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Synthesis and Reactivity of Six‐Membered Oxa‐Nickelacycles: A Ring‐Opening Reaction of Cyclopropyl Ketones
Author(s) -
Tamaki Takashi,
Nagata Midue,
Ohashi Masato,
Ogoshi Sensuke
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200900929
Subject(s) - cyclopentane , chemistry , cycloaddition , ketone , reactivity (psychology) , ring (chemistry) , methyl vinyl ketone , nickel , medicinal chemistry , stereochemistry , catalysis , organic chemistry , medicine , alternative medicine , pathology
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 °C to give η 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 μ‐η 2 :η 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 °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‐η 2 :η 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 η 1 ‐nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric η 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 η 1 ‐nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate η 3 ‐oxa‐allylnickel complexes ( 8 , 9 ), whereas insertion into dimeric η 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 η 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 η 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.