PALLADIUM-CATALYZED REACTIONS FOR FINE CHEMICAL SYNTHESIS, 4 - PHOSPHAPALLADACYCLE-CATALYZED HECK REACTIONS FOR EFFICIENT SYNTHESIS OF TRISUBSTITUTED OLEFINS - EVIDENCE FOR PALLADIUM(0) INTERMEDIATES

The coupling reaction of 1,1-disubstituted olefins (alpha-methylstyren e, n-butyl methacrylate) with various aryl bromides (Heck reaction) ha s been studied as a new concept to synthesize trisubstituted olefins. Surprisingly, the nature of the base dramatically influences the produ ct distribution. Thus, a systematic investigation on the role of base in Heck reactions of 1,1-disubstituted olefins was performed. Less coo rdinating bases like NaOAc, NaOBz or Na2CO3 yield a statistical distri bution of regioisomers with the terminal olefin 10 as the major produc t. However, by using amines like Bu3N or diisopropylethylamine (DIPEA) as base internal olefins can be synthesized with high selectivities. With phosphapalladacycle 3 as catalyst precursor, we were able to obta in catalyst turnover numbers up to 1000, while Pd(OAc)(2)/2PPh(3) was one order of magnitude less active. Analysis of the reaction profile b y kinetic investigations led to the postulation of a reduction and sub sequent oxidative addition of the catalyst precursor 3 to form 12 as c atalytically active intermediate.