A practical reversed-polarity alternative to organocuprate conjugate addition chemistry. Halocarbon coupling reactions of enal- and enone-derived allylnickel reagents

Article abstract of DOI:10.1021/ja00016a037

A direct method for the conversion of conjugated enals and enones into [1-[(trialkylsilyl)oxy]allyl]nickel(II) chloride complexes is described. Reaction of R¹CH=CR²CHO (R¹ = H, Me, Ph; R² = H and R² = H; R² = Me) with Ni(COD)₂ (COD = 1,5-cyclooctadiene) and ClSiMe₂R³ (R³ = Me or t-Bu) affords bis[[1,2,3-η ³-1-[(trialkylsilyl)oxy]-2-propenyl](μ-chloro)nickel(II)] complexes 1a-f as burgundy-red crystalline solids in high to excellent yields (86-98%). The X-ray crystal structure of 1c confirms the η³-bonding mode for the allyl ligand and provides evidence of significant overlap of oxygen lone pair electron density with the allyl π-system. Reaction of 3-buten-2-one with Ni(COD)₂ and Me₃SiCl similarly affords bis[[1,2,3-η ³-1-methyl-1-[(trimethylsilyl)oxy]-2-propenyl](μ-chloro)nickel(II) ] (1g) in 72% yield, while the corresponding 2-cyclopentenone reaction fails, giving rise to a nickel metal mirror. The latter reaction does, however, succeed when conducted in the presence of pyridine, giving bis(pyridine)(chloro)[1,2,3-η ³-1-[(tert-butyldimethylsilyl)oxy]-2-cyclopentenyl]nickel(II) complex (1h) in 95% yield. Irradiation of 1e (R¹ = R² = H; R³ = t-Bu) in the presence of 2-iodopropane affords tert-butyldimethylsilyl (E)-enol ether 2a in 78% yield (E:Z = 10:1). Similar results are observed with [1-[(trialkylsilyl)oxy]allyl]nickel(II) chloride complexes generated in situ from propenal, 2-methylpropenal, 2-butenal, 2-methyl-2-butenal, hexenal, and 3-buten-2-one, in coupling reactions with 1-iodobutane, 2-iodopropane, 1-bromoethene, 2-bromopropene, bromobenzene, o-bromobenzaldehyde, and isobutyryl chloride, affording the corresponding silyl (E)-enol ethers in 60-78% isolated, purified yield, based on Ni(1,5-COD)₂. High C(3) regioselectivities (>20:1) are observed for all of the C(3)-unsubstituted enals, while lower regioselectivities, in the range of 4-6:1 before purification, are observed for the C(3)-substituted enals; regioisomerically pure materials are obtained upon chromatography. Moderate to high E:Z stereoselectivities (E:Z = 4-15:1 before purification; E:Z = 20-50:1 after chromatography) are observed in all cases, save the reaction of isobutyryl chloride with 1e and the reaction of the 3-buten-2-one-derived allylnickel complex with 2-bromopropene, both of which give 1:1 E:Z ratios. In a more convenient, economical, and general variant, Ni(COD)₂ is generated in situ via the sodium metal reduction of NiCl₂(pyridine)₄ (in turn prepared from NiCl₂(H₂O)₆) in the presence of cyclooctadiene and reacted with the enal or enone in the presence of ClSiMe₂R³ to afford the [1-[(trialkylsilyl)oxy]allyl](chloro)(pyridine)nickel(II) complex, which is treated with the halocarbon and irradiated to afford the usual silyl enol ether products. Yields of 63-84%, based on enal or enone, were observed in reactions with propenal, (E)-2-hexenal, 2-cyclopentenone, and 2-cyclohexenone in reactions with bromoethene, bromobenzene, and 1-iodobutane using this procedure.