Metal-free hydrogenation catalysis of polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1039/c2cc37190a

The frustrated Lewis pair, B(C₆F₅) ₃/Ph₂PC₆F₅, acts as an efficient catalyst for the hydrogenation of the polycyclic hydrocarbons including anthracene derivatives, tetracene and tetraphene, at 80 °C and 100 atm H₂ pressure via a mechanism involving protonation of polyaromatic species followed by hydride transfer. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc37190a

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COMMUNICATION
Metal-free hydrogenation catalysis of polycyclic aromatic hydrocarbonsw
a
b
Yasutomo Segawa and Douglas W. Stephan*
Received 2nd October 2012, Accepted 25th October 2012
DOI: 10.1039/c2cc37190a
The frustrated Lewis pair, B(C F ) /Ph PC F , acts as an efficient
2
catalytic reduction. In an effort to develop an FLP-based catalytic
approach to aromatic reductions, we have explored the reduction
of polycyclic aromatic hydrocarbons (PAHs) using FLP catalysts
based on weakly basic phosphines. In this communication, we
demonstrate that several anthracene derivatives, tetracene and
tetraphene are readily catalytically hydrogenated to the partially
reduced products using this metal-free strategy.
6
5 3
6 5
catalyst for the hydrogenation of the polycyclic hydrocarbons
including anthracene derivatives, tetracene and tetraphene, at
8
0 8C and 100 atm H2 pressure via a mechanism involving
protonation of polyaromatic species followed by hydride transfer.
Polycyclic aromatic hydrocarbons (PAHs) are potent atmospheric
1
pollutants generated by fuel combustion but they also occur
The hydrogenation of anthracene (1) using H
using 10 mol% of an FLP catalyst and 102 atm H
80 1C. Initially, a series of phosphines were used in combination
with B(C as the catalyst. Use of the phosphines Ph P, tBu
or 1,8-bis(PPh )(C H ) showed no catalytic reduction. In
2
was explored
2
pressure at
naturally in oil, coal, and tar deposits as well as in comets and
2
meteorites. While these compounds are found in grilled meats or
6
F
5
)
3
3
3
P
smoked fish and are known to exhibit carcinogenic, mutagenic,
3–8
and teratogenic effects, these polyaromatic compounds are also
2
10
6
9
10
finding applications in energy storage, photosensitizers for solar
contrast, the hydrogenated product, 9,10-dihydroanthracene
(2), was obtained in 16% and 3% yields, respectively, when the
catalyst employed either PhP(C F ) or P(C F ) , respectively,
1
1
cells and electronic devices. Partial hydrogenation of PAHs has
1
4
2–17
been achieved by employing Birch reductions, LiAlH
18–22
transition metal-based catalysts
or
6
5 2
6 5 3
as the base component of the FLP. However, use of Ph
2 6 5
PC F
and this can be exploited as
23
6 5 3
and B(C F ) as the FLP catalyst resulted in the isolation of 2
a strategy for ‘‘protection’’. Nonetheless selective avenues to
partially hydrogenated PAHs are still rare.
in 97% yield in 10 hours at 80 1C and 102 atm H
2
(Table 1).
A comparatively new strategy for hydrogenation involves
the use of frustrated Lewis pairs (FLPs). These are combinations
of Lewis acids and bases in which steric demands inhibit wholly
a
Table 1 FLP hydrogenation of PAHs
Substrate
Time (h) Product
Yield (%)
97
24
or partially the formation of a Lewis acid–base adduct. This
unquenched acidity and basicity affords the ability of such
combinations to activate a variety of small molecules including
1
1
0
0
25,26
dihydrogen (H2).
The latter finding has resulted in a number
of reports on FLP systems for catalytic hydrogenation of polar
27–38
double bonds in imines, enamines, or silyl enol ethers.
More
95
recently, we reported stoichiometric aromatic hydrogenations of
3
9
anilines to cyclohexylamines using B(C
6
F
5
)
3
and H
2
.
These
of the
reactions are terminated by the heterolytic splitting of H
2
resulting alkyl-amine in combination with borane, yielding the
40
corresponding cyclohexylammonium salt. Also recently we
b
80
have reported the catalytic hydrogenation of 1,1-disubstituted
olefins by FLP catalysts. The key discovery in the latter work
was that weak Lewis basic partners (Ph PC F , P(2,6-Cl C H ) ,
48
2
6
5
2
6
3 3
P(1-naphthyl) , or MeN(p-tol) ) yielded sufficiently acidic salts
3
2
to protonate olefins prompting hydride transfer and affording
1
0
90
30
a
Department of Chemistry, Graduate School of Science,
Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
b
Department of Chemistry, University of Toronto, 80 St George St,
Toronto, Ontario M5S3H6, Canada.
48
E-mail: dstephan@chem.utoronto.ca; Tel: +1-416-946-3294
w Electronic supplementary information (ESI) available: Experimental
procedures and details of X-ray crystallography and computational
study. CCDC 900243. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc37190a
a
Reactions in C
b
2
H
2
Cl
2
2 6 5 6 5 3
, 10 mol% Ph PC F /B(C F ) , 80 1C, 102 atm
H
2
.
Cis/trans = 75 : 25.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11963–11965 11963

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2 6 5
This successful reduction suggests that Ph PC F provides the
optimum balance between steric bulk and Lewis basicity for
FLP hydrogenation catalysis. In this regard, it is noteworthy
that the same combination of B(C F ) and Ph PC F was
6
5 3
2
6 5
previously employed to catalytically reduce 1,1-disubstituted
0
4
ethylenes.
Employing Ph
2 6 5 6 5 3
PC F /B(C F ) as the catalyst and conditions
analogous to those described above, the catalytic hydrogenation
of alkyl- or aryl-substituted anthracenes was examined. 9-Methyl-
anthracene (3) was hydrogenated to 9-methyl-9,10-dihydro-
anthracene (4) in 95% yield in 10 hours whereas the disubstituted
anthracene, 9,10-dimethylanthracene (5), reacted more slowly,
yielding the cis and trans isomers of 9,10-dimethyl-9,10-dihydro-
anthracene (6) in a 3 : 1 ratio in 80% yield after 48 hours
(Table 1). The PAHs tetracene (benz[b]anthracene, 7) and tetra-
phene (benz[a]anthracene, 9) were hydrogenated to afford 5,12-
dihydrotetracene (8) and 7,12-dihydrotetraphene (10) in 90% and
30% yields after 10 and 48 hours, respectively (Table 1). Other
Fig. 2 Proposed mechanism for FLP-catalyzed hydrogenation of
PAHs such as 9-phenylanthracene and 9,10-diphenylanthracene,
naphthalene, pyrene, phenanthrene and perylene were not
reduced under these conditions, suggesting that only PAHs
containing sterically unencumbered anthracene moieties could
be reduced.
anthracene.
formation of 6 results from the steric congestion inhibiting
protonation at the 10-position. Similarly the observation that
phenyl substituted anthracenes were unreactive is consistent
with both the elevated steric congestion and the expected
increased acidity of the purported intermediate cation.
Further insight concerning this reactivity of PAHs was pursued
The nature of the products was confirmed by comparison of
NMR spectra with previously reported data (see ESIw) and in
the case of product 8, X-ray data confirmed the formulation
4
2
(
Fig. 1). The structural data demonstrate a bent conformation
via theoretical calculation performed using Gaussian 09 using
43,44
the B3LYP6-31G(d)
with a bent angle of 1361. Presumably it is similar features that
account for the improved solubility of the hydrogenated
products in organic solvents.
basis set. Heats of hydrogenation (DH)
of PAH derivatives were calculated (see ESIw). Hydrogenation of
4
5
anthracene at 9,10-positions is known to be exothermic. Methyl
groups at the 9- and 10-positions have little impact on the heat
of hydrogenation although the formation of the trans-isomer
of 6 is calculated to be slightly more exothermic (DDH =
We have previously shown that Ph
form an FLP and that Ph PC is sufficiently basic to
participate in the FLP activation of H in combination with
At the same time, the resulting phosphonium
cation was shown to be sufficiently acidic to deliver protons
2 6 5 6 5 3
PC F and B(C F )
2
6 5
F
2
4
.
0
À1
B(C
6
F
5
)
3
1.2 kcal mol ). The experimentally observed preference for
the cis-isomer of 6 indicates that hydride delivery, the last
4
0
to 1,1-disubstituted olefins. In the present reductions, this suggests
a proposed mechanism in which anthracene and related PAH
substrates are protonated following FLP heterolysis of H2
affording the phosphonium borate salt [Ph PHC F ][HB(C F ) ].
step in the proposed mechanism, is not reversible. Phenyl
À1
substituents reduce the exothermicity to ca. 6 kcal mol
.
This is presumably associated with the loss of conjugation
between the anthracene ring and the phenyl group as well as
the impact of steric interactions on reduction. The heat of
2
6
5
6 5 3
Subsequent protonation of the anthracene followed by irreversible
hydride delivery affords the reduced product and regenerates the
FLP for catalysis (Fig. 2). This proposition is also consistent with
the observation that FLPs comprised of more basic phosphines
À1
hydrogenation of 7 (À17.2 kcal mol ) is higher than that of
À1
the anthracene derivatives, while that of 9 (À7.3 kcal mol ) is
lower. These values parallel the yields of the hydrogenated
products suggesting that the reductions are thermodynami-
cally driven.
6 5 3
and B(C F ) fails to effect catalytic reduction even though
these FLPs are known to effect reduction of polar substrates.
The reactivity of the present catalyst system showed an
apparent selectivity for anthracene fragments and no reactivity
with naphthalene, consistent with the lower computed pKa
value of protonated anthracene (À13.3) compared to that of
In conclusion, we have discovered the first metal-free catalytic
hydrogenation of aromatic systems. Specifically, PAHs including
anthracene derivatives, tetracene and tetraphene are reduced to
the corresponding dihydro-derivatives using the FLP catalyst
derived from the combination of B(C F ) and Ph PC F .
4
1
protonated naphthalene (À20.4). Presumably, the slower
6
5 3
2
6 5
The proposed mechanism involves initial protonation with
subsequent hydride transfer affording the dihydro-products.
While the present catalyst system appears to be selective for
PAHs containing anthracene fragments, current mechanistic
and catalyst studies are probing catalyst modifications with
the view to the production of desirable products via aromatic
reductions.
The authors thank NSERC of Canada for financial support.
DWS is grateful for the award of a Canada Research Chair.
Fig. 1 ORTEP drawing of 8 with 50% thermal ellipsoids.
1
1964 Chem. Commun., 2012, 48, 11963–11965
This journal is c The Royal Society of Chemistry 2012

View Online
YS is grateful for the JSPS Strategic Young Researcher
Overseas Visits Program for Accelerating Brain Circulation.
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