NOVEL REARRANGEMENT REACTIONS .1. MOLECULAR REARRANGEMENT VIA METATHESIS BETWEEN INTRAMOLECULAR SI-SI AND FE-FE BONDS

Thermal treatment of t-BuC5H4Me2SiSiMe2C5H4-t-BU with Fe(CO)(5) in xyl ene gave the diiron complex (Me2SiSiMe2)[(eta(5)-t-BuC5H3)Fe(CO)](2)(m u CO)(2) (3) existing as a mixture of cis and trans isomers (3c and 3t ) which were separated by preparative TLC. When the two isomers were h eated respectively in xylene, the cis substrate (Sc) rearranged to the trans product [Me2Si(eta(5)-t-BuC5H3)Fe(CO)(2)](2) (4t), while the tr ans substrate (3t) afforded the cis product [Me2Si(eta(5)-t-BuC5H3)Fe( CO)(2)](2) (4t). This indicates that the rearrangement reaction is ste reospecific. Complexes [(eta(5):eta(5)-C5H4Me2SiSiMe2C5R3R')Fe(CO)](2) (mu . CO)(2)(R=H, R=t-Bu, 5; R=R=Me, 6), containing different cyclopen tadienyl groups, were prepared similarly. Their respective rearrangeme nt reactions only yielded the corresponding unsymmetrical products [(M e2Si)(eta(5)-C5H4)Fe(CO)(2) Pi(Me2Si)(eta(5)-C5R3R')Fe(CO)(2)](R=H,R'= t-Bu, 7;R=R'=Me,8); no symmetrical disproportionation products were de tected. This indicates that the reaction proceeds via an intramolecula r rather than intermolecular pathway. The conclusion is further confir med by the fact that no crossover product was formed in subsequent rea rrangement between two different substrates. Probing the reaction by E SR spectroscopy shows that the reaction proceeds via an intermediate c ontaining iron-centered radicals. On the basis of these experimental f acts, an alternative mechanism for the rearrangement reaction is propo sed. Molecular structures of 3c, 3t, 4t, and 4c were determined by X-r ay diffraction.