Lithium ion pairs

Due to the high interest in metalation reactions with lithium amide or alkyllithiums, an indicator scale of lithium ion pairs in THF has been developed119. Aggregation studies have indicated that organolithium species exist predominantly, if not exclusively, as monomers in the 10-3-10-4 M concentration range. Particular attention has been devoted to the lithium and caesium ion-pair acidities of diphenylamine in THF120 that, at 25 °C, have been found to be 19.05 and 24.20, respectively. 

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... 

DMSO. Filled squares are for cesium ion pairs and fiUed diamonds are for lithium ion pairs in THF. Open circles are for cesium ion pairs in CHA. For convenience, a line with slope = 1 and intercept = 0 has been placed on the plot. 

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. 

Conductometric measurements were feasible down to 50 volume percent of THF, and the dissociation constants of lithium ion-pairs were determined from those data. The log Kdiss were shown to be linear with reciprocal of the appropriate dielectric constant - a relation commonly reported284) although not obvious. The solvating power of a medium is not determined uniquely by the bulk dielectric constant, D the more sophisticated Kirkwood function (D - l)/(2 D + l)329) could provide a better measure of that property. However, since the Kirkwood function is nearly linear with 1/D in the investigated range of dielectric constants, the discrimination between them is not practical. 

The reasons for the effectiveness of silylated amide bases might consist in an internal protection of the reactive functionalities, either via the formation of tightly associated lithium ion pairs following the deprotonation, or via the formation of silylated alcohols or silylated amides operating as protecting groups. Both pathways would prevent the alkoxide or amide coordination to palladium resulting in catalyst deactivation. 

P. Johansson, S. Beranger, M. Armand, H. Nilsson, P. Jacobsson, SoUd State Ionics 2003, 156, 129-139. Spectroscopic and theoretical study of the l,2,3-triazole-4,5-dicarbonitrile anion and its lithium ion pairs. 

An ab initio study of the addition of lithium aluminium hydride (LAH) to acetonitrile and malononitrile is reported the free anions generated by hydride addition show clear preferences for the enamide (RCH CH=NH RCH=CHN H) over the imide (RCH2CH=N ). Lithium ion pair formation stabilizes both tautomers, the localized imide is stabilized slightly more than the enamide, and the enamide preference is somewhat reduced but persists. The alane-complexed lithium ion pairs result in a small imide preference for the LAH adduct of acetonitrile and a dramatically reduced enamide preference for the LAH adduct of malononitrile. Alane affinities were determined for the lithium ion pairs formed by LiH addition to the nitriles. The alane binding greatly affects the imide-enamide equilibria such that alane complexation might even provide... 

Solvent effects on the C n.m.r. spectra of indenyl-lithium have been investigated. The pyrolysis of 9-Li-fluorene at 180 C provides 9,9-Li2 fluorene. An equilibrium lithium ion-pair indicator scale has been established for a series of fluorenederivatives which form solvent separation ion-pairs in THF AS values were found to be small for the indicator equilibria, studied by u.v.-vis spectroscopy. ... 

Reich HJ, Borst JP, Dykstra RR, Green DP. A nuclear-magnetic-resonance spectroscopic technique for the characterization of lithium ion-pair stmctures in Thf and Thf/Hmpa solution. 7. Am. Chem. Soc. 1993 115(19) 8728-8741. 

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