Zwei die zusammenpassen: Ausgezeichnete Regio‐, endo/exo‐, Diastereo‐, und/oder Enantioselektivitäten sind das Markenzeichen einer Cu‐katalysierten Cycloaddition von Nitronen an α′‐Hydroxyenone (siehe Schema). Die Addukte können in die entsprechenden Isoxazolidin‐Aldehyde, ‐Ketone und ‐Carbonsäuren umgewandelt werden.
A range of novel heterocyclic cations have been synthesized by the Rh(III)-catalyzed oxidative C-N and C-C coupling of 1-phenylpyrazole, 2-phenylpyridine, and 2-vinylpyridine with alkynes (4-octyne and diphenylacetylene). The reactions proceed via initial C-H activation, alkyne insertion, and reductive coupling, and all three of these steps are sensitive to the substrates involved and the reaction conditions. Density functional theory (DFT) calculations show that C-H activation can proceed via a heteroatom-directed process that involves displacement of acetate by the neutral substrate to form charged intermediates. This step (which leads to cationic C-N coupled products) is therefore favored by more polar solvents. An alternative non-directed C-H activation is also possible that does not involve acetate displacement and so becomes favored in low polarity solvents, leading to C-C coupled products. Alkyne insertion is generally more favorable for diphenylacetylene over 4-octyne, but the reverse is true of the reductive coupling step. The diphenylacetylene moiety can also stabilize unsaturated seven-membered rhodacycle intermediates through extra interaction with one of the Ph substituents. With 1-phenylpyrazole this effect is sufficient to suppress the final C-N reductive coupling. A comparison of a series of seven-membered rhodacycles indicates the barrier to coupling is highly sensitive to the two groups involved and follows the trend C-N(+) > C-N > C-C (i.e., involving the formation of cationic C-N, neutral C-N, and neutral C-C coupled products, respectively).
The N-heterocyclic carbenes (NHCs) 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene (alkyl = ethyl, IEt2Me2; isopropyl, IiPr2Me2) and 1,3-di-isopropylimidazol-2-ylidene (IiPr2) cleave Ru3(CO)12 at room temperature to give the mononuclear complexes Ru(NHC)2(CO)3 (NHC = IEt2Me2
1; IiPr2Me2
2; IiPr2, 3
). X-ray crystallography reveals that 1 contains trans-NHC ligands whereas 2 and 3 have an unexpected cis arrangement of carbenes. Exposure of 1−3 to air in the solid state or upon reaction with O2 in solution yields the carbonato complexes Ru(NHC)2(CO)2(CO3) (NHC = IEt2Me2
4, IiPr2Me2
5, IiPr2, 6).
Reaction of the bulky 6-membered N-heterocyclic carbene 6-Mes with Ni(cod)(2) gives a C-H activated 6-Mes nickel(II) complex, but the novel three-coordinate Ni(I) species Ni(6-Mes)(PPh(3))Br if the reaction is performed in the presence of Ni(PPh(3))(2)Br(2). The Ni(I) complex is a precursor for the catalytic hydrodehalogenation of aryl halides.
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