111196-49-7Relevant articles and documents
Carbon-carbon bond forming reactions of rhenium enolates with terminal alkynes. Evidence for an alkyne C-H oxidative addition mechanism and observation of highly stereoselective base-catalyzed proton transfer reactions in rhenium metallacycles
Stack, Jeffrey G.,Simpson, Robert D.,Hollander, Frederick J.,Bergman, Robert G.,Heathcock, Clayton H.
, p. 2716 - 2729 (2007/10/02)
The acetonitrile-substituted complexes fac-(MeCN)(OC)3(PPh3)ReCH(R2)CO(R1) (4, R1 = OEt R2 = H; 5, R1 = OEt R2 = Me) and the chelating amide complex 6 react with terminal alkynes (R3 C≡CH) to afford five-membered metallacycles 7-9 The metallacycles possess an exocyclic double bond which exists in the less thermodynamically stable Z stereochemistry, in which the alkyl group R3 is oriented proximate to the metal center. These complexes rearrange in the presence of a Lewis base to form the endocyclic isomers 10-12. The structure of the metallacycle 10g was determined by a single-crystal X-ray diffraction study. Labeling studies demonstrated that the 1,3-hydrogen shift involved in this rearrangement is intramolecular and stereospecific with respect to the rhenium center, even though it is mediated by an external reagent. Deuterium incorporation into the 7-position of the endo metallacycles can also be induced by base and once again occurs stereospecifically (anti to the phosphine ligand). Treatment of the metallacycles with acid results in removal of the metal and yields the unsaturated esters as a mixture of stereo- and regioisomers. Kinetic studies demonstrated that the reaction of 4 with terminal alkynes involves reversible dissociation of the coordinated acetonitrile to form an unsaturated intermediate 29 (R1 = CO2Et) This intermediate reacts faster with more electron-rich alkynes (HC≡CCMe3) than electron-deficient alkynes (HC≡CCO2Me) and displays a R3C≡CH/R3C≡CD kinetic isotope effect kH/kD of 2.0. On the basis of these results and information about reactions of related rhenium complexes, we suggest that formation of the metallacycles is best accommodated by the mechanism shown in Scheme VIII: insertion of 29 into the alkyne C-H bond leading to a 7-coordmate rhenium acetylide hydride 31, 1,3-migration of the hydride to the acetylide β-carbon, providing vinylidene complex 32, and then migration of the enolate ligand to the a-carbon of the vinylidene group, giving the exo metallacycle.
