, 1H), 7.89–7.95 (m, 2H),
7.78–7.86 (m, 2H), 7.60 (d, 1H), 7.51 (d, 1H), 7.17–7.25 (m, 2H),
6.62–6.69 (m, 2H), 2.92 ppm (s, 3H). 13C NMR (CDCl3): d=133.3,
133.1, 132.1, 131.5, 130.9, 130.2, 130.1, 128.6, 128.4, 128.3, 127.9,
127.8, 127.6, 127.5, 126.6, 126.5, 125.7, 125.6, 124.8, 124.7, 123.3,
122.3, 20.1 ppm. [M+1] (ESI) calculated: 343.15; found: 343.15.
[6]Hexahelicene-7-carboxylic acid (1): Methylhelicene 15 (1 g,
2.9 mmol) and SeO2 (810 mg, 7.3 mmol, 2.5 equiv) were heated in
Dowtherm A (10 mL) on a preheated plate at 2608C (external tem-
perature) while blowing air over the mixture. After 10 min all
sample has evaporated and after 1.5 h total, NMR analysis showed
full conversion of 15. The mixture was cooled to room temperature
and CHCl3 was added to the residue. The resulting mixture was fil-
tered over Celite and concentrated to give 1.2 g crude acid 1. The
obtained material was purified by column chromatography over
150 mL SiO2 (toluene:acetone 5:1) to give 130 mg of pure acid
1 and 290 mg product containing mix fractions. The mix fractions
were purified by column chromatography over 60 mL SiO2 (tolue-
ne:acetone 5:1) to give an additional 70 mg of pure acid 1. The
pure fractions were combined to give acid 1 (190 mg, 18%) as
a brown solid.
1H NMR (CDCl3): d=9.13 (d, 1H), 8.88 (s, 1H), 8.04–8.11 (m, 2H),
7.93–8.03 (m, 2H), 7.83 (t, 2H), 7.51 (dd, 2H), 7.18–7.29 (m, 3H),
6.68 ppm (q, 2H). 13C NMR (CDCl3): d=170.3, 132.9, 132.1, 131.6,
131.2, 129.9, 129.5, 129.3, 129.2, 128.9, 128.3, 128.0, 127.8, 127.7,
127.5, 127.4, 127.3, 126.3, 126.2, 125.3, 125.1, 123.1 ppm. [MÀ1]
(ESI) calculated: 371.12; found: 371.0.
1-(1-Methylphenanthren-4-yl)-2-naphthaldehyde (12): Suzuki
product 11 (6 g, 17.2 mmol) and 2,3-dichloro-5,6-dicyano-1,4-qui-
none (DDQ) (11.7 g, 51.7 mmol, 3 equiv) were heated at 1208C (ex-
ternal temperature) for 2 h in chlorobenzene (100 mL). After NMR
analysis revealed complete conversion, the mixture was filtered
over Celite and washed with toluene. The resulting mother liquor
was washed with 1m NaOH (3ꢁ300 mL) and the combined aque-
ous layers were washed with toluene (300 mL) and CHCl3 (300 mL,
2ꢁ150 mL). The combined organic layers were washed with water
(300 mL), dried over Na2SO4, and concentrated to give 6.1 g crude
28 as a black oil. The obtained material was purified by column
chromatography over 360 mL SiO2 (ethyl acetate:heptanes 1:9) to
give pure aromatized product 12 (3.3 g, 55%) as a yellow oil,
which solidified.
1H NMR (CDCl3): d=9.72 (s, 1H), 8.14 (t, 2H), 8.05 (d, 1H), 7.98 (d,
1H), 7.87 (d, 1H), 7.83 (d, 1H), 7.58 (m, 2H), 7.48 (d, 1H), 7.35 (d,
2H), 7.24–7.34 (m, 2H), 6.87 (m, 1H), 2.91 ppm (s, 3H). 13C NMR
(CDCl3): d=192.9, 150.1, 136.8, 136.2, 133.3, 132.6, 132.3, 132.2,
131.0, 130.8, 130.6, 130.3, 129.3, 129.0, 128.6, 128.5, 128.4, 127.9,
127.4, 127.3, 127.2, 126.4, 126.2, 123.3, 122.9, 20.9 ppm. [M+Na]
(API/ES) calculated: 369.13; found: 369.13.
CCDC 1457936 (15) contains the supplementary crystallographic
data for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
Acknowledgements
4-(2-Ethynylnaphthalen-1-yl)-1-methylphenanthrene (14): To al-
dehyde 12 (4.4 g, 12.7 mmol) in CH3OH (300 mL) was added K2CO3
(3.7 g, 26.7 mmol, 2.1 equiv), then dimethyl (1-diazo-2-oxopropyl)-
phosphonate 13 (3.2 g, 16.9 mmol, 1.33 equiv) and the resulting
mixture was stirred at room temperature overnight. After NMR
analysis revealed complete conversion, water (1 L), sat. NaHCO3
(500 mL), ethyl acetate (500 mL), and heptane (500 mL) were
added. The layers were separated and the aqueous layer was ex-
tracted with ethyl acetate (500 mL, 400 mL). The combined organic
layers were dried over Na2SO4 and concentrated and the residue
was filtered over SiO2 (DCM) to give 3.2 g crude 13 as a yellow
foam. This was combined with 700 mg crude 13 from another
batch and was purified over 250 mL SiO2 (toluene) to give 14
(3.1 g, 61%) as a light yellow solid.
The analytical department at Syncom BV is acknowledged for
carrying out the analyses. Ms. Tiemersma-Wegman of the
University of Groningen is acknowledged for the mass determi-
nation of 7-methyl[6]helicene 15. We are grateful to the
European Commission(Seventh Framework Program NMP4-SL-
2008-214340) for partial funding of this work. The Leuven
group thanks KU Leuven (GOA) and the Fund of Scientific Re-
search—Flanders.
Keywords: acid functionalization · cross-coupling · helicene ·
metal catalyzed cyclizations · scanning tunneling microscopy
1H NMR (CDCl3): d=8.09 (d, 1H), 7.91 (t, 2H), 7.83 (s, 1H), 7.80 (m,
1H), 7.72 (d, 1H), 7.56 (d, 1H), 7.41–7.47 (m, 2H), 7.31–7.37 (m,
2H), 7.15–7.28 (m, 2H), 6.89 (1H), 2.88 (s, 3H), 2.73 ppm (s, 1H).
13C NMR (CDCl3): d=147.3, 135.2, 134.5, 133.8, 132.9, 132.2, 131.1,
131.1, 130.0, 129.7, 128.5, 128.2, 127.7, 127.6, 127.1, 127.0, 126.8,
126.1, 125.9, 123.4, 119.183.5, 81.1, 20.9 ppm. [M+1] (API/ES) calcu-
lated: 343.14; found: 343.0
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Received: June 27, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 8
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FULL PAPER
A winding road: A synthesis of 7-car-
boxyhexahelicene 1 has been devel-
oped using a convergent cross-coupling
synthesis that provides the 7-methyl de-
rivative in good yield. The methyl group
proved to be remarkably recalcitrant to
oxidation to the carboxylic acid of 1.
Self-assembly of 1 from the liquid on
solid Au(111) provided monolayers of
well-ordered racemic two-dimensional
crystals.
&
Cross-Coupling
M. W. van der Meijden, T. Balandina,
O. Ivasenko, S. De Feyter, K. Wurst,
R. M. Kellogg*
&& – &&
Synthesis, Properties, and Two-
Dimensional Adsorption
Characteristics of [6]Hexahelicene-7-
carboxylic acid
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Chem. Eur. J. 2016, 22, 1 – 8
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!