Download First palladium- and nickel-catalyzed oxidative

Pure Appl. Chem., Vol. 80, No. 5, pp. 1089–1096, 2008.
doi:10.1351/pac200880051089
© 2008 IUPAC
First palladium- and nickel-catalyzed oxidative
diamination of alkenes: Cyclic urea, sulfamide,
and guanidine building blocks*
Kilian Muñiz‡, Claas H. Hövelmann, Jan Streuff, and
Esther Campos-Gómez
Laboratory for Homogeneous Catalysis and Molecular Synthesis, Institute of
Chemistry, UMR 7177, Université Louis Pasteur, 4 rue Blaise Pascal, F-67000
Strasbourg Cedex, France
Abstract: We recently reported the first catalytic diamination of alkenes. This protocol calls
for the use of Pd(II) as catalyst in combination with PhI(OAc)2 as terminal oxidant and furnishes the final diamines as cyclic ureas. It consists of an unprecedented two-step reaction of
aminopalladation and Csp3-N-bond formation involving a Pd(IV) species. Introduction of
Ni(II) catalysts for homogeneous oxidation allows for an efficient diamination with sulfamides, which lead to convenient liberation of the free diamines. In related protocols, the
substrate scope of the diamination has been broadened to the formation of cyclic guanidines.
Keywords: catalysis; oxidation; homogeneous; diamines; palladium; nickel.
INTRODUCTION
Vicinal diamines constitute an important functional group that is encountered in organic compounds
ranging from natural products to pharmaceutically relevant molecules [1] and inorganic compounds
such as the Noyori hydrogenation and transfer hydrogenation catalysts [2]. A particularly interesting
common approach to this functional group of 1,2-diamines would make use of selective catalytic oxidation of alkenes. Such a procedure would have to establish defined conditions for a compatible combination of alkene, nitrogen source, metal catalyst, and oxidant (Fig. 1).
Fig. 1 Diamination of alkenes.
Early work from our laboratory employed preformed imidoosmium(VIII) reagents, which display
unprecedented reactivity and selectivity toward selective diamination [3]. In addition, chiral versions of
these reactions allowed for the development of diastereoselective [4] and enantioselective [5] alkene diamination as well as the construction of novel stereogenic metal centers [6]. However, due to the in-
*Paper based on a presentation at the 14th International Symposium on Organometallic Chemistry Directed Towards Organic
Synthesis (OMCOS-14), 2–6 August 2007, Nara, Japan. Other presentations are published in this issue, pp. 807–1194.
‡Corresponding author
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herent properties of the osmium-diamine chelate unit in the products, a catalytic variant could not be
accomplished. Related uses of transition-metal promoters such as thallium [7a], mercury [7b,c], palladium [7d,e], or copper [7f,g] also require stoichiometric amounts due to related formation of stable transition metal-diamine complexes or metal reoxidation incompatibility.
The drawback of stoichiometric metal complexes to achieve alkene diamination was finally overcome with the development of group 10 metal catalysis in the presence of iodosobenzene diacetate as
oxidant.
CONCEPT FOR INTRAMOLECULAR ALKENE DIAMINATION
In 1975, Bäckvall reported a Pd-promoted diamination of alkenes consisting of a successive reaction of
Pd-alkene complexes with excess N-alkyl amines as nitrogen source followed by administration of a
strong oxidant such as lead tetraacetate or bromine [7c,d].
In order to overcome deactivation of the Pd by the diamine product, we envisioned tethering of
the two transferable nitrogen atoms by a group X as a promising approach toward realization of a catalytic variant (Fig. 2).
Fig. 2 Conceptual approach toward intramolecular diamination of alkenes.
The reaction would then be expected to proceed via a two-step process of aminometallation followed by a second C–N bond formation upon interaction with the oxidant.
The initial step of aminometallation has ample precedence for Pd(II), and a variety of subsequent
diversification reactions are available as discussed in a recent review [8]. What remained to be established were the exact conditions for the second hitherto unrealized C–N bond-forming step, which had
to proceed selectively overriding potentially competing processes such as β-hydride elimination, protonation, or alkoxylation.
PALLADIUM-CATALYZED DIAMINATION
In view of the successful development of intramolecular aminopalladation of alkenes and its subsequent
chemistry, this approach was considered first. Urea groups were employed as tethered nitrogen sources.
Upon administration of iodosobenzene, clean diamination was observed for an aniline-derived test substrate (Scheme 1) [9].
© 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096
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Scheme 1 Development of intramolecular diamination.
In this initial reaction, arene overoxidation was a significant side reaction, which could be controlled in favor of selective diamination by accelerating the overall reaction upon addition of an equimolar amount of base.
The reaction conditions proved general, and a variety of different precursors could be oxidatively
transformed into the cyclic urea products under the general reaction conditions (Scheme 1). As for all
examples from Scheme 2, these reactions proceed under concomitant formation of a five-membered
ring annelation [9].
Scheme 2 Intramolecular diamination of alkenes: pyrrolidine annelation.
Changing the substrate in order to achieve six-membered ring annelation, the reactions suffered
a significant decrease in rate, and base acceleration did not prove to occur any longer (Scheme 3).
Hence, the catalyst loading had to be adjusted to 25 mol % in order to provide reasonably high conversion [9].
Overall, the use of urea as a source for transferable nitrogen atoms in oxidation of alkenes led to
very successful results and established the first methodology for intramolecular diamination. With regards to liberation of the free diamines, deprotection of N-tosylated cyclic ureas were met with difficulties. In order to provide free access to more convenient nitrogen sources, alternative tethers had to
be considered.
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Scheme 3 Intramolecular diamination of alkenes: piperazine annelation.
At this stage, our earlier occupation with sulfamides had resulted in the development of an intermolecular Buchwald–Hartwig coupling of sulfamides to aryl halides employing Pd-based catalysts
(Scheme 4) [10–12]. The apparent compatibility between Pd(II)-derived catalysts and sulfamides rendered this functional group a promising candidate for the development of an intramolecular diamination.
Scheme 4 Pd-catalyzed N-arylation of sulfamide.
NICKEL-CATALYZED DIAMINATION
Within this context, initial attempts on the realization of a Pd-catalyzed diamination with sulfamide precursors did not lead to a general diamination procedure, despite long-standing efforts. Instead,
aminoalkoxylation [13] was always encountered. Gratifyingly, replacement of Pd by Ni(II) salts resulted in selective diamination. Careful optimization of the reaction conditions led to the development
of suitable conditions for a high-yielding protocol based on Ni(II) catalysis (Scheme 5) [14].
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Scheme 5 Intramolecular diamination employing sulfamides: process optimization.
Extension of this protocol to a series of sulfamide precursors showed that the reaction is general.
As in the examples from Scheme 3, these reactions proceed under concomitant formation of pyrrolidine
annelation (Scheme 6) [14].
Scheme 6 Intramolecular diamination employing sulfamides.
In addition to these examples, chiral substrates led to the formation of more elaborate structures.
The advantage of sulfamides over the related urea products is immediately visible from the ease of deprotection as outlined in Scheme 7. Depending on the individual carbamate, standard removal employ© 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096
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Scheme 7 Diamine liberation from cyclic sulfamides.
ing either basic hydolysis, hydrogenolysis of acidic cleavage, respectively, affords free sulfamides,
which can be converted to the free pyrrolidine-2-methylamines by LAH reduction. Alternatively, directly removal of both protection groups is feasible [14,15].
Recent efforts dealt with the development of related diamination reactions for the construction of
cyclic guanidines from 1,2-diamination (Scheme 8). A first substrate showed that such an approach is
indeed feasible and that the more electron-demanding substituted nitrogen is incorporated in the second
step. The reaction outcome is unaffected by the chosen protocol since both Ni and Pd catalysis give
clean diamination. Current efforts aim to exploit this transformation in the synthesis of higher functionalized molecules [16].
Scheme 8 Alkene guanidination by diamination.
MECHANISTIC UNDERSTANDING
The reactions of Ni- and Pd-catalyzed diamination are believed to proceed as outlined in Fig. 2. Details
on the mechanism are currently under investigation [16]. All data points to a sequence of aminometallation followed by a second alkyl-nitrogen bond formation. No radical intermediates are involved
in the overall process as selective deuteration at the terminal position of the alkene leads to diastereomerically pure products in all cases.
Key to the realization of the present reactions is the use of the powerful oxidant iodosobenzene
diacetate [17]. This oxidant promotes rapid and irreversible oxidation of the metal at the intermediary
alkyl-metal state, presumably to Pd(IV) [18] and Ni(III). This high metal oxidation state is the prerequisite to the realization of the previously unprecedented catalytic alkyl-nitrogen bond formation with
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group 10 metals at room temperature [19–21]. This outcome represents a marked difference to related
amination of aryl and alkenyl groups [22], which usually requires higher reaction temperatures, especially for electron-demanding nitrogen groups. In addition, the present protocols for diamination do not
require ligand fine-tuning, but rather proceed under ligand-free conditions.
In summary, we have developed the first transition-metal-catalyzed diamination reactions of
alkenes [9,14,23]. These reactions are characterized by complete selectivity and ease of operation.
ACKNOWLEDGMENTS
The work described within was supported by generous grants from the Fonds der Chemischen Industrie,
the French Agence Nationale de la Recherche (ANR), the Deutsche Forschungsgemeinschaft (DFG),
the Deutscher Akademischer Austauschdienst (DAAD), and the German-Israeli Foundation (GIF).
E. C.-G. is a Ph.D. student from Oviedo University.
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© 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096