Chapter 1
Structure–Reactivity Relationship in Organolithium Compounds

Elena Carl and Dietmar Stalke

1.1 Structural Principles in Organolithium Compounds

Owing to the versatile application of organolithiums compounds in syntheses, the identification of their molecular structure is vital to deduce structure–reactivity relationships. In reaction schemes, the organolithium compounds are often depicted as monomeric species although it is known since 1963 that the real structure of these compounds is much more complicated. Back then, Dietrich [1] determined the first solid-state structure of soluble ethyllithium from single crystals (and the first solid-state structure of a lithium compound that was determined via experimental X-ray diffraction analyses ever). In the solid state, basic organolithium reagents such as n-BuLi, i-PrLi, and LiCH2SiMe3 form hexameric aggregates, while t-BuLi and MeLi aggregate to tetramers [2]. The basic building principle in these deltahedra is the arrangement of the lithium cations in a Li3 triangle capped by a carbanionic Cα atom (Figure 1.1). The Li3 triangle is the building block for deltahedral metal cores, and further aggregation leads to tetramers or hexamers where the lithium cation reaches its preferred coordination number of four [3].

Figure 1.1 Aggregation of the μ3-Cα-capped Li3 triangle to give deltahedral metal cores.

The molecular structure in organolithium compounds is not only defined by the electrostatic interactions between the counter charged atoms (Li+ and −CH2R). The choice of the used solvent or co-solvent also has an immense effect on the molecular structure [4]. Our focus is on the disaggregation of lithium oligomers by adding Lewis donor bases and on the structural differences in organolithium compounds with a silicon next to the Cα carbon anion (Li–Cα–R3, Li–Cα–SiR3). We present experiments that demonstrate the enhanced reactivity of smaller disaggregated fragments as, for example, the benzylic deprotonation of toluene employing n-BuLi, which is only feasible on the addition of tetramethylethylenediamine (TMEDA) [5]. Alkyllithium compounds such as MeLi and n-BuLi are the most famous and commonly used representatives and their disaggregation has been investigated thoroughly for many years [6]. In addition, LiCH2SiMe3 is a commonly used reagent in syntheses [7] and interesting studies about its disaggregation and reactivity with different donor bases have been published recently.

1.2 Donor-Base-Free Structures

1.2.1 Tetramers

[MeLi]4 [8] (1), [EtLi]4 [9] (2), and [t-BuLi]4 [2a] (3) are the only donor-base-free tetrameric structures of organolithium compounds known so far. The characteristic core of these structures is built by four of the Li3 triangles joining together to create the tetrahedron. Each of the four Li3 triangles is μ3-capped by a Cα atom above the center of the equilateral metal triangle. In this way, each carbanion coordinates to three lithium atoms so that every lithium cation reaches at least its preferred coordination number of four. Even in the solid state, none of the three tetramers adopts ideal Td symmetry (Figure 1.2).

Figure 1.2 Solid-state structures of the basic [RLi]4 tetramers [MeLi]4 (1), [EtLi]4 (2), and [t-BuLi]4 (3).

However, the crystallographically independent Li···Li distances within the individual tetramers are similar within the estimated standard deviations (esds). They decrease from 256 pm in 1 to 253 pm in 2 and 241 pm in 3. One would expect the opposite considering the increasing steric demand of the organic groups. But along this line, the t-butyl group has the highest electron-releasing capability and provides the most charge to the single lithium cations so that they can get in closer proximity than in methyl lithium with a considerably higher positive charge and hence higher repulsion.

The Li–Cα bond lengths are almost invariant at 226 ± 2 pm and close to the mean Li–C bond distance of 230 pm from the CCDC (The Cambridge Crystallographic Data Centre) [10] (Table 1.1). [EtLi]4 and [t-BuLi]4 display relatively short Li···Cβ distances so that by the arrangement of the methyl groups in close proximity to a lithium cation some extra charge density can be provided to the lithium cation. In the [t-BuLi]4 tetramer, the t-butyl groups are even arranged ecliptically relative to the Li3 triangle. Because there is no Cβ atom in [MeLi]4, this gives rise to long-range interactions of the methyl groups of adjacent tetramers (Figure 1.3). As both the apical lithium cation and the basal lithium atoms point toward a nearby methanide group, the sum of all long-range interactions give the unit cell a tetramer in the center and the centroid of a tetramer at each corner. The related Li···“Cβ” distances (236 pm) are only 10 pm longer than the Li–Cα bonds. Their considerable contribution to the overall lattice energy leaves [MeLi]4,∞ insoluble in non-donating solvents while [EtLi]4 and [t-BuLi]4 are soluble even in nonpolar hydrocarbons such as hexane [3].

Table 1.1 Distances in the alkyllithium tetramers (pm).

Compounds

Li···Li

Li–Cα

Li–Cβ

References

[MeLi]4

1

259

226

236

[8]

[EtLi]4

2

253

228

250

[9]

[t-BuLi]4

3

241

225

237

[2a]

Figure 1.3 Long-range interactions between the [MeLi]4 tetramers.

1.2.2 Hexamers

The most important and prominent representatives of the octahedral Li6 structural motif are [Me3SiCH2Li]6 [11] (4) and [n-BuLi]6 [2a] (5), followed by others such as [i-PrLi]6 [12] (6), [c-HexLi]6 [13] (7), and [(t-Bu)2C6H3Li]6 [14] (8) (Figure 1.4). In the Li6 octahedra, only six of the eight Li3 triangles are μ3-capped by Cα atoms. The two remaining uncapped Li3 triangles are arranged oppositely and show elongated Li···Li distances of 294–318 pm. This elongation is due to a stronger electrostatic repulsion of the uncapped lithium cations because the attraction and electron density of a capped anion is missing. The six carbanions of the octahedron form a “paddle-wheel” along the noncrystallographic threefold axis through the midpoint of these uncapped triangles.

Figure 1.4 Solid-state structures of [RLi]6 hexamers: [Me3SiCH2Li]6 (4), [n-BuLi]6 (5), [i-PrLi]6 (6), and [c-HexLi]6 (7).

On average, the bond distances of the capped Li3 triangles in the hexamers are equal to those of the tetramers (240–251 pm vs 240–246 pm) (Table 1.2). As in the tetramers, in all structures 5–7, secondary electron donation exists by the methyl group in β position to the metallated carbon atom or in γ position for [Me3SiCH2Li]6 (4), respectively.

Table 1.2 Distances in the alkyllithium hexamers (pm).

Compounds

Li···Li

Li–Cα

Li–Cβ

References

[Me3SiCH2Li]6

4

246(318)a

219(227)

267b

[11]

[n-BuLi]6

5

243(294)

216(227)

229

[2a]

[i-PrLi]6

6

240(296)

218(231)

231

[12]

[c-HexLi]6

7

240(297)

218(230)

249

[13]

[(t-Bu)2C6H3Li]6

8

251(314)

215(221)

—

[14]

a Values in brackets represent the Li···Li distances in the unoccupied Li3 triangles.

b Li–Cγ distance.

1.2.3 Comparison of [Me3SiCH2Li]6 and [n-BuLi]6

Both compounds 4 and 5 have the same central structural motif, the hexameric Li6 octahedron capped by six carbanions. A closer look reveals small but appreciable structural differences between 4 and 5 due to the different types of carbanions. Structures 4 and 5 vary in the position of the Cα carbon atoms relative to the triangles. In [n-BuLi]6, the Cα is located more in the center of the lithium triangle while in [Me3SiCH2Li]6, the carbon atom is shifted to one side (Figure 1.5a) [15]. The Si–Cα bond is almost parallel to the closely approached Li···Li vector. This is reflected in a Si–C–Li bond angle close...