Reactions of cationic and neutral iron polyene complexes with nucleophiles

M. Azad Hossain, Marquette University

Abstract

While nucleophilic attack on coordinated polyenes is a paradigm of organotransition metal chemistry, not all polyene ligands have been examined. Two such important ligands are the cross-conjugated pentadienyl and trimethylenemethane ligands. The parent as well as substituted (cross-conjugated pentadienyl)Fe(CO)$\sb3$ cations have been generated at low temperature and reacted only with weak hetero atom nucleophiles such as water and methanol. While trimethylenemethane iron complexes has been prepared, their reactivity toward nucleophiles has not been examined. Therefore a study to develop methods for the formation of C-C bonds via coupling of these iron complexed ligands with various nucleophiles was undertaken. The parent as well as several substituted cross-conjugated pentadienyl cations have been generated in situ by reaction of the corresponding (2-acetoxymethylenebutadiene) iron complexes (186, 209, 206, 217a, 217b, 223a, 223b and 237) with BF$\sb3$.Et$\sb2$O. Without isolation these were reacted with various weak carbon and hetero atom nucleophiles such as allyltrimethylsilane, trimethylaluminum, triethylaluminum, triethylsilane, water and methanol. In general the reaction with carbon nucleophiles gave predominantly the corresponding substituted trimethylenemethane complexes due to the attack at C4 of the cross-conjugated pentadienyl cation along with the corresponding (diene)iron complexes due to the attack at C5 of the cation. An alkyl or aryl substitutent at C5 of the cross-conjugated pentadienyl cation influenced more nucleophilic attack at this site, i.e. more diene products compared to the TMM products by stabilizing the partial $\delta\sb+$ at C5. On the otherhand water and methanol exclusively attacked at C5 affording (diene) complexes as the only product. The known tricarbony(trimethylenemethane) iron complexes (122, 190a and 265) were prepared by reaction of the corresponding allyl dihalides with Fe$\sb2$(CO)$\sb9$. These were treated with various reactive carbon nucleophiles (266, 267, 268, 269, methyl lithium, n-butyl lithium and phenyl lithium) in THF/HMPA (4:1) followed by work up with electrophiles. In most cases, the corresponding allylated products were obtained. However, the reaction of 122 and 265 with phenyl lithium and methyl lithium respectively predominantly gave products which incorporate a molecule of carbon monoxide, while the reaction of 190a under similar conditions produced isomeric (isoprene)Fe(CO)$\sb3$ complex 190b. Mechanisms for the formation of both the CO inserted products and products without CO insertion have been proposed. A novel methodology for the liberation of the TMM ligand, involving C-C bond formation has been developed.

Recommended Citation

M. Azad Hossain, "Reactions of cationic and neutral iron polyene complexes with nucleophiles" (January 1, 1992). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. Paper AAI9318925.
http://epublications.marquette.edu/dissertations/AAI9318925

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