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Relevant academic research and scientific papers

Zirconium-catalyzed intermolecular hydroamination of alkynes with primary amines

A simple catalyst system generated in situ by combination of 5 mol-% [Zr(NMe2)4] and 10 mol-% of a sulfonamide catalyzes the intermolecular hydroamination of alkynes with primary amines. At elevated temperatures, hydroamination is achieved with both internal and terminal alkynes as well as with sterically demanding and less demanding primary amines. In contrast, secondary amines do not react under identical conditions. Of the sulfonamide additives investigated, sterically demanding tosylamides such as N-(tert-butyl)-p-toluenesulfonamide give the best results. The regioselectivity of the addition of amine to unsymmetrically substituted internal as well as terminal alkynes is significantly influenced by the nature of the sulfonamide additive. In particular, the successful use of sterically less demanding primary amines such as n-hexyl-or benzylamine clearly indicates that the assumption that Zr catalysts generally form catalytically inactive bridging μ2-imido dimers or other unreactive intermediates in the presence of these amines is not correct. Intermolecular hydroamination reactions of alkynes with primary amines that are sterically less demanding than 2,6-dimethylaniline can be achieved with in situ generated zirconium catalysts. This finding is highly important for a better and more general understanding of the mechanism of group 4 metal-catalyzed hydroamination reactions. Copyright

Titanium Catalysts with Linked Indenyl-Amido Ligands for Hydroamination and Hydroaminoalkylation Reactions

Titanium complexes containing a bridging indenylethylamido ligand have been synthesized and used as catalysts for hydroamination and hydroaminoalkylation reactions. All dichloro titanium complexes (η5:η1-Ind-C2H4-NR)TiCl2 (R = i-Pr (2a), t-Bu (2b), Cy (2c), Ph (2d)), which were prepared by reacting TiCl4(Et2O)2 with Li2[Ind-C2H4-NR], were fully characterized by single-crystal X-ray analysis. Reaction of 2a-c with methyllithium gave the thermally sensitive corresponding dimethyl titanium complexes [η5:η1-Ind-C2H4-N(alkyl)]TiMe2 (alkyl = i-Pr (3a), t-Bu (3b), Cy (3c)), while the N-aryl-substituted dimethyl titanium complexes [η5:η1-Ind-C2H4-N(aryl)]TiMe2 (aryl = Ph (3d), p-MeOC6H4 (3e)) were directly prepared by reacting Li2[Ind-C2H4-N(aryl)] with in situ generated Cl2TiMe2. In the case of complex 3d, the molecular structure could be determined by single-crystal X-ray analysis. All dimethyl titanium complexes (η5:η1-Ind-C2H4-NR)TiMe2 (R = i-Pr (3a), t-Bu (3b), Cy (3c), Ph (3d), p-MeOC6H4 (3e)) were finally used as precatalysts for the intermolecular hydroaminoalkylation of 1-octene (4), the intramolecular hydroamination and hydroaminoalkylation reactions of aminoalkenes, and the intermolecular hydroamination of 1-phenylpropyne (12). These experiments showed that the N substituent of the indenylethylamido ligand strongly influences the activity of the catalysts in the individual reactions.

Iron-catalysed 1,2-aryl migration of tertiary azides

1,2-Aryl migration of α,α-diaryl tertiary azides was achieved by using the catalytic system of FeCl2/N-heterocyclic carbene (NHC) SIPr·HCl. The reaction generated aniline products in good yields after one-pot reduction of the migration-resultant imines.

Iron-catalyzed reductive coupling of nitroarenes with olefins: Intermediate of iron-nitroso complex

Using a single half-sandwich iron(II) compound, CpFe(1,2-Ph2PC6H4S)(NCMe) (Cp- = C5Me5-, 1) as a catalyst, reductive coupling of nitroarenes with olefins has been achieved by a well-defined iron(II)/(EtO)3SiH system. Through either inter- or intramolecular reductive coupling, various branched amines and indole derivatives have been directly synthesized in one-pot. Mechanistic studies showed that the catalysis is initiated by activation of nitroarenes by the iron(II) catalyst with silane, generating iron-nitrosoarene intermediate for the C-N bond coupling.

Remote sp3 C–H Amination of Alkenes with Nitroarenes

Direct installation of a functional group at remote, unfunctionalized sites in an alkyl chain is a synthetically valuable but rarely reported process. The remote relay hydroarylamination of distal and proximal olefins, and of olefin isomeric mixtures, has been achieved through NiH-catalyzed alkene isomerization and sequential reductive hydroarylamination with nitroarenes. This provides an attractive approach to the direct installation of a distal arylamino group within alkyl chains. The single-step conversion of simple olefins and nitro(hetero)arenes to value-added arylamines is a practical strategy for amine synthesis as well as the remote activation of sp3 C–H bonds. The value of this transformation is further supported by the regioconvergent arylamination of isomeric mixtures of olefins. Modern organic synthesis requires more efficient strategies, such as C–H functionalization, with which to construct complex molecules from readily available chemicals. Undirected functionalization of remote aliphatic C–H bonds is a synthetically valuable but largely unknown process. Synergistic combination of metal-catalyzed chainwalking (migration of a double bond along the hydrocarbon chain, a process involving repeated migratory insertions and β-hydride eliminations) and cross-coupling chemistry offers a general approach to the remote functionalization of easily accessed unsaturated hydrocarbon substrates. In this paper, we demonstrate that direct installation of a distal arylamino group can be achieved from two common feedstock chemicals (olefins and nitroarenes) via nickel hydride chemistry. It is anticipated that the strategy could inspire the development of other remote functionalizations with different regioselectivity as well as asymmetric transformations. Zhu and colleagues describe the remote hydroamination of alkenes with nitro(hetero)arenes through nickel-catalyzed alkene isomerization and sequential reductive relay hydroamination process. Using two common feedstock chemicals, olefins and nitroaromatics, in an operationally simple procedure, this attractive protocol provides efficient and practical access to a wide range of arylamines under mild conditions.

Enantioselective radical addition reactions to imines using binaphthol-derived chiral N-triflyl phosphoramides

Binaphthol-derived chiral phosphoric acid catalysts were applied to enantioselective radical addition reactions of imines and provided chiral amines with good enantioselectivities (73-84% ee). Furthermore, the enantioselectivities were not affected by eit

Titanium hydroamination catalysts bearing a 2-aminopyrrolinato spectator ligand: Monitoring the individual reaction steps

A series of new titanium half sandwich complexes, containing a 2-aminopyrrolinato ligand {NXylN}- as the ancillary ligand, have been prepared and are shown to be pre-catalysts for the hydroamination of alkynes. The coordination of {NXylN}- to titanium was achieved by reaction of [Cp*TiMe3] with the protioligand NXylNH giving [Cp*Ti(NXylN)(Me) 2] (1). Upon reaction of complex 1 with an excess of tert-butylamine, the imido complex [Cp*Ti(NXylN)(NtBu)(NH 2tBu)] (2) was formed. The latter provided the preparative entry to the synthesis of a range of N-aryl substituted imido complexes. Imido ligand exchange with 2,6-dimethylaniline, 2,4,6-trimethylaniline as well as 2,6-diisopropylaniline gave the corresponding arylimido complexes 3-5 in clean reactions. Reaction of the titanium imido complex [Cp*Ti(N XylN)(NtBu)(NH2tBu)] 2 with terminal arylacetylenes, such as phenylacetylene and tolylacetylene, led to C-H activation and the formation of alkynyl/amido complexes, whereas the arylimido complexes 3 and 5 cleanly underwent {2 + 2} cycloaddition, giving the azatitanacyclobutene derivatives. A single-crystal X-ray structure analysis of the azatitanacyclobutene [Cp*Ti(NXylN){κ2N(2, 6-C6H3Me2)CTolCH}] (11) provided the first crystallographically characterized Markovnikov cycloaddition product of an imidotitanium complex with a terminal alkyne. The mechanistic aspects of the hydromanination of alkynes with the new Ti half sandwich complexes were studied and established a reversible {2 + 2} cycloaddition step and the cleavage of the metallacyclic intermediate as the rate determining step in the catalytic cycle. The titanium half sandwich imido complexes were found to be active catalysts for the inter- and intramolecular hydroamination of a broad range of alkynes and ω-aminoalkynes.

Triazole-Au(I) complexes: A new class of catalysts with improved thermal stability and reactivity for intermolecular alkyne hydroamination

(Chemical Equation Presented) A series of 1,2,3-triazole-bound cationic Au(I) catalysts have been synthesized, and their structures have been characterized by X-ray crystallography. Variable-temperature NMR studies revealed dynamic triazole-Au cation coor

Neutral Ti complexes as catalysts for the hydroamination of alkynes and alkenes: Do the labile ligands change the catalytic activity?

A detailed comparison between three four-coordinate Ti complexes featuring the general bidentate ligand [η5-(C5H 4)-SiMe2-NtBu]2- and two ligands X (NMe 2, Me, Cl) as catalyst precursors (I-III) for the intermolecular hydroamination of alkynes and the intramolecular hydroamination of alkenes is presented. The results strongly suggest that the catalytically active species are only identical for reactions performed with the bis(dimethylamido) complex I or the dimethyl complex II. Under the reaction conditions, the labile ligands X are proteolytically removed by the reacting amine to form catalytically active imido or amido complexes, together with dimethylamine or methane. Although both catalyst precursors can be used successfully for many substrate combinations, the preparative and kinetic studies clearly indicate that dimethylamine, which is formed from the bis(dimethylamido) catalyst precursor I and the reacting amine, is able to convert the catalytically active imido or amido complexes back into the catalyst precursor and therefore inhibits the reactions. As a consequence, the bis(dimethylamido) catalyst precursor I shows a poorer catalytic performance than the corresponding dimethyl complex II. Additionally, it is shown that the dichloro complex III is only a suitable catalyst precursor for selected hydroamination reactions. Corresponding reactions that are more difficult to achieve - such as reactions of diarylalkynes or amino alkenes - do not work efficiently with this complex. A possible explanation for this observation is the finding that the dichloro catalyst precursor III is obviously converted into a different catalytically active species. This can happen if the [η5-(C5H4)-SiMe2-NtBu]-ligand system of the catalyst precursor is being destroyed and removed from the Ti center under the reaction conditions. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

[Ind2TiMe2]: A general catalyst for the intermolecular hydroamination of alkynes

[Ind2TiMe2] (Ind=indenyl) is a highly active and general catalyst for the intermolecular hydroamination of alkynes. It catalyzes the reaction of primary aryl-, tert-alkyl-, sec-alkyl-, and nalkylamines with internal and terminal alkynes. In the case of unsymmetrically substituted 1-phenyl-2-alkylalkynes, the reactions occur with modest to excellent regioselectivities, whereby formation of the anti-Markovnikov regioisomers is favored. While the major product of hydroamination reactions of terminal arylalkynes is always the anti-Markovnikov isomer, alkylalkynes react with arylamines to preferably give the Markovnikov products. To achieve reasonable rates for the addition of sterically less hindered n-alkyland benzylamines to alkynes, these amines must be added slowly to the reaction mixtures. This behavior is explained by the fact that the catalytic cycle proposed on the basis of an initial kinetic investigation includes the possibility that the rate of the reaction increases with decreasing concentration of the employed amine. Furthermore, no dimerization of the catalytically active imido complex is observed in the hydroamination of 1-phenylpropyne with 4-methylaniline in the presence of [Ind2TiMe2] as catalyst. In general, a combination of [Ind2TiMe2]-catalyzed hydroamination of alkynes with subsequent reduction leads to the formation of secondary amines with good to excellent yields. Particularly impressive is that [Ind 2TiMe2] makes it possible for the first time to perform the reactions of n-alkyl- and benzylamines with 1-phenylpropyne in a highly regioselective fashion.