Rearrangement of a Propargyl Vinyl Rhenium Complex to a Rhenium Allenyl Vinyl Ketone Complex

An η¹- to η³-propargyl rearrangement promotes vinyl migration to CO in trans-C₅H₅(CO)₂Re(CH₂C≡CH)((E)-CH=CHCH₂CMe₃) (7) which eventually leads to the formation of allenyl vinyl ketone complex C₅H₅(CO)Re(η²,η²-H₂C=C=CHCOCH=CHCH₂CMe₃) (8). This allenyl vinyl ketone complex 8 undergoes additional transformations at room temperature leading to the diastereomeric allenyl vinyl ketone complex C₅H₅(CO)Re(t)(η²,η²-H₂C=C=CHCOCH=CHCH₂CMe₃) (9) by complexation of rhenium to the opposite face of the vinyl group and to the π-allyl σ-vinyl rhenium complex C₅H₅(CO)Re[η³(exo,syn)-H₂CCHCHCOCH=CCH₂CMe₃] (10) by a net [1,5] hydrogen migration. Upon heating at 105 °C, allenyl vinyl ketone complex 9 rearranged to the isomeric π-allyl σ-vinyl rhenium complex C₅H₅(CO)Re[η³(endo, syn)-H₂CCHCHCOCH=CCH₂CMe₃] (11). When the first formed π-allyl σ-vinyl rhenium complex 10 was heated at 105 °C, it first rearranged to allenyl vinyl ketone complex 9 and then to the more stable π-allyl σ-vinyl rhenium complex 11. Two different mechanisms can account for the interconversion of allenyl vinyl ketone complexes with π-allyl σ-vinyl rhenium complexes. One involves a [1,5] sigmatropic hydrogen shift over the unsaturated organic fragment and the second involves transfer of hydrogen via a rhenium hydride intermediate.