Photochemistry of metalloporphyrin carbene complexes

Full text of DOI:10.1021/ja954097e

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306
J. Am. Chem. Soc. 1996, 118, 5306-5307
Photochemistry of Metalloporphyrin Carbene
Complexes
Christopher J. Ziegler and Kenneth S. Suslick*
School of Chemical Sciences
UniVersity of Illinois at Urbana-Champaign
Urbana, Illinois 61801
ReceiVed December 6, 1995
The photochemistry of metalloporphyrins and other tetrapy-
1
rrolic systems continues to attract considerable interest. There
have been numerous studies on the photoactivity of various
porphyrin complexes with single-bonded axial ligands, including
2
halogens, oxo-anion species, molecular oxygen, and a variety
of metals, including chromium, manganese, and iron. The
3
2
Figure 1. Photolysis of Fe(TPP)CCl in benzene under argon at 25
°
C.
photochemistry of metalloporphyrins with multiple-bonded axial
ligands has not been explored. We report here the photochem-
istry of several iron porphyrin carbene and vinylidene com-
plexes. Irradiation of these complexes with visible light cleaves
the iron-carbon double bond, resulting in a four-coordinate iron-
2
Table 1. Photolysis of Fe(TPP)CX in the Presence of Alkene
Substrates
(II) porphyrin and a free carbene. This chemistry is unique in
the area of transition metal carbene or alkylidene compounds.
Porphyrins with multiple-bonded axial ligands are especially
4
relevant to both biological and catalytic oxidative processes.
Iron porphyrin carbene and nitrene species are isoelectronic with
a terminal iron-oxo complex, the proposed intermediate of the
cytochrome P-450 monooxygenases and its synthetic analogs.
Most recently, the catalytic cyclopropanation of olefins by iron
porphyrins using ethyl diazoacetate was thought to involve a
porphyrin carbene complex.5
We prepared Fe(TPP)CCl2, Fe(TPP)CBr2, Fe(TPP)CClF, and
Fe(TPP)CC(p-C6H4Cl)2 under anaerobic conditions following
Mansuy’s procedure using iron metal as the reductant (TPP )
5
,10,15,20-tetraphenylporphyrinate).6 These compounds were
further purified via crystallization from benzene/pentane under
argon in an inert atmosphere box and then fully characterized.
7
To examine the photochemistry of these complexes, degassed
-
4
benzene solutions (∼1 × 10 M) were irradiated with a 300
W Xe arc lamp, which was filtered to remove both infrared
and ultraviolet (<360 nm) light in order to prevent sample
heating and porphyrin bleaching, respectively. In addition, the
photolysis cells were thermostated to an internal solution
temperature of 20 °C.
In all cases, irradiation of the parent compounds cleanly
produced a new porphyrin identified as Fe(TPP) (λmax ) 419,
442, and 537 nm), as shown in Figure 1 for Fe(TPP)CCl2. Both
the Soret and Q-bands of the parent compounds are photoactive.8
The formation of Fe(TPP) was confirmed by addition of pyridine
after photolysis of the carbene or vinylidene complexes, yielding
the bispyridine complex, Fe(TPP)(py)2. Also as expected,
exposure of photolysed solutions to oxygen resulted in the
formation of the µ-oxo dimer species, [Fe(TPP)]2O.
(1) (a) Photochemistry of Polypyridine and Porphyrin Complexes;
Kalyanasundaram, K., Ed.; Academic: New York, 1992 . (b) Blauer, G.;
Sund, H. Optical Properties and Structure of Tetrapyrroles; W. De
Gruyter: Berlin, 1985. (c) Mauzerall, D. In The Porphyrins; Dolphin, D.,
Ed.; Academic: New York, 1979; Vol. 5, p 29 ff.
The production of Fe(TPP) in this reaction is indicative of
homolytic cleavage of the metal-carbon double bond and
production of a carbene, as in the following reaction:
(
2) (a) Imamura, T.; Jin T.; Suzuki, T.; Fujimoto, M. Chem. Lett. 1985,
8
47. (b) Hendrickson, D. N.; Kinnaird, M. G.; Suslick, K. S. J. Am. Chem.
Soc. 1987, 109, 1243. (c) Jin, T.; Suzuki, T.; Imamura, T.; Fujimoto, M.
Inorg. Chem. 1987, 26, 1280. (d) Suslick, K. S.; Watson, R. A. Inorg. Chem.
1
1
991, 30, 912. (e) Suslick, K. S.; Watson, R. A.; Wilson, S. R. Inorg. Chem.
991, 30, 2311. (f) Proniewicz, L. M.; Bajdor, K; Nakamoto, K; J. Phys.
hν
Fe(TPP)CX2
9
8 Fe(TPP) + :CX2
Chem. 1986, 90, 1760.
(
3) (a) Suslick, K. S.; Watson, R. A. New J. Chem. 1992, 16, 633. (b)
Suslick, K. S.; Acholla, F. A.; Cook, B. R. J. Am. Chem. Soc. 1987, 109,
818.(c) Buchler, J. W.; Dreher, C. Z. Naturforsch. 1984, 39b, 222.
4) Sheldon, R. A., Ed. Metalloporphyrins in Catalytic Oxidations;
Marcel Dekker: New York, 1994.
5) Wolf, J. R.; Hamaker, C. G.; Djukic, J-P.; Kodadek, T.; Woo, L. K.
J. Am. Chem. Soc. 1995, 117, 9194.
6) (a) Mansuy, D.; Lange, M.; Chottard, J. C.; Guerin, P.; Morliere, P.;
The transient formation of free carbenes was confirmed by
addition to various substrates (Tables 1 and 2), with products
monitored by capillary GC and GCMS. The reactivity of free
carbenes with alkene substrates has been thoroughly investi-
2
(
(
(
(8) The quantum yields for these compounds are comparable to those of
other metalloporphyrins with axial ligands. These yields upon irradiation
Brault, D.; Rougee, M. J. Chem. Soc., Chem. Commun. 1977, 648. (b)
Mansuy, D.; Lange, M.; Chottard, J. C.; Bartoli, J. F.; Chevrier, B.; Weiss,
R. Angew. Chem., Int. Ed. Engl. 1978, 17, 780. (c) Mansuy, D. Pure Appl.
Chem. 1980, 52, 681. (d) Mansuy, D.; Lange, M.; Chottard, J. C. J. Am.
Chem. Soc. 1978, 100, 3214.
-
4
-3
of the Soret band ranged from 1 × 10 to 4 × 10 with CBr2 < CCl2 <
CC(C6H4Cl)2 < CClF, which tracks the relative stabilities of the four
resulting carbene fragments. The Q-bands, upon selective irradiation, were
also found to be photoactive.
(
7) UV-Vis, 1H NMR, FDMS, and elemental analysis were used to
(9) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic: New York,
1971; pp 267-362. (b) Parham, W. E.; Schweizer, E. E. Org. React. 1963,
13, 55.
confirm the identity and purity of these species, which agreed completely
with the prior literature.
S0002-7863(95)04097-2 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5307
Table 2. Photolysis of Fe(TPP)CX
2
under Competitive Conditions
In the photochemistry of Fe(TPP)CX2, cyclopropanation
could conceivably occur either through free dihalocarbene
production or through a metal-mediated carbenoid intermediate.
In the first case, photoexcitation would liberate the carbene
fragment, which would then go on to react with the alkene
substrate. In the second possibility, the metal center would play
a distinct role in the reaction, perhaps via a metallocycle
intermediate. The mechanism of photochemical cyclopropa-
nation with these complexes was determined by competitive
substrate addition, from either intra- or intermolecular selectivity.
Similar approaches have been found useful in mechanistic
studies of epoxidations by metalloporphyrin oxo complexes.11
In the irradiation of Fe(TPP)CCl2, both the selectivity of an
intramolecular competition, (S)-(-)-limonene (first entry in
Table 2), and an intermolecular competition, cyclohexene vs
1
-hexene (second entry in Table 2), were examined. In both
cases, the product ratios matched exactly that of free CCl2
produced from the reaction of base with CHCl3 conducted at
the same temperature (20 °C). In addition, photolysis of Fe-
(TPP)CClF with alkene substrates produced both syn-fluoro and
syn-chloro products, whose ratios could also be compared to
the reaction of CHCl2F with base. As before, no significant
difference in product ratios was found between the metallopor-
phyrin complex photolysis and the free carbene reactions. This
similarity in selectivity indicates that upon photoactivation the
parent compound undergoes homolytic dissociation with the
formation of free carbene.
Unlike the photolyses of the dihalocarbene iron porphyrins,
the vinylidene complex Fe(TPP)CC(p-C6H4Cl)2 did not liberate
a reactive fragment that added to alkene substrates. Instead,
upon photolysis the vinylidene complex produced di(p-chlo-
rophenyl)acetylene:
gated,9,10 and the selectivity of product formation can be used
as a test of the formation of free carbenes. A number of alkenes
were selected to corroborate the formation of carbenes from
the photolysis of the metalloporphyrin carbene complexes.
Irradiation of Fe(TPP)CCl2, Fe(TPP)CBr2, or Fe(TPP)CClF in
the presence of alkenes produced dihalocyclopropanes (Table
1
). All of these additions occurred in good yield in dilute
solution, and the relative yields from the three complexes reflect
the relative reactivity of the three free carbene fragments in
solution. The photolysis of more concentrated solutions of these
three species resulted in the formation of Fe(TPP)X in addition
to the cyclopropanes and Fe(TPP). The production of Fe(TPP)X
in solution results from halocarbene attack on unreacted Fe-
hν
Fe(TPP)CC(p-C H Cl)
9
8
6
4
2
Fe(TPP) + (p-C H Cl)CtC(p-C H Cl)
6
4
6
4
The yield of the above acetylene, confirmed by GCMS, occurred
in quantitative yield (99%). This phenyl migration has been
seen before in a number of other organic reactions that produce
the vinylidene fragment as an intermediate.
The photochemical generation of a free carbene fragment
(TPP)CX2 and can be inhibited by photolysis in the presence
of high alkene substrate concentration.
1
2
(
10) (a) Skell, P. S.; Cholod, M. S. J. Am. Chem. Soc. 1969, 91, 6035.
b) Skell, P. S.; Cholod, M. S. J. Am. Chem. Soc. 1969, 91, 7131.
11) (a) Cook, B. R.; Reinert, T. J.; Suslick, K. S. J. Am. Chem. Soc.
(
(
from a transition metal carbene complex has not been observed
1
986, 108, 7281. (b) Suslick, K. S.; Cook, B. R. J. Chem. Soc., Chem.
13
previously. Although the photochemistry of both Fischer and
Schrock type carbene complexes has been investigated, no
examples of homolytic carbene dissociation have been found.
In Fischer type carbenes, photoexcitation often leads to loss of
carbonyl and subsequent reaction with substrates, or migration
and insertion of a carbonyl into the metal-carbon bond, as seen
in the recent chemistry of Hegedus and co-workers. In other
carbene and alkylidene compounds, such as (CO)5WdC(OR)-
R′ and [Cp(NO)(PPh3)RedChPh] , isomerization of the metal-
carbon double bond occurs. In the case of the metallopor-
Commun. 1987, 200. (c) Suslick, K. S. In ActiVation and Functionalization
of Alkanes; Hill, C. L., Ed.; John Wiley and Sons: New York, 1989; pp
2
19-241. (d) Collman, J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.;
Brauman, J. I. Science 1993, 261, 1404. (e) Suslick, K. S.; Van Deusen-
Jeffries, S. In ComprehensiVe Supramolecular Chemistry; Lehn, J. M., Ed.;
Elsevier: London, 1996; Vol. 5, in press.
14
(
12) Hartzler, H. D. J. Am. Chem. Soc. 1964, 86, 526.
(
13) (a) Pourreau, D. B.; Geoffroy, G. L. Photochemistry of Alkyl,
1
5
Alkylidene, and Alkylidyne Complexes of the Transition Metals. AdVances
in Organometallic Chemistry; Academic: New York, 1985; Vol. 24, pp
3
26-340.
+
(14) (a) Foley, H. C.; Strubinger, L. M.; Targos, T. S.; Geoffroy, G. L.
16
J. Am. Chem. Soc. 1983, 105, 3064. (b) Fong, L. K.; Cooper, N. J. J. Am.
Chem. Soc. 1984, 106, 2595. (c) Ofele, K; Roos, E.; Herberhold, M. Z.
Naturforsch., B: Anorg. Chem., Org. Chem. 1976, 31B, 1070. (d) Rieke,
R. D.; Kojima, H.; Ofele, K. J. Am. Chem. Soc. 1976, 98, 6735. (e) Casey,
C. P.; Shusterman, A. J.; Vollendorf, N. W.; Haller, K. J. J. Am. Chem.
Soc. 1982, 104, 2417.
phyrin carbene complexes, which should be referred to as
“Mansuy type” carbene complexes, the porphyrin excitation
initates the metal carbon bond cleavage. We believe that this
is due to the mixing between the π* orbitals of the porphyrin
ring and the iron-carbon orbitals, which is also responsible for
(
15) (a) McGuire, M. A.; Hegedus, L. S. J. Am. Chem. Soc. 1982, 104,
5
538. (b) Hegedus, L. S.; McGuire, M. A.; Schultze, L. M.; Yujin, C.;
17
the hypso type spectrum (i.e., blue-shifted) seen in these
Anderson, O. P. J. Am. Chem. Soc. 1984, 106, 2680. (c) Borel, C.; Hegedus,
L. S.; Krebs, J. Satoh, Y. J. Am. Chem. Soc. 1987, 109, 1101. (d) Hegedus,
L. S.; deWeck, G.; D’Andrea, S. J. Am. Chem. Soc. 1988, 110, 2122. (e)
Lastra, E.; Hegedus, L. S. J. Am. Chem. Soc. 1993, 115, 87. (f) Hegedus,
L. S. Acc. Chem. Res. 1995, 28, 299.
1
8
complexes.
Hypso spectra are a common trait in other
photodissociative porphyrin complexes, most notably CO
1
complexes.
(
16) (a) Rooney, A. D.; McGarvey, J. J.; Gordon, K. C. Organometallics
995, 14, 107. (b) McCormick, F. B.; Kiel, W. A.; Gladysz, J. A.
Organometallics 1982, 1, 405.
17) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic:
New York, 1979; Vol. 3, pp 1-165.
18) Tatsumi, K; Hoffmann, R. Inorg. Chem. 1981, 20, 3771.
Acknowledgment. This work was supported by a grant from the
National Institutes of Health (HL25934) and a National Science
Foundation Graduate Fellowship (C.J.Z.).
1
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JA954097E