C-H arylation of triphenylene, naphthalene and related arenes using Pd/C

Article abstract of DOI:10.1039/c4sc03051f

A highly selective arylation of a number of polyaromatic hydrocarbons (PAHs) with aryliodonium salts and Pd/C as the only reagent is reported. The first C-H functionalization of triphenylene is explored, and proceeds at the most sterically hindered position. This non-chelate assisted C-H functionalization extends the reactivity profile of Pd/C and provides controlled access to π-extended PAHs, an important aspect of work towards the preparation of nanographenes. Mechanistic studies suggest in situ formation of catalytically active insoluble nanoparticles, and that the reaction likely proceeds via a Pd(0)/Pd(II) type reaction manifold.

Full text of DOI:10.1039/c4sc03051f

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C–H Arylation of Triphenylene, Naphthalene and
Related Arenes using Pd/C
Cite this: DOI:
10.1039/x0xx00000x
Karl D. Collins, Roman Honeker,‡ Suhelen VásquezꢀCéspedes,‡ DanꢀTam D.
Tang, and Frank Glorius*
Received 00th January 2012,
Accepted 00th January 2012
A highly selective arylation of a number of polyaromatic hydrocarbons (PAHs) with
aryliodonium salts and Pd/C as the only reagent is reported. The first C–H functionalization of
triphenylene is explored, and proceeds at the most sterically hindered position. This nonꢀ
chelate assisted C–H functionalization extends the reactivity profile of Pd/C and provides
controlled access to πꢀextended PAHs, an important aspect of work towards the preparation of
nanographenes. Mechanistic studies suggest in situ formation of catalytically active insoluble
nanoparticles, and that the reaction likely proceeds via a Pd(0)/Pd(II) type reaction manifold.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Pd/C has long been established as an efficient catalyst in
hydrogenation and crossꢀcoupling reactions.1 Although
supported catalysts are formally heterogeneous in nature,
studies of Pd/C suggest it typically acts as a reservoir of
homogeneous active catalytic species, following leaching of
palladium from the support into solution.1c,1e,2 Our recent work
identified that Pd/C can mediate highly selective direct C–H
functionalizations of heteroaromatic systems, representing
Previous work:
Oi and Inoue10
PdCl2 (5 mol%)
CuCl (4 equiv.)
Phenanthrene
Fluoranthene
mono-arylated
PAHS
ArSnCl3
DCE, 80
C
Itami
Pyrene7d
Phenanthrene7d
Benzo[a]-
anthracene7d
Fluoranthene11
Pd(OAc)2 (2.5 mol%)
o-chloranil (1 equiv.)
novel reactivity beyond its traditional applications.3
A
mono-arylated
PAHS
B
DCE, 80
C
surprising facet of these works was that studies suggested a
heterogeneous active catalytic species despite the high
temperature or strongly oxidizing nature of the reaction
conditions.4,5 Furthermore, Pd/C was clearly superior in terms
of regioselectivity and yield to all homogeneous palladium
sources screened, and no ligands were employed. Following
these unexpected observations, we were encouraged to explore
the more challenging selective C–H functionalization of nonꢀ
heteroatom containing arenes,6 with a view to unveiling further
surprising selectivity or reactions exploiting heterogeneous
catalytic species.
O
3
Sanford11
Ar
2 (5 mol%)
Ar
N
N Ar
[Ar2I]BF4
PhNO2
130
Pd
C
Cl Cl
2
Ar = 2,6-dichlorophenyl
1 5-20 equiv.
3
>98:2
Limitations: electron-rich coupling partners
not tolerated and reaction stoichiometry
This work:
Novel transformation:
The direct arylation of PAHs is an important and unmet
challenge for the ‘bottom up’ preparation of π
ꢀextended PAHs.7
H
H
Ar
[Ar2I]BF4
or
[ArI(TRIP)]BF4
Controlled access to such defined graphene like twoꢀ
dimensional sheets of sp2ꢀhybridized carbon atoms could have
significant implications for the preparation of larger graphene
structures,8 which are of immense interest to numerous
scientific fields.9 Despite the potential applications, only
limited precedent for the direct arylation of larger PAHs is
reported. Seminal work by Oi and Inoue reported an effective
Pd/C (2.5-5 mol%)
DME, 70-100
C
4
5
>96:4
Triphenylene
(+ other PAHs)
Improved transformation:
Ar
[Ar2I]BF4
or
[ArI(TRIP)]BF4
Pd/C (2.5 mol%)
arylation
of
phenanthrene
and
fluoranthene
with
aryltintrichlorides.10 One example of phenanthrene arylation
has also been reported by Shi.11 Important studies by the group
of Itami demonstrated a more general solution (Scheme 1)7
using prepared boroxines as coupling partners.
DME, 80
C
1 2 equiv.
3
>95:5
Electronicaly diverse coupling partners tolerated
Scheme 1. C–H functionalization of polyaromatic hydrocarbons (PAH). TRIP is
2,4,6-triisopropylphenyl.
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A [Ar2I]BF4
Pd/C (5 mol%)
polyaromatic
hydrocarbon
2 equiv.
mono-arylated products
or
100 C, DME
16 - 24 h
5-10
B [ArI(TRIP)]BF4
Br
Ar
Ph
Cl
NO2
C9
C1
7a-c
8
A Ar = Ph 72% >99:1b,c
A Ar = oMeC6H4 66%c
B Ar = pClC6H4 59%ca
A 51%, 90:10b,d,e
X-ray
5a
X-ray
5b
A 58%
5c
A 41%
5d
A 45%
6a
6b
B 49%
A 54%, C1:C2 80:20
A 65%
97:3
F
Cl
Ar
CO2Et
Ar
CO2Et
9a-d
A Ar = Ph 54%, 93:7b,d
A Ar = mCO2EtC6H4 44%f
A Ar = pBrC6H4 47%f
10a-b
A Ar = Ph 64%90:10b,c,e
A Ar = pFC6H4 47%c
5e
A 63%
5f
A 51%
5g
B 64%
6c
A 44%
Scheme 2. Arylation of larger polyaromatic hydrocarbons. Yields given are isolated yields for the major regioisomer unless noted otherwise. aConditions: anthracene
(5 equiv.), Pd/C (2.5 mol%), at 80 °C and
at 100 °C. bRatio of major regioisomer to any other regioisomers. cConditions: 80 °C, Pd/C (2.5 mol%). dConditions: 70 °C,
Pd/C (2.5 mol%). eYield is for major isomer only, though isolated as a mixture. f100 °C. TRIP is 2,4,6-triisopropylphenyl.
A
B
reported conditions3b identified traces of an arylated product.
ꢀchloroanil as oxidant, they have Following an initial solvent screen, we then explored the
described arylations of pyrene, phenanthrene and reactivity of larger PAHs and were very excited to observe a
benzo[ ). Optimization
]anthracene at the more reactive olefin like Kꢀregion7d highly selective arylation of triphenylene (
of these PAHs. Notably, the more challenging CꢀH arylation of established that 5 mol% of Pd/C in DME at 100°C gave 5a in
Using Pd(OAc)2 and
o
a
4
triphenylene (4), which does not contain a Kꢀregion is not
65% yield with excellent
α:β selectivity (>97:3), with no
reported. Selective coupling of fluoranthene12 and the
polyarylation of corannulenes13 and perylenes14 have also been
efforts made to exclude air or moisture. The suppression of
polyarylation in direct CꢀH functionalization of PAHs is still an
unmet challenge, and accounts in part for the remaining
material. Xꢀray studies confirmed that the arylation occurs at
demonstrated. C–H arylation of naphthalene (
explored more extensively, though typically proceeds with
moderate yields and poor
Sanford group employing a bulky Pd(II) diimine catalyst (
nitrobenzene at 130˚C represents the only highly selective
arylation of naphthalene ( ), and scope is limited to a small
1), has been
α:β
selectivity.15 A study from the the significantly more hindered position. The scope of the
2
) in reaction was explored and we were pleased to find this
challenging transformation proceeds effectively with a range of
functionalized diaryliodonium salts (Scheme 2). The diversity
of functionalities tolerated in the reaction provides multiple
options for further derivatization of these novel structures
αꢀ
1
number of electronꢀdeficient or electronꢀneutral aryliodonium
salts as coupling partners (Scheme 1).16,17
In this work we first present a Pd/C (Heraeus: 5 wt% Pd/C towards more complex
π
ꢀextended systems. The arylation of
type Kꢀ0219, <3% H2O) catalyzed arylation of triphenylene (4)
related
πꢀextended PAHs was also demonstrated: We observed
with aryliodonium salts.18 To the best of our knowledge this
represents both the first C–H functionalization of triphenylene,
an unprecedented selectivity for the arylation of anthracene at
the C1ꢀH position, as confirmed by Xꢀray crystallographic
analysis. The Kꢀregion7d of pyrene and phenanthrene is
and
a rare example of CꢀH functionalisation of nonꢀ
heteroaromatic systems using a simple heterogeneous catalyst
(Scheme 1).19 Notably, the reaction occurs with complete
selectivity at the most hindered position. The arylation of
several other PAHs is also demonstrated, including the hitherto
unknown direct C–H functionalization of anthracene at C1–H.20
This simple catalytic system also enabled a highly selective
preferentially arylated to give
7 and 8 respectively. The high
yields observed for the monoꢀarylation of pyrene when
compared to the literature precedent are particullary noteworthy
considering the value of these structures.7d Selective arylation
of the zigzag edge of both acenaphthene and fluoranthene to
give
9
and 10 respectively
, was also achieved. For the arylation
arylation (α:β >95:5) of naphthalene (1) with electronically of naphthalene (
1) a reduced catalyst loading of 2.5 mol% was
diverse diaryliodonium salts. Empirical data suggests the in situ employed, and reaction proceeded at 80 °C with no additional
generation of catalytically active insoluble nanoparticles from a reagents or ligands required to achieve consistently excellent
Pd/C precatalyst, and further mechanistic studies support a
Pd(0)/Pd(II) reaction manifold.
α
:
β
selectivities of >94:6 (Scheme 3). We observed
synthetically useful yields with only equivalents of
2
naphthalene, providing practical access to arylated products;
previous studies15 have typically employed 5–100 equivalents
of naphthalene to achieve acceptable yields, which significantly
impedes purification. If required, the yield of the reaction can
be further increased by using a greater excess of naphthalene.
Results and Discussion
Initially exploring the arylation of simple arenes using
Pd/C, we found subjecting naphthalene (1) to our previously
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Notably, this simple catalytic system enabled the reaction of
2) A hot filtration test indicated that all active catalytic
electronically diverse iodonium salts, with
unsymmetrical iodonium salts containing
o
ꢀsubstitution and species are removed by the filtration process, which is
cheap, nonꢀ consistent with a catalytically active heterogeneous species.
a
transferable arene (2,4,6ꢀtriisopropylphenyl) also well tolerated.
3) Several threeꢀphase tests using polymerꢀbound
naphthalene were undertaken, and in no instance was any
arylated product detected on the polymer support.22 Should
Pd/C act simply as a reservoir for homogeneous palladium,
reaction of the supported naphthalene would be expected.
Control experiments indicated that the support does not inhibit
reaction.
Ar
A [Ar2I]BF4
Pd/C (2.5 mol%)
or
DME, 80
16 h
C
B [ArI(TRIP)]BF4
1, 2 equiv.
3
F
A
While these methods for evaluating the nature of the active
catalyst have individual limitations, taken as a whole these
NO2
F
results are strongly indicative of
a catalytically active
heterogeneous component. Considering the high temperature
and oxidative nature of our reaction conditions it is probable
that palladium does leach from the support into solution,1c,1e,2
though does not generate catalytically active homogeneous
palladium species. These experiments provide no indication of
whether the heterogeneous species is bound to the carbon
support or not.
3b
3e
54%, 95:5 a
3a, 59%, 95:5
3c
51%, 95:5b
3d
53%, 95:5 2.5 mmola 60%, 96:4a
40%, 97:3b
75%, 94:6 (5 equiv. 1)a
OMe
B
CO2Et
Cl
Preliminary studies of the active catalytic species
For the reaction of naphthalene, kinetic studies indicated a
significant induction period and a sigmoidal reaction profile,23
which is consistent with the formation of the active catalytic
species in situ (Figure 1).
3a
57%, 95:5c
3f
54%, 95:5c
3g
40%, 96:4c
3h
48%, 97:3b
3i
3j
40%, 94:6b
45%, 95:5b
Scheme 3. Scope of naphthalene arylation. Yields given are isolated yields for the
major regioisomer. aGC yield. bReaction temperature 100 °C. TRIP is 2,4,6-
triisopropylphenyl.
A: Reaction kinetics for standard conditions
We have also investigated the reaction of a large number of
substituted naphthalenes, though unfortunately we observed either
no reactivity or poor selectivity (see supporting information for all
examples). Electronically distinct arylated naphthalenes 2a, 2f and 2i
were subjected to the reaction conditions a second time to identify
any electronic influence on the selectivity of a second arylation,
though all substrates yielded a near identical distribution (7:3) of bisꢀ
arylated products. Despite significant effort, we were unable to
isolate bisꢀarylated triphenylene or anthracene structures: monoꢀ
arylated compounds were subjected to the reaction conditions for a
second time resulting in conversion of the starting materials, though
gave intractable mixtures of compounds.
80
60
40
20
0
0
2
4
6
8
10
12
14
Reaction time h
B: Reaction kinetics with pre-activated catalyst
60
40
20
0
Heterogeneity studies
Our preliminary experiments in this study were indicative of a
reaction mechanism distinct to that of our previously reported
C–H functionalization of heteroaromatic systems using Pd/C
and aryliodonium salts.3b Consequently, investigation of the
heteroꢀ or homogeneous nature of the active catalytic species
and the reaction mechanism were undertaken to gain further
insight. Studying the arylation of triphenylene
(4) and
0
2
4
6
8
10
naphthalene ( ) with Ph2IBF4 we investigated the nature of the
1
active catalytic species. A heterogeneous catalytic species was
Reaction time h
indicated by several established methods:1d,4,21
Figure 1. Reaction profile for the reaction of naphthalene (
1) and Ph2IBF4: A
1) A standard Hgꢀpoisoning test resulted in inhibition of the
reaction, suggesting the active catalytic species is likely to be
heterogeneous in nature.
Standard reaction conditions and B After pre-activation of the catalyst with PIDA.
Leaching of palladium into solution and formation of
homogeneous active catalytic species could account for this
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observation, though is contrary to our studies. Consequently,
we investigated the potential origins of this induction period.
A more extensive catalyst screening was undertaken,
importantly highlighting significant differences in the reactivity
We found that pretreatment of the catalyst under the of several commercial Pd/C sources. Interestingly, we found
standard reaction conditions for 2 h with diaryliodonium salts that in addition to Pd/C and Pd(OAc)2, Pd/Al2O3 proved a
or diacetoxyiodobenzene (PIDA) prior to introduction of proficient catalyst for arylation, and we subsequently undertook
naphthalene, resulted in a significant reduction of the induction a short comparison of all three systems (Table 2).
period. We hypothesized that pretreatment of the catalyst with
Table 2. Comparative study of catalysts for the arylation of naphthalene
these strong oxidants may result in oxidation of Pd(0) to Pd(II)
and initiation of a Pd(II)/Pd(IV) catalytic cycle as proposed by
Sanford.13a Alternatively, a physical modification of Pd/C
resulting in the formation of the active catalyst may also
account for the induction period. To investigate the likelihood
of a Pd(II)/Pd(IV) catalytic cycle we investigated the reactivity
of six homogeneous or heterogeneous Pd(II) sources for the
arylation of naphthalene (Table 1).
b
Pd/C
het
het
63%
75%
Pd(OAc)2
het
Pd/Al2O3
het
het
64%
79%
Hg poisoning
Hot filtration test
Standard reaction
5 equiv 1
het
52%
50%
Table 1. Screening of Pd(II) catalysts for reactivity
Reaction with
MeOPhI(TRIP)]BF4
Reaction with
54%
60%
30%
nd
trace
trace
[
p
α
:
βa
Yield %a selectivity
[oꢀMePh2I]BF4
Catalyst
K2PdCl4
PdCl2(dppe)
PdCl2(PPh3)3
Pd(OH)2/Cb
Pd(OH)2/Fe3O4
8
3
9
2
12
95:5
82:18
96:4
nd
Standard conditions: Naphthalene (1) (2 equiv.), iodonium salt (1
equiv.), catalyst (2.5 mol%), DME (0.1 M), 80 °C, 16 h. Yields given
are isolated yields for the major regioisomer unless noted otherwise. het
indicates that the results suggest a heterogeneous active catalytic
species. aGC yield. b5 mol% of catalyst used.
b
96:4
Pd(OAc)2
52
93:7
For all catalysts Hg poisoning and hot filtration studies indicate
a heterogeneous active catalytic species, supporting the earlier
hypothesis that Pd(OAc)2 is reduced to Pd(0) nanoparticles
Standard conditions: Naphthalene (0.2 mmol), Ph2IBF4 (0.5 eq),
Catalyst 5.0 mol%, DME (1 mL), 80 °C, 16 h. aDetermined by GCꢀFID
b2.5 mol% catalyst. nd is not determined.
under the reaction conditions. When
5 equivalents of
naphthalene are used, both supported systems give a higher
yield than in the standard reaction, but with Pd(OAc)2 this is not
the case. This indicates that the presence of a support is not
crucial for reactivity but it appears to increase the turnover
number of the catalyst. Finally, although Pd/Al2O3 proved
highly proficient for the standard reaction, only trace reactivity
was observed with electronꢀrich or orthoꢀsubstituted arenes. It
would appear that while several palladium sources can mediate
the basic transformation, Pd/C is typically superior.
With the exception of Pd(OAc)2 only negligible reactivity was
identified, which does not support that the catalytic cycle is
initiated by a Pd(II) species. The reactivity observed with
Pd(OAc)2 can readily be explained by the wellꢀestablished
24
thermal reduction of Pd(OAc)2 to form catalytically active
heterogeneous Pd(0) nanoparticles. Kinetic studies employing
Pd(OAc)2 as the catalyst demonstrated an induction period
consistent with the generation of nanoparticles, and the
formation of heterogeneous material could be observed
visually. Further comparative studies (vide infra) support this
hypothesis. A preliminary analysis of the results presented so
Reaction mechanism
While postulating traditional ‘homogeneous’ type reaction
far indicates that the activation of the catalyst with PIDA or an mechanisms for processes involving nanoparticles is unlikely to
be reflective of the true processes, we sought we investigate the
feasibility of a number of reaction pathways:
iodonium salt is unlikely to be due to an oxidation of Pd(0) to
Pd(II)/Pd(IV). The formation of a new heterogeneous Pd(0)
species following leaching of Pd into solution25 possibly via an
Ostwald ripening mechanism,26 or catalyst modification
without leaching of the metal could account for the
observations reported. The reaction is shown to be first order in
palladium, and a reduced induction period with increased
catalyst loading is also observed. These results are consistent
with the formation of the active catalyst being rateꢀlimiting.
1) Undirected
C–H
activation
of
naphthalene/triphenylene.6y
2) Oxidation of Pd(0) to Pd(II) and initiation of
Pd(II)/Pd(IV) reaction manifold.11
3) Radical reaction pathways.13a
4) Insertion of an ArPd(II)X species.7d
5) Electrophilic attack of naphthalene/triphenylene
(SEAr/electrophilic palladation).7d,27
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We immediately discounted a mechanism that proceeds initially
Radical coupling pathways have been proposed by
via an undirected C–H activation. Firstly, an unusual inverse Rodriguez for related reactions (Figure 3).15a In our studies we
kinetic isotope effect (KIE) of 0.54 was determined by parallel have found that the reaction is inhibited by TEMPO, BHT and
initial rate studies of naphthalene and d8ꢀnaphthalene, ruling out BQ which is consistent with a radical component within the
a rateꢀlimiting C–H activation (Scheme 4).28
reaction mechanism (Figure 3). Identification of a PhꢀBHT
adduct further supports this proposition, and suggests that an
aryl radical is derived from the iodonium salt. We feel that
Ph
direct addition of aryl radicals to naphthalene (Figure 2,
A) is
Pd/C
DME
Ph2IBF4
Ph2IBF4
unlikely in this instance, as this addition is well precedented to
occur with regioselectivites dependent on the electronic nature
of the aryl radical,15a,29 something we do not observe. The
formation of PAH radical species via hydrogen atom
parallel
experiments
1
1
Ph
kH/kD=0.54
Pd/C
DME
abstraction (Figure 1,
B) is also unlikely when bond
1-d8
2-d7
dissociation energies (BDE) and/or energetics of the radical
formation are considered.30 Firstly, the C–H BDE of benzene is
highly comparable to that of naphthalene and therefore would
be likely to react under these reaction conditions.30a,b Under our
conditions several monoarenes gave only trace reactivity.
Secondly, as selectivity of the hydrogen atom abstraction will
be governed by the BDE/stability of the formed radical,
arylation of anthracene would be expected at the bisꢀbenzylic
C9ꢀH in contrast to the C1ꢀH selectivity observed in our study
D
H
Activated Pd/C
MeOD
Deuterium
incorporation
0%
DME
100 C, 24 h
4
5
2
D
H
Activated Pd/C
MeOD
Deuterium
incorporation
0%
DME
100 C, 24 h
(Scheme 2).30a,c,d The relative BDE for the
α and β CꢀH bonds
1
in naphthalene are still debated, though are very similar and
selectivity on this basis seems unlikely.30 Consequently we
propose should a radical process be in operation, palladium is
nevertheless involved in the bond forming step.
H
Ph/Ar
Activated
Pd/C
Ph2IBF4
(1 equiv.)
DME
Ar2IBF4
(1 equiv.)
Ph = 8%, 2 h; 19%, 4 h
Ar = 3%, 2 h; 10%, 4 h
Ar = m-EtCO2Ph
Scheme 4. Determination of KIE, deuteration and competition experiments.
Secondly, the reaction is shown to be zeroꢀorder in naphthalene
precluding its involvement in the rateꢀdetermining step. Finally,
deuteration experiments employing preactivated catalyst
indicated no deuterium incorporation into naphthalene or
triphenylene, suggesting that a Pd(0) catalysed C–H activation
is unlikely. We next considered a Pd(II)/Pd(IV) manifold for
the arylation of naphthalene (
1) with diaryliodonium salts as
proposed by Sanford13 (Figure 2).
Figure 3. Previously proposed radical pathways for the arylation of naphthalene
and radical inhibitor studies.
Figure 2. Pd(II)/Pd(IV) reaction manifold proposed by Sanford.
Based on additional experiments and the data presented so
far,
a speculative mechanism using naphthalene as a
representative substrate is proposed (Figure 4). As previously
noted, it is important to recognize the limitations in attributing
homogeneous type reaction mechanisms to heterogeneous
processes. We hypothesize that Pd/C acts as a metal reservoir,
and following leaching of Pd from the support, catalytically
Sanford proposed a rateꢀdetermining oxidation of Pd(II) to
Pd(IV) accounts for the failed reaction of electronꢀrich
iodonium salts. In contrast to her report, our studies show facile
reaction of electronꢀrich iodonium salts, with electronꢀdeficient
coupling partners often requiring higher temperatures. A
competition experiment also demonstrated more electronꢀrich
iodonium salts react more rapidly, indicating a rateꢀlimiting
oxidation of palladium is unlikely (Scheme 4). Furthermore,
our described catalyst activation studies suggested entry into a
Pd(II)/Pd(IV) catalytic cycle was unlikely. An alternative rateꢀ
limiting reductive elimination form Pd(IV) is also improbable.
proficient insoluble nanoparticles
A are formed. While we
cannot rule out modification of the catalyst without leaching of
palladium, the product formation when employing Pd(OAc)2
indicates that the support is not essential for reactivity.
Favorable interactions between extended
nanoparticles suggest saturation of the surface to give
π
ꢀsystems and
is
B
likely, particularly in the presence of a large excess of
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naphthalene
(1). Experimental evidence supports this release of a reactive phenyl species, helping to minimise the
hypothesis: under the standard reaction conditions PhI is formation of biphenyl. Unlike in the reaction of only Ph2IBF4
generated throughout the duration of the reaction in proportion and Pd/C (Figure 3), trace amounts of biphenyl are formed
to product formation. In contrast, when Ph2IBF4 is treated with under the standard reaction conditions, suggesting a low
catalytic Pd/C in the absence of the arene, 80% yield of PhI is effective concentration of the reactive phenyl component.
identified after only 2 h (Figure 4). This result suggests that Following formation of an ArꢀPd(II) species
naphthalene inhibits the direct reaction of the diaryliodonium electrophilic palladation followed by reꢀaromatisation and
salt and Pd. That the reaction is shown to be zero order in reductive elimination, or an insertion and subsequent ꢀhydride
B, a facile
β
naphthalene is also consistent with saturation type kinetics elimination would provide access to the product. Similar
associated with heterogeneous catalysis in the presence of a mechanisms have been proposed by Itami for the arylation of
large excess of reagent (pseudo first order conditions).31
pyrene.7d
The creation of a vacant coꢀordination via reversible
The selective transfer of the least sterically hindered arene
dissociation of naphthalene to give species
C, would then when employing unsymmetrical diaryliodonium salts is
enable reaction with the iodonium salt. Single electron consistent with an electrophilic arylation using transition metal
reduction of the iodonium salt results in a controlled release of catalysis,27 though does not rule out an insertion pathway.
a phenyl radical, and recombination with Pd (I) would generate Alternatively, the reactivity of naphthalene in contrast to simple
a formal Pd(II) cationic species
D
.
monoꢀarenes could be accounted for when considering an
insertion pathway. It is reported that naphthalene has a greater
propensity than simple benzenes to form η2ꢀcomplexes with
metals, a prerequisite for activation of the arene towards
insertion.34 Finally, as the reaction is shown to be zero order in
naphthalene, neither palladation process would be rateꢀlimiting,
though the unusual inverse KIE is yet to be accounted for.
Although the fundamental origin is unclear, Perin has
demonstrated decreasing acidity of benzoic acids and phenols
for deuterated species relative to the corresponding proteated
compounds.35 This would suggest an effect equivalent to
increasing the electronꢀdensity at the oxygen atom.
Consequently, a slightly faster reductive elimination or
βꢀ
hydride elimination when employing d8 due to the cumulative
1
ꢀ
effect of several deuterium atoms increasing the electronꢀ
denisty at palladium could account for the unusual KIE, should
this be the rateꢀlimiting step of the catalytic cycle. Inverse KIEs
for reductive eliminations are reported.36
Conclusions
We have reported the first general C–H arylation of
triphenylene. This reaction proved applicable to several other
PAHs and naphthalene, and without recourse to ligands or other
additives provides a practical and highly selective arylation
using electronically and sterically diverse aryliodonium salts.
Mechanistic studies are consistent with the formation of
insoluble nanoparticles following leaching of Pd into solution
from Pd/C. The reaction has been demonstrated as likely to
proceed via a Pd(0)/Pd(II) manifold rather than the commonly
proposed Pd(II)/Pd(IV) often cited for similar transformations.
Acknowledgements
‡Authors contributed equally. Generous financial support by the
Deutsche Forschungsgemeinschaft (Leibniz award; SFB 858)
and the Fonds der Chemischen Industrie (R.H.) is gratefully
acknowledged. We thank Steffen Greßies for preliminary
experiments and Theresa Olyschläger for experimental support.
We also thank greatly Constantine G. Daniliuc for Xꢀray
crystallographic analysis.
Figure 3. Proposed reaction mechanisms for the arylation of PAHs. Naphthalene
is used as a representative substrate.
Single electron processes involving hypervalent iodine (III)
reagents are well established32 and the ability of Pd to partake
in single electron processes is widely discussed.33 Furthermore,
our mechanistic studies are consistent with the formation of an
aryl radical, though no evidence for a free radical in solution
has been established. A 2ꢀelectron oxidative addition process to
Notes and references
a Dr. K. D. Collins, R. Honeker, S. VásquezꢀCéspedes, Dr. D.ꢀT. D. Tang,
and Prof. Dr. F. Glorius.
give
B directly from Pd(0) and the iodonium salt cannot be
ruled out. As discussed, the presence of polyaromatic species
appears to inhibit the reaction of diaryliodonium salt and Pd; an
important consequence of this appears to be the controlled
Westfälische WilhelmsꢀUniversität Münster
OrganischꢀChemisches Institut
6 | J. Name., 2012, 00, 1-3
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ARTICLE
DOI: 10.1039/C4SC03051F
Mochida, K. Kawasumi, Y. Segawa, K. Itami, J. Am. Chem. Soc.
,
Corrensstraße 40, 48149 Münster, Germany
Eꢀmail: glorius@uniꢀmuenster.de
2011, 133, 10716. The Kꢀregion is defined by Itami as an exposed
outer πꢀbond of PAHs. Although aromatic, these outer πꢀbonds
possess partial olefinic character and are less stabilized, and therefore
more reactive, than the other πꢀbonds within the PAH.
Electronic Supplementary Information (ESI) available: details of any
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This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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Page 8 of 8
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C4SC03051F
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8 | J. Name., 2012, 00, 1-3
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