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-Z n, NiBr2(PPh3)2-Zn, or Pd-(OAc)2/PPh3-Zn catalytic systems induced ald ol-type condensation of isopropenyl acetate with aldehydes to produce 4-substituted (E)-3-buten-2-ones. PdCl2(PhCN)2-SnCl2 was utilized in t he aldol-type reaction with any arenecarbaldehyde, bearing either an e lectron-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 e ster or ketone, (2) olefinic aldehydes only reacted in the presence of saturated aliphatic aldehydes, (3) reactivity in the condensation wit h substituted benzaldehydes is the inverse of that in usual nucleophil ic addition to carbonyl compound, and (4) 2-methoxybenzaldehyde only r eacted in the presence of 4-methoxybenzaldehyde. Using PdCl2(PhCN)2-Sn Cl2, 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 6 0 -degrees-C in dioxane selectively produced 5-substituted 4-acetyl-2( 5H)-furanones via dehydrogenation of gamma-substituted beta-acetyl-gam ma-butyrolactones.