Quantification of the β‐Stabilizing Effect of the Dicarbonyl(η⁵‐cyclopentadienyl)iron Group

Abstract

Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday Kinetic investigations of the reactions of (prop-2-enyl)dicarbonyl(cyclopentadienyl)iron complexes 1 with benzhydrylium ions 3, and of dicarbonyl(cyclopentadienyl)[(1,2-h)propene]iron(II) tetrafluoroborate (9 ¥ BF 4 ) with p-nucleophiles have been performed to elucidate the magnitude of the b-effect of the [(CO) 2 FeCp] group (Fp group). Introduction of the Fp group into the allylic position of propene and 2-methylpropene increases the nucleophilicity of the p-bonds by nine and six orders of magnitude, respectively, with the result that the allyl-Fp complexes 1a (N 6.78) and 1b (N 8.45) are among the strongest neutral p-nucleophiles. Replacement of one b-H-atom in the isopropyl cation by the Fp group reduces the electrophilicity by more than 20 orders of magnitude, so that 9 ranks among the weakest cationic C-electrophiles (E À 11.2).

Recent chemistry of the η5-cyclopentadienyl dicarbonyl iron anion

“…Researchers Mayr et al have experimentally prepared KFp in 81% yield by reaction of Fp 2 with potassium-graphite laminate (KC 8 ) in THF (Scheme 3) [40]. A similar procedure has also been performed by Fischer et al [41].…”

“…Mayr and coworkers studied [40] the effect that the Fp moiety had on the reactivity of the bond in allyl complexes. They synthesized (Scheme 19) the allyl complexes and reacted them with various electrophiles to discover that there was a six-to ninefold increase in nucleophilicity.…”

Determination of the Electrophilicity Parameters of Diethyl Benzylidenemalonates in Dimethyl Sulfoxide: Reference Electrophiles for Characterizing Strong Nucleophiles

“…In recent years, large efforts have been made to develop nucleophilicity scales for comparing the reactivities of structurally different compounds, such as alkenes and arenes, [1][2][3] alcohols and amines, [4][5][6] carbanions, [7,8] and organometallics [1,9] or hydride donors. [1,10] To characterize the nucleophilicities of these compounds, the kinetics of their reactions with benzhydrylium ions (Ar 2 CH + ) and structurally related quinone methides have been investigated and evaluated by using the linear free energy relationship given in Equation (1), in which N and s are nucleophile-dependent parameters and E is an electrophile-dependent parameter.…”

Towards a General Scale of Nucleophilicity?

“…[8,9] The most extensive nucleophilicity scale, [10,11] presently available, has been derived from the rate constants of the reactions of benzhydrylium ions with alkenes, arenes, enol ethers, ketene acetals, [12] enamines, [13] allyl element compounds, [14] transition-metal p-complexes, [15] diazoalkanes, [16] and delocalized carbanions [17][18][19] (p-nucleophiles), amines, [20] alcohols, [21] alkoxides, [22] phosphanes, [23] inorganic anions [24][25][26][27] (n-nucleophiles), and a variety of hydrides [28][29][30][31] (s-nucleophiles); these rate constants were subjected to a least-squares minimization on the basis of Equation (3) [10,11] in which k is the second-order rate constant at 20 8C and E is a nucleophileindependent electrophilicity parameter.…”