Oxidative, Iodoarene-Catalyzed Intramolecular Alkene Arylation for the Synthesis of Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1002/chem.201804786

A catalytic, metal-free and chemoselective oxidative intramolecular coupling of arene and alkene C?H bonds is reported. The active hypervalent iodine (HVI) reagent, generated catalytically in situ from iodotoluene and meta-chloroperoxybenzoic acid (m-CPBA), reacts with o-vinylbiphenyls to generate polyaromatic hydrocarbons in up to 95 % yield. Experimental evidence suggests the reactions proceed though vinyliodonium and, possibly, vinylenephenonium intermediates.

Full text of DOI:10.1002/chem.201804786

A Journal of
Accepted Article
Title: Oxidative, Iodoarene-Catalyzed Intramolecular Alkene Arylation
for the Synthesis of Polycyclic Aromatic Hydrocarbons
Authors: Zhensheng Zhao, Liam H. Britt, and Graham K. Murphy
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To be cited as: Chem. Eur. J. 10.1002/chem.201804786
Link to VoR: http://dx.doi.org/10.1002/chem.201804786
Supported by

10.1002/chem.201804786
Chemistry - A European Journal
COMMUNICATION
Oxidative, Iodoarene-Catalyzed Intramolecular Alkene Arylation
for the Synthesis of Polycyclic Aromatic Hydrocarbons
Zhensheng Zhao, Liam H. Britt and Graham K. Murphy*[a]
Dedicated to Dr. Fred. G. West, on the occasion of his 60th birthday.
Abstract: A catalytic, metal-free and chemoselective oxidative
A. Selected strategies for phenanthrene synthesis
intramolecular coupling of arene and alkene C-H bonds is reported.
1. Oxidative biaryl coupling:
3. Cycloisomerization
The active hypervalent iodine (HVI) reagent, generated catalytically in
situ from iodotoluene and m-CPBA, reacts with o-vinylbiphenyls to
generate polyaromatic hydrocarbons in up to 95% yield. Experimental
evidence suggests the reactions proceed though vinyliodonium and,
possibly, vinylenephenonium intermediates.
Al, Fe, Mo
Pd, Pt, Fe, Ag,
Au, Ga, In
Ti, Pb, Tl, Cu
PhI(OTFA)2
R
X
Ti, Zn, Cu
PhI(OTFA)2
Zr,V, Mo, W
• iodoiranium
•6-endo-trig
Al, Rh, Ru, Fe
Polycyclic aromatic hydrocarbons (PAHs), including
phenanthrene, chrysene and picene, are important organic
motifs[1] in natural products[2] and organic materials,[3] with
applications as optical photoswitches,[4] chiral ligands[5] and
myriad others. There are numerous approaches to the
phenanthrene scaffold,[6] such as forging the biaryl bond in cis-
stilbenes under metal-catalyzed[7] or photocyclization[8] conditions
(Figure 1a, 1); olefination via metal-mediated via McMurray
coupling,[9] carbene dimerization,[10] carbonyl-olefin metathesis[11]
or RCM (Figure 1a, 2);[12] and metal-catalyzed cycloisomerization
(Figure 1a, 3).[13] These metals can be highly effective as
stoichiometric reagents or catalysts in these reactions, but the
development of an efficient, catalytic and metal-free strategy for
PAH synthesis would be valuable.
EDG
Y
2. Olefination: McMurray coupling,
carbene dimerization, metathesis
4. Oxidative alkene arylation
B. This work: catalytic, oxidative alkene arylation
bond
formed
vinylene-
phenonium
R
R
R
cat. Tol-I
m-CPBA
R
R
R
R
R
R
1
2
Figure 1. A. Metal- and HVI-mediated phenanthrene synthesis. B. This work:
catalytic oxidative alkene arylation.
(Figure 1b). Numerous substitution patterns and functional groups
are tolerated, and experimental evidence suggests the reactions
proceed via vinyliodonium and vinylenephenonium intermediates.
Hypervalent iodine (HVI) reagents are viable surrogates for
metals in several such reactions; however, the mechanisms are
still described using terms such as reductive elimination, oxidative
addition, and ligand exchange. For example, PhI(OTFA)2 has
been used to effect the oxidative biaryl coupling reaction,[14] and
also in a new oxidative arene-alkene coupling, wherein an
iodoiranium ion is intercepted via a 6-endo-trig SEAr reaction
(Figure 1a, 4).[15] This latter discovery illustrates how HVI reagents,
as environmentally benign oxidants, can be used to form C-C
bonds that are typically associated with cross-coupling processes.
As the synthesis of ortho-vinylbiaryls is easier than many other
phenanthrene precursor types (see Figure 1a), and given the
inherent value of an effective, chemoselective, and metal-free
PAH synthesis, this reaction holds tremendous potential. The
modern frontier of HVI-mediated reactivity are processes catalytic
in aryl iodide,[16] wherein the iodane is (re)generated in situ by an
oxidant, though to date, very few of these enable C-C bond-
formation.[17] We report here the use of iodotoluene and m-CPBA
in the catalytic, oxidative alkene arylation for PAH synthesis
Our study began by subjecting ortho-vinylbiphenyl 1a,
prepared in two steps from 2-bromobenzaldehyde, to a mixture of
m-CPBA and p-iodotoluene in CH2Cl2, from which phenanthrene
2a was isolated in 30% yield (Table 1, entry 1). Heating and
testing other solvents led to a reaction in PhCl at 110 °C, which
gave 2a in 65% yield (entries 2-5; also see Tables SI1-SI5). The
loading of BF3•OEt2 was optimal at 30 mol%, giving 2a in 79%
yield (entries 5-7). Twelve iodoarene catalysts were tested,
including iodobenzene, p-iodoanisole and p-iodonitrobenzene
(entries 8-10), and while all were viable, p-iodotoluene was best.
Varying the loading of catalyst and m-CPBA showed 0.25 eq. and
1.5 eq. to be highest yielding, respectively (entries 11-14). Control
experiments showed m-CPBA to be crucial, and also that
conversion to 2a could occur even without the iodoarene (entries
15, 16), albeit in low yield.[18] Thus, an iodotoluene-catalyzed
synthesis of phenanthrene can be rapidly achieved in good yield,
as a highly chemoselective oxidative process.
Mono- and poly-substituted ortho-vinylbiphenyls (1a-1at)
were subjected to the optimized reaction conditions (Scheme 1).
b-Methyl derivative 1b, as a 3.3:1 mixture of cis:trans isomers,
gave 2b as the sole product in 75% yield. The related ethyl (1c),
iso-propyl (1d) and b-hydrocinnamyl (1e) derivatives, as cis:trans
mixtures, were subjected to the reaction, and gave products 2c-e
in 14% to 66% yield. The electron- rich b-methoxy derivative 1f
gave 2f in 53% yield; however, substitution of the vinyl moiety
[a]
Dr. Z. Zhao, Mr. Liam H. Britt and Prof. Dr. G.K Murphy
Department of Chemistry
University of Waterloo
200 University Ave. W., Waterloo, ON, Canada, N2L3G1
E-mail: graham.murphy@uwaterloo.ca
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201804786
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COMMUNICATION
Table 1. Reaction optimization for synthesis of 2a[a]
R
25 mol% p-Tol-I
1.5 eq. m-CPBA
R
cat. Ar-I
m-CPBA
0.3 eq. BF3•OEt2
PhCl, 110 °C
2 h
R
R
R
R
R
1a-1atb
2a - 2at
BF3•OEt2
solvent, T °C, 2 h
1a
2a
Me
R
m-CPBA
(eq.)
BF3•OEt2
(mol%)
T
(°C)
entry
Ar-I
solvent
yield
2n (X-ray)
2b, 95%c
Me Me
2a, R = H, 81%
2b, R = Me, 75%
2c, R = Et, 66%
2d, R = i-Pr, 57%
2e, R = (CH2)2Ph, 14%
2f, R = OMe, 53%
2g, R = Ac, 0%
1
2
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.2
2.0
1.5
1.5
–
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
Ph
20
20
20
20
20
10
30
30
30
30
30
30
30
30
20
20
CH2Cl2
CH2Cl2
DCE
rt
30%
45%
52%
45%
65%
54%
79%
63%
48%
70%
70%
60%
72%
81%
0%
40
2k, R = H, 71%d
2k, R = H, 45%e
2l, R = Me, 51%
3
84
2h, R = CO2Me, 0%
2j, 77%
2m, R = OMe, 51%
2n, R = Cl, 52%
2o, R = NO2, 0%
4
toluene
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
110
110
110
110
110
110
110
110
110
110
110
110
110
R
2p, R = Me, 47%f
2s, R = Cl, 40%f
5
R
Me
R
2q, R = Me, 68%
2t, R = Cl, 36%
2v, R = F, 66%
2w, R = OCF3, 59%
2x, R = NO2, 0%
2y, R = CN, 0%
2z, R = CF3, 0%
2r, R = Me, 86%
2u, R = Cl, 42%
2aa, R = Ph, 84%
6
F
2ac, 52%
7
Me
2ab, 83%
F
8
OMe
R
2ad, 46%
OMe
9
p-NO2-Ph
p-anisyl
p-tolyl
p-tolyl
p-tolyl [b]
p-tolyl [c]
p-tolyl
–
2af, R = Cl, 59%
2ag, R = OMe, 72%
2ah, R = OBn, 39%
Me
Ph
2ae, 73%
MeO
10
11
12
13
14
15
16
R
2ai, 42%
2ao, 24%
MeO
R
Me
2aj, R = Cl, 66%
2ak, R = OMe, 56%
2al, R = OBn, 28%
Cl
2am, R = OMe, 56%
2an, R = OBn, 45%
Cl
Me
Me
MeO
MeO
Me
R
2as, R = Me, 11%
2at, R = Ph, 10%
Me
2ap, R = Me, 46%
2aq, R = Ph, 49%
Me
2ar, 57%
1.5
35%
Scheme 1. Synthesis of substituted phenanthrenes.[a] [a] Conditions: 1a (0.1
mmol), 25 mol% p-Tol-I and 1.5 eq. m-CPBA in PhCl at 110 °C, add BF3•OEt2
(30 mol%) and stir for 2 h. Isolated yields. [b] Mix of cis:trans isomers, where
appropriate. [c] From b-Me 1i. [d] From cis-1f. [e] From trans-1f. [f] NMR yield
using hexamethyldisiloxane as internal standard.
[a] Conditions: 1a (0.1 mmol), Ar-I (20 mol%) and m-CPBA in solvent (1 mL)
at 110 °C, add BF3•OEt2 and stir for 2 h. Isolated yields. [b] 0.15 eq. p-Tol-
I. [c] 0.25 eq. p-Tol-I.
with electron-withdrawing groups (1g or 1h) proved unsuccessful.
The isomeric a-methyl (1i) and a,b-dimethyl (1j) substrates gave
phenanthrenes in 95% and 77% yield, respectively. The
separable b-phenyl isomers cis-1k and trans-1k allowed us to
observe a stereochemical effect on the reactivity, in that cis-1k
was converted to 2k in significantly higher yield (71%) than was
trans-1k (45%). Other b-aryl derivatives, such as p-tolyl (1l), p-
anisyl (1m) and p-chlorophenyl (1n), resulted in the 9-
arylphenanthrenes 2l-n in 51-52% yield, though the reaction
again failed with a strongly electron-deficient substrate (p-
nitrophenyl, 1o).
(1ac) or methoxy (1ad) substituents resulted in the corresponding
phenanthrenes 2ab-2ad in 46%-83% yield. Thus, the substituents
on the arene undergoing coupling needed not be strongly
electron-donating ,[15] and were only limited by very electron-
withdrawing groups. The effect of substitution on the remaining
arene was also investigated, and we found methyl- (1ae), chloro-
(1af), methoxy- (1ag), benzyloxy- (1ah) and dimethoxy (1ai)
substrates to all be tolerated, giving the corresponding 2-, 3- or
2,3-substituted phenanthrenes (2ae - 2ai) in 39%-73% yield.
Substrates with phenyl, methyl, methoxy, benzyloxy or chlorine
groups on both arenes gave either the 2,6- or 2,7-disubstituted
phenanthrenes 2aj-2aq in 28%-66% yield. Lastly, three tri-
substituted phenanthrenes were prepared with a combination of
either methyl, phenyl and methoxy substituents, and the products
(2ar-2at) were obtained in 10%-57% yield, which was significantly
lower than the related systems 2j and 2ai, which lacked the C7
substituent.
Various phenanthrene substitution patterns were achieved
using a combination of differing groups on the arenes and the
alkene. Substituting the arene undergoing oxidative coupling with
methyl or chlorine groups at positions ortho-, meta- and para- to
the biphenyl bond led to products 2p-2u in 36% – 86% yield, with
the low yields 2p and 2s attributed to the increased strain of ortho-
substitution in co-planar arenes. Substituents such as fluorine (2v,
66%) and -OCF3 (2w, 59%) and phenyl (2aa, 84%) were also
feasible, but strongly electron-poor substrates at this position (1x-
z) failed. Substrates bearing either two methyl (1ab), fluorine
We also targeted tetra- and pentaarene scaffolds, and found that
2-naphthyl-substituted styrene 1au could be converted to
chrysene (2au) in 42% yield. Two divinylterphenyl derivatives
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25 mol% p-Tol-I
1.5 eq. m-CPBA
(1av and 1aw) were prepared, and while both provided the
pentaarene products in similar yield, their major products differed.
Unsubstituted 1av gave benzo[k]tetraphene 2av as the major
isomer and picene 2av’ (X-ray) as the minor, whereas dimethyl-
terphenyl 1aw gave 3,10-dimethylpicene 2aw’ (X-ray) as the
major isomer. Though this differing reactivity is not understood,
these results suggest that in addition to restrictive substrate
design, substitution effects might be exploitable for influencing the
reaction outcome.
Me
Me
Me
0.3 eq. BF3•OEt2
PhCl, 110 °C
2 h
R
R
R
2ax
2ay
2az
2.1:1
2.5:1
2.7:1
2ax’ 88% yield
2ay’ 84% yield
2az’ 88% yield
1ax, R = Me
1ay, R = Ph
1az, R = Cl
25 mol% p-Tol-I
1.5 eq. m-CPBA
Me
Me
Me
0.3 eq. BF3•OEt2
PhCl, 110 °C
2 h
Me
Me
Me
1ba 3.5:1 cis:trans
2ax
3.0:1
2ax’ 45% yield
BF4
I-Ph
25 mol% p-Tol-I
1.5 eq. m-CPBA
30 mol% BF3•OEt2
PhCl, 110 °C, 2 h
Ref. 21
0.3 eq. BF3•OEt2
PhCl, 110 °C
3
2a, 90% yield
2 h
2au, 42% yield
1au
R
X
I-Tol
D
D
R
50 mol% p-Tol-I
3 eq. m-CPBA
D
30 mol% BF3•OEt2
PhCl, 110 °C, 2 h
Me
Me
R
+
0.6 eq. BF3•OEt2
PhCl, 110 °C
2 h
–
[10-d]-2r
50% (1H NMR)
4 X = BF4
X = H2O, HRMS: 430.0772
5 (or related isomers)
HRMS: 194.1084
R
R
2av
2aw
3.3:1
1:2.5
R
2av’ 50% yield
2aw’ 46% yield
1av, R = H
1aw, R = Me
Scheme 3. Control experiments as mechanism probes.
2av’
(X-ray)
2aw’
(X-ray)
toluene, which was activated by BF3•OEt2.[22] Attack by the
electron-rich olefin of 1ax gave benzylic cation A, whose
deprotonation led to vinyliodonium B.[23] We attempted to explain
the conversion of 1ax into 2ax’ by invoking an equilibration
between cationic intermediates
A and C (en route to
Scheme 2. Synthesis of tetra- and penta-arenes.
vinyliodonium D), but we could not rationalize the formation of
higher energy cation C from A. As substrates 1ax and 1ba each
led to 2ax and 2ax’ in ratios favouring 2ax, it is possible that their
respective, initially-formed vinyliodonium intermediates (B and D)
were converging into the common vinylenephenonium ion[24] E,
which is analogous to a phenyl-stabilized isomer of 5. The
reaction outcomes might be explained by the pendant arene then
attacking at either end of the cyclopropene, with the major product
deriving from attack at the least sterically-congested site.
Various control experiments were carried out to shed light on
a possible reaction mechanism. First, our experience with 1,2-
phenyl shifts[19] in HVI-mediated reactions compelled us to test for
them here. a-Methylvinyl substrates (1ax-az), bearing p-Me, p-Ph
or p-Cl groups, were prepared to permit observation of any
scrambling of the substituents’ relative positions. The products
were recovered in 84%-88% yield, and consisted of both 2,9- and
2,10-disubstituted phenanthrenes in 2.1-2.7:1 ratios. The b-
methylvinyl isomer 1ba (cis:trans 3.5:1) also gave two
phenanthrenes in 45% yield, as a 3.0:1 ratio, again favouring 2ax
over the anticipated major isomer 2ax’.[20] One possible
explanation for these consistent results involves conversion to
product via a common intermediate. We showed vinyliodonium
ions to be possible reaction intermediates by preparing 3 from 2-
ethynyl biphenyl[21] (see Supporting Information) and subjecting it
to the reaction, where 2a was recovered in 90% yield. Similarly,
vinyliodonium 4 was converted to [10-d]-2r in 50% NMR yield
without deuterium scrambling. Dissolving 4 in acetonitrile and
analysing by ESI-MS allowed us to observe and trap its water
Me
X
I
Me
H
X
I
1,2-phenyl
shift
BF3•OEt2
Tol
Me
Tol
H
Me
Ar
X = OBF3
Ar
Ar
Tol-I=O
-H+
X
1ax
A
C
Ar = p-Tol
Me
-H+
m-CPBA
I
X
Tol
Tol-I
I
Me
Tol
Ar
2ax
SNvinylic
D
-H+
Ar
B
E
E
1ba
2ax’
Scheme 4. Proposed reaction mechanism.
adduct (4, X=H2O), whose fragmentation gave
a
peak
corresponding to vinyl cation-type structures (eg. 5), or isomers
thereof. Attempts to characterize vinyliodonium or other
intermediates from the catalytic reaction, by either mass
spectrometry or crystallization, proved unsuccessful.
In conclusion, we report here the first synthesis of polycyclic
aromatic hydrocarbons via an iodoarene-catalyzed, oxidative C-
H coupling reaction between arene and alkene. This metal-free
reaction involved in situ oxidation of the iodotoluene catalyst by
m-CPBA, which reacted to generate vinyliodonium then
vinylenephenonium ions as intermediates, the latter of which was
trapped by the pendant arene. Products accessible from this
Given the control experiments, and the lack of reactivity with
strongly electron-poor substrates, we propose the following
mechanism, illustrated with isomers a-Me-1ax and b-Me-1ba
(Scheme 4). m-CPBA oxidation of iodotoluene gave iodoso-
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10.1002/chem.201804786
Chemistry - A European Journal
COMMUNICATION
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functional groups tolerated ranged from electron rich to
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iodoarene catalyzed C-C bond-forming reaction, and also
appears to be the first ever example of a C-C bond being formed
[9]
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a vinylenephenonium ion. Mechanistic and systematic
functional group tolerability studies, as well as further applications
of this methodology are currently underway, and will be disclosed
in due course.
[11] C. C. McAtee, P. S. Riehl, C. S. Schindler, J. Am. Chem. Soc. 2017, 139,
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Supporting Information
Optimization and experimental details, NMR characterization data and ion
trapping MS experimental data. CCDC 1829170 (2av’), 1829171 (2aw’)
and 1879172 (2n) contain the supplementary crystallographic data for this
paper. These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
[14] a) B. T. King, J. Kroulík, C. R. Robertson, P. Rempala, C. L. Hilton, J. D.
Korinek, L. M. Gortari, J. Org. Chem. 2007, 72, 2279; b) T. Dohi, M. Ito,
K. Morimoto, M. Iwata, Y. Kita, Angew. Chem., Int. Ed. 2008, 47, 1301;
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[17] For examples of such precesses see a) T. Dohi, A. Maruyama, M.
Yoshimura, K. Morimoto, H. Tohma, Y. Kita, Angew. Chem., Int. Ed. 2005,
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[18] Under strongly acidic conditions, an acid-catalyzed dehydrative
cyclization of epoxides can generate PAHs. The background reaction
herein may be an example of this. See a) C. K. Bradsher, J. Am. Chem.
Soc. 1939, 61, 3131; b) C. K. Bradsher, A. S. Burhans, J. Am. Chem.
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Acknowledgements
The Natural Sciences and Engineering Research Council
(NSERC) and the Early Researcher Award from the Province of
Ontario are acknowledged for financial support.
Keywords: hypervalent iodine • organocatalysis • polycyclic
aromatic hydrocarbons • vinyliodonium ion • vinylenephenonium
ion
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suggesting a difference in reaction mechanism. Equilibrating iodoiranium
intermediates (See Ref. 15), or even
a conjugate addition/a-
[4]
[5]
[6]
T. J. Wigglesworth, D. Sud, T. B. Norsten, V. S. Lekhi, N. R. Branda, J.
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10.1002/chem.201804786
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Zhensheng Zhao Liam H. Britt and
Graham K. Murphy*
Page No. – Page No.
Oxidative, Iodoarene-Catalyzed
Intramolecular Alkene Arylation for
the Synthesis of Polycyclic Aromatic
Hydrocarbons
Reacting a mixture of p-iodotoluene, m-CPBA and BF3•OEt2 with ortho-vinylbiphenyls
results in a metal-free, oxidative alkene arylation reaction. Various polycyclic
aromatic hydrocarbon scaffolds are accessible by this reaction, which appears to
proceed via the intermediacy of a vinylenephenonium ion intermediate.
R
R
R
X
R
R
X
I
cat. Tol-I
m-CPBA
R
R
Tol
R
R
BF3•OEt2
R
Tol-I
Ar
R
catalytic • metal-free • 52 examples • up to 95% yield • tris-, tetra- and pentaarenes
This article is protected by copyright. All rights reserved.