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 PBu3 at 100 °C to give η2‐enonenickel complexes (2 a–c). In the presence of PCy3 (Cy=cyclohexyl), 1 a and 1 b reacted with [Ni(cod)2] to give the corresponding μ‐η2:η1‐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 PCy3, [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 °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 μ‐η2:η1‐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 η1‐nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric η1‐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 η1‐nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate η3‐oxa‐allylnickel complexes (8, 9), whereas insertion into dimeric η1‐nickelenolate complex 6 ai did not take place. The diastereoselectivity of the insertion of an enone into 6 c having PCy3 as a ligand differs from that into 6 ci having IPr as a ligand. In addition, the stereochemistry of η3‐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 Ni0/IPr‐catalyzed [3+2] cycloaddition reactions of cyclopropyl ketones with enones. In contrast, reductive elimination from the η3‐oxa‐allylnickel having PCy3 as a ligand proceeds with inversion of stereochemistry. This is probably due to rapid isomerization between syn and anti isomers prior to reductive elimination. Rings all round: The oxidative addit