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 + .

Experimental differentiation between pure C = N double bond rotation and nitrogen inversion in N-arylimines is possible with a single compound (13b) under the proviso of slow rotation about the N-aryl single bond. Labelling by 'H and 13C nuclei at the diastereotopic faces of the C = N moiety as well as of the N-aryl group is the clue to a successful stereodynamic analysis, as performed by variable-temperature NMR spectroscopy of 13b, a sterically congested and chiral model compound. Interpretation of similar measurements on a second model (13d) is less straightforward. The experimental observation of time-averaged C , symmetry by NMR coalescences is only compatible with a mechanism of ( E / Z ) stereomutation either by pure inversion at sp2 nitrogen or by a contribution from C = N rotation together with a synchronized (geared) contrarotation about the N-aryl single bond. However, the latter combination is concluded to be predominantly inversion-like by comparisons with related imines.The conformational lability at a C =X double bond in 1 and 2 is measured by the rate of (E,Z) diastereoisomerization, or of (E,Z) diastereotopomerization if the substituents e and .f are constitutionally equal. This stereomutation might occur by a 180"-rotation about the C = X double bond via a transition state 3 but if the key atom X carries a lone electron pair (like X = N or C-Li+), the alternative mechanism of "planar" inversion (or "lateral shift"[21) is more probable with linear coordination of X in a transition state 4. Quantum-mechanical calculations on imines (X = N) indicate a much higher energetic barrier of rotation (3) than of inversion (4) for iminomethane (1, R 5Definite experimental evidence to differentiate between these mechanisms has apparently not been published. The methyl groups within each one of the o-isopropyl substituents are diastereotopic (inequivalent) in the ground state of the quinone imine 6 a and should become enantiotopic (equivalent by mirror symmetry) during inversion (4); but the attempt to demonstrate this was thwarted by faster rotation about the N-aryl single bond [","] which leads to premature positional equilibration of these methyl groups. Kessler and Leibfritz ["] had to use guanidines to show that the diastereotopomerization of the groups e and f in 6 b occurred as fast as isopropyl enantiotopomerization; but comparisons with 6c and further derivatives were required to exclude contributions from N -aryl single-bond rotation in 6b. c I MQN--NMeCH2PhThus all available evidence accumulates to a "consen-S U S " [~~] with "many arguments"['31 for the inversion mechanism of imine stereomutation. However, comparisons within a series of related compounds would be time-consuming in other systems["] (e. g., X = C-Li+) and may be quantitatively questionable for reasons of possible electronic[31 and c~nformational[~~ differences in the ground states. Guided by synthetic and kinetic experien~e ['~~'~] with the conformationally quite rigid imine 7, we therefore considered the construction of a mode...

Exhaustive α‐Alkylation of Five‐Membered Ring Ketones by Sodium Hydride and Dimethyl Sulfate or Ehtyl Iodide Cα‐Permethylation may be carried out by addition of a basesensitive ketone like cyclopentanone to the inexpensive and stable mixture of sodium hydride and dimethyl sulfate. Exhaustive Cα‐ethylation is exemplified with sodium hydride by simultaneous introduction of ketone 4 and ethyl iodide. The 13C‐NMR absorptions of 1,3‐alkylated 2‐indanones 3–5 are assigned and some of the CH coupling constants reported.

'H and 13C NMR signals were assigned and CH coupling constants ('J, 'J, '4 determined for a series of a-monoand a,a-disubstituted (1,1,3,3-tetramethyl-2-indanylidene)methanes with the following a-substituents: (mesityl),B, n-propyl, phenyl, rerr-butyl-C(=NH), cyano, (rert-butyl),C(OH), pivaloyl, H,N-CO, PhNH-CO, carboxy, nitro, acetoxy, Me,SiO, Me,Si, PhS, PhSMe+, PhSO, PhSO, , bromo and trimethylstannyl. The ' J couplings with the oleíinic proton span the range 124.3-193.7 Hz. Substituent-induced chemicai shifts (SCS) of most of the nuclei with respect to the a-unsubstituted olefin obey simple additivity in the a,a-disubstituted compounds and are very similar to the SCS values along the C=N double bond in the isoelectronic (1,1,3,3-tetramethyl-2-indanylidene)amines within the error limits. The exceptions concern nuclei in the immediate vicinity of the perturbing substituent. A dominant mechanistic contrihution of electric field effects appears likely for the more distant aromatic part of the indanylidene moiety. The chemical shifts of two (2,2,5,5-tetramethylcyclopentyiidene)methanes are shown to be compatible with the SCS parameters from the indanylidene series.