Rapid catalytic oxygenation of hydrocarbons by ruthenium pentafluorophenylporphyrin complexes: Evidence for the involvement of a Ru(III) intermediate

Full text of DOI:10.1021/ja9542092

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J. Am. Chem. Soc. 1996, 118, 8961-8962
8961
Table 1. Hydrocarbon Oxidationa Catalyzed by [RuIITPFPP)(CO)]
Rapid Catalytic Oxygenation of Hydrocarbons by
Ruthenium Pentafluorophenylporphyrin
Complexes: Evidence for the Involvement of a
Ru(III) Intermediate
time
(min)
product
yieldc
(%) (turnovers/min)
max rated
No.
substrate
(% conv.)b
1e adamantane
2f adamantane
3 cis-decalin
20 1-adamantanol (76.2)
adamantane-1,3-diol (7.3)
120 1-adamantanol (61.0)
adamantane-1,3-diol (6.0)
25 (Z)-9-decalol (79.6)
(Z)-decal-9,10-diol (4.2)
60 (E-9-decalol (25.8)
secondary alcohols (4.3)
ketones (13.9)
91
97
90
70
72
800
64
John T. Groves,* Marcella Bonchio,†
Tommaso Carofiglio,‡ and Kirill Shalyaev
Department of Chemistry, Princeton UniVersity
4 trans-decalin
4.4
Princeton, New Jersey 08544
ReceiVed December 15, 1995
5f cyclohexane
180 cyclohexanol (1.6)
cyclohexanone (6.7)
95
22
New methods for the catalytic oxygenation of hydrocarbons
continue to be of fundamental interest1 and potential economic
value.2 Among the metalloporphyrin-mediated oxidations,1,3
ruthenium catalysts display remarkable activity for aerobic
oxidations3b,4 and promising reactivity with N2O.5 Recently,
Hirobe et al.6 have reported efficient oxygenation reactions with
ruthenium porphyrin complexes and aromatic N-oxides in the
presence of strong mineral acids. Here we describe the very
rapid oxygenation of a variety of substrates including alkanes,
terminal olefins, and benzene, catalyzed by perfluorinated
ruthenium porphyrin complexes7 in aprotic media and a dis-
section of the mechanism of this process.
Carbonyl (5,10,15,20-tetrapentafluorophenylporphyrinato)-
ruthenium(II) [RuII(TPFPP)(CO)], 1,8 has shown remarkable
activity9 with 2,6-dichloropyridine-N-oxide [pyCl2NO] as the
oxygen donor. Hydroxylations of adamantane and cis-decalin
were achieved with high selectivity, complete stereoretention,
high rates (up to 800 turnovers/min), and high efficiency (400-
14600 turnovers, Table 1, entries 1-3). Oxygenation of less
reactive substrates proceeded with lower but still significant
turnover numbers (100-3000, Table 1, entries 4, 5, and 7).10
Analysis of the adamantane hydroxylation catalyzed by 1
provided revealing information with respect to the mechanism.
The evolution of products over time (curve A, Figure 1) showed
an initial induction period followed by a fast, zero-order phase
6 1-octene
7 1-octene/
60 1,2-epoxyoctane (96)
1,2-epoxyoctane (54)
96
90
11 (36)g
9.5
4.8
adamantane 60 1-adamantanol (28)
8h benzene
12h 1,4-benzoquinone (13.3)
40
a [substrate] ) [pyCl2NO] ) 0.02 M, [1] ) 50 µM. All reactions in
CH2Cl2 at 65 °C in sealed containers. No difference was registered in
reactions performed under aerobic or anaerobic conditions. b Percent
conversion based on substrate consumed. Products were identified by
GC-MS and compared to authentic samples. c Percent yield based on
pyCl2NO consumed. d Maximum oxidation rate measured as the slope
of the zero-order phase of the kinetic plot (see text). Turnover numbers
based on the substrate conversion. e Similar conversions were obtained
with [RuVI(TPFPP)(O)2]18-22 and [RuVI(TPFPBr8P)(O)2]21 with pyCl2NO.
f [substrate] ) [pyCl2NO] ) 0.2 M, [1] ) 10 µM. g Cf. ref 16.
h [benzene] ) 2 M, [pyCl2NO] ) 0.02 M, [1] ) 50 µM.
* Author to whom correspondence should be addressed.
† Current address: Dipartimento di Chimica Organica, Universita` di
Padova, Italy.
‡ Current address: Dipartimento di Chimica Inorganica, Metallorganica
ed Analitica, Universita` di Padova, Italy.
(1) (a) Groves, J. T.; Han, Y.-Z. In Cytochrome P-450. Structure,
Mechanism and Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum
Press: New York, 1995; pp 3-48. (b) Groves, J. T.; Nemo, T. E. J. Am.
Chem. Soc. 1983, 105, 6243-6248. (c) ActiVation and functionalization
of alkanes; Hill, C. L., Ed.; John Wiley & Sons: New York, 1989.
(2) SelectiVe Hydrocarbon ActiVation: Principle and Progress; Davies,
J. A., et al., Eds; VCH: New York, 1994.
(3) (a) Metalloporphyrins in Catalytic Oxidations; Sheldon, R. A., Ed.;
M. Dekker: New York, 1994. (b) Mlodnika, T.; James, B. R. In
Metalloporphyrin Catalyzed Oxidations; Montanari, F., Casella, L., Eds.;
Kluwer: Dordrecht, The Netherlands, 1994; p 121.
(4) (a) Groves, J. T.; Quinn, R. Inorg. Chem. 1984, 23, 3844-3846.
(b) Groves, J. T.; Quinn, R. J. Am. Chem. Soc. 1985, 107, 5790-5792. (c)
Groves, J. T.; Ahn, K.-H. Inorg. Chem. 1987, 26, 3831-3833. (d) Groves,
J. T.; Quinn, R. U.S. Patent 4,822,899, April 18, 1989.
(5) Groves, J. T.; Roman, J. S. J. Am. Chem. soc. 1995, 117, 5594-
5595.
Figure 1. Time course for adamantane hydroxylation by 1 in CH2Cl2 at
65 °C; adamantane (0.02 M), pyCl2NO (0.02 M), 1 (50 µM); 1-
adamantanol determined by GC ((1%). Curve A: typical kinetic trace with
1 as catalyst, r ) 0.999 for the linear section, 68 ( 4 turnovers/min for
triplicate runs. Curve B: reaction A recharged with pyCl2NO and adamantane
to 0.02 M at t ) 20 min. Curve C: 4a used as the catalyst (see text).
with maximum rate and the highest efficiencies (column 6, Table
1). Deviation from linear behavior was observed only after 90%
of the oxygen donor and 80% of the substrate had been
consumed. Recharge of the reaction mixture with pyCl2NO and
adamantane afforded another catalytic burst with a similar rate
but with no induction period (curve B, Figure 1). Thus, the
active form of the catalyst was still fully efficient.
The transformation of 1 to the active catalyst proceeded Via
an adduct 2 [K ) 350 M-1, at 25 °C], as detected by changes
in the IR (1959-1950 cm-1) and UV-vis (404-406 nm)
spectra. Photostimulation with red-orange light (>560 nm) was
observed,11 consistent with photoejection of the carbonyl ligand.
Ruthenium porphyrin radical cations are known to be formed
from the corresponding carbonyl compounds by chemical or
electrochemical oxidation.12 Further, these species show a
characteristic absorption band in the 600-700 nm region.12c The
radical cation 3 was quantitatively generated by reaction of 1
with ferric perchlorate.13 The shift of ν CO (from 1950 to 1979
cm-1), the broadening of the Soret band, the appearance of a
(6) (a) Higuchi, T.; Satake, C.; Hirobe, M. J. Am. Chem. Soc. 1995, 117,
8879-8880. (b) Ohtake, H.; Higuchi, T.; Hirobe, M. Heterocycles 1995
and references therein. (c) Higuchi, T. J. Synth. Org. Chem. Jpn. 1995,
53, 644-644.
(7) (a) Ellis, P. E.; Lyons, J. E. Coord. Chem. ReV. 1990, 105, 181-
193. (b) Birnbaum, E. R.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E.,
Gray, H. B. Inorg. Chem. 1995, 34, 1751-1755. (c) Murahashi, S.-I.;
Naota, T.; Komiya, N. Tetrahedron Lett. 1995, 36, 8059-8062.
(8) 1 was prepared by metalation of the free base with Ru3(CO)12 in
refluxing o-dichlorobenzene (55% yield). 1H-NMR (CDCl3, δ) 8.70. IR
(KBr, cm-1) 1965 (ν CdO). UV-vis (CHCl3, nm) 404, 524, 554. Cf. ref
7c.
(9) Vis-a`-vis RuVI-dioxo and RuIV-oxo porphyrin based systems (cf. ref
4 and Ho, C.; Leung, W.-H.; Che, C.-M. J. Chem. Soc., Dalton Trans.
1991, 2933-2939.)
(10) Degradation of the active catalyst at higher catalyst concentration,
which is more pronounced with the less reactive substrates, would account
for the high activity at lower catalyst concentration (Table 1, entry 2). Also
the identity of the axial ligand X, in Scheme 1, is probably concentration
dependent, thus further affecting the reaction rates.
(11) Unfiltered irradiation (tungsten lamp, 300 W) caused the rapid
degradation of the porphyrin ligand.
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8962 J. Am. Chem. Soc., Vol. 118, No. 37, 1996
Communications to the Editor
Scheme 1
substrates displayed the same rate in the fast kinetic phase in
separate reactions. Similar results were obtained for decalin
(kH/kD ) 5.4 at 40 °C). These observations can be explained if
a reactive intermediate, such as 5, is formed in the rate-
determining step. The temperature dependence of adamantane
hydroxylation suggested a significant barrier for the N-O bond
cleavage step.15
The unusually low reactivity of 1-octene with respect to
adamantane and cis-decalin appeared to result from catalyst
inhibition by this substrate. Thus, 1-octene was twice as reactive
as adamantane in competitive oxidation, and the efficiency of
the 1-octene epoxidation was improved 3-fold when excess
pyCl2NO was employed.16 Coordination of 1-octene to the RuIII
center would explain this observation, but no such adduct was
observed.17
The results are consistent with the rate-determining formation
of a RuV-oxo species or a RuIV-oxo porphyrin radical cation12a,b
(5) as the reactive species in these reactions. A similar
transformation of a RuIII octaethyl porphyrin (OEP) complex
to a reactive oxo-RuIV (OEP) cation radical has been proposed
by Dolphin and James;3b,12a,b however, the stability reported for
that species at 25° 12f stands in contrast to the reactivity of 5.
Significantly, pyCl2NO was not able to oxidize [RuIV(TPFPP)-
(O)] to [RuVI(TPFPP)(O)2]18 rapidly, and this oxidant is an
unlikely intermediate.19 Oxygen donors such as iodosylbenzene,
magnesium monoperoxyphthalate, oxone, and tetrabutylammo-
nium periodate were able to produce the trans-dioxo species
but were ineffective for rapid catalysis under the conditions
described here.20
Figure 2. Superimposed UV spectra (CH2Cl2, 25 °C) of 2 (s) (1 ) [50
µM] + pyCl2NO ) [0.02 M]), 3 (- ‚ -), and 4a (‚ ‚ ‚). Inset: X-
band EPR of 3 (inset a) and 4b (inset b) recorded in CH2Cl2 at 135 K.
635 nm absorption (Figure 2, dotted curve), and the EPR signal
(g ) 2.00) (Figure 2, inset a) agree with the proposed structure
for 3.12c Dichloromethane solutions of compound 3 were stable
at low temperature, but 3 converted slowly to 1 with clean
isosbestic behavior (IR and UV-vis) upon standing at 25 °C.
Reaction of 3 with an excess of the N-oxide at -50 °C
induced a color change from green to orange. Evolution of
CO was observed as the solution reached 25 °C, leading to
species 4a (Figure 2, line-dotted curve). The occurrence of an
internal electron transfer process from ruthenium(II) to the
porphyrin radical cation upon ligation has been reported.12c
Indeed, compound 4b (OD ) 2,6-lutidine-N-oxide) showed an
EPR signal typical of a ruthenium(III) species12d,e (Figure 2,
inset b, g ) 2.55, g| ) 2.05). Significantly, compound 4a
(OD ) pyCl2NO) catalyzed adamantane hydroxylation with no
induction period.14 However, the zero-order regime occurred
with a similar rate (Figure 1, curve C). Moreover, 4a was the
species observed by UV-vis spectroscopy during the rapid
phase of catalysis.
Rapid, efficient hydrocarbon oxygenation can be achieved
with a perfluorophenyl RuIII porphyrin catalyst Via the stepwise
oxidation of the ruthenium(II) carbonyl complex. A search for
accesses to the key RuIII-RuV cycle from O2, N2O, and H2O2
is underway.
Acknowledgment. Support of the National Science Foundation and
fellowship grants to M.B. and T.C. from the University of Padova are
gratefully acknowledged.
On the basis of the above observations, we propose the
catalytic cycle in Scheme 1 for this reaction. Such a mechanism
accounts both for the slow initiation leading to the formation
of 4a and for the zero-order phase of the oxygenation process.
Significantly, the hydroxylation of adamantane showed a kinetic
deuterium isotopic effect (kH/kD ) 3.05 at 65 °C) in a
competitive experiment, while deuterated and undeuterated
JA9542092
(15) An Eyring plot of the zero-order rate constants determined over
the range 318-338 K was linear indicating a ∆Hq of 28.6 kcal/mol and a
small but positive ∆Sq of 7.0 cal mol-1 K-1; (ln k ) -9.73, -10.76, -12.39
at 338, 328, and 318 K, respectively).
(16) [1-octene] ) 0.01 M, [pyCl2NO] ) 0.04 M, [1] ) 50 µM, in CH2-
Cl2 at 65 °C, maximum rate ) 36 turnovers/min (entry 6, Table 1).
(17) Groves, J. T.; Ahn, K.-H.; Quinn, R. J. Am. Chem. Soc. 1988, 110,
4217-4220.
(12) (a) Dolphin, D.; James, B. R.; Leung, T. Inorg. Chim. Acta 1983,
79, 25-27. (b) Leung, T.; James, B. R.; Dolphin, D. Inorg. Chim. Acta
1983, 79, 180-181. (c) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin,
D.; James, B. R. Can. J. Chem. 1983, 61, 2389-2396. (d) James, B. R.;
Dolphin, D.; Leung, T. W.; Einstein, F. W.; Willis, A. C. Can. J. Chem.
1984, 62, 1238-1245. (e) James, B. R.; Mikkelsen, S. R.; Leung, T. W.;
Williams, G. M., Wong, R. Inorg. Chim. Acta B 1984, 85, 209-213. (f)
For a review, see: James, B. R. In Fundamentals of Research in
Homogeneous Catalysis; Shilov, A. E., Ed. Gordon Breach: New York,
1986; pp 309-324.
(13) An emerald green solution of 3 was obtained upon oxidation of 1
with Fe(ClO4)3‚nH2O in CH2Cl2; IR (CH2Cl2, -50 °C,) 1979 cm-1 (ν
CdO).
(14) PyCl2NO was added to a solution of 3 at -50 °C. The substrate
was added after 4a formed at 25 °C. The reaction mixture was then heated
at 65 °C.
(18) [RuVI(TPFPP)(O)2] was obtained by oxidation of 1 with m-
chloroperbenzoic acid (m-CPBA) in CH2Cl2. 1H-NMR (CDCl3, δ) 9.16.
IR (KBr, cm-1) 826 (ν RudO). UV-vis (CHCl3, nm) 412, 506. RuIV
-
(TPFPP)(O) was prepared from [RuVI(TPFPP)(O)2] by treatment with
17
PPh3
.
1H-NMR (CDCl3, δ) -8.70. Excess pyCl2NO did not afford [RuVI-
(TPFPP)(O)2] in 48 h.
(19) No products were observed upon spontaneous decomposition of
[RuVI(TPFPP)(O)2] in the presence of adamantane.
(20) The hydroxylation of steroids and similar substrates was observed
when [RuVI(TPFPP)(O)2] was used as starting catalyst in the presence of
pyCl2NO; however, turnover was slower, and no variation of the UV-vis
spectrum of [RuVI(TPFPP)(O)2] was observed during catalysis.
(21) [RuVI(TPFPBr8P)(O)2] was obtained by oxidation of the correspond-
ing Ru(II)-carbonyl complex7b with m-CPBA in CH2Cl2. IR (KBr, cm-1
830 (ν RudO), UV-vis (CHCl3, nm) 440, 524, 560.
)