Synthesis of η³-Propargyl Rhenium Complexes

Protonation of Rhenium Alkyne Complexes Produces η3-Allyl Rhenium Complexes via Observable 1-Metallacyclopropene Intermediates

“…Reinvestigation of the Reaction of Diisopropyl Acetylene Complex 2 with Acid. Previously we had shown that protonation of C 5 Me 5 (CO) 2 Re(η 2 -Me 2 CHC⋮CCHMe 2 ) ( 2 ) with HBF 4 led to the formation of a 1:1 mixture of η 3 -allyl complexes 3-exo and 3-endo and had suggested the possibility of rhenium−alkyne hydride and 1-metallacyclopropene intermediates (Scheme ) . Our success in observing related intermediates in the reactions of butyne complex 4 led us to investigate the low-temperature protonation of 2 .…”

“…In a second case, protonation of C 5 Me 5 (CO) 2 Re(η 2 -Me 2 CHC⋮CCHMe 2 ) ( 2 ) led to the formation of a 1:1 mixture of the η 3 -allyl complexes C 5 Me 5 (CO) 2 Re(η 3 - exo , syn -Me 2 C−CH−CHCHMe 2 ) + PF 6 - ( 3-exo ) and C 5 Me 5 (CO) 2 Re(η 3 - endo , syn -Me 2 C−CH−CHCHMe 2 ) + PF 6 - ( 3-endo ) . A mechanism involving formation of 1-metallacyclopropene intermediate B followed by hydride migration was suggested to explain the formation of the η 3 -allyl complexes 3-exo and 3-endo (Scheme ).…”

Acid-Catalyzed Isomerization of Rhenium Alkyne Complexes to Rhenium Allene Complexes via 1-Metallacyclopropene Intermediates

“…We have found three ways to generate 1-metallacyclopropenes: (1) addition of electrophiles to alkyne complexes, (2) addition of electrophiles to allene complexes, and (3) ionization of complexed haloalkenes or enol trifluoroacetates. Important reactions of 1-metallacyclopropenes include, in order of decreasing relative rates: (1) addition of anions to give haloalkene or enol trifluoroacetate complexes, (2) deprotonation to give complexed allenes, (3) degenerate 1,2-hydride migrations, (4) hydride migrations to give η 3 -allyl complexes, , and (5) deprotonation to give complexed alkynes. The electron-deficient nature of the carbene-like carbon of 1-metallacyclopropenes explains their propensity to add nucleophiles.…”

Quantitative Determination of the Regioselectivity of Nucleophilic Addition to η3-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium between η3-Propargyl Rhenium Complexes and Rhenacyclobutenes

“…Nucleophiles have been observed to attack each of the three carbon centers of η 3 -propargyl rhenium complexes. We found that while kinetic addition of nucleophiles occurs at the center carbon of the η 3 -propargyl ligand to give rhenacyclobutene complexes, this addition is sometimes reversible and can be followed by addition at either of the other two metal-bound carbon atoms to give η 2 -allene or η 2 -alkyne complexes 4a,10,11. The methyl substituted η 3 -propargyl complex [C 5 Me 5 (CO) 2 Re( η 3 -CH 2 C≡CMe)][BF 4 ] (1-Me) reacted irreversibly with carbon nucleophiles such as malonates and acetylides to give metallacyclobutenes (Scheme 2); protonation of the metallacyclobutenes led to η 3 -allyl complexes.…”

“…4 Other methods for the synthesis of isolable η 3 -propargyl complexes include halide abstraction from η 1 -propargyl or η 1 -allenyl metal halide complexes,5 reaction of metal halides with propargyl nucleophiles,6 reaction of propargyl ether complexes with Lewis acids,7 rearrangement of η 1 -homopropargyl metal complexes,8 and oxidative addition of propargyl halides or tosylates to metal complexes 6e,9…”