Lithium-carbanion pair

Phenylpropionic acid (10) was obtained in low yield and low enantiomeric purity. Although not necessarily detrimental, here configurational instability of the intermediate lithium carbanion pairs 9/epi-9 is the major reason for the insufficient result. This issue will be examined more closely in Section III.A. 

Enantioselective synthesis with lithium/(-)-sparteine carbanion pairs 97AG(E)2282. 

The carbanionic carbon of a-amino-organolithium compounds is often tetrahedral and the carbon-lithium ion pair is sometimes tight enough that V Li- C couplings can be observed in NMR spectra. If the lithium-bearing carbon is secondary, it is stereogenic. If one stereoisomer predominates over the other, this fact can be used to advantage in asymmetric synthesis. The predominance may be because of a diastereomeric bias... 

The reactive species in fluoride-mediated carbon-carbon bond-forming reactions has been investigated.146 The regio- and diastereo-selectivities of silanes reacting with cyclohexenone in the presence of a catalytic amount of fluoride have been compared with the reactivity of analogous solvent-separated lithium ion pairs. Closely analogous behaviour has been observed, showing that carbanions and not siliconate complexes are the reactive species in the fluoride-catalysed reactions. 

Hoppe, D. Hense, T. Enantioselective synthesis with lithium/(-)-sparteine carbanion pairs. Angew. Chem. Int. Ed. 1997, 36, 2282-2316. 

Usually, the selectivities of both the individual steps - formation of the lithium intermediate and electrophilic substitution - cannot be monitored separately, and the product of the two selectivities is measured. Fortunately, it turned out, for most carbanion pairs and electrophilic reagents, that the substitution step is completely stereospecific (mostly retention, in some cases inversion), within the hmits of detection. Larger errors only may occur when the rates of... 

The effects of electrophiles and solvents on the stereochemistry of electrophilic substitution of a lithium carbanion generated from (5, )-l-phenylbut-2-en-l-yl diiso-propylcarbamate have been examined using various acids and carbon electrophiles. The stereochemical outcomes have been described in terms of the existence of an equilibrium involving a solvent-separated ion pair and three different kinds of contact ion pairs in which the ligand coordinated to the lithium atom differs. 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

An aspect of general interest in organometallic chemistry is the equilibrium between contact and solvent-separated ion pairs, because metal cations which are sun ounded by an individual solvent cage are expected to show different reactivity towards basic centres than those closely attached to carbanions or amines. At the same time, the anionic centre is less shielded in an SSIP than in a CIP and thus expected to be more reactive. In solution, the differentiation by NMR methods between both structural motifs relies in most cases on chemical shift interpretations and, if possible, on heteronuclear Overhauser (NOE) measurements. The latter method is especially powerful in the case of lithium organic compounds, where H, Li or even H, Li NOE can be detected by one- and two-dimensional experiments. ... 

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

---