Lithiumorganics

Lithium organic compounds have gained in importance ever since their discovery. Today they are indispensable reagents in organic as well as inorganic syntheses. They are readily applied in various preparative protocols, ranging from deprotonation of weakly acidic reagents to bond formation in organic group transfers as well as in industrial large-scale anionic polymerisation reactions. The structure-reactivity-relationship is still the Holy Grail to be found in this class of compounds as it is commonly accepted that the lithiated species determine the composition, yield and stereo- chemistry of the product^[1].


Charge density investigations can provide insight into the bonding and reactivity of these labile molecules. The picolyllithium com- pound [(C₆H₆NLi·NC₆H₇)]₂ (Fig. 2 top) is an excellent candidate for a detailed structure-reactivity-relationship investigation. The com- pound is obtained by deprotonating an excess of 2-picoline with n-butyllithium in THF and consists of a dimer with unreacted pico- line as donor molecules and internal structural standard^[2]. Look- ing at the plain connectivities the question of whether the com- pound is a carbanion or a lithium amide cannot be answered decisively. The three-dimensional distribution of the electrostatic potential (ESP) displayed in Figure 2 bottom provides further evidence. Opposite to the Li–N bond, we find a vast region of negative ESP above the picolyl-anion plane. Areas of negative ESP are preferred reaction sites for electrophiles and indeed, an electrophilic attack on 2-picolyllithium generally occurs at the methylene group. Hence, although the Li–C bond is quite long the anion reacts as a carbanion.


It is known that in hydrocarbon solvents lithiumorganics form oligomers and that their size significantly affects the reactivity. For the two compounds most commonly used in synthesis – nBuLi and tBuLi – the degree of oligomerization in solution was specified rather early by cryoscopical and spectroscopical measurements. It was found that nBuLi forms a hexamer while tBuLi forms a tetramer in non-donating solvents. Hence their solution structure is identical to the solid state structure^[3]. By addition of ethers like diethyl ether or THF and especially by addition of tertiary amine donor bases like TMEDA or PMDETA, these oligomers can be disaggregated to smaller fragments resulting in an enhanced reactivity. In practice, mixtures of different donor bases are frequently used, however, the exact percentage of the single donor bases to give the most suit- able complex is mostly determined empirically and the exact constitution of the reactive complex is unknown. It is most com- monly presumed that the entire coordination sphere of the lithium atom is occupied by the strongest donor molecules available.
In a recent study we could monitor the selective and consecutive donor base addition and exchange in the same lithiumaryl carb- anion complex by structural determination for the first time (Fig. 3). Unequivocally, it is feasible by stoichiometric addition of a second donor base to fine-tune the composition of a mixed base complex. Surprisingly, however, the Li−C-bond lengths and hence the re- activity do not linearly scale to the increasing amount of the better donor base in the complex. The required amounts of donor bases to enhance the reactivity can be adjusted far more accurately than anticipated in the past. In this example only three equivalents of THF are required instead of the expected four. Thus, the formation of hetero-donor base complexes should be taken into account when tailoring the reactivity of an organolithium compound^[4].


Lithium cyclopentadienyl derivatives are one of the most applied starting materials in metal organic synthesis. They are compounds par excellence to generate a huge variety of sandwich or half- sandwich d-block metal organics via transmetalation or salt eli- mination reactions. We have shown recently that addition of ammonia to THF generates lithocene or even naked Cp anions (Fig. 4). The strong donating and deaggregating properties of ammonia observed herein are the most likely reason for the high reaction rate of many organometallic species in liquid ammonia. The use of ammonia can be the key for reactions with a high potential barrier caused by steric repulsion. In contrast to reactivity enhancing additives like TMEDA or PMDETA the absence of a hydrocarbon skeleton can improve the performance of many reactions^[5].

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[1] T. Stey, D.Stalke „Lead structures in lithium organic chemistry“ in The Chemistry of Organolithium Compounds, eds. Z. Rappoport, I. Marek, John Wiley & Sons New York, 2004, 47-120.

[2] H. Ott, U. Pieper, D. Leusser, U. Flierler, J. Henn, D. Stalke Angew.Chem. 2009, 121, 3022, Angew.Chem. Int. Ed. 2009, 48, 2978 and highlighted by P. Macchi, Angew.Chem. 2009, 121, 5905, Angew. Chem. Int. Ed. 2009, 48, 5793.

[3] T. Kottke, D. Stalke Angew. Chem. 1993, 105, 619; Angew. Chem. Int. Ed. Engl. 1993, 32, 580.

[4] D. Stern, N. Finkelmeier, K. Meindl, J. Henn, D. Stalke Angew. Chem. 2010, 122, 7021; Angew.Chem. Int. Ed. 2010, 49, 6869.

[5] R. Michel, R. Herbst-Irmer, D.Stalke Organometallics 2011, 30, 4379.