A slippery slope: Mechanistic analysis of the intramolecular scholl reaction of hexaphenylbenzene

Article abstract of DOI:10.1021/ja046513d

DFT calculations support an arenium cation-based mechanism for the Scholl reaction converting hexaphenylbenzene to hexa-peri-benzocoronene. The curve connecting fully benzenoid intermediates on the potential energy diagram is convex. This "slippery slope" provides an explanation for the ease of this cascade Scholl reaction. The calculated reaction coordinate predicts that intermediates will not accumulate; this prediction is verified by experiment. Copyright

Full text of DOI:10.1021/ja046513d

Published on Web 10/28/2004
A Slippery Slope: Mechanistic Analysis of the Intramolecular Scholl Reaction
of Hexaphenylbenzene
Pawel Rempala, Ji rˇ ´ı Kroul ´ı k, and Benjamin T. King*
Department of Chemistry, UniVersity of NeVada, Reno, NeVada 89557
Received June 12, 2004; E-mail: king@chem.unr.edu
1
The Scholl reaction is the acid-catalyzed oxidative condensation
of aryl groups. In a series of papers,2 M u¨ llen and co-workers
have demonstrated that the intramolecular variation is useful for
the synthesis of large polycyclic aromatic hydrocarbons (PAHs)
from dendritic arene precursors. Many C-C bonds between
unfunctionalized aryl moieties can be formed; the current record
Scheme 1. Hexaphenylbenzene Condensation
-4
Scheme 2. Arenium Cation and Radical Cation o-Terphenyl
Condensation Pathways
4
is 126 bonds. The Scholl reaction also holds yet-unrealized promise
for the synthesis of carbon nanotube segments and has been used
to prepare hexaalkoxytriphenylenes from o-dialkoxybenzenes. The
Kovacic conditions, which use transition metal salts such as MoCl
5
6
7
5
,
2 3
CuCl , or FeCl , are popular. Recently, it was reported that, in the
course of the Scholl oxidation of hexaphenylbenzene (1), intermedi-
ate 2 was produced along with the expected product hexa-peri-
benzocoronene (3) (Scheme 1).3 While the identification of 2 is
important, little else is known about the mechanism of this reaction.
Some related reactions, like the oxidative coupling of phenols, are
,8
9
satisfactorily explained by radical processes. Other related reac-
On the basis of the o-terphenyl results, the arenium cation-based
mechanism was calculated for the conversion of 1 to 3. Energies
tions, like the oxidative coupling of aryl- or alkoxy-substituted
arenes, are proposed to occur via radical cations
cations.11
We present a computational12 and experimental study on the
mechanism of a prototypical intramolecular Scholl condensation,
the conversion of 1 to 3 (Scheme 1).
The o-terphenyl (4) model system (Scheme 2) was examined
first to determine which of the proposed11 mechanistic pathways,
arenium or radical cation based, is most likely.
Since the exact nature of the proton transfer and the oxidative
dehydrogenation transition states (TSs) is not obvious, these TSs
were not calculated. Instead, literature values were adopted. For
proton transfers, we used the average value of 4.0 kcal/mol
calculated for the hydronium ion/benzene system.13 For the oxida-
tions, we used the value of 6.8 kcal/mol measured for dehydroge-
nation of 9,10-dihydroanthracene by nitrobenzene.14
10,11
+
or arenium
are based on protonated 1 (1‚H ) as the acid and evolution of H .
2
Each C-C bond-forming step is consummated, restoring aromatic
stabilization, before the next begins. Aryl-aryl bonds are introduced
along the pathway having the least strained fully benzenoid inter-
mediates; this pathway happens to be contiguous. Since the Curtin-
Hammet principle17 is likely to apply, C-C bond-forming TSs re-
sulting from protonation at different sites were considered. Scheme
3 illustrates two paths from 12 to 13. In the first, ortho protonation
of a phenyl group results in arenium ion 14. In the second, the more
basic PAH core is protonated at the δ position, leading to 15.
The o-phenyl and δ-PAH protonation pathways, which intersect
at the fully aromatic points a
, and a ) 3, continue until fully condensed 3 is formed (Figure
2). The energies of the C-C bond-forming TSs in the δ-PAH series
1
) 1, a
2
) 12, a
3
) 13, a
4 5
, a ) 2,
a
6
7
δ
δ
(d
i i
) with respect to the most stable arenium ion (c ) decrease in
To compare the two reaction pathways, the same net energetic
outcome is required. We chose protonated o-terphenyl (B‚H ) as
the δ-PAH series, with the exception of the last step. The C-C
bond-forming TS energies of the o-phenyl pathway (d ) are greater
i
+
o
the acid, which corresponds to a catalytic amount of a strong Brøn-
sted acid in a weakly basic solvent. Hence, 4 f 5 is degenerate.
Since the radical cation pathway requires two individual one-elec-
tron oxidations and the arenium cation pathway requires the single-
than those of the δ-PAH pathway, indicating that the δ-PAH
protonation pathway is preferred.
It is noteworthy that consecutive condensation steps are increas-
ingly exergonic. Likewise, the energy of the dehydrogenation TSs
step formal loss of H
2
, the following schemes were applied: 4 +
h
i
relative to a
as it is estimated to lie 6.8 kcal/mol above the proceeding dihydro
compound g . The increasing exergonicity correlates reasonably with
i
also decreases in this series. This is not unexpected,
+
O f 6 + R; deprotonated 7 + 2O + 2B f 8 + 2R + 2 B‚H ,
where O is an one-electron oxidant, R is its reduced form, and B is
the conjugate base, o-terphenyl. In addition, the first step, 4 + O
f 6 + R, is set so ∆G ) 0, which will require a strong oxidant,
as 4 has high ionization potential (vacuum adiabatic, B3LYP/
i
increasing resonance energy per π electron18 (REPE) of growing
PAH core (3.43, 3.53, 3.55, 3.50, 3.63 kcal/mol for i ) 2, 3, 4, 5,
6, 7, respectively; cf. benzene, 3.34 kcal/mol). Since the curve
6
-31G(d), 7.22 eV; B3LYP/6-311+G(d,p)//B3LYP/6-31G(d), 7.53
i
connecting points a is convex, the reaction contour resembles a
slippery slope.
The Hammond postulate19 is manifest in this reaction. For both
pathways, the nascent CC bond is shortest in the least exothermic
first step (Table 1).
15
eV; photoelectron spectroscopy, 7.99 eV; this discrepancy is not
unusual ).
The trans C-C bond-forming TSs 9 and 10 are significantly
lower in energy than their cis counterparts. Both vacuum and
solvated data suggest that the arenium cation pathway is preferred
over the radical cation pathway (Figure 1).
16
The identity of the rate-determining TS is determined by the
strength of the catalytic acid. Stronger acids will stabilize the proto-
15002
9
J. AM. CHEM. SOC. 2004, 126, 15002-15003
10.1021/ja046513d CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
In our laboratories, oxidation of hexaphenylbenzene with sub-
stoichiometric oxidant (CuCl , PhI(O CCF ) , FeCl , or MoCl )
2 2 3 2 3 5
affords only 3 and unreacted 1 with nearly quantitative mass
balance. After separation of insoluble 3, HPLC/diode array UV
analysis of reaction mixtures revealed unreacted hexaphenylbenzene
containing small quantities of compounds exhibiting a UV spectrum
consistent with chlorinated 1, or, in one instance, a small quantity
of a compound exhibiting a UV spectrum consistent with a PAH.
These experiments are consistent with the calculated arenium cation
reaction profile.
Figure 1. Free energy diagrams (red, vacuum; blue, solvated) for arenium
cation and radical cation o-terphenyl condensation pathways.
The unexpectedly clean intramolecular hexacyclization of hexakis-
20
(
chloroacetamido)benzene also occurs without accumulation of
intermediates. This reaction was investigated by analogous sub-
stoichiometric mass balance experiments.
In summary, the intramolecular Scholl reaction of hexaphenyl-
benzene likely proceeds by protonation, electrophilic attack, depro-
tonation, and subsequent oxidation. C-C bonds are likely formed
stepwise and contiguously, with the first being formed the slowest.
The reaction is increasingly exergonic, possible due to increasing
REPE. It is likely that the utility of the Scholl reaction for the
formation of large PAHs arises from the slippery slope phenomenon.
Acknowledgment. This work was supported by an award from
the Research Corporation and by the University of Nevada.
Acknowledgment is made to the Donors of the American Chemical
Society Petroleum Research Fund for partial support of this research.
We thank Prof. S. H. Gellman for alerting us to ref 20.
Figure 2. Reaction coordinate diagram for 1 f 3. Labels: ai as shown; bi
protonation TS; ci arenium cation; di C-C bond formation TS; ei protonated
dihydro intermediate; fi deprotonation TS; gi neutral dihydro intermediate;
hi oxidation TS.
Supporting Information Available: Computational details, ener-
gies, coordinates, and experimental procedures. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 3. Hexaphenylbenzene Condensation
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