Aerobic catalytic photooxidation of olefins by an electron-deficient pacman bisiron(III) μ-oxo porphyrin

Article abstract of DOI:10.1021/jo048570v

(Chemical Equation Presented) The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) μ-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) μ-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions.

Full text of DOI:10.1021/jo048570v

1
2
Aerobic Catalytic Photooxidation of
Olefins by an Electron-Deficient Pacman
Bisiron(III) µ-Oxo Porphyrin
Oxygen atom transfer (OAT) from porphyrin as with
other metal-oxo platforms1
3-15
is preferred for the side-
on approach of substrate to the metal-oxygen bond. The
architecture of the cofacial cleft, formed from attaching
the diiron(III) µ-oxo bisporphyrins to a single rigid pillar,
can sterically confine substrate attack to the electroni-
cally favored side-on geometry. Traditional cofacial bispor-
Joel Rosenthal, Bradford J. Pistorio,
Leng Leng Chng, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of
Technology, 77 Massachusetts Avenue,
phyrin systems, however, have little vertical flexibility
Cambridge, Massachusetts 02139-4307
16,17
(
ca. 1 Å),
and consequently pose the complication that
nocera@mit.edu
the photogenerated terminal metal oxo is difficult to
access by substrate. To circumvent this problem, we have
synthesized cofacial bisporphyrins bearing a dibenzofu-
Received August 16, 2004
18-20
ran (DPD) spacer,
which exhibits significant vertical
flexibility.2 X-ray crystallography establishes that the
diiron(III) µ-oxo DPD complex, (DPD)Fe O, is a clamped
the metal-metal distance of the (DPD)Fe
1,22
2
spring:2
3,24
2
O
complex is 3.5 Å whereas the metal-metal distance of
the relaxed DPD pocket is >7.5 Å. We have shown by
transient absorption spectroscopy25 that the strongly
clamped DPD Pacman is opened by photocleavage of the
III
III
Fe-O bond, induced by excitation of the Fe -O-Fe
core. A superior photocatalytic activity of the DPD
framework, as compared to traditional xanthene or
anthracene bridged Pacman porphyrins (ca. 10000-fold)
is derived from the presentation of an extended cofacial
cleft, enabling the ferryl intermediate to be accessed
easily by substrate from a side-on approach. To date, the
photocatalytic cycle has been turned over with easily
oxidized substrates (e.g., phosphines and sulfides) owing
to the modest redox potential of the etioporphyrin ferryl
subunit.2
The synthesis and oxygen atom transfer (OAT) photoreac-
tivity of a diiron(III) µ-oxo meso-tripentafluorophenyl bispor-
phyrin appended to a dibenzofuran spacer are presented.
Reaction of 4,6-diformyldibenzofuran under standard Lind-
sey conditions furnishes the parent cofacial porphyrin
architecture in a single step. These cofacial porphyrins
photocatalyze the oxidation of sulfides and olefins using
visible light and molecular oxygen as the terminal oxidant.
High turnover numbers reflect the enhanced stability of the
electron-deficient diiron(III) µ-oxo bisporphyrin core ap-
pended to a dibenzofuran spacer under aerobic conditions.
6,27
It is known that the oxidizing power of metalloporphy-
rins can be increased by introducing electron-withdraw-
28,29
ing groups onto the porphyrin periphery.
Toward this
(
10) Balch, A. L.; Chan, Y.-W.; Cheng, R.-J.; La Mar, G. N.; Latos-
Grazynski, L.; Renner, M. W. J. Am. Chem. Soc. 1984, 106, 7779.
(
11) Balch, A. L. Inorg. Chim. Acta 1992, 198-200, 297.
Diiron bisporphyrins may utilize O
oxidation without the need for an external co-reductant.
2
for substrate
(12) Groves, J. T.; Myers, R. S. J. Am. Chem. Soc. 1983, 107, 5791.
(13) Veige, A. S.; Slaughter, L. M.; Wolczanski, P. T.; Matsunaga,
N.; Decker, S. A.; Cundari, T. R. J. Am. Chem. Soc. 2001, 123, 6419.
1
-8
The overall cycle relies on a photon to cleave the ther-
(
14) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421.
mally inert Fe-O bonds of a diiron(III) µ-oxo bisporphy-
(15) Brock, S. L.; Mayer, J. M. Inorg. Chem. 1991, 30, 2138.
(16) Collman, J. P.; Wagenknecht, P. S.; Hutchinson, J. E. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1537.
II
IV
rin to generate a geminal PFe /PFe dO pair (P )
porphyrin). The active catalyst, the ferryl intermediate,
is capable of oxygenating substrates with the concomitant
formation of two equivalents of reduced iron(II) porphy-
rin. Reaction of the two separate ferrous porphyrin
(
17) Chang, C. K.; Liu, H. Y.; Abdalmuhdi, I. J. Am. Chem. Soc.
1984, 106, 2725.
18) Chang, C. J.; Yeh, C.-Y.; Nocera, D. G. J. Org. Chem. 2002, 67,
403.
19) Chng, L. L.; Chang, C. J.; Nocera, D. G. J. Org. Chem. 2003,
(
1
(
9-11
subunits with O
2
reforms the diiron(III) µ-oxo complex
68, 4075.
(
20) Chang, C. J.; Deng, Y.; Heyduk, A. F.; Chang, C. K.; Nocera,
for re-entry into a photocatalytic cycle.
D. G. Inorg. Chem. 2000, 39, 959.
(
21) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.;
(
1) Richman, R. M.; Peterson, M. W. J. Am. Chem. Soc. 1982, 104,
795.
2) Peterson, M. W.; Rivers, D. S.; Richman, R. M. J. Am. Chem.
Nocera, D. G. Chem. Commun. 2000, 1355.
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Am. Chem. Soc. 2004, 126, 10013.
5
(
Soc. 1985, 107, 2907.
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122, 410.
(
3) Peterson, M. W.; Richman, R. M. Inorg. Chem. 1985, 24, 722.
4) Ercolani, C.; Gardini, M.; Pennesi, G.; Rossi, G. Chem. Commun.
(
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2003, 42, 8262.
1
1
983, 549.
(
5) Weber, L.; Haufe, G.; Rehorek, D.; Hennig, H. Chem. Commun.
991, 502.
6) Weber, L.; Hommel, R.; Behling, J.; Haufe, G.; Hennig, H. J.
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Inorg. Chem. 2003, 42, 8270.
(
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2002, 124, 7884.
Am. Chem. Soc. 1994, 116, 2400.
(
(
7) Hennig, H.; Luppa, D. J. Prakt. Chem. 1999, 341, 757.
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Miller, S. E.; Carpenter, S. D.; Nocera, D. G. Inorg. Chem. 2002, 41,
3102.
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Soc. 1987, 109, 1243.
(
9) Latos-Grazynski, L.; Cheng, R.-J.; La Mar, G. N.; Balch, A. L.
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1
0.1021/jo048570v CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/09/2005
J. Org. Chem. 2005, 70, 1885-1888
1885

CHART 1
SCHEME 1a
FIGURE 1. Changes in the absorption profile for the pho-
tolysis (λexc > 425 nm) of benzene solutions of (DPDF)Fe
2
O
-
6
(
ca. 10 M) in the presence of DMS (ca. 0.2 M) at 293 K. The
arrows indicate the disappearance of (DPDF)Fe O and the
. Spectra were recorded over
2
2 2
appearance of (DPDF)Fe (DMS)
the span of 30 min at ∼5 min intervals.
TABLE 1. Turnover Numbers and Conversion Yields
for Photocatalytic DMS Oxidationa
photoproduct
catalyst
TP P)Fe
DMSO TON (% yield)
2
DMSO TON (% yield)
(
(
(
F
2
O
750 ( 25 (96%)
9635 ( 165 (∼100%)
620 ( 31 (90%)
32 ( 4 (4%)
trace
68 ( 3 (10%)
DPDF)Fe O
2
b
DPD)Fe
2
O
a
a
Key: (a) (1) BF3‚OEt2, CH2Cl2, (2) 2,3-dichloro-5,6-dicyano-
exc
Determined for a 4 h photolysis (λ > 425 nm) of solutions under
1
(
,4-benzoquinone (DDQ); (b) (1) FeBr2, CH3CN, reflux, (2) aq HCl;
c) 2N NaOH.
2
1 atm of O (298 K) but otherwise of the same composition as those
b
used in stoichiometric photolysis experiments. Reference 26.
is consistent with the more electron-withdrawing per-
fluorinated phenyl periphery of the porphyrin platform.
Consistent with these spectral trends, the photooxidation
end, we now describe the synthesis and characterization
of an electron-deficient DPD platform appended with
meso-pentafluorophenyl porphyrins (DPDF) (Chart 1).
chemistry of (DPD)Fe
80 nm whereas the photochemistry of (DPDF)Fe
proceeds smoothly with λexc ) 420-460 nm.
Although (DPDF)Fe O is thermally inert in dry ben-
2
O is promoted with λexc ) 360-
The photooxidation chemistry of Fe
pared to that of (DPD)Fe O and nonpillared tetrakis-
pentafluorophenyl) bisporphyrin, (TP P)Fe O. We show
O complex is an active catalyst for
2
O(DPDF) is com-
3
2
O
2
(
F
2
that the (DPDF)Fe
2
2
-6
zene (∼10 M), a prompt photoreaction is observed when
the compound is photolyzed (λexc > 425 nm) in the
presence of dimethyl sulfide (DMS) (0.2 M). As shown in
Figure 1, isosbestic points are maintained during the
course of the photoreaction (Figure 1); no spectral changes
are observed with continued irradiation at times longer
than that for which the final trace is shown. The
photoproduct spectrum is identical to that of (DPDF)-
the photoinduced aerobic oxygenation of sulfides and
olefins under mild conditions and with the highest turn-
over numbers (TONs) yet achieved for a cofacial bispor-
phyrin photocatalyst.
The synthesis of the (DPDF)Fe
. The tripentafluorophenyl cofacial bisporphyrin (DP-
DF)H (1) is obtained in one step by reacting 4,6-
2
O is outlined in Scheme
1
4
diformyldibenzofuran with pyrrole and pentafluoroben-
zaldehyde under standard Lindsey conditions.30 The
Fe
2
(DMS)
2
, which was independently prepared by the Na/
Cl in benzene in the presence
Hg reduction of (DPDF)Fe
2
2
major byproduct, tetrakis(pentafluorophenyl)porphyrin,
is separated from 1 by column chromatography. Iron
2
insertion into 1 with FeBr followed by treatment with
of DMS (λabs,max ) 416, 526, 554(sh) nm). The organic
oxidation photoproduct was identified as DMSO by GC/
MS analysis.
HCl furnishes the corresponding (dichloro)diiron(III)
complex 2 (91%). The (DPDF)Fe O complex is obtained
The photooxidation (λexc > 425 nm) of DMS by (DPDF)-
2
Fe
formed in the presence of O
for a given photolysis period and overall conversion yields
for DMS photooxidation performed under 1 atm of O are
listed in Table 1. The TON for DMS photooxidation by
DPDF)Fe O is more than an order of magnitude greater
than that obtained for (DPD)Fe O and unbridged (TP
P)Fe O photocatalysts. Moreover, (DPDF)Fe O is selec-
tive for the oxidation of DMS to DMSO; no dimethyl
sulfone (DMSO ) was observed by GC/MS analysis. For
comparison, 10% DMSO is produced by the (DPD)Fe
photocatalyst. The increased catalytic efficacy of (DPDF)-
Fe O is attributed to the increased stability of the meso-
substituted photocatalyst, as (DPDF)Fe O remained
2
O in benzene solutions becomes catalytic when per-
in 93% yield by metathesis in benzene with 2 N NaOH.
Figure S1 (Supporting Information) compares the
ground-state electronic absorption spectrum of (DPDF)-
2
. Turnover numbers (TONs)
2
Fe
Fe
2
O to its nonfluorinated DPD parent complex, (DPD)-
O. The prominent Soret absorption of (DPDF)Fe O at
P)-
but red-shifted from that of (DPD)Fe
max ) 392 nm); the energies of the Q-bands for all three
2
2
(
2
4
02 nm is energetically comparable to that of (TD
F
3
1
2
F
-
(
(
TF
λ
5
PP)Fe
2
O
2
O
2
2
compounds are similar. The overall red-shift of the
2
absorptionprofileof(DPDF)Fe Oascomparedto(DPD)Fe O
2
2
2
2
O
(
30) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828.
31) Jayaraj, K.; Gold, A.; Toney, G. E.; Helms, J. H.; Hatfield, W.
2
(
E. Inorg. Chem. 1986, 25, 3516.
2
1886 J. Org. Chem., Vol. 70, No. 5, 2005

TABLE 2. Turnover Numbers and Conversion Yields
for Olefin Oxidation Using (DPDF)Fe2O as the
Table 2 is consistent with an autoxidation process and
no different than that typically obtained for other cofacial
a
Photocatalyst
5,6
porphyrin photocatalysts. It might be expected that the
substrate
product
TON
% yield
lack of easily abstracted hydrogen atoms in styrene would
lead to simpler photoproduct mixtures. However, as
shown in Table 2, large amounts of benzaldehyde, as well
as acetophenone and phenylacetaldehyde, are obtained
along with the epoxide product. This product distribution
parallels that previously observed for the stoichiometric
cyclohexene
cyclohexene oxide
89 ( 17
69 ( 12
437 ( 74
15
12
73
2
2
-cyclohexen-1-ol
-cyclohexen-1-one
styrene
styrene oxide
37 ( 6
1609 ( 340
150 ( 27
54 ( 11
2
87
8
benzaldehyde
acetophenone
phenylacetaldehyde
IV
3
oxidation of styrene derivatives by electron rich Fe dO
39
a
intermediates produced by nonphotochemical methods.
Determined for a 4 h photolysis (λexc > 425 nm) of solutions under
1
2
atm of O (298 K) but otherwise of the same composition as those
The products are derived from a carbon radical, which
is postulated to form from the reaction of the ferryl
intermediate with alkene.
used in stoichiometric photolysis experiments.
active under photocatalytic conditions more than twice
2
Figure 2 adapts this mechanism to (DPDF)Fe O pho-
as long as the (DPD)Fe
2
O. The latter is susceptible to
tocatalysis. Epoxide and aldehyde photoproducts form
along separate paths with photogenerated benzylic radi-
cal I as a common origin. The epoxide is produced from
decomposition by hydroxylation of the unsubstituted
32-34
meso-positions of the porphyrin ring;
is circumvented by the presence of the ancillary pen-
tafluorophenyl groups of (DPDF)Fe O, which lower the
susceptibility of the porphyrin to oxidative damage.
this pathway
2
direct carbon-oxygen ring closure of I; the (DPDF)Fe O
photocatalyst reforms from reaction of oxygen with the
cofacial bisporphyrin photoproduct. Competing with ring
closure to give the epoxide, oxygen attack of I yields II,
which presumably converts to benzaldehyde via the
2
35-37
Additionally, the DPD spacer and meso-substitutents
result in increased steric congestion about the ferryl
oxygen. Hence, if a second Pacman molecule is considered
as a potential substrate, it is not surprising that the rate
of oxidative decomposition is reduced dramatically for
oxetane. (DPDF)Fe
likely by a Balch-type mechanism.
possible that a µ-peroxo intermediate, formed from the
reaction of O with (DPDF)Fe , could act as an oxidant
2
is converted to (DPDF)Fe
2
O most
1
0,40-42
While it is
(
DPDF)Fe
The increased electrophilicilicity of the ferryl interme-
diate of (DPDF)Fe O prompted us to consider the more
2
O as compared to the unbridged congener.
2
2
toward substrate, it is more likely that the unimolecular
O-O bond ruptures to give the bis-ferryl Pacman inter-
mediate occurs under catalytic conditions (i.e., inert
solvent at room temperature). Indeed, in the absence of
2
challenging task of olefin oxidization. A prompt photo-
reaction is observed upon irradiation (λexc > 425 nm) of
-
6
substrate, (DPDF)Fe
2
is converted quantitatively to
(DPDF)Fe
2
O (∼10 M) in deaerated, dry benzene con-
(
DPDF)Fe O upon addition of 0.5 equivalents of O ,
2
2
taining either styrene or cyclohexene (1.0 M) and tert-
-
7
indicating that substrate is not necessary to effect µ-oxo
formation from the presumed µ-peroxo intermediate.
Electron transfer from olefin substrate to the highly
oxidizing ferryl accounts for the other products of styrene
oxidation. Ketones have been observed for styrene oxida-
butyl isonitrile (1.5 × 10 M) as a capping ligand for
the Fe(II) photoproduct. The time course for the absorp-
tion profile for this photoreaction is virtually identical
to that shown in Figure 1 for DMS oxidation. For pyridine
solvent, a clean photoreaction is observed in the absence
of external capping ligand.
4
3
tion by dioxygen using redox active rhodium and
palladium44 catalysts. Although electron transfer is most
Photocatalytic epoxidation of cyclohexene and styrene
was accomplished in a fashion similar to that described
for DMS photooxidation. Table 2 lists the TON for
photoepoxidatation of the two substrates. We note that
no photoreaction is observed in the absense of (DPDF)-
IV
effectively promoted by an Fe dO porphyrin cation
radical, it is likely that the highly electron deficient
nature of the DPDF platform enables the charge-transfer
chemistry that leads to the observed acetophenone and
phenylacetaldehyde oxidation products. Indeed, the fact
that no ketone oxidation product has been previously
reported for stoichiometric styrene oxidation by the more
2
Fe O photocatalyst. Cyclohexene photoconversion pro-
ceeds with modest turnover but multiple products are
obtained as a result of OAT (cyclohexene oxide) and
allylic oxidation (cyclohexen-1-ol and 2-cyclohexen-1-one).
Cyclohexene is generally susceptible to radical chain
autoxidations that can give rise to yields of 2-cyclohexen-
IV
39
electron rich (TMP)Fe dO oxidant, lends credence to
this contention.
(
2
DPDF)Fe O does not oxidize cis-cyclooctene or cis/
trans-stilbene under photocatalytic conditions. We at-
tribute this lack of reactivity to the inability of bulky
substrates to access the sterically hidered cleft of the
tetra-meso-substituted Pacman architecture. Time-re-
1
-ol and/or 2-cyclohexene-1-one that exceed the amount
38
of epoxide product. The product distribution shown in
(
32) Kamachi, T.; Shestakov, A. F.; Yoshizawa, K. J. Am. Chem. Soc.
004, 126, 3672.
33) Kalish, H. R.; Camp, J. E.; Stepien, M.; Latos-Grazynski, L.
Balch, A. L. J. Am. Chem. Soc. 2001, 123, 11719.
34) Torpey, J.; Lee, D. A.; Smith, K. M.; Ortiz de Montellano, P. R.
J. Am. Chem. Soc. 1996, 118, 9172.
35) Chang, C. K.; Ebina, F. J. Chem. Soc., Chem. Commun. 1981,
78.
36) Traylor, P. S.; Dolphin, D.; Traylor, T. G. J. Chem. Soc., Chem.
Commun. 1984, 279.
2
(
(39) Groves, J. T.; Gross, A.; Stern, M. K. Inorg. Chem. 1994, 33,
5065.
(
(40) Chin, D.-H.; La Mar, G. N.; Balch, A. L. J. Am. Chem. Soc. 1980,
102, 5947.
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(41) Chin, D.-H.; La Mar, G. N.; Balch, A. L. J. Am. Chem. Soc. 1980,
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102, 4344.
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1
(
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2
J. Org. Chem, Vol. 70, No. 5, 2005 1887

2
FIGURE 2. Photocycle for styrene oxidation by (DPDF)Fe O that accounts for epoxide and aldehyde photoproduction.
solved transient absorption studies of phosphine oxida-
tion by bis-iron(III)-µ-oxo Pacman catalysts have shown
that the rate and efficiency of OAT decrease as the steric
demand of phosphines increase,25 illustrating the impor-
tance of substrate access to the short-lived ferryl photo-
intermediate. For the DPDF Pacman, amine photooxi-
dation (i.e., trimethylamine, pyridine, etc.) is not observed.
The cone angle of trimethylamine is 132°.45 Previous
studies of phosphine oxidation establish that substrates
possessing cone angles in the range of 130° have access
to the DPD Pacman cleft.27 Accordingly, the lack of
substrate oxidation appears to be of thermodynamic and
not of kinetic origin.
TABLE 3. Thermodynamic Reactivity Scale and X-O
5
1
Bond Energies for X(g) + O(g) f XO(g)
X
XO
PO
∆H (kcal/mol) X(g) + O(g) f XO(g)
Ph
Me
3
P
2
Ph
Me
3
-133
-86
S
2
SO
styrene
styrene oxide
∼ -86
Me
O
3
N
Me
O
3
NO
-67
2
-59
are collected in Table 3. Using the thermodynamic cycle,
the opposing OAT reactivity displayed by DMS and
styrene versus trimethylamine allows us to bracket the
IV
approximate Fe dO BDE between 65 and 85 kcal/mol.
As has been done previously for transition metal oxo
Although OAT to substrates that would be expected to
be thermodymically favorable such as stillbene and
cyclooctene is not observed, it must be concluded that the
barrier to these oxidations is kinetic in origin and related
to the sterically restricted molecular cleft.
46-50
species,
the overall OAT reaction may be partitioned
into the following simple thermodynamic steps:
1
X + / O f XO ∆H1
(1)
2
2
1
A f B + / O
∆H2
(2)
(3)
2
2
In summary, we have shown that the multielectron
oxidation photocatalysis of olefins may be achieved using
an electron-deficient Pacman bisporphyrin (DPDF)Fe O
2
A + X f B + XO ∆H ) ∆H + ∆H
2
3
1
The approximate strength of the iron-oxygen bond of
and molecular oxygen without the need for an external
coreductant. The electron deficiency of this bisporphyrin
photocatalyst is reflected in our ability to drive the
photochemistry using relatively long-wavelength light in
the visible region.
the photochemically generated ferryl intermediate (spe-
2
cies A in eq 2) may be determined (∆H ) with the
enthalpic expressions for O-O bond homolysis and
substrate (X) oxidation in place. Enthalpic values for the
oxidation of a number of organic oxygen atom acceptors
Acknowledgment. J.R. thanks the Fannie and
John May Hertz Foundation for a predoctoral fellow-
ship. This work was supported under grants from the
National Science Foundation (CHE-0132680) and the
National Institutes of Health (GM 47274).
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47) Harlan, E. W.; Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1986,
Supporting Information Available: Experimental meth-
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2
1
08, 6992.
(
(
(
48) Craig, J. A.; Holm, R. H. J. Am. Chem. Soc. 1989, 111, 2111.
(
2
DPD)Fe O. This material is available free of charge via the
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1888 J. Org. Chem., Vol. 70, No. 5, 2005