Reactivity of Model Complexes
FULL PAPER
Table 1. Values of the second-order rate constants for the oxidation of
selected organic substrates by Cpds 0, I, and II (direct measurements at
priate way. That is why in biological systems its generation
is controlled by a proton relay from the amino acid residues
at the active site. On the one hand, acid catalysis promotes
the conversion from Cpd 0 to Cpd I, and on the other hand,
À158C) produced from [FeIII
ACHTUNGTREN(NUGN TMP)(OH)] in MeCN.
Substrate
kCpd 0 [mÀ1 sÀ1
]
kCpd I [mÀ1 sÀ1
]
kCpd II [mÀ1 sÀ1
]
cis-stilbene[a]
DMS[b]
0.142Æ0.006
9.7Æ0.1
66Æ2
0.063Æ0.004
6.4Æ0.2
À
it suppresses a possible homolytic cleavage of the O O
(1.7Æ0.1)ꢁ104[f]
bond to form Cpd II. Therefore, this high-valent oxoiro-
TBPH[c]
(0.50Æ0.04)ꢁ103
2.2Æ0.3
R
ACHTUNGTRENNUNG
n(IV)–porphyrin p-cation radical is commonly accepted as
DHA[d]
ACHTUNGTRENNUNG
xanthene[d]
4.3Æ0.3
AHCTUNGTRENNUNG
5.3Æ0.1
the main reactive intermediate in iron
ACHTUNGERTN(NUNG III)–porphyrin-cata-
fluorene[d]
0.09Æ0.02
6.1Æ1.0
0.140Æ0.002
lyzed reactions.[1]
[e]
AcrH2
(5.6Æ0.3)ꢁ103
(0.87Æ0.08)ꢁ102
(1.50Æ0.03)ꢁ104
A completely different situation was observed for the hy-
dride transfer reaction with 10-methyl-9,10-dihydroacridine
(AcrH2) as the nicotinamide adenine dinucleotide (NADH)
analogue. As known from recent findings,[10] this kind of re-
action can be described as a hydrogen-atom transfer process
regarded as proton-coupled electron transfer (PCET) from
À
À
[a] Epoxidation. [b] Sulfoxidation; [c] O H abstraction. [d] C H abstrac-
tion. [e] Hydride transfer. [f] At À358C.
idation reactions, since the latter are 2eÀ-oxidation process-
es. Thus, for the reaction of Cpd II with cis-stilbene or di-
methyl sulfide (DMS), a multistep redox process must oper-
ate. As a consequence, the epoxidation of cis-stilbene with
Cpd 0 is more than twice as fast as the reaction with Cpd II.
This effect is smaller in sulfoxidation reactions, since sulfides
are electronically more versatile reaction partners, which
can form more stable intermediate states. In general, the
sulfoxidation reaction turned out to be very fast, whereas
epoxidation proved to be one of the slowest of all the stud-
ied oxygenation reactions. This is actually not surprising
since cis-stilbene is known to be an inert substrate.[8]
the NADH analogue to Cpd II, with a subsequent electron
+
C
transfer from AcrH to Cpd II to form AcrH as the final
product (see Scheme 4).
Scheme 4. General reaction scheme for the sequential ꢂhydride abstrac-
tionꢃ from AcrH2.
À
A comparison of the rate constants for the C H abstrac-
tion reactions between Cpds 0, I, and II and the different
substrates used for this type of reaction reveals a close cor-
relation between the resulting reactivity order (namely, fluo-
rene < 9,10-dihydroanthracene (DHA) < xanthene; see
In this eÀ/H+/eÀ sequence, the proton transfer is the rate-
determining step,[11] therefore the reactivity order of Cpds 0,
I, and II can be accounted for in terms of the basicity of the
formed intermediates. In this case, Cpd II appears to be the
most reactive species as a consequence of the mechanism of
the hydride-transfer process. In the initial electron-transfer
Table 1) and the corresponding bond dissociation energies
À1
À
of the C H bond (namely, 80.1, 76.3, and 74.2 kcalmol for
fluorene, DHA, and xanthene, respectively).[9] This coher-
step, Cpd II is reduced to an ironACTHNGUTERNNU(G III)–oxo species, whereas
À
ence between the strength of the C H bond and the result-
ing rate constant is also a strong indication that Cpds 0, I,
Cpd I is reduced to the iron(IV)-oxo species Cpd II. Since
the lower-charged iron center has a higher ability to pro-
+
À
C
and II can indeed promote C H abstractions as postulated
above (see Figure 6, as well as Figures S2 and S3 in the Sup-
porting Information). Our results concerning C H abstrac-
tion and hydride-transfer reactions are fully consistent with
recent findings by Nam and Fukuzumi et al., who investigat-
ed the reactivity of Cpd II for related porphyrin systems.[10]
mote the subsequent proton abstraction from AcrH2 , the
reaction of Cpd II is significantly faster. It is worth noting
that in the case of Cpd I the reaction solution is slightly
acidified by the excess of m-CPBA that also delivers the
À
À
necessary protons for the heterolytic cleavage of the O O
bond in the conversion of Cpd 0 to Cpd I as stated above.
Due to this effect, even Cpd 0 turned out to be a more ef-
fective ꢂhydride abstractionꢃ agent than Cpd I. Apart from
the fact that again a lower-charged iron species is involved
in the proton abstraction, Cpd 0 is produced by the addition
of a subequivalent amount of m-CPBA to prevent the
acidification of the solution and thereby the conversion to
Cpd I. The latter process implies that also the
À
A special situation can be observed in O H abstraction
reactions, in which second-order rate constants are quite
high and rather similar to each other (ca. one order of mag-
nitude difference between Cpds 0, I, and II). This may be
due to the fact that hydrogen abstraction in this case is very
easy, and the substrate 2,4,6-tri-tert-butylphenol (TBPH) is
commonly used as a very effective radical scavenger.
The most-discussed and best-analyzed reactive intermedi-
ate is surely the high-valent iron(IV)–oxo p-cation radical
(Cpd I), due to its outstanding oxidizing capabilities. As ex-
pected, it also turned out to be the most effective catalytic
species in our experimental studies. In some cases it outper-
forms Cpd 0 and Cpd II by a few orders of magnitude as far
as the rate constants are concerned (see Table 1). The reac-
tion of Cpd I with DMS even had to be studied at À358C to
enable us to follow the course of the reaction in an appro-
acylperoxoironACTHNUTRGNEUNG(III)–porphyrin complex (Cpd 0) can act as a
+
C
.
base and contribute to the proton abstraction from AcrH2
Conclusion
In conclusion, the exceptional possibility to selectively pro-
duce, identify, and stabilize Cpd 0, Cpd I, and Cpd II in solu-
tion enabled us to carry out direct kinetic studies on the re-
Chem. Eur. J. 2009, 15, 13435 – 13440
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13439