Intramolecular hydroarylation of alkynes catalyzed by platinum or gold: mechanism and endo selectivity

The cyclization of differently substituted aryl alkynes with PtII or AuI catalysts proceeds by endo-dig pathways. When AgI was used to generate reactive cationic AuI catalysts, 2H-chromenes dimerize to form cyclobutane derivatives by a AgI-catalyzed process. A DFT study on the cyclization mechanism shows a kinetic and thermodynamic preference for 6-endo-dig versus 5-exo-dig cyclizations in PtII-catalyzed processes. Calculations indicate that although Friedel-Crafts and the cyclopropanation processes via metal cyclopropyl carbenes show very similar activation energies, platinum cyclopropyl carbenes are the stationary points with the lowest energy.     

Reaction of enol ethers with alkynes catalyzed by transition metals: 5-exo-dig versus 6-endo-dig cyclizations via cyclopropyl platinum or gold carbene complexes

The intramolecular reaction of enol ethers with alkynes in methanol is catalyzed by electrophilic Pt(II), Pd(II), and Au(III) chlorides and by a Cu(I) complex to give five- or six-membered rings bearing dimethyl acetals. The reaction takes place by an anti addition of the enol ether and the metal to the alkyne. The possible involvement of vinylidene complexes in this reaction is excluded. In addition to the usual 5-exo-dig (or 6-exo-dig) pathways, a 6-endo-dig pathway has also been found to take place with certain enynes. One case of 5-endo-dig cyclization has also been found. A general scheme for the alkoxycyclization of enynes catalyzed by transition metals based on DFT calculation of PtCl(2) and AuCl(3) complexes that includes exo and endo cyclizations is presented.     

An efficient heterogeneous gold(I)-catalyzed intermolecular cycloaddition of 2-aminoaryl carbonyls and internal alkynes leading to polyfunctionalized quinolines

The first heterogeneous intermolecular cycloaddition of 2-aminoaryl carbonyls and internal alkynes was realized in DMF at 100?°C by using a triphenylphosphine-functionalized MCM-41-supported gold(I) complex [MCM-41-PPh₃-AuCl] and AgOTf as catalysts, yielding a variety of polyfunctionalized quinolines in good to excellent yields. This heterogeneous gold(I) complex could easily be prepared via a simple two-step procedure from commercially available reagents and recovered by filtration of the reaction mixture. The recovered catalyst could be reused at least seven times with almost consistent activity without addition of AgOTf as a cocatalyst.     

N‐Heterocyclic Carbene Gold(I) Catalyzed Transformation of N‐Tethered 1,5‐Bisallenes to 6,7‐Dimethylene‐3‐azabicyclo[3.1.1]heptanes

Tying up loose ends : The reaction of bisallenes tethered with N‐(p‐tolylsulfonamide) in the presence of a cationic gold N‐heterocyclic carbene catalyst gave new cycloisomerization products, 6,7‐dimethyleneazabicyclo[3.1.1]heptanes, in high yields (see scheme; IPr=N,N′‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene).

     

Scope and mechanism of the Pd(II)-catalyzed arylation/carboalkoxylation of unactivated olefins with indoles

Treatment of 1-methyl-2-(4-pentenyl)indole (5) with a catalytic amount of [PdCl2(MeCN)2] (2; 5 mol %) and a stoichiometric amount of CuCl2 (3 equiv) in methanol under CO (1 atm) at room temperature for 30 min gives methyl (9-methyl-2,3,4,9-tetrahydro-4-carbazolyl)acetate (6), which was isolated in 83% yield. A number of 2- and 3-alkenyl indoles undergo a similar palladium-catalyzed cyclization/carboalkoxylation to give the corresponding polycyclic indole derivatives in moderate to excellent yields with excellent regio- and diastereoselectivity. Under similar conditions, vinyl arenes undergo intermolecular arylation/carboalkoxylation with indoles to give 3-(1-aryl-2-carbomethoxyethyl) indoles in moderate yield with high regioselectivity. Stereochemical analyses of the palladium-catalyzed cyclization/carboalkoxylation of both 2- and 3-alkenyl indoles are in agreement with mechanisms involving outer-sphere attack of the indole on a palladium-olefin complex followed by alpha-migratory insertion of CO and methanolysis of the resulting acyl palladium intermediate. CuCl2 functions as the terminal oxidant in this palladium-catalyzed cyclization/carboalkoxylation of alkenyl indoles and also significantly increases the rate of reaction of 2 with the alkenyl indole to form the corresponding acyl palladium complex. Spectroscopic studies are in agreement with the intermediacy of a heterobimetallic Pd/Cu complex as the active catalyst in this reaction.     

Ind(2)TiMe(2)]: a general catalyst for the intermolecular hydroamination of alkynes

[Ind(2)TiMe(2)] (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 n-alkylamines 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-alkyl- and 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 [Ind(2)TiMe(2)] as catalyst. In general, a combination of [Ind(2)TiMe(2)]-catalyzed hydroamination of alkynes with subsequent reduction leads to the formation of secondary amines with good to excellent yields. Particularly impressive is that [Ind(2)TiMe(2)] makes it possible for the first time to perform the reactions of n-alkyl- and benzylamines with 1-phenylpropyne in a highly regioselective fashion.