Reactivity of (Pentadienyl)iron(1+) Cations: Nucleophilic Addition by Phosphines Is Reversible in Certain Cases

The reaction of (pentadienyl)Fe(CO)g+ cations 1,5, and 11 with phosphines was examined. At short reaction times, attack of PPhg on la 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 (lb), kinetic attack by PPhg occurs exclusively at the unsubstituted pentadienyl terminus. The complex 2a was characterized crystallographically, and the stereochemistry of PPhg attack was established to be opposite to Fe(CO)g by use of a stereoselectively labeled cation, dexo-la. The reaction of PPhg with 11 initially gives £,Z-12, which isomerizes to £,£-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 £,Z-12 to ß reversible phosphine addition