Tetramers BuLi 4 structure

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

FIGURE 6. Solid-state structures of the basic [RLi]4 tetramers [MeLiL (1), [EtLiL (2) and [t-BuLi]4 (3)... 

The gas-phase structure of f-BuLi has also been probed by McLean and coworkers using photo ionization mass spectrometry". At room temperature, they detected only tetramers and determined the ionization potential of (f-BuLi)4 to be 6.2 eV. Other ions were detected above 8 eV with general formulae R Li4+(n = 1-3) and RLi2+(R = f-Bu). Both R3Li4+ and RLi2 had also been observed by Brown and coworkers in the H spectra of f-BuLi vapor, while R2Li4 and RLi4+ had not . 

There is similar controversy over whether the monomer (M-2) or open-dimer (OD-2) transition state governs the mode of addition of organolithium compounds to carbonyl compounds (Fig. 2). Although a four-center transition state M-2 involving the monomer has long been believed to participate in the alkylation [34], some ab initio calculation evidence shows that an open dimer OD-2 or related dimeric structure is more likely to be involved in the reactions, especially when (MeLi)2 participates [35, 36]. McGarrity and co-workers showed that the dimer of n-BuLi is tenfold more reactive than the tetramer toward benzaldehyde in THF. Even at high dilution there is no detectable concentration of the monomer [37],... 

Organolithium reagents (RLi) are tremendously important reagents in organic chemistry. In recent years, a great deal has been learned about their structure in both the solid state and in solution. X-ray analysis of complexes of n-butyllithium with A,A,A, A -tetramethylethylenediamine (TMEDA), THF, and 1,2-dimethoxyethane (DME) shows them to be dimers and tetramers [e.g., (BuLi DME)4]. X-ray analysis of isopropyllithium shows it to be a hexamer. 

Organolithium reagents are oligomers (i.e., dimers, trimers, and higher species) in nondonor solvents such as alkanes LiMe is a tetramer with a cubane structure 14.1, for example. RLi forms solvates with THE Addition of the chelating ligand Me2NCH2CH2NMe2 (TMEDA) leads to formation of a monomer, and this increases the reactivity. n-BuLi can deprotonate toluene... 

---