Reactivity of (pentadienyl)iron(1+) cations: Nucleophilic addition by phosphines is reversible in certain cases

Article abstract of DOI:10.1021/om00011a034

The reaction of (pentadienyl)Fe(CO)₃⁺ cations 1, 5, and 11 with phosphines was examined. At short reaction times, attack of PPh₃ on 1a proceeds at both the unsubstituted and substituted pentadienyl termini to give 2a and 3a, respectively (2:1); however, over a period of ca. 21 h, the minor product 3a isomerizes to the more thermodynamically stable 2a. As the steric bulk of the 1-substituent is increased from methyl to ethyl (1b), kinetic attack by PPh₃ occurs exclusively at the unsubstituted pentadienyl terminus. The complex 2a was characterized crystallographically, and the stereochemistry of PPh₃ attack was established to be opposite to Fe(CO)₃ by use of a stereoselectively labeled cation, d_exo-1a. The reaction of PPh₃ with 11 initially gives E,Z-12, which isomerizes to E,E-12 over a 22 h period. Reaction with 5 with ((S)-neomenthyl)diphenylphosphine gives two optically active diastereomers, 15a and 15b (3:2 ratio); fractional crystallization gives 15a in greater than 80% mass recovery. In solution, pure 15a isomerizes to a mixture of 15a and 15b (3:2). The interconversions of 3a to 2a, of E,Z-12 to E,E-12, and of 15a and 15b are rationalized by reversible phosphine addition.