REACTIVITY OF (PENTADIENYL)IRON(1- NUCLEOPHILIC-ADDITION BY PHOSPHINES IS REVERSIBLE IN CERTAIN CASES() CATIONS )

The reaction of (pentadienyl)Fe(CO)(3)(+) cations 1, 5, and 11 with ph osphines was examined. At short reaction times, attack of PPh(3) on 1a proceeds at both the unsubstituted and substituted pentadienyl termin i 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 s table 2a. As the steric bulk of the 1-substituent is increased from me thyl to ethyl (1b), kinetic attack by PPh(3) occurs exclusively at the unsubstituted pentadienyl terminus. The complex 2a was characterized crystallographically, and the stereochemistry of PPh(3) attack was est ablished to be opposite to Fe(CO)(3) by use of a stereoselectively lab eled cation, d(exo)-1a. The reaction of PPh(3) 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 di astereomers, 15a and 15b (3:2 ratio); fractional crystallization gives 15a in greater than 80% mass recovery. In solution, pure 15a isomeriz es 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 reversib le phosphine addition.