Marked Changes in Relative Nucleophilicity in Comparing S_N2Ar Reactions of Free Arenes and Coordinated Arenes. Kinetic Studies of Reactions of (η⁶-Chlorobenzene)(η⁵-cyclopentadienyl)iron(II) Tetrafluoroborate with Anionic and Neutral Nucleophiles

Abstract

Quantitative kinetic studies have been made of reactions of the (η6-chlorobenzene)(η5-cyclopentadienyl)iron(II) cation, (C6H5Cl)Fe(C5H5)+ (1), with methoxide, phenoxide, methanethiolate, benzenethiolate, and azide ions in methanol, piperidine, morpholine, aniline, and thiourea in methanol, and guanidine in ethanol. The results were compared with the same nucleophile−solvent combinations in reactions with 1-chloro-2,4-dinitrobenzene, C6H3(NO2)2Cl (2). In general, the reactivity decreases on passing from 2 to 1. The decrease is small (≤10-1) for phenoxide (PhO-), methanethiolate (MeS-), and guanidine (gua) and large (ca. 10-5 ) for benzenethiolate (PhS-), azide (N3-), piperidine (pip), morpholine (morph), aniline (anil), and thiourea (thiou). The differences in reactivity when comparing the SN2Ar reactions of 1 and 2 is discussed in terms of the different location of the negative charge generated in the transition state by the electrons displaced from the reaction center by the entering groups (arenide electrons).