The empirical expression (1)J(CLi) = L[n(a + d)](-1) is proposed; it claims a reciprocal dependence of the NMR coupling constant (1)J((13)C, Li) in a C-Li compound on two factors: (i) the number n of lithium nuclei in bonding contact with the observed carbanion center and (ii) the sum (a + d) of the numbers a of anions and d of donor ligands coordinated at the Li nucleus that generates the observed (1)J(CLi) value. The expression was derived from integrations of separate NMR resonances of coordinated and free monodentate donor ligands (t-BuOMe, Et2O, or THF) in toluene solutions of dimeric and monomeric 2-(alpha-aryl-alpha-lithiomethylidene)-1,1,3,3-tetramethylindan at moderately low temperatures. This unusually slow ligand interchange is ascribed to steric congestion in these compounds, which is further characterized by measurements of nuclear Overhauser correlations and by solid-state structures of the dimers bearing only one donor per lithium atom (d = 1). Increasing microsolvation numbers d are also accompanied by typical changes of the NMR chemical shifts delta (positive for the carbanionic (13)C(alpha), negative for C(para) and p-H). The aforementioned empirical expression for (1)J(CLi) appears to be applicable to other cases of solvated monomeric, dimeric, or tetrameric C-Li compounds (alkyl, alkenyl, alkynyl, and aryl) and even to unsolvated (d approximately 0) trimeric, tetrameric, or hexameric organolithium aggregates, indicating that (1)J(CLi) might serve as a tool for assessing unknown microsolvation numbers. The importance of obtaining evidence about the (13)C NMR C-Li multiplet splitting of both the nonfluxional and fluxional aggregates is emphasized.

The trans/cis stereoinversion of the trigonal carbanion centers C-α in a series of monomeric 2-(α-aryl-αlithiomethylidene)-1,1,3,3-tetramethylindanes (known to be trisolvated at Li) is rapid on the NMR time scales (400 and 100.6 MHz) in THF solution. The far-reaching redistribution of electric charge in the ground-state molecules caused by lithiation (formal replacement of α-H by α-Li) is illustrated through NMR shifts, Δδ. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett ρ = +5.2), pointing to a mechanism that involves heterolysis of the C−Li bond via a solventseparated ion pair (SSIP). This requires immobilization of only one additional (the fourth) THF molecule at Li + , which accounts for part of the apparent activation entropies of ca. −23 cal mol −1 K −1 and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp 2 -stereoinversion process is "catalyzed" by the solvent THF; its mechanism is monomolecular with respect to the ground-state species because the pseudo-first-order rate constants, measured through NMR line shape analyses, are independent of the concentrations (inclusive of decomposition) of the dissolved species (hence no associations and no dissociation to give free carbanion intermediates). In the deduced pseudomonomolecular mechanism (bimolecular through solvent participation), the angular C-α of the SSIP undergoes rehybridization (approximately in-plane inversion) through a closeto-linear transition state; this motion occurs with a concomitant "conducted tour" migration of Li + (THF) 4 and is unimpaired by additional ortho-methylations at α-aryl. The synthetic route started with preparations of three α-chloro congeners through the carbenoid chain reaction, followed by vinylic substitution of α-Cl by α-SnMe 3 (most efficient in THF despite steric congestion). The final Sn/Li interchange reaction afforded the new 1-aryl-1-alkenyllithium samples, initially uncontaminated by free Li + .

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.

(EZ) Equilibria, 15111. -Syntheses and Lability of the Configuration of 2-Iminoindan Derivatives with Front StrainSyntheses and properties are described for sterically shielded imines R,C=NR (3c, e, g-k), which are rather inert toward nucleophiles. Nucleophilic attack a t the nitrogen atom of 3k is indicated by the formation of the azine 4. (E,Z) Configurational diastereotopomerization (antilsyn) is strongly dependent on N substituents [CH3, phenyl, I-naphthyl, acetyl, Si(CH3)3, cyano, SC6HJ, SOC6H5, S02C6H5, and nitro]. It is accelerated by front strain along the C = N bond in 3a-c, e, g, h, j, and k and provides an energetic basis (by AG* and AAG* values) for the gauging of force-field parameters. This (E,Z) stereomutation is characterized by a vanishing solvent dependence and a positive volume of activation (+ 10.2 cm3 mol-' for 3a).In contrast to other n: acceptor substituents, the N-nitro group in 31 retards (E,Z) interconversion.

NMR spectra in combination with pH measurements are shown to provide a simple and convenient procedure for determining the basicities of imines which are either slowly hydrolyzed (4, 5) or completely stable but inconveniently weak bases (11-13). The method has the advantage that it does not require any precise knowledge of the concentrations of the substrates or reagents. N-Alkyl (4, 15) and N-unsubstituted imines (5, 8, 9, 14) show quite similar basicities with a weak solvent dependence (from pure water to 99.5% methanol) which is akin to that of pyridine. The N-aryl imines 10-13 are less basic by ca. 4 pK units. It is concluded that spatial solvent exclusion by bulky substituents has remarkably little effect on the pKL values.Protonated pyridine (pK, = 5.14) is a slightly weaker acid than protonated aniline (4.62) in water whereas the order is reversed (5.37r2] versus 5.80r31) in anhydrous methanol; minimal pK, values for pyridinium (3.6r4]) and anilinium (4.0[3,51) were reported in aqueous (ca. 20:80) methanol. These rather modest pK, variations contrast with the strong solvent-dependence for an uncharged acid like acetic acid (4.76r6l in water, but 9. 61' 1 in methanol). In the aprotic but very polar media DMSO and acetonitrile the cations pyridinium (pK, = 3.4[','] and ca. 12.4[9-111, respectively) and anilinium (3.6['.91 and 10.6[93L01) are much more acidic than acetic acid ( 12.4['~'0] and 22.3r91). This trend continues into the gas phase [I2] where the acidity difference from acetic acid grows to 125 kcal/mol for pyridinium and 136 kcalhol for anilinium, corresponding to roughly 100 pK units. Obviously, solvation of the carboxylate anion is a more critical factor[s] and actually best achieved in water. But although solvent (e.g., DMSO) stabilization of protonated nitrogen is energetically more eficient[I21, it does not reduce the acidity in a corresponding proportion as long as the possibly more important[I3] solvation of the dissociated protons can operate; this accounts for the weaker solvent response of the nitrogen basicity.The extent of iminium cation solvation may be impeded by spatial restraints to the molecular approach of a given solvent (S). 2,6-Di-tert-butylpyridinium (1) is a well-studied example with the expected acidity in the gas p h a~e [ '~~'~] . Compared with sterically congested aliphatic ammonium ions ['6], the pK, value of 1 is much less diminished from 5.0 in ~a t e r [ '~, '~] to 3.6 in 50% aqueous The proposed explanation[l41 suggests that bonding of a single water molecule (S) at the cationic site might suffice to obtain the usual solvation energy at the expense of ca. 6 entropy units. 3With such background information, we report on the acidity constants and their solvent dependencies for iminium ions 2 and 3, expecting that bulky substituents X and Y andlor R should cause only moderate changes with respect to the sterically less congested analogs from the literature. Steric shielding in our model substances minimizes the possibility of interfering imine hydrolysis and appea...