Entropies of Organolithium Aggregation Based on Measured Microsolvation Numbers

Abstract

The recent measurement (J. Am. Chem. Soc. 2008, 130, 14179–14188) of the microsolvation numbers of monodentate, nonchelating ethereal donor ligands coordinating to the monomers and dimers of two sterically shielded C(aryl)–Li compounds permits the determination of well-founded dimerization enthalpies (ΔH 0) and entropies (ΔS 0) from properly formulated equilibrium constants, which must include the concentrations of the free donor ligands. The monomers are found to dimerize endothermically (ΔH 0 > 0) in [D8]toluene solution in the presence of the donor tBuOMe or THF, but only slightly exothermically (ΔH 0 = −0.5 kcal per mol of dimer) with the donor Et2O. The dimerization entropies ΔS 0 (in cal mol–1 K–1) with the respective equivalents of released donor ligands are 7.2 and 11.0 (with 2 equiv of tBuOMe in the two cases), 6.1 (with 2 Et2O), and 34.1 (with 4 THF). It is shown that the improper omission of microsolvation from the equilibrium constant (a usual practice when the ligand numbers are not known) can lead to “contaminated” aggregation entropies ΔS ψ, which may deviate considerably from the “true” entropies ΔS 0. A method is provided for estimating the required microsolvation numbers from 13C/Li NMR coupling constants 1 J C,Li for less congested organolithium types whose coordinated and free donor ligands cannot be distinguished by NMR integration.

Microsolvation and sp2-stereoinversion of monomeric α-(2,6-di-tert-butylphenyl)vinyllithium as measured by NMR

“…−11 cal mol −1 K −1 [10], as also known [11] for the diastereotopomerization of 1a &3THF and 2 &3THF whose Δ S ψ ‡ values (entries 1 and 2, Table 2) are similar to those of 4 &3THF (entry 3, Table 2), which suggests a similar course of the stereoinversion processes. In contrast, the disolvated monomers 4 &2 t -BuOMe and 4 &2Et 2 O would have to immobilize two additional ligands at the expense of doubled entropy contributions (ca.…”

“…−22 cal mol −1 K −1 ) if generating SSIPs (like 12 ) with tetrasolvated Li + , which explains why their diastereotopomerization rates remained below our NMR time scales even though the doubled THF fixation would provide an increased negative contribution to Δ H ψ ‡ . For a critical evaluation of these pseudoactivation parameters, it may be recalled [10–11] that the “true” activation enthalpies Δ H ‡ will be almost equal to Δ H ψ ‡ , whereas true Δ S ‡ will be more negative than Δ S ψ ‡ by a mathematical correction of roughly R ln[free THF] = up to 5 cal mol −1 K −1 .…”

“…However, our sterically congested model system [ 9 ] 1 exhibited separate resonances of the latter three donor ligands (“Don” in Scheme 1 ) above the melting points of the solvent mixtures, so that the microsolvation numbers d could be measured by simple NMR integrations. On this basis, dimerization equilibria of 1a and 1b were analyzed [ 10 ] with a proper allowance for the differing microsolvation of the monomeric and dimeric components.…”

“Conducted Tour” Migration of Li+ during the cis/trans Stereoinversion of α‐Arylvinyllithiums

“…The DS y°v alue of 1 was also strongly negative, but it was not included in entry 4b ecause it wasn umerically less reliable due to the competing decomposition of 1 in THF. This appeared to apply also to the presence of the second ortho-isopropyl substituent in 6 (entries5 and 6) because it must be admitted that the monoisopropyl congener 5 apparently began to dimerizei nT HF at the exceptionally high temperatures of up to 92 8C( ac ommon trend of the usually [21] endothermic dimerizations). Comparison of 4 with its 2-methyl congener 3 (entries 8a nd 7, Ta bleS7 [11] )e stablished that the presenceo ft he second ortho-methyl substituent in 4 had almostn oe ffect on the barrier.…”

“…Comparison of 4 with its 2-methyl congener 3 (entries 8a nd 7, Ta bleS7 [11] )e stablished that the presenceo ft he second ortho-methyl substituent in 4 had almostn oe ffect on the barrier. This appeared to apply also to the presence of the second ortho-isopropyl substituent in 6 (entries5 and 6) because it must be admitted that the monoisopropyl congener 5 apparently began to dimerizei nT HF at the exceptionally high temperatures of up to 92 8C( ac ommon trend of the usually [21] endothermic dimerizations). This becamev isible for 5 in THF through the published [22] concentration dependence of the rate constants k y at 88 8Ca nd the extrapolated, apparent DG y°( 0 8C) values that increased with increasing concentrations of 5 (footnotes" b" in Ta blesS8-S10 [11] ).…”