Mechanistic dichotomy in CpRu(CH(3)CN)(3)PF(6) catalyzed enyne cycloisomerizations.
Enynes are easily accessible building blocks as a result of the rich chemistry of alkynes and thus represent attractive substrates for ring formation. A ruthenium catalyst for cycloisomerization effects such reaction of 1,6- and 1,7-enynes typically at room temperature in acetone or DMF under neutral conditions. The reaction is effective for forming five- and six-membered rings of widely divergent structure. The alkyne may bear both election-donating and election-withdrawing substituents. The alkene may be di- or trisubstituted. Introduction of a quaternary center at the propargylic position of an ynoate, however, completely changes the nature of the reaction. In the case of a 1,6-enynoate, a seven-membered ring forms in excellent yield under equally mild conditions. Evidence is presented to indicate a complete change in mechanism. In the former case, the reaction involves the intermediacy of a ruthenacyclopentene. In the latter case, a C-H insertion to form a pi-allylruthenium intermediate is proposed and supported by deuterium-labeling studies. A rationale is presented for the structural dependence of the mechanism.
Trost
Barry M
BM
Department of Chemistry, Stanford University, Stanford, California, 94305,USA.
Toste
F Dean
FD
D000480:Alkynes / Q000737:chemistry*; D000498:Allyl Compounds / Q000737:chemistry; D002384:Catalysis; D006844:Hydrocarbons, Cyclic / Q000138:chemical synthesis*; D007536:Isomerism; D009942:Organometallic Compounds / Q000737:chemistry; D012428:Ruthenium / Q000737:chemistry*
D000480:Alkynes; D000498:Allyl Compounds; D006844:Hydrocarbons, Cyclic; D009942:Organometallic Compounds; D012428:Ruthenium