Palladium catalysts hydroamination

Chang et al. reported a mild tandem intramolecular hydroamination of yne amines to form an endo-adduct intermediate, which reacts with electron-deficient azides to produce cyclic amidines <06JA12366>. Selected examples of an interesting synthetic route to tropene derivatives 165 via a dual hydroamination strategy is shown below. This one-step reaction makes use of a palladium catalyst and takes place by sequential intermolecular hydroamination of cycloheptatriene with aryl, heteroaryl, and primary alkyl amines to generate intermediate 166, followed by transannular intramolecular hydroamination <06JA8134>. 

Several catalytic systems have been investigated for hydroamination of unsaturated bonds [16]. Takahashi et al. reported the telomerization of 1,3-dienes in the presence of an amine leading to octadienylamine or allylic amines when palladium catalysts are used in association with monodentate or bidentate phosphine ligands, respectively [17]. Dieck et al. demonstrated the beneficial effect of addition of an amine hydroiodic salt in the hydroamination reaction of 1,3-dienes in which the allylic amines are produced via an intermediate Jt-allyl palladium complex [18]. Coulson reported the Pd-catalyzed addition of amines to allenes where dimerization is incorporated [4]. This reaction presumably proceeds via a cyclic palladium intermediate in which the Pd activates the olefinic bond for nucleophilic attack the reactions are therefore different from pronucleophilic additions. 

Recent studies on the C—C cleavage reaction of diynes proceeded smoothly with Pd(NO3)3 as the best among all palladium catalysts tested [67]. Substituted benzoxazoles 115 and 116 along with alkanones were derived from C—C bond cleavage of diynes through hydroamination [68]. 

Intermolecular hydroamination of alkynes, which is a process with a relatively low activation barrier, has not been used for the synthesis of chiral amines, since the achiral Schiff base is a major reaction product. However, protected aminoalkynes may undergo an interesting intramolecular allylic cyclization using a palladium catalyst with a chiral norbomene based diphosphine ligand (Eq. 11.9) [115]. Unfor tunately, significantly higher catalyst loadings were required to achieve better enantioselectivities of up to 91% ee. 

The group of Hartwig has shown that dienes imdergo hydroamination with anihnes in the presence of the palladium catalyst formed in situ from [ Pd( 7r-allyl)Cl 2] and Trost s ligand (2.190). Under optimum conditions cyclohexa-... 

The hydroamination of 1,3-dienes has been known for many years, but these reactions have often generated mixtures of products. More recently, the intermolecular and intramolecular hydroamination of dienes has been reported with lanthanide and palladium catalysts to generate allylic amines in high yields. These reactions have also occurred with high enantioselectivity in some cases. 

The hydroamination of alkenes and alkynes has been of longstanding interest in organometallic chemistry [26]. Much of the early work in this area focused on early transition metal or lanthanide metal catalyst systems. However, much recent progress has been made in late-metal catalyzed hydroamination chemistry, and several interesting hydroamination reactions that afford nitrogen heterocycles have been developed using palladium catalysts. 

The first example of a heterogeneously catalyzed hydroamination of an alkene appeared in a 1929 patent in which it is claimed that NHj reacts with ethylene (450°C, 20 bar) over a reduced ammonium molybdate to give EtNH2 [24]. An intriguing reaction was also reported by Bersworth, who reacted oleic acid with NH3 in the presence of catalysts like palladium or platinum black or copper chromite to give the hydroamination product in quantitative yields [25]. However, this result could not be reproduced [26]. 

The hydroamination of allene with morpholine or allylamines has been attempted with palladium-based catalysts. Usually, a mixture of 1 1 telomers (hydroamination products) and 1 2 telomers is obtained, the latter being the major [308, 309] or only... 

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

In 2003, O Shaughnessy and Scott reported the first example of rare-earth metal complexes supported by biaryl diamide ligands as the catalysts for the hydroamination reactions [159]. A series of C2 symmetric secondary diamine proligands L37-L40 were prepared by arylations of (7 )-2,2 -diamino-6,6 -dimethybiphenyl under palladium catalysis. L37 reacted with complexes [Ln N(SiHMe2)2 3(THF)2] to form the biaryl diamide complexes [Ln(L37) N(SiHMe2)2 (THF)2] (Ln = Y (190), La (191), Sm (192)). Deprotonation... 

Palladium-based catalysts for enantioselective hydroamination reactions were discovered by extending the results of a high-throughput screening approach directed toward the discovery of non-selective hydroamination catalysts, reported by the Hartwig group in 2001. The approach taken by Hartwig and co-workers is described in Section 1.13.2.5. ... 

Although Pd-catalyzed intramolecular hydroamination reactions of alkynes have been known for ten years, analogous transformations of unactivated alkenes have only recently been developed [33]. Key to the success of these studies was the use of a cationic palladium complex bearing a pyridine-derived P-N-P pincer ligand (29). For example, treatment of 26 with catalytic amounts of 29, AgB F4, and Cu(OTf)2 led to the formation of pyrrolidine 27 in 88% yield with 4 1 dr (Eq. (1.13)). Detailed mechamstic studies have indicated these transformations proceed via alkene coordination to the metal complex followed by outer-sphere aminopaUadation to provide 28. Protonolysis ofthe metal-carbon bond with acid generated in situ leads to formation of the product with regeneration of the active catalyst. 

This reaction constitutes a special type of process in which a hydrogen and a nucleophile are added across the diene, with formation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20], hydroamination [21, 22], and addition of active methylene compounds [21a, 23, 24]. These reactions are initiated by an oxidative addition of H-Nu to the palladium(O) catalyst, which produces a palladium hydride species 1 in which the nucleophile is coordinated to the metal (Scheme 11.1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a JT-allylpalladium intermediate. Now, depending... 

Trimetallic palladium and gold earbene complexes structured around a triphenylene scaffold (13) have been synthesized and used in ct-arylation of propiophenone, Suzuki-Miyaura reactions and hydroamination of acetylene. They exhibited an enhanced reactivity compared to other trimetallic as well as more conventional monometallic complexes. This enhancement is thought to arise from the specific topological properties that might favour the interaction between the catalysts and the aromatic substrates as... 

Intramolecular hydroamination of protected amino-1,3-dienes has been shown to produce five- and six-membered heterocyclic homoallylic amines when the tridentate PNP pincer complex of palladium(II) (68) is employed as a catalyst (5 mol%). " ... 

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