A facile palladium-mediated contraction of benzene to cyclopentadiene: Transformations of palladium(II) p-benziporphyrin

Article abstract of DOI:10.1002/anie.201102218

Feeling the contractions: Addition of palladium(II) and a hydroxide ion to a C-C double bond, β elimination, and competing cheletropic extrusion of carbon oxide and 1,2-hydride shift reactions lead to the contraction of p-phenylene to form a cyclopentadie

Full text of DOI:10.1002/anie.201102218

DOI: 10.1002/anie.201102218
Porphyrinoids
A Facile Palladium-Mediated Contraction of Benzene to
Cyclopentadiene: Transformations of Palladium(II)
p-Benziporphyrin**
´
Bartosz Szyszko, Lechosław Latos-Graz˙ynski,* and Ludmiła Szterenberg
The contraction of benzene and its derivatives to form a
cyclopentadiene ring has rarely been reported. Pioneering
studies on the photooxidation of benzene led to the con-
clusion that cyclopentadienecarboxyaldehyde was formed in
this reaction.[1] Since then, research on photoinduced reac-
tions of hydroxy- and dihydroxybenzene revealed interesting
mechanistic features, including ring contraction from benzene
to cyclopentadiene.[2] A similar structural motif was detected
in the course of thermal decomposition of anisole or
dihydroxybenzene.[3] The oxidation of phenol with dioxygen
in the presence of metallic copper resulted in the aromatic
ring contraction to afford substituted cyclopentenes.[4] Carbo-
cycle contraction to benzvalene followed by opening of the
ring to form benzene was postulated in theoretical studies on
the high-temperature intramolecular topomerization of [1,2-
13C2]benzene to [1,3-13C2]- and [1,4-13C2]benzene.[5] In more
general terms, the benzene contraction belongs to an exclu-
sive group of reactions where the cleavage of aromatic
structures is of fundamental importance. Significantly, oxida-
tive ring cleavage is a key metabolic step in the biodegrada-
tion of aromatic compounds by bacteria. The common
metabolic pathway is a ring fission by catechol dioxygenases
that contain a nonheme iron(II) center in the active site.[6] The
representative examples where such a challenge has been
chemically addressed include cleavage of the aromatic rings
with formation of metallacyclopentadiene complexes accord-
geometry and unique reactivity. Herein we report the
contraction of the benzene ring embedded in palladium(II)
p-benziporphyrin 1. This process affords palladium(II) 21-
formyl-21-carbaporphyrin 4 and palladium(II) 21-carbapor-
phyrin 5, and proceeds via palladium(II) 22-hydroxycyclo-
hexadieneporphyrin 3 as
a spectroscopically detectable
intermediate.
Reaction of palladium(II) chloride with p-benziporphyrin
1 in acetonitrile results in the formation of the four-coordinate
palladium(II) p-benziporphyrin 2 (Scheme 1). The geometry
Scheme 1. Synthesis of 2. Reaction conditions: palladium(II) chloride
(3.3 equiv), acetonitrile, 293 K, 48 h, 72%.
of 2 as determined by X-ray crystallography[16] (Figure 1)
resembles the structure of palladium(II) vacataporphyrin or
nickel(II) and cadmium(II) p-benziporphyrins,[17–19] and
reflects the balance between the constraints of the macro-
ing to a retro-alkyne cyclotrimerization mechanism,[7]
a
reductive silylation of silylsubstituted arenes,[8] or insertion
of tungsten into unstrained aromatic rings.[9] Recently, an
À
impressive room-temperature C C bond fission of an arene
by a metallacarborane was reported.[10]
Porphyrinoids (including carbaporphyrinoids) provide a
unique macrocyclic platform that is suitable for exploring
organometallic chemistry confined to a particular macrocyclic
environment.[11–15] Often C H or C C bonds are held close to
the metal center, thus enforcing an unusual coordination
À
À
´
[*] B. Szyszko, Prof. L. Latos-Graz˙ynski, Dr. L. Szterenberg
Department of Chemistry, University of Wrocław
14 F. Joliot-Curie St., 50-383 Wrocław (Poland)
Fax: (+48)71-328-2348
E-mail: llg@wchuwr.pl
Homepage: http://llg.chem.uni.wroc.pl
[**] Financial support from the Ministry of Science and Higher
Education (Grant N N204 021 939) is kindly acknowledged. DFT
Figure 1. Crystal structure of 2 (top) and interaction geometry between
the palladium(II) ion and the p-phenylene moiety (bottom). Thermal
ellipsoids represent 50% probability. Selected bond lengths [ꢀ]: Pd–
N(23) 2.075(2), Pd–N(24) 2.035(2), Pd–N(25) 2.082(3).
´
calculations were carried out at the Poznan Supercomputer Center.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102218.
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Communications
cyclic ligand, the size of the palladium(II) ion, and the
predisposition of the palladium(II) for square-planar geom-
etry.
by the 13C chemical shift difference between C(2) and C(21)
carbon atoms in 2 (Dd = 33.1 ppm).
A solution of palladium(II) p-benziporphyrin 2 in aceto-
nitrile underwent a reaction over 12 h after addition of
potassium carbonate at 293 K to form a heterogeneous
mixture. Palladium(II) 21-formyl-21-carbaporphyrin 4 and
palladium(II) 21-carbaporphyrin 5 were identified as the final
products of this reaction (Scheme 2). These species are
reproducibly formed in a 3.5:1 (4/5) molar ratio under the
The distinct feature of the structure is the very pro-
nounced bending of the chloride ligand toward the annulenic
unit. Significantly, the N(23)-Pd-N(25) fragment (168.9(1)8) is
slightly distorted from linearity, while N(24)-Pd-Cl is bent
(150.0(1)8) as a consequence of strain induced by incorpo-
ration of a metal–chloride bond into a macrocyclic ring. In
particular, the p-phenylene approaches palladium(II) at a
distance much shorter (Pd···C(21) 2.83 ꢀ, Pd···C(22) 2.85 ꢀ)
than the expected van der Waals contact (Pd···C 3.3 ꢀ),[20] but
still larger than commonly observed Pd–C bond lengths, as
the typical values for palladium(II)–C(h2) alkene complexes
are in the order of 2.18 ꢀ.[21,22] The palladium(II) ion interacts
with the benzene ring in a h2 fashion. The projection of the
palladium(II) ion onto the C(2)C(3)C(21)C(22) plane (C4
plane) lies close to the center of the C(21)–C(22) bond.
The coordination of palladium(II) constrains the libration
of p-phenylene, and the 1H NMR spectrum of 2 contains
sharp p-phenylene signals (d2,3 = 9.04 ppm, d21,22 = 1.40 ppm)
with no signs of conformational exchange at the 220 K–298 K
range (Figure 2a). The palladium(II)···p-phenylene interac-
tion in solution can be readily confirmed by analysis of the 13
C
chemical shift difference of the C(21) resonances for 2 and 1
(Dd = 25.5 ppm). This interaction might be also be evidenced
Scheme 2. Contraction of palladium(II) p-benziporphyrin 2.
reaction conditions. Importantly, independent experiments
have shown that the direct transformation of 4 into 5 has not
been detected in acetonitrile solution in the presence of
potassium carbonate. On the synthetic (16.5 mg) scale, 4 and 5
were obtained in 42% and 12% yields, respectively, after
chromatographic workup. In fact, when the reaction was
carried out in a variety of solvents, it was clear that the
composition of contracted products is dependent on the
choice of solvent. Compound 5 is preferentially formed in
protic solvents (methanol and ethanol), whereas only 4 was
detected in chloroform or dichloromethane solutions. A
mixture of 4 and 5 was typically formed in polar aprotic
solvents (DMSO, THF, acetonitrile).
The progress of the contraction from 2 to 4 was directly
followed by 1H NMR spectroscopy under conditions that
allowed the best spectroscopic monitoring of the reaction
(i.e., in CDCl3 saturated with water in the presence of solid
potassium carbonate). Initially an unstable intermediate,
namely aromatic palladium(II) 22-hydroxycyclohexadiene-
porphyrin 3, was produced at 298 K and results from the
stereoselective anti-addition of palladium(II) and a hydroxide
ion across the C(21)–C(22) double bond. After 0.5 h (298 K),
Figure 2. 1H NMR spectra of a) 2 (CDCl3, 215 K), b) 4 (CDCl3, 280 K),
and c) 5 (CDCl3, 270 K). Resonance assignments follow the typical
numbering of p-benziporphyrin (a) or N-confused porphyrins (b,c).
S=solvent.
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the transient 3 converts into the aromatic 4, which is the sole
stable product of the reaction.
The 1H NMR spectra of 3, 4, and 5 (Figures 2 and 3) show
the basic features of aromatic carbaporphyrinoids.[12,15,21]
Figure 3. 1H NMR spectra of a) 6-H; b) 3 (both CDCl3, 220 K). The
inset shows the upfield part of the spectrum of 6-D (CDCl3, 220 K) as
obtained in reaction of 2 with sodium borodeuteride in [D4]methanol.
The residual resonances of 4 in (b) and 6 (inset) are marked with
asterisks. Resonance assignments follow the numbering of p-benzipor-
phyrin. S=solvent.
Scheme 3. Palladium(II)-mediated contraction mechanism.
Complete assignments of resonances have been made on
the basis of relative intensities and through a combination of
homonuclear (COSY, NOESY, ROESY) and heteronuclear
(HMQC, HMBC) correlation techniques. The 1H NMR
spectrum of transient 3, which was efficiently trapped at
220 K to prevent any further transformation (Figure 3b),
shows an increased multiplicity of resonances compared to
the spectra of Cs-symmetric 2, 4, and 5 (Figure 2). The spectra
of 3 and palladium(II) cyclohexadieneporphyrin 6 (Scheme 4)
show some similarities (Figure 3), as both species contain the
cyclohexadiene moiety.
The most notable structural feature of 3 is the coordina-
tion of palladium(II) by the tetrahedrally hybridized C(21)
center. A complete set of resonances that correspond to OH,
(d = 0.69 ppm, readily exchangeable with deuterium after
addition of D2O), H(21) (d = À1.40 ppm), and H(22) (d =
À0.36 ppm) serve as the fingerprint of the unprecedented
22-hydroxycyclohexadienyl moiety. The structural model of 3,
generated by DFT optimization (Figure 4), reflects the
structural constraints determined by NOE measurements. In
fact, the cyclohexadiene ring adopts the highly strained half-
chair conformation. The palladium(II) and hydroxy units are
located at the vicinal carbon atoms and occupy axial positions
to result in an anti arrangement. The NOE correlations
(Figure S3 in the Supporting Information), which result from
through-space interactions OH···H(2), OH···H(3), OH···20-o-
Ph, and H(21)···20-o-Ph, confirm unambiguously the axial
Figure 4. DFT-optimized structure of 3. Selected bond lengths [ꢀ] and
angles [8]: Pd–C(21) 2.088, Pd–N(23) 2.173, Pd–N(24) 2.120, Pd–
N(25) 2.063; N(23)-Pd-N(25) 172.0, N(25)-Pd-C(21) 161.0. C dark gray,
H light gray, N blue, O red, Pd orange.
position of the hydroxy group in the cyclohexadiene unit; this
conformation results from the anti-addition of palladium(II)
and a hydroxide ion to 2.
1
The specific H (d = 2.43 ppm) and 13C (d = 170.0 ppm)
resonances of the 21-formyl unit in 4 and H(21) (d =
À3.86 ppm) in 5 readily confirmed the identity of these
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complexes. Significantly, the 13C chemical shifts determined
for the C(21) atoms of 3 (d = 22.3 ppm), 4 (d = 59.9 ppm), 5
(d = 34.0 ppm), and 6 (d = 14.2 ppm) reflect the tetrahedral
geometry around the coordinating carbon atom.
The coordination environment of the palladium(II) center
corresponds to the conventional square-planar structure with
the N(22), N(23), N(24), and C(21) atoms occupying equa-
torial positions. The macrocycle reveals the bond-length
pattern expected for the aromatic 21-carbaporphyrin.[15] The
specific localization of the 21-formyl substituent results in the
relatively short Pd···C(formyl) distance (2.531(2) ꢀ). Signifi-
cantly, in spite of the intensive exploration of metallocarba-
porphyrinoids, 4 (Figure 5) and 5 are the very first represen-
tatives of 21-carbaporphyrin complexes.[13,15,23–25]
Scheme 4. Addition to 2. Reaction conditions: a) sodium borohydride,
methanol, b) sodium borodeuteride, [D4]methanol, and c) sodium
ethoxide in ethanol.
addition of alkoxides is reversible as 2 forms from 7 during
column chromatography on silica gel. The addition of the
ethoxide ion to form 7 competes with the reduction of 2 to
form 6-H. The ethoxide ion in ethanol acted as the reducing
agent, as equimolar amounts of 6-H and ethanal were
1
detected by H NMR spectroscopy in the reaction mixture
(Figure S5).
In conclusion, palladium(II) p-benziporphyrin provides a
unique environment to alter the fundamental reactivity of the
benzene unit. The possibility of a metal···(carbon–carbon)
interaction by encapsulating the specific donor center
(CCNNN) in the porphyrinic core is of fundamental
importance.[12,17,18] By taking advantage of the additional
stabilization that arises from geometrical constraints of the
porphyrin macrocycle, a cascade of intramolecular rearrange-
ments has been efficiently promoted. Accordingly, the
remarkable, facile palladium(II)-mediated contraction of p-
phenylene to cyclopentadiene affords the first reported
complex of 21-carbaporphyrin.
Figure 5. Crystal structure of 4 (top: perspective view, bottom: side
view with phenyl groups omitted for clarity). Thermal ellipsoids
represent 50% probability. Selected bond lengths [ꢀ]: Pd–N(22)
2.012(2), Pd–N(23) 2.048(2), Pd–N(24) 2.023(2), Pd–C(21) 2.084(2).
The side view shows the geometry of interaction between palladium(II)
and the formyl substituent.
A feasible mechanism of contraction consistent with
formation of 3, 4, and 5 is shown in Scheme 3 and comprise
the following major steps: 1) addition of palladium(II) and a
hydroxide ion to the C(21)–C(22) double bond,[26] 2) b elimi-
nation,[27] and 3) competing contractions by 1,2-hydride shift
or cheletropic extrusion of carbon oxide. The contraction is
accompanied by a relief of strain energy of the embedded
conjugated 1,3-cyclohexadiene ring and finally by the for-
mation of a new aromatic porphyrinoid system.
Received: March 30, 2011
Published online: May 31, 2011
À
Keywords: C C activation · carbaporphyrinoids · palladium ·
porphyrinoids · ring contraction
.
Activation of the p-phenylene moiety of 2 toward a
combined addition of palladium(II) and a nucleophile of
choice is of particular importance and has been proven to be a
more general reactivity route. In fact, reaction of 2 with
sodium borohydride (in methanol), sodium borodeuteride (in
[D4]methanol), and sodium ethoxide afforded 6-H, 6-D, and 7
respectively (Scheme 4). Significantly, 6 and 7 do not undergo
further conversion, thus directly confirming the unique role of
the hydroxy group in the contraction mechanism. The
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