Cyclopropyl phenyl ketone underwent oxidative addition to Ni(PCy3) generated from Ni(cod)2 and PCy3 to give a nickeladihydropyran, which is a key intermediate for the Ni(0)-catalyzed homo- or heterocycloaddition to give cyclopentane compounds having two carbonyl substituents at the 1,3-position.

Cyclopropanecarboxaldehyde (1 a), cyclopropyl methyl ketone (1 b), and cyclopropyl phenyl ketone (1 c) were reacted with [Ni(cod)(2)] (cod = 1,5-cyclooctadiene) and PBu(3) at 100 degrees C to give eta(2)-enonenickel complexes (2 a-c). In the presence of PCy(3) (Cy = cyclohexyl), 1 a and 1 b reacted with [Ni(cod)(2)] to give the corresponding mu-eta(2):eta(1)-enonenickel complexes (3 a, 3 b). However, the reaction of 1 c under the same reaction conditions gave a mixture of 3 c and cyclopentane derivatives (4 c, 4 c'), that is, a [3+2] cycloaddition product of 1 c with (E)-1-phenylbut-2-en-1-one, an isomer of 1 c. In the presence of a catalytic amount of [Ni(cod)(2)] and PCy(3), [3+2] homo-cycloaddition proceeded to give a mixture of 4 c (76%) and 4 c' (17%). At room temperature, a possible intermediate, 6 c, was observed and isolated by reprecipitation at -20 degrees C. In the presence of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), both 1 a and 1 c rapidly underwent oxidative addition to nickel(0) to give the corresponding six-membered oxa-nickelacycles (6 ai, 6 ci). On the other hand, 1 b reacted with nickel(0) to give the corresponding mu-eta(2):eta(1)-enonenickel complex (3 bi). The molecular structures of 6 ai and 6 ci were confirmed by X-ray crystallography. The molecular structure of 6 ai shows a dimeric eta(1)-nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric eta(1)-nickelenolate structure, and the nickel(II) 14-electron center is regarded as having "an unusual T-shaped planar" coordination geometry. The insertion of enones into monomeric eta(1)-nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate eta(3)-oxa-allylnickel complexes (8, 9), whereas insertion into dimeric eta(1)-nickelenolate complex 6 ai did not take place. The diastereoselectivity of the insertion of an enone into 6 c having PCy(3) as a ligand differs from that into 6 ci having IPr as a ligand. In addition, the stereochemistry of eta(3)-oxa-allylnickel complexes having IPr as a ligand is retained during reductive elimination to yield the corresponding [3+2] cycloaddition product, which is consistent with the diastereoselectivity observed in Ni(0)/IPr-catalyzed [3+2] cycloaddition reactions of cyclopropyl ketones with enones. In contrast, reductive elimination from the eta(3)-oxa-allylnickel having PCy(3) as a ligand proceeds with inversion of stereochemistry. This is probably due to rapid isomerization between syn and anti isomers prior to reductive elimination.

dipolar zwitterionic form. The Cu(II) complexes seem to crystallize as isolated entities since only intramolecular H-bonds are found. However two disordered water positions can be found in a Fourier difference map, and two extra oxygen atoms were included in the refinement so that their occupancy adds to unity.

A perspective view…︁ of a six‐membered oxa‐nickelacycle with the bulky ligand IPr is shown (Ni: green, N: cyan, O: magenta, C: white, H omitted). This structure shows a monomeric η1‐nickelenolate complex; the coordination geometry of the nickel(II) 14‐electron center is regarded as “an unusual T‐shaped planar” as described by S. Ogoshi et al. in their Full Paper on This complex is a key intermediate of the nickel‐catalyzed [3+2] cycloaddition reaction of cyclopropyl ketones with enones; the insertion of enones into a six‐membered oxa‐nickelacycle occurred at room temperature to generate the η3‐oxa‐allylnickel complex.