C5H5(CO)2FE-ETA-1-HOMOALLYLIDENE TO C5H5(CO)2FE-ETA-1-ALLYLIDENE REARRANGEMENT, A NEW ROUTE TO VINYLMETHYLIDENES VIA LIGAND REARRANGEMENT - SYNTHESIS AND 1,4-ADDITIONS TO (C5H5(CO)2FE-ETA-1-[2-(BICYCLO[3.2.1]OCT-3-ENYLIDENE)])+CF3SO3-

An epimeric mixture of [η1-(1-bicyclo[2.2.1]hept-2-enyl)methoxymethyl]C5H5(CO)2Fe, 4 and 5, was prepared by sequential treatment of 1-bicyclo[2.2.1]hept-2-enecarbonyl chloride with sodium dicarbonyl(η5-cyclopentadienyl)ferrate, methyl triflate, and sodium borohydride in basic methanol. Treatment of 4 and 5 in methylene chloride at ‒78 °C with fluoroboric acid/diethyl ether or with trimethylsilyl triflate yields C5H5(CO)2Fe-η1-(2-bicyclo[3.2.1]oct-3-enylidene) tetrafluoroborate or triflate, 9+BF4- or 9+TfO-, respectively. Attempts to detect the putative η1-1-(bicyclo[2.2.1]hept-2-enyl)methylidene intermediate 6+ by monitoring the conversion of 4 and 5 to 9+ at ‒78 °C by1H and/or13C NMR were unsuccessful. This iron(II) homoallylidene to allylidene rearrangement constitutes a new route to β,γ-unsaturated iron(II) methylidenes. Molecular mechanics calculations (PCMODEL/MMX) employing ametallic carbocations as models for the iron(II) methylidenes are utilized to predict the course and relative facility of the rearrangement. A variety of nucleophiles (Nu), viz., lithium triethylborohydride, methyllithium, methyllithium in the presence of Cu(I) or Ce(III), lithium dimethylcuprate, dilithium dimethylcyanocuprate, methanol/NaHCO3, p-methylthiophenoxide, dimethyl lithiomalonate, and vinyllithium react stereoselectively with 9+ at the γ-position to yield a single complex, 10‒15, respectively, in each case. Addition is exo with methyllithium, lithium dimethylcuprate, and dilithium dimethylcyanocuprate and is presumed to be so with methanol/sodium bicarbonate, dimethyl sodiomalonate, and vinyllithium as well. When protonated with trifluoroacetic acid in methylene chloride at ‒78 °C, both 10 (Nu = H) and 11 (Nu = Me) yield C5H5(CO)2Fe-exo-η2-(exo-4-Nu-bicyclo[3.2.1]oct-2-ene) trifluoroacetates, 9+ (Nu = H) and 19+CF3CO2- (Nu = Me), respectively, which rearrange when warmed above ‒40 °C to C5H5(CO)2Fe[exo-η2-(4-Nu-bicyclo-[3.2.1]oct-2-ene)], 21+CF3CO2- (Nu = H), or exo-4-Nu-bicyclo[3.2.1]oct-2-ene, 22 (Nu = Me), respectively. Methyl triflate alkylates 10 (Nu = H) at the β-position to form a new alkylidene (η1-2-(3-methylbicyclo-[3.2.1]octylidenyl)dicarbonyl(η5-cyclopentadieneyl)iron triflate?), which rearranges and dissociates—3-methylbicyclo[3.2.1]oct-2-ene (23) is the final product. © 1990, American Chemical Society. All rights reserved.