Photoassisted oxygenation of alkane catalyzed by ruthenium complexes using 2,6-dichloropyridine N-oxide under visible light irradiation

Article abstract of DOI:10.1039/b316970g

The chloro(Me₂SO)ruthenium(II) complexes with tris(2-pyridylmethyl)amine or its derivative catalyses the selective, stereospecific, and photoregulative alkane oxidation in the presence of 2,6-dichloropyridine N-oxide under visible light irradiation.

Full text of DOI:10.1039/b316970g

Photoassisted oxygenation of alkane catalyzed by ruthenium complexes
using 2,6-dichloropyridine N-oxide under visible light irradiation†
Motowo Yamaguchi,* Takashi Kumano, Dai Masui and Takamichi Yamagishi
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University,
Minami-osawa, Hachioji, Tokyo 192-0397, Japan
Received (in Cambridge, UK) 5th January 2004, Accepted 6th February 2004
First published as an Advance Article on the web 23rd February 2004
The chloro(Me2SO)ruthenium(II) complexes with tris(2-pyr-
idylmethyl)amine or its derivative catalyses the selective,
stereospecific, and photoregulative alkane oxidation in the
presence of 2,6-dichloropyridine N-oxide under visible light
irradiation.
and an appearance of the new peak assigned as a free Me2SO
molecule (Fig. S-2†). A single isomer of the chloro(MeCN)
complex with TPA was obtained. Fast-atom bombardment mass
spectra of the photoreaction products clearly indicated the forma-
tion of [RuCl(TPA)(Me2SO-d6)]Cl and [RuCl(CD3CN)(T-
PA)]Cl.10 It was concluded that excitation in the MLCT band
resulted in the selective substitution of the S-bound Me2SO ligand
by a solvent molecule. It is noteworthy that no isomerization in
regard to the geometry was observed in Me2SO.
Photocatalysts have drawn much attention, since they are capable
of activating the unreactive C–H bonds to functionalize saturated
hydrocarbons.1 Dissociation of a ligand is often a trigger to initiate
the reaction catalyzed by transition metal complexes.1,2 Photo-
chemical ligand exchange and/or isomerizations of ruthenium
complexes have been well known. Photosubstitution reactions of
polypyridyl ruthenium(II) complexes are generally assumed to
occur by excitation in the MLCT region via a dissociative
mechanism.3
Development of the efficient oxidation catalyst for saturated
hydrocarbons is a challenging goal for chemists.4,5 We have studied
oxidation of alkanes catalyzed by non-heme ruthenium com-
plexes.6 Recently, we have found a new ruthenium-catalyzed
oxidation reaction of alkanes utilizing 2,6-dichloropyridine N-
oxide as co-oxidant and m-CPBA (m-chloroperbenzoic acid) or
Ce(IV) ion as initiator.6c It is assumed that the role of the initiator is
to oxidize a Ru(II) ion resulting in labilization of a ligand and the
key step of the reaction is a dissociation of a ligand in the ruthenium
complex. The further reaction with N-oxide may form the oxo
species.6c Katsuki and co-workers reported that the oxidation of
alcohols and epoxidation of alkenes catalyzed by nitrosyl(salen)
ruthenium complexes under photoirradiation including photo-
release of NO.7,8 In the present study, the light-driven alkane
oxygenation reaction catalyzed by ruthenium complexes using N-
oxide has been investigated.
Photosubstitution reactions of trans(Cl,
Namino)-[RuCl{5-
(MeOCO)3-TPA}(Me2SO)]Cl (2), §which showed high catalytic
ability than 1 on alkane oxidation,6c have also been examined and
the substituted products were obtained as shown below (Fig. S-3, S-
4, S-5†).11,12 A quantum yield for substitution of the Me2SO ligand
of 2 irradiated at 415 nm (Xe lamp) in MeCN was 0.013 ± 0.001
determined by actinometry. In the presence of 4-picoline (10
equiv.) in 1,2-dichloroethane similar UV-vis spectral changes of 2
Photosubstitution reactions of chloro(Me2SO)ruthenium(II
complex with tris(2-pyridylmethyl)amine ( = TPA), trans(Cl,
amino)-[RuCl(TPA)(Me2SO)]Cl (1), have been investigated in
Me2SO and in MeCN.†6a,9 Although the UV-vis spectrum of 1 in
Me2SO showed no change by photoirradiation, the Me2SO peak in
the NMR spectrum of 1 in Me2SO-d6 collapsed by irradiation (Fig.
S-1†). The UV-vis spectra in MeCN have changed by photoirradia-
tion with isosbestic points at 264 and 370 nm and become constant
after five-min irradiation, as shown in Fig. 1. The NMR spectra in
CD3CN clearly showed a collapse of the coordinated Me2SO peak
)
Fig. 1 UV-vis spectral change of chloro((Me2SO)ruthenium(II) complex,
trans(Cl, Namino)-[RuCl(TPA)(Me2SO)]Cl (1), under photoirradiation in
MeCN. 6.7 3 1025 mol l21. 0, 3, 8, 15, 30 sec, 1, 1.5, 2, 3, 5 min. lmax
(before irradiation): 318 nm(sh); 365 nm. lmax (after irradiation): 336
nm(sh); 368 nm(sh); 411 nm.
N
† Electronic supplementary information (ESI) available: syntheses of the
complexes, crystallographic data of complex 4 (Table S-1), 1H NMR
spectra before and after irradiation (Fig. S-1, S-2, S-3, S-5), UV-vis spectral
change (Fig. S-4, S-6). See http://www.rsc.org/suppdata/cc/b3/b316970g/
Fig. 2 ORTEP drawing of the cation of trans(Cl, Namino)-[RuCl{5-
(MeOCO)3-TPA}(Me2SO)]PF6 (4). Thermal ellipsoids are drawn at the
50% probability level.‡
798
C h e m . C o m m u n . , 2 0 0 4 , 7 9 8 – 7 9 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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with isosbestic points have been observed under photoirradiation
(Fig. S-6†). The photoproduct was the corresponding 4-picoline
complex.13
We are grateful to Professor Haruo Inoue and Dr. Shinsuke
Takagi for help in measurements of the quantum yield and fruitful
discussions.
Since the above experiments clearly demonstrates that the
Me2SO ligand of 1 and 2 selectively dissociates by photoirradiation
in the MLCT region, catalytic alkane oxidation catalyzed with
ruthenium complex 1 or 2 using 2,6-dichloropyridine N-oxide
under visible light irradiation ( > 385 nm) has been examined.14 The
catalytic oxidation of adamantane using 1 or 2 under irradiation
gave 1-adamantanol and adamantane-1,3-diol selectively in good
yields (Table 1). No 2-adamantanol was obtained, and only trace
amounts of adamantan-2-one and 1-chloroadamantane were de-
tected. The substrate was selectively oxidized at a tertiary carbon.
Induction periods were observed: 3 h for 1 and 0.5 h for 2. No
oxidation product was detected without the ruthenium complexes
or irradiation. Although efficient alkane oxidation reactions
catalyzed by Ru porphyrins with 2,6-dichloropyridine N-oxide
have been reported,15 there was no previous report of those
catalyzed by non-heme type complexes except ours, as far as we
know.6c,d Initiators such as m-CPBA or Ce(IV) ion were required in
the dark reactions, however, no initiator was required in the
photoirradiated reactions. It is noteworthy that the reaction under
irradiation was significantly faster than that without irradiation in
the presence of initiator. Furthermore, hydroxylation of trans- or
cis-decalin at tertiary carbon was also achieved with complete
retention of the configuration, giving trans- or cis-9-decalinol
stereospecifically,16 which suggests that the involvement of the
radical species can be excluded as in the case with the Ru porphyrin
complexes.15
Katsuki et al. have reported that irradiation was required only at
the initial stage of the reaction in the photooxidation of alcohol
catalyzed by a (nitrosyl)ruthenium complex.7c If the irradiation is
required only for the initiation of the reaction, once the oxidation
reaction starts, the reaction proceeds without irradiation. To testify
it, the reaction under irradiation with ON/OFF switching was
examined. It was found that the reaction was suppressed con-
siderably when the irradiation was turned off, and the oxidation
restarted by irradiation. It clearly showed that the reaction is not a
photoinitiated one but a photoassisted reaction. This behavior
suggests that the irradiation may play a significant role not only in
the initiation of the catalytic reaction but also in generating the
active species for alkane oxidation. It is for the first time to realize
the photoassisted catalytic oxygen transfer from 2,6-dichloropyr-
idine N-oxide to alkanes, as far as we know.17
Notes and references
‡ CCDC 230225. See http://www.rsc.org/suppdata/cc/b3/b316970g/ for
crystallographic data in .cif or other electronic format.
§ 2: FAB-MS: (M 2 Cl)+ 679, (M 2 Cl 2 Me2SO)+ 601. 1H NMR:
d(CDCl3, 270 MHz) 2.93 (6H, s, Me2SO), 3.95 (6H, s, CH3OCO), 3.98 (3H,
s, CH3OCO), 5.38 (2H, s, CH2(ax)), 5.42 (2H, d, J = 15.5 Hz, CH2(eq)),
5.81 (2H, d, J = 15.5 Hz, CH2(eq)), 7.64 (1H, d, J = 8.2 Hz, py-H3(ax)),
7.82 (2H, J = 8.2 Hz, py-H3(eq)), 8.17 (1H, dd, J = 2.0, 8.2 Hz, py-
H4(ax)), 8.29 (2H, dd, J = 2.0, 8.2 Hz, py-H4(eq)), 9.33 (2H, d, J = 2.0 Hz,
py-H6(eq)), 10.29 (1H, d, J = 2.0 Hz, py-H6(ax)). The X-ray structure of
the PF6 salt, complex 4, is shown in Fig. 2.
1 (a) R. H. Crabtree, Photosensitization and Photocatalysis Using
Inorganic and Organometallic Compounds, ed. K. Kalyanasundaram
and M. Gräzel, Kluwer Academic Publishers, Dordrecht, 1993, p. 391;
(b) A. N. Druzhinina, L. S. Shul’pina and G. B. Shul’pin, Bull. Acad.
Sci. USSR Div. Chem. Sci., 1991, 40, 1492; (c) A. E. Shilov and G. B.
Shul’pin, Chem. Rev., 1997, 97, 2879.
2 (a) T. Sakakura and M. Tanaka, Chem. Lett., 1987, 249; (b) M. Tanaka
and T. Sakakura, Homogeneous Transition Metal Catalyzed Reactions,
ed. W. R. Moser and D. W. Slocum, American Chemical Society,
Washington, DC, 1992, p. 181.
3 (a) B. Durham, J. V. Caspar, J. K. Nagle and T. J. Meyer, J. Am. Chem.
Soc., 1982, 104, 1803; (b) C. R. Hecker, P. E. Fanwick and D. R.
McMillin, Inorg. Chem., 1991, 30, 659.
4 (a) A. E. Shilov, Metal Complexes in Biomimetic Chemical Reactions,
CRC Press, Boca Raton, 1997, p. 176; (b) Z. Hu and S. M. Gorum,
Biomimetic Oxidations Catalyzed by Transition Metal Complexes, ed.
B. Meunier, Imperial College Press, London, 2000, p. 269.
5 (a) M. Ito, K. Fujisawa, N. Kitajima and Y. Moro-oka, Oxygenase and
Model Systems, ed. T. Funabiki, Kluwer Academic Publishers, Dor-
drecht, 1997, p. 345; (b) T. Naota, H. Takata and S.-I. Murahashi, Chem.
Rev., 1998, 98, 2599.
6 (a) M. Yamaguchi, H. Kousaka and T. Yamagishi, Chem. Lett., 1997,
769; (b) M. Yamaguchi, T. Iida and T. Yamagishi, Inorg. Chem.
Commun., 1998, 1, 299; (c) M. Yamaguchi, Y. Ichii, S. Kosaka, D.
Masui and T. Yamagishi, Chem. Lett., 2002, 434; (d) M. Yamaguchi, ,
S. Izawa, , D. Masui and and T. Yamagishi, 34th International
Conference on Coordination Chemistry, Edinburgh, 2000, PE009; (e)
X-ray structure of trans(Cl, Namino)-[RuCl(TPA)(Me2SO)][RuCl-
3(Me2SO)3] (3); D. Masui, M. Yamaguchi and T. Yamagishi, Acta
Cryst., 2003, E59, m308.
In conclusion, we have developed stereospecific and photo-
regulative catalytic alkane oxidation reactions using chloro-
(Me2SO) ruthenium(II) complexes with tris(2-pyridylmethyl)amine
or its derivative in the presence of 2,6-dichloropyridine N-oxide
under visible light irradiation. Further studies on application of this
reaction and mechanistic investigations are now in progress.
7 (a) T. Takeda, R. Irie, Y. Shinoda and T. Katsuki, Synlett, 1999, 1157;
(b) K. Matsutani, T. Uchida, R. Irie and T. Katsuki, Tetrahedron Lett.,
2000, 41, 5119; (c) A. Miyata, M. Murakami, R. Irie and T. Katsuki,
Tetrahedron Lett., 2001, 42, 7067.
8 C. F. Works, C. J. Jocher, G. D. Bart, X.-H. Bu and P. C. Ford, Inorg.
Chem., 2002, 41, 3728.
9 Complex 1 was obtained by fractional recrystallizations from MeOH
and AcOEt. The solution of 1 in MeCN or Me2SO was irradiated by a
Pyrex-filtered 500 W ultra-high pressure mercury lamp.
10 FAB MS: (M 2 Cl)+ 505 for 1; (M 2 Cl)+ 511 for [RuCl(TPA)(Me2SO-
d6)]Cl; (M 2 Cl)+ 471 for [RuCl(CD3CN)(TPA)]Cl.
Table 1 Catalytic oxidation of adamantane catalyzed by complex 1 or 2 with
2,6-dichloropyridine N-oxide under visible light irradiationa
11 FAB MS: (M 2 Cl)+ 685 for [RuCl{5-(MeOCO)3-TPA}(Me2SO-
d6)]Cl; (M 2 Cl)+ 645 for [RuCl(CD3CN){5-(MeOCO)3-TPA}]Cl.
12 The structure of the chloro(MeCN) complexes is tentative, since the
position of the MeCN ligand is inconclusive, thus far.
1-Adamantanol Adamantane-
Catalyst
Time/h
(%)b
1,3-diol (%)b
TON
13 FAB MS: M+ 694 for [RuCl{5-(MeOCO)3-TPA} (4-Mepy)]+.
14 Visible light ( > 385 nm) was irradiated, since 2,6-dichloropyridine N-
oxide decomposed by UV irradiation.
15 (a) H. Ohtake, T. Higuchi and M. Hirobe, J. Am. Chem. Soc., 1992, 114,
10660; (b) J. T. Groves, M. Bonchio, T. Carofiglio and K. Shalyaev, J.
Am. Chem. Soc., 1996, 118, 8961.
16 The oxidation of decalin was done under visible light irradiation using
2,6-dichloropyridine N-oxide for 5 h in 1,2-dichloroethane at room
temperature. With complex 2, trans-9-decalinol from trans-decalin
(conv. = 13%) and cis-9-decalinol from cis-decalin (conv. = 71%)
were obtained stereospecifically.
1
24
4
24
24
46
78
0
9
13
0
128
208
0
2
2c
2d
65
18
202
a Conditions: the reaction was carried out under visible light irradiation
( > 385 nm) in 1,2-dichloroethane under nitrogen at room temperature.
[adamantane] = 0.04 mol l21. The ratio of adamantane–2,6-dichloropyr-
idine N-oxide–catalyst was 200 : 300 : 1. b Determined by GC-MS with
internal standard based on the substrate. c The reaction without irradiation.
d The reaction without irradiation in the presence of m-CPBA. Ada-
mantane–2,6-dichloropyridine N-oxide–catalyst–m-CPBA was 200 : 300 : 1
: 10.
17 Groves et al. reported the photostimulation ( > 560 nm) of the
transformation of [Ru(TPFPP)(CO)] to the active catalyst, however, no
result of catalytic alkane reaction was provided15b
.
C h e m . C o m m u n . , 2 0 0 4 , 7 9 8 – 7 9 9
799