Adamantane Selective Hydroxylation by 2,6-Dichloropyridine N-Oxide and Organoruthenium(II) Polyoxometalates as Catalyst Precursors

Article abstract of DOI:10.1002/1615-4169(200209)344:8<841::AID-ADSC841>3.0.CO;2-L

Organoruthenium polyoxometalates with general formula [{Ru(C₆Me₆)}₃M₅O₁₈], M=Mo, W, serve as catalyst precursors, together with 2,6-dichloropyridine N-oxide, to effect the hydroxylation of adamantane with conversion up to 94%, and C³-H/C²-H selectivity > 100. Under analogous conditions, hydroxylation of cis-decalin occurred with complete stereoretention. Control experiments and kinetic evidence suggest the in-situ formation of a high valent Ru-oxo species as the competent oxidant.

Full text of DOI:10.1002/1615-4169(200209)344:8<841::AID-ADSC841>3.0.CO;2-L

COMMUNICATIONS
Adamantane Selective Hydroxylation by 2,6-Dichloropyridine
N-Oxide and Organoruthenium(II) Polyoxometalates as Catalyst
Precursors
Marcella Bonchio,a,* Gianfranco Scorrano,a Paolo Toniolo,a Anna Proust,b Vincent
Artero,b Valeria Contec
a
¡
CNR-CMRO, Dipartimento di Chimica Organica, Universita di Padova, Via Marzolo 1, 35131 Padova, Italy
Fax: ( ‡ 39)-049-8275239, e-mail: marcella.bonchio@unipd.it
b
c
¬
Universite Pierre et Marie Curie (Paris 6) LCIM, 4 place Jussieu, 75252 Paris cedex 05, France
Fax: ( ‡ 33)-1-44273841, e-mail: proust@ccr.jussieu.fr
¡
Dipartimento di Scienze e Tecnologie Chimiche, Universita di Roma ∫Tor Vergata, via della Ricerca Scientifica,
00133 Roma, Italy
Fax: ( ‡ 39)-06-72594328, e-mail: valeria.conte@uniroma2.it
Received: May 8, 2002; Accepted: July 12, 2002
Dedicated to Prof. Roger Sheldon on the occasion of his 60th birthday.
metalates;[8] (ii) functionalisation of the polyoxoanion
Abstract: Organorutheniumpolyoxometalates with
general formula [{Ru(C6Me6)}3M5O18], M Mo, W,
surface leading to ruthenium-supported polyoxometa-
lates.[9] The latter approach has been used for the
synthesis of new organometallic species providing the
rutheniumcentre with a hybrid set of ligands comprising
both organic residues and polyoxometalates.[10]
ˆ
serve as catalyst precursors, together with 2,6-
dichloropyridine N-oxide, to effect the hydroxyla-
tion of adamantane with conversion up to 94%, and
C3-H/C2-H selectivity >100. Under analogous con-
ditions, hydroxylation of cis-decalin occurred with
complete stereoretention. Control experiments and
kinetic evidence suggest the in-situ formation of a
high valent Ru-oxo species as the competent
oxidant.
We now present preliminary results obtained in the
oxyfunctionalisation of adamantane with 2,6-dichloro-
pyridine N-oxide and organoruthenium(II) polyoxome-
talates with general formula [{Ru(C6Me6)}3M5O18],
ˆ
M Mo, W. These complexes are lacunary Lindqvist-
type polyoxoanions supporting three rutheniumcentres
(Figure 1).[9]
Keywords: C-H activation; 2,6-dichloropyridine N-
oxide; hydrocarbon hydroxylation; oxidation; poly-
oxometalates; ruthenium
Among the metal-mediated oxidations, ruthenium
catalysts have shown some unmatched performances
in terms of dioxygen activation, selectivity, and turnover
rate.[1 3] Outstanding results have been obtained in many
diverse transformations, including alcohol[4] and ether
oxidation,[5] epoxidation,[6] and hydroxylation of unac-
tivated alkanes.[3] Considering the catalyst design, a
twofold issue to be addressed relates to the electronic
tuning of the catalytic centre and the inertness of the
metal coordination sphere under oxidising conditions. A
promising perspective is to provide the active ruthenium
centre with a totally inorganic ligand systemderived
[7]
frompolyoxometalates (POM).
With this aim, two different routes have been pursued:
(i) rutheniumincorporation within the polyoxoanion
framework, leading to ruthenium-substituted polyoxo- [{Ru (C6Me6)}3M5O18] M Mo, W.
Figure 1. Structure of organorutheniumpolyoxometalates
II
ˆ
Adv. Synth. Catal. 2002, 344, No. 8
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/02/34408-841 844 $ 17.50+.50/0
841

Marcella Bonchio et al.
COMMUNICATIONS
For comparison purposes we have also investigated nium). In the absence of ruthenium no attack to the
the catalytic behaviour of two organometallic precur- substrate is detected. Scheme 1 illustrates the reaction
sors used for the synthesis of POM derivatives, i.e., conditions and classifies the observed products high-
[Ru(C6Me6)Cl2]2 and [Ru(p-cymene)Cl2]2. No reaction lighting the regioselectivity.
was observed with the rutheniumincorporated polyox-
As reported in Table 1, addition of acid promotes the
oxidation rate with no appreciable influence on the
otungstate [Ru(DMSO)PW11O39]5 .[8]
In our study, to prevent the incursion of radical product distribution (entry 1). The oxidation catalysed
oxidation pathways, likely occurring when peroxidic by [{Ru(C6Me6)}3Mo5O18] (catalyst A) has been taken as
oxidants are used with transition metal catalysts, a reference to investigate both the effect of 2,6-dichloro-
heteroaromatic N-oxide was chosen as oxygen do- pyridine N-oxide concentration (entries 2 6) and the
nor.[1,11] Furthermore, 2,6-dichloropyridine N-oxide is catalyst loading (entries 2, 7, and 8). Substrate conver-
simply generated by reaction of the parent pyridine with sion up to 94% was obtained by using an excess of the
hydrogen peroxide in acidic media, thus a global oxygen donor while lowering the catalyst results in a
catalytic process may be envisaged where the pyridine
by-product is recycled and the primary oxidant is
ultimately H2O2.[12]
OH
Cl
The hydroxylation of aliphatic hydrocarbons is
certainly one of the most interesting oxidations from
tertiary
attack
both a synthetic and mechanistic point of view.[11]
A
1-Cl
1-OH
typical substrate widely used for studying these oxida-
tive processes is adamantane (AdH). Owing to its
peculiar nature it is a very useful probe to evaluate both
the efficiency of a catalytic systemand its selectivity on
the basis of the product distribution. The careful analysis
of the selectivity, apart fromthe substantial meaning
connected with the synthetic aspect of the oxidation, is
also essential in mechanistic studies aimed at obtaining
information on the nature of the active species.
OH
OH
cat.,PyCl2NO
DCE, HCl, 80
OH
O
Cl
C
1,3-diol
1,3-(OH)Cl
AdH
secondary
attack
Adamantane hydroxylation was performed in acidic
1,2-dichloroethane at 808C in the presence of 2,6-
dichloropyridine N-oxide (1 3 equivalents) and the
2-one
organoruthenium complex (2 8 mol % based on ruthe- Scheme 1. Reaction conditions and observed products.
Table 1. Adamantane hydroxylation by organoruthenium catalysts and 2,6-dichloropyridine N-oxide.[a]
#
Catalyst[b]
[mol %]
Sub./O.D.[c]
Conv.[d]
[%]
Product Distribution[e] [%]
TOF[f]
[TON h ]
À
1
1[g]
2
3
4
5
6
7
8
9
A [4]
A [4]
A [4]
A [4]
A [4]
A [4]
A [2]
A [8]
B [4]
C [4]
D [4]
1:1
1:1
1:0.75
1:1.5
1:2
1:3
1:1
1:1
1:1
56
59
52
85
86
94
62
64
51
60
55
1-OH [81] 1-Cl [8.5] 2-one [3] 1,3-diol [7]
1-OH [85] 1-Cl [6] 2-one [2.5] 1,3-diol [6.5]
1-OH [84] 1-Cl [7] 2-one [3] 1,3-diol [6]
1-OH [72] 1-Cl [8.5] 2-one [2.5] 1,3-diol [15.5] 1,3-(OH)Cl [1.5]
1-OH [69] 1-Cl [12] 2-one [3] 1,3-diol [14] 1,3-(OH)Cl [2]
1-OH [54] 1-Cl [20] 2-one [3] 1,3-diol [18] 1,3-(OH)Cl [5]
1-OH [85] 1-Cl [6.5] 2-one [2.5] 1,3-diol [6.5]
1-OH [82] 1-Cl [7] 2-one [2.5] 1,3-diol [7.5]
1-OH [84] 1-Cl [8] 2-one [3] 1,3-diol [5.5]
n.d.
13
8
17
18
15
30
6
9
5
40
10
1:1
1:1
1-OH [87] 1-Cl [3.5] 2-one [2] 1,3-diol [7.5]
1-OH [82] 1-Cl [9] 2-one [2.5] 1,3-diol [6.5]
11[h]
[a]
In all reactions: adamantane 0.07 mmol, DCE (1 mL), concentrated HCl (2 mL) at 808C; products analysed after complete
consumption of 2,6-dichloropyridine N-oxide (ca. 3 h).
[b]
A ˆ [{Ru(C6Me6)}3Mo5O18], B ˆ [{Ru(C6Me6)}3W5O18], C ˆ [Ru(C6Me6)Cl2]2, D ˆ [Ru(p-cymene)Cl2]2.Catalyst loading
based on the total rutheniumamount.
[c]
[d]
[e]
[f]
Adamantane/2,6-dichloropyridine N-oxide (Oxygen Donor) ratio.
Conversion based on the substrate.
For product abbreviations see Scheme 1.
Calculated on the basis of the maximum oxidation rate (zero-order phase of the kinetics).
Reaction performed in the absence of added acid. Reaction time 22 h.
Reaction time 0.5 h.
[g]
[h]
842
Adv. Synth. Catal. 2002, 344, 841 844

Adamantane: Selective Hydroxylation
COMMUNICATIONS
froma subsequent fast oxidation of 2-hydroxyadaman-
tane, is considered as a single attack to a secondary
position.
ꢀ1†
Values for the selectivity in the oxidation of adamantane
with several oxidants are reported in the literature,
interestingly, radical oxidations are characterised by low
C3-H/C2-H values in the range 1 25[13] while catalytic
systemsbasedonRuderivativesexhibitC3-H/C2-Hvalues
ꢁ 100.[3,14]
Figure 2. Time course for adamantane hydroxylation by
[{RuII(C6Me6)}3Mo5O18] and 2,6-dichloropyridine N-oxide
(entry 6 in Table 1).
higher turnover frequency (TOF), so that the overall
oxidation rate is not much affected. The kinetic
In that respect the important point which emerges
behaviour of the model oxidation (entry 4) has been fromthe data collected in Table 1 is related to the
monitored by quantitative GC analysis of adamantane somewhat high value of the ratio C3-H/C2-H observed in
conversion and product formation over time (Figure 2). all oxidations. Such a number, which does not vary much
A sigmoidal curve, typical of an auto-catalytic process, in the different reaction conditions, is in the range 100
was indeed detected in all reactions explored. As it can 130. This evidence speaks against the occurrence of a
be seen in Figure 2, the kinetic trace can be dissected in radical process, and indicates a ruthenium-centred
two phases where the first lag phase, corresponding to a species as the active oxidant.[3,14]
marked induction time (tind), is followed by a fast zero-
order phase. Fromthis linear part we extracted a
To further substantiate this conclusion, hydroxylation
of cis-decalin was used as mechanistic probe. The
turnover frequency (TOF) as detailed in Table 1. The oxidation catalysed by [{Ru(C6Me6)}3Mo5O18] (cata-
presence of an induction time likely rules out the lyst A), proceeds up to 18% conversion in 1.5 hours,
starting rutheniumcomplex as the competent catalyst.
leading to (Z)-9-decalol with complete stereoretention
Inspection of Table 1 reveals that all the Ru(II) systems and no trace of isomerisation products [trans-decalin,
promote the oxidation with similar product distribution (E)-9-decalol]. Once again this behaviour is consistent
and turnover frequency. So both POM-based species with the involvement of a high valent oxo-ruthenium
(catalysts A and B) and organometallic precursors complex as reactive species in the system. [1,3,14]
(catalysts C and D) in the presence of 2,6-dichloropyr-
The nature of the competent oxidant can be addressed
idine N-oxide appear to evolve to the same active by considering previously reported spectroscopic and
intermediate which is able to hydroxylate adamantane. mechanistic evidence acquired on related systems.
The p-cymene derivative (catalyst D) promotes the
The interaction of 2,6-dichloropyridine N-oxide with
fastest reaction (entry 11). It is evident therefore that the rutheniumpentafluorophenylporphyrin [Ru-
the active species derives fromthe decomposition of the
(TPFPP)(CO)] was found to produce the extremely
original catalyst introduced in the reaction mixture. In reactive RuV-oxo intermediate characterised by high
this case, a structure-reactivity correlation based on the selectivity and an unprecedented turnover fÀrequencyfor
polyoxometalate nature becomes pointless, moreover adamantane hydroxylation (TON 48,000 h ). [3a] In the
1
the acceleration obtained with the molybdate complex systemunder investigation, which yields analogous
with respect to the parent tungstate derivative is very chemo- and stereoselectivities, this kind of rapid
modest (entries 2 and 9 in Table 1). However, it should catalysis was not observed. The role of the porphyrin
be also pointed out that commercially available RuCl3 ligand could however play a major role on the reactivity
or Ru(acac)3 precursors show poor reactivity (10% of the active species.[3b] On the other hand, selective
conversion).
alkane hydroxylation with non-porphyrin ruthenium
To address the nature of the active oxidant formed in catalysts has been generally ascribed to oxo-ruthenium
solution, we should consider the selectivity parameter intermediates generated by reaction of low valent
defined as the relative reactivity of tertiary to secondary precursors with bulk oxidants.[1] Furthermore, the
CH groups calculated per H atom(4 tertiary and
reactivity pathway delineated so far, presents strong
12 secondary in adamantane). The C3-H/C2-H factor analogies with the rutheniumtetraoxide (RuO 4)-medi-
can be calculated on the basis of the product distribution ated oxidations,[14] namely: (i) scarce influence of the
(see Scheme 1), using Equation (1): where 1,2-dihy- precursor ligand system; (ii) C3-H/C2-H selectivity
droxyadamantane and 1-chloro-3-hydroxyadamantane, around 100; (iii) retention of configuration in decalin
are considered as deriving froma double attack to a
oxidations; (iv) production of chloroalkanes (1-Cl/1-
tertiary C-H, while 2-adamantanone, which derives OH ˆ 1/10) in the presence of chloride ions. In catalytic
Adv. Synth. Catal. 2002, 344, 841 844
843

Marcella Bonchio et al.
COMMUNICATIONS
processes, RuO4 is usually generated in situ by reaction Acknowledgements
of RuCl3 or RuO2 with NaIO4, HIO4, NaOCl or NaBrO3.
[1,14] These reactions are carried out in a biphasic medium
and require a delicate balance of a solvent mixture:
usually CCl4, CH3CN and water to prevent catalyst
deactivation. It is therefore difficult to correlate the
catalyst efficiency under so diverse conditions, but for
the sake of comparison AdH hydroxylation mediated by
RuO4 has been reported to proceed with ca. 60%
conversion in 19 h.[14a]
Financial support from Italian National Research Council is
gratefully acknowledged. This work has been done in the frame
of Cost D12 WG ™New Catalysts for environmentally friendly
oxidations with H2O2 or O2∫.
References
[1] a) W. P. Griffith, Chem. Soc. Rev. 1992, 21, 179 185;
b) T. Naota, H. Takaya, S.-I. Murahashi, Chem. Rev.
1998, 98, 2599 2660; c) A. E. Shilov, G. B. Shul×pin,
Chem. Rev. 1997, 97, 2879 2932.
Although precise mechanistic information remains to
be acquired in order to address the nature of active
oxidant, our results extend the protocol for the use of
2,6-dichloropyridine N-oxide as primary oxidant, which
represent an attractive alternative to both peroxidic and
halogenated oxidants. The fact that its reduction
product is recovered unaltered and potentially recycled
makes it a valuable tool to activate hydrogen peroxide
overcoming the problem of catalase-like induced
decomposition and radical reactions. Both aspects are
fundamental for lowering the environmental ∫E∫ factor
of the oxidation process.[15]
Work in progress aims at finding an access to rapid Ru-
mediated hydroxylation with polyoxometalate-based
systems, thus mimicking the perfluoroporphyrin behav-
iour with highly robust, environmentally benign cata-
lysts.
[2] a) J. T. Groves, R. Quinn, J. Am. Chem. Soc. 1985, 107,
5790 5792; b) R. Neumann, M. Dahan, J. Am. Chem.
Soc. 1998, 120, 11969 11976 and references therein.
[3] a) J. T. Groves, M. Bonchio, T. Carofiglio, K. Shalyaev, J.
Am. Chem. Soc. 1996, 118, 8961 8962; b) T. Shingaki, K.
Miura, T. Higuchi, M. Hirobe, T. Nagano, Chem.
Commun. 1997, 861 862 and references therein.
[4] A. Dijksman, A. Marino-Gonzalez, A. M. I. Payeras,
I. W. C. E. Arends, R. A. Sheldon, J. Am. Chem. Soc.
2001, 123, 6826 6833.
[5] L. Gonsalvi, I. W. C. E. Arends, R. A. Sheldon,
Chem.Commun. 2000, 202 203.
[6] J. L. Zhang, H. B. Zhou, J. S. Huang, C. M. Che, Chem.
Eur. J. 2002, 8, 1554 1562.
[7] a) M. T. Pope, A. Muller, Angew. Chem. Int. Ed. Engl.
1991, 30, 34 48; b) C. L. Hill, C. M. Prosser-McCharta,
Coord.
Chem.
Rev.
1995,
143,
407 455;
c) Polyoxometalates, (Ed.: C. L. Hill), Chem. Rev.
(Special Issue) 1998, 98, 1 390.
[8] A. Bagno, M. Bonchio, A. Sartorel, G. Scorrano Eur. J.
Inorg. Chem. 2000, 17 20.
Experimental Section
[9] V. Artero, A. Proust, P. Herson, P. Gouzerh, Chem. Eur.
J. 2001, 7, 3901 3910.
[10] R. Villaneau, R. Delmont, A. Proust, P. Gouzerh, Chem.
Eur. J. 2000, 6, 1184 1192.
General Oxidation Procedure
Oxidation reactions were carried out (duplicate runs) by
dissolving [AdH] ˆ 0.071 mol LÀ in distilled 1,2-dichloro-
1
[11] R. A. Sheldon, J. K. Kochi, Metal catalysed oxidation of
organic compounds Academic Press, New York, 1981.
[12] G. E. Chivers, H. Suschitzky, J. Chem. Soc. D. 1971, 28.
[13] a) C. R¸chardt, Angew. Chem. Int. Ed. Engl. 1970, 9,
830 843; b) D. H. R. Barton, J. Boivin, W. B. Mother-
well, N. Ozbalik, K. M. Schwartzentruber, Nouv. J. Chim.
1986, 10, 387 398.
[14] a) J. M. Bakke, J. E. Braenden Acta Chem. Scand. 1991,
45, 418 423; b) J. M. Bakke, D. Bethell, Acta Chem.
Scand. 1992, 46, 644-649.
ethane (1 mL) together with 2,6-dichloropyridine N-oxide
(1 3 equivalents vs. AdH), a GC internal standard (C16H34),
the rutheniumcatalyst (2 8% v s. AdH based on ruthenium
amount) and 37% aqueous HCl (2 mL). The reaction mixture
was then thermostatted at 808C. Products were identified by
GC (HP 5890 series II instrument equipped with a 15 m,
0.5 mm I.D, 0.25 m AT-1701 capillary column) and GC-MS (HP
5890 series II instrument equipped with HP 5970 Mass
selective detector and a 30 m, 0.25 mm I.D, 0.25 m EC-1
capillary column) analysis. Chemical yields and conversions
were determined by quantitative GC analysis with respect to
the internal standard.
¬
[15] R. A. Sheldon, C. R. Acad. Sci. Paris, Serie IIc 2000, 3,
541 551.
844
Adv. Synth. Catal. 2002, 344, 841 844