Platinum-catalysed aerobic 1,2-aminooxygenation of alkenes

Article abstract of DOI:10.1039/b912139k

Platinum(ii) salts in combination with copper salts and molecular oxygen catalyse an unprecedented intramolecular transfer of heteroatoms to alkenes to yield aminooxygenation products under sustainable conditions.

Full text of DOI:10.1039/b912139k

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Platinum-catalysed aerobic 1,2-aminooxygenation of alkeneswz
˜
Kilian Muniz,*y Alvaro Iglesias and Yewen Fang
Received (in Cambridge, UK) 19th June 2009, Accepted 18th August 2009
First published as an Advance Article on the web 28th August 2009
DOI: 10.1039/b912139k
Platinum(II) salts in combination with copper salts and mole-
cular oxygen catalyse an unprecedented intramolecular transfer
of heteroatoms to alkenes to yield aminooxygenation products
under sustainable conditions.
dichloride and copper dibromide already promotes the
oxidation of alkenyl urea 1a. However, under these relatively
harsh conditions the yield remains low due to significant
amounts of decomposed starting material. Reducing the
copper dibromide to a catalytic amount of 30 mol% leads to
a significant increase in yield when benzoquinone is employed
as terminal oxidant (Scheme 2, entry 2).
1,2-Difunctionalization of alkenes represents a powerful
synthetic tool in the synthesis of organic building blocks. In
particular, Sharpless devised seminal catalytic transformations
based on osmium(VIII) catalysts for dihydroxylation1 and
aminohydroxylation.2
To our delight, experiments under aerobic conditions
quickly led to the realization of suitable reaction conditions.
A combination of 10 mol% PtCl2, 30 mol% CuBr2 and
atmospheric dioxygen in dmso as the solvent provides an
efficient catalyst system for the oxidation of 1a (entry 5).
Apparently, copper bromide concentration is of key
importance, as attempts to lower its amount further to 0.15
equivalents resulted in a significantly reduced rate (entry 6).
Other copper salts, such as copper(II) chloride and acetate, had
less efficient results.
Recent efforts in the area of high oxidation state catalysis with
noble metals have culminated in the development of palladium-
catalysed transformations that include diacetoxylation,3
aminoalkoxylation4 and diamination.5
These reactions share a common feature regarding the use
of the strong oxidant iodosobenzene diacetate. While this
reagent enables a rapid and reliable oxidation, it produces
iodobenzene as an undesired by-product. We recently reported
the first catalysis protocols for intramolecular diamination
of unfunctionalised alkenes, which employed ureas and
sulfamides as the respective nitrogen sources.5,6 This initial
work employed high oxidation palladium and nickel catalyst
states derived from iodosobenzene diacetate as a stoichiometric
oxidant. In an attempt to devise alternative reoxidation
conditions, we recently reported protocols with copper(II)
halides as suitable oxidants for the case of palladium-catalysed
1,2-diamination reactions (Scheme 1).7
Less than 10% conversion, with no formation of 2a,
was obtained in the absence of the platinum catalyst. This
observation confirms that the oxidation from 1a to 2a is
indeed platinum-catalysed.
The outcome of this reaction is, however, unexpected, as the
obtained product is not the expected cyclic urea, but rather
its isomeric isourea. Obviously, under the given reaction
conditions, the carbonyl oxygen displays a more pronounced
nucleophilicity than the tosylamide nitrogen. This is not
surprising given that the high nucleophilicity of the tosylamide
in the related palladium reactions results from deprotonation
at the outset of the reaction.5 Scheme 3 shows the results
for eight different reactions of platinum-catalysed aerobic
oxidation, demonstrating that the transformation from
eqn (1) is general under the chosen reaction conditions of
platinum catalysis. For this set of substrates, the reaction
proceeded with complete chemoselectivity in favour of isourea
formation. In particular, the efficient oxidation to six-membered
bicyclic products 2a–f is without precedence in all currently
available alkene oxidation protocols employing N-tosyl urea
as the heteroatom source.5,7 Oxidation of 1b displayed an
interesting rate acceleration in the presence of carboxylic
acids. More work is necessary to determine its basis, which
was not found in other cases. In addition, complete chemo-
selectivity in favour of isourea formation11 is noteworthy as
Still, these protocols call for the use of 2–3 equivalents of
copper salts. Attempts to employ aerobic reaction conditions
could not meet with success due to the requirement of basic
reaction conditions in the initial step of aminopalladation.8
Here, we describe a new protocol for oxidative 1,2-
difunctionalisation of alkenes employing platinum catalysis9
under aerobic conditions. As aminoplatination represents
a well-established process under neutral or even acidic
conditions,10 we considered the application of platinum salts
an attractive possibility for oxidation under base-free conditions.
As outlined in Scheme 2, a simple combination of platinum
´
Institut de Chimie, UMR 7177, Universite de Strasbourg, 4 rue Blaise
Pascal, F-67000 Strasbourg Cedex, France.
E-mail: muniz@chimie.u-strasbg.fr
w This article is part of a ChemComm ‘Catalysis in Organic Synthesis’
web-theme issue showcasing high quality research in organic
chemistry. Please see our website (http://www.rsc.org/chemcomm/
organicwebtheme2009) to access the other papers in this issue
z Electronic supplementary information (ESI) available: Experimental
procedures and characterisation, and spectral reproduction for new
compounds. CCDC 737012. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b912139k
y New address: Institut Catala d’Investigatio
´
Quımica (ICIQ),
´
Av. Paısos Catalans, 16, E-43007 Tarragona, Spain.
Scheme 1 Diamination of alkenes with copper bromide as reoxidant.
Chem. Commun., 2009, 5591–5593 | 5591
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Scheme 4 Transformation of 2e into 4.
Scheme 2 Platinum-catalysed oxidation of alkenes: optimization of
The isourea products may constitute interesting building
blocks for further organic transformation. For example,
acid-mediated cleavage of isourea 2e led to clean isolation of
the corresponding amino alcohol 3 in very good yield.
Subsequent oxidative transformation under standard
conditions14 gave rise to the piperidine-2-carboxylic acid, 4
(Scheme 4). This route provides convenient access to the motif
of substituted piperidine-2-carboxylic acids. Its synthesis
should be of interest in view of the recent successful application
of proline and related derivatives in organocatalysis.
the oxidant. BQ = 1,4-benzoquinone.
related pyrrolidine annelation reactions yield 2 : 1 mixtures of
ureas and isoureas.12 The isourea structure was established for
compound 2h by X-ray crystallography,13 and deduced for all
other products 2 from the characteristic 13C carbon signal for
CH2O at 72–73 ppm and the typical carbonyl IR band at
1610–1614 cmꢁ1
.
In order to understand the underlying features of this new
aerobic catalysis, we chose compound 1d for a competition
study regarding the oxidation state of the involved platinum
catalyst. Since the oxidation potential of platinum(II) salts is
rather low (0.74 V for PtCl6 + 2eꢁ/PtCl4 + 2Clꢁ],15 we
reasoned that steady oxidation state of +IV might be involved
throughout the whole catalysis.16 However, a comparison
of reactions using platinum dichloride and tetrachloride,
respectively, as catalyst precursors revealed a significantly
faster conversion in case of the former one (Fig. 1). After a
total of 12 h, the reaction with the PtCl2 catalyst precursor
yielded 90% of 2d, while the one from PtCl4 led to only 55%
yield. On the basis of this investigation, we favour a catalytic
cycle involving Pt(II)/Pt(IV) catalysis.
2ꢁ
2ꢁ
We suggest the following catalytic cycle for the new
transformation (Fig. 2). It starts from a Pt(II) complex,
which, via a transient p-alkene complex of 1, undergoes
intramolecular aminoplatination to A. This step has ample
Scheme 3 Platinum-catalysed aerobic aminooxygenation of alkenes
a
under concomitant piperidine annelation. With 50 mol% tartaric
Fig. 1 Monitored reaction progress for the oxidation of 1d to 2d in
acid as additive.
the presence of PtCl2 and PtCl4, respectively.
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128, 7179; L. Desai and M. S. Sanford, Angew. Chem., Int. Ed.,
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5 J. Streuff, C. H. Hovelmann, M. Nieger and K. Muniz, J. Am.
¨
¨
Chem. Soc., 2005, 127, 14586; K. Muniz, C. H. Hovelmann and
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Chem. Soc., 2007, 129, 14542.
6 K. Muniz, J. Streuff, C. H. Hovelmann and A. Nu´
nez, Angew.
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Chem., Int. Ed., 2007, 46, 7125.
7 K. Muniz, C. H. Hovelmann, E. Campos-Go
´
mez, J. Barluenga,
¨
J. M. Gonza
776; C. H. Hovelmann, J. Streuff, L. Brelot and K. Muniz, Chem.
´
lez, J. Streuff and M. Nieger, Chem.–Asian J., 2008, 3,
¨
Commun., 2008, 2334; K. Muniz, J. Streuff, P. Cha
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´
vez and
¨
8 A. Minatti and K. Muniz, Chem. Soc. Rev., 2007, 36, 1142.
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´
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Fig. 2 Proposed catalytic cycle. X = Cl, Br.
10 Aminoplatination is the initial step in catalytic hydroamination.
For leading references see: J. L. McBee, A. T. Bell and T. D. Tilley,
J. Am. Chem. Soc., 2008, 130, 16562; C. F. Bender and
R. A. Widenhoefer, J. Am. Chem. Soc., 2005, 127, 1070;
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precedence in platinum-catalysed hydroamination chemistry.10
Even under employed neutral conditions, protonolysis of A
does not constitute a feasible pathway, as no hydroamination
product was ever detected. Instead, rapid oxidation of the
alkyl-platinum complex A to generate the corresponding
platinum(IV) complex B may occur in the presence of copper
bromide.17,19,20 This step characterises our present protocol as
an oxidatively intercepted hydroamination pathway. Alkyl
platinum(IV) catalyst state B undergoes intramolecular reductive
elimination to generate the C–O bond. This step is reminiscent
of related alkoxylation reactions at Pt(IV).17–19 Regeneration
of the copper(II) oxidant is then accomplished under aerobic
conditions in accordance with the established Wacker
conditions.20
11 For earlier reports on pyrrolidine-annelated isourea formation see
the first reference in ref. 7, and B. M. Cochran and F. E. Michael,
Org. Lett., 2008, 10, 5039.
12 Please see the ESIz for details.
13 Single crystals of 2h were recrystallised from methanol at room
temperature. CCDC 737012. For crystallographic data in CIF or
other electronic format, see DOI: 10.1039/b912139k.
14 D. Kalch, N. De Rycke, X. Moreau and C. Greck, Tetrahedron
Lett., 2009, 50, 492.
15 Value from R. C. West, Handbook of Chemistry and Physics,
The Chemical Rubber Co., Cleveland, 50th edn, 1969, p. D-110.
16 For some rare examples of platinum(IV) catalysis: A.
In summary, we have developed a new catalytic approach
in oxidative homogeneous platinum catalysis using aerobic
conditions. The reaction employs catalytic amounts of
copper(II) oxidant without any additives and represents a
significant advance in oxidative 1,2-difunctionalisation of
alkenes.
Dieguez-Vazquez, C. C. Tzschucke, Wing-Ye Lam and
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S. Kobayashi, K. Kakumoto and M. Sugiura, Org. Lett., 2002, 4,
1319.
17 For methanol elimination from neutral Pt(IV) complexes under
basic conditions: R. A. Periana, D. J. Taube, H. Gamble,
H. Taube, T. Satoh and H. Fujii, Science, 1998, 280, 560;
J. R. Khusnutdinova, L. L. Newman, P. Y. Zavalij, Y.-F. Lam
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E. Khaskin, P. Y. Zavalij and A. N. Vedernikov, J. Am. Chem.
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This work was supported by the Fonds der Chemischen
Industrie, the International Centre for Frontier Research in
Chemistry and the Agence Nationale de Recherche.
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