The site of Michaeladditionof ~-heteroatom substitutedthiols (such as mercaptoethanol)to unsubstituted2, 4-diectoneand 2, 4, 6-trienonesis at the terminal(en)position. This site preferenceis transferred to the (3site when the addition is accomplishedusing Ti+4 complexation. These ~site additionproductsrapidly rearrangeto the m positionon bsse treatment. 0 1997Elsevier Science Ltd.
Unsubstituted, alien-and trienones undergo Michael type' addition of thiols predominately at their terminal position [cosite]. Just on the basis of the relative steric effects at the available attachment sites [13, & m, etc.], this mode of bonding would be anticipated. Nevertheless, the interplay of kinetic, electronic, resonance and steric effects is complex and subtle variations modifi the result. MetaI atom chelation can markedly determine the site of Michael addition in such systems. Mercaptoethanol addition to dienone 12 in methanol/THF using diisopropylethyhunine IDIPEA] catalyst, at 45"C, yields 5-(2-hydroxyethylthio)-nr-nitro-3(E)-penteneophenone 23 contaminated with only trace amounts of the isomenc 13addition product 3-(2hydroxyethylthio)-m-nitro-4-penteneophenone [4]4 (observed by both TLC [hexane/EtOAc, 3:1] and NMR).