Synthesis and reactivity of six-membered oxa-nickelacycles: a ring-opening reaction of cyclopropyl ketones.

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.