NICKEL-CATALYZED AND PALLADIUM-CATALYZED ALDOL-TYPE CONDENSATION BY ENOL ESTERS

PdCl2(PhCN)2-SnCl2, bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2]/PPh3-Zn, NiBr2(PPh3)2-Zn, or Pd-(OAc)2/PPh3-Zn catalytic systems induced aldol-type condensation of isopropenyl acetate with aldehydes to produce 4-substituted (E)-3-buten-2-ones. PdCl2(PhCN)2-SnCl2 was utilized in the aldol-type reaction with any arenecarbaldehyde, bearing either an electron-donating group or an electron-withdrawing group, at 50-degrees-C in acetonitrile. Catalytic systems using zinc, in particular Ni(cod)2/PPh3-Zn, exhibited some high chemoselectivities at 60-degrees-C in dioxane: (1) an aldehyde only reacted in the presence of an internal ester or ketone, (2) olefinic aldehydes only reacted in the presence of saturated aliphatic aldehydes, (3) reactivity in the condensation with substituted benzaldehydes is the inverse of that in usual nucleophilic addition to carbonyl compound, and (4) 2-methoxybenzaldehyde only reacted in the presence of 4-methoxybenzaldehyde. Using PdCl2(PhCN)2-SnCl2, Ni(cod)2/PPh3-Zn, or Pd(OAc)2/PPh3-Zn, cyclic enol ester, namely 5-methyl-2(3H)-furanone also caused aldol-type reaction with carbonyl compounds. The reaction with Ni(cod)2/PPh3-Zn or Pd(OAc)2/PPh3-Zn at 60 -degrees-C in dioxane selectively produced 5-substituted 4-acetyl-2(5H)-furanones via dehydrogenation of gamma-substituted beta-acetyl-gamma-butyrolactones.