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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 1089 1090 K. MUÑIZ et al. 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 Diamination of alkenes 1091 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. © 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096 K. MUÑIZ et al. 1092 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]. © 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096 Diamination of alkenes 1093 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 K. MUÑIZ et al. 1094 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 © 2008 IUPAC, Pure and Applied Chemistry 80, 1089–1096 Diamination of alkenes 1095 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. REFERENCES AND NOTES 1. For a general review on chiral diamines, see: (a) D. Lucet, T. Le Gall, C. Mioskowski. Angew. Chem., Int. Ed. 37, 2580 (1998); for a review on trans-1,2-diamino cyclohexane and chiral derivatives thereof, see: (b) Y. L. Bennani, S. Hanessian. Chem. Rev. 97, 3161 (1997). 2. (a) R. Noyori. Angew. Chem., Int. Ed. 41, 2008 (2002); (b) R. Noyori. Adv. Synth. Catal. 345, 15 (2003); (c) R. Noyori, T. Ohkuma. Angew. Chem., Int. Ed. 40, 40 (2001); (d) R. Noyori, M. Yamakawa, S. Hashiguchi. J. Org. Chem. 66, 7931 (2001); (e) R. Noyori, S. 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