TMEDA complexes with organolithium

It is of interest to note that Kminek, Kaspar and Trekoval have made calorimetric measurements on the interaction of n-butyllithium with several Lewis bases at RLi/base ratios of one. The largest enthalpy changes (in agreement which the results of Quirk and Kester were found for TMEDA and dimethoxyethane and the smallest with diethyl ether and anisole. Their results for anisole clearly show that even aromatic ethers will interact and complex with organolithium species. Thus, their findings serve to fortify the viscometric findings regarding the influence of aromatic ethers on the poly(styryl)lithium association state. 

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. ... 

The dynamic behavior of various solid organolithium complexes with TMEDA was investigated by variable-temperature and CP/MAS and Li MAS NMR spectroscopies. Detailed analysis of the spectra of the complexes led to proposals of various dynamic processes, such as inversion of the five-membered TMEDA-Li rings and complete rotation of the TMEDA ligands in their complex with the PhLi dimer (81), fast rotation of the ligands in the complex with cyclopentadienyllithium (82) and 180° ring flips in the complex with dilithium naphthalene (83) °. The significance of the structure of lithium naphthalene, dilithium naphthalene and their TMEDA solvation coiMlexes, in the function of naphthalene as catalyst for lithiation reactions, was discussed . ... 

Remarkably, the closely related benzyllithium 304 is configurationally unstable even at -78 °C.138 Transmetallation of 303 (88% ee) at -78 °C gave an organolithium which reacted to give racemic product 305 in the presence or absence of TMEDA. Furthermore, the reaction of the 304-(-)-sparteine complex with each of racemic or enantiomerically pure 2 in a Hoffmann test gave the same 1 1.6 ratio of diastereoisomers. It is not yet clear whether this unexpected difference between 301 and 304 is due to an electronic difference between the naphthyl and phenyl systems, or whether it arises from the difference in steric hindrance, and therefore the dihedral angle between the ring and the amide, in the two compounds. 

Activation of organolithium compounds. n-Butyllithium and phenyllithium react very slowly with diphenylacetylene. However, the 1 1 complex of either lithium compound and TMEDA reacts with diphenylacetylene at room temperature. For example, the reaction of /-butyllithium under these conditions followed by carbonation gives cis-4,4-dimethyl-2,3-diphenyl-2-pentenoic acid (1) and a trace of 2-phenyl-3-f-butylindone (2). Thus addition takes place as well as metallation.1... 

It might be expected that in the presence of TMEDA or other tertiary diamines anomalous reaction products might be obtained with organolithium compounds such as benzyllithium. A number of reports in the literature disclose instances of the expected reaction products from reactions such as carbonation to the carboxylic acid and addition to benzophenone (I, 3, 4, 12). The phenyllithium-TMEDA (1 1) complex in benzene was allowed to react with benzophenone to give a 95% yield of triphenylcarbinol and with cyclohexanone to yield 59% of the 1-phenylcyclohexanol. The reaction with excess trimethylsilyl chloride is apparently quantitative. The main consideration in using these complexes is to use low temperatures for reaction and aqueous washes of ammonium chloride solution in the work-up to remove all of the tertiary diamine (the odor can be detected in low concentrations.)... 

Conjugated Dienes and Other Monomers. Alkyllithiums such as n-butyllithium—and even the growing polyethylene carbon-lithium bond complexed with chelating diamines such as TMEDA—are effective initiators for the polymerization of conjugated dienes such as 1,3-butadiene and isoprene. A polybutadiene of high 1,2-content can be produced from butadiene in hydrocarbon solvents using these N-chelated organolithium catalysts. 

Unsaturated elastomers can be readily metallated with activated organolithium compounds in the presence of chelating diamines or alkoxides of potassium or sodium. For example, polyisoprene, polybutadiene, styrene-butadiene copolymers, and styrene-isoprene copolymers can be metallated with n-butyllithium TMEDA complexes (1/1 or 1/2 ratio) to form allylic or benzylic anions. The resulting allylic anion can be employed as an initiator site to grow certain branched or comb polymer species. These polymers can include polystyrene, which would form hard domains, or polybutadiene, which forms soft domains. 

In experiments on metallation of silacyclopentenes by organolithium compounds, the ROP was observed [100]. Weber et al. [101-108] showed that polymerization can be performed by treating monomer with catalytic amounts of alkyllithium compounds complexed with polar compounds (HMPA, TMEDA) in THF at -78°C. In terms of phenomenology, symmetrically [101-103] and un-symmetrically [101, 103, 105] disubstituted derivatives of l-silacyclopent-3-ene with methyl, phenyl, and vinyl radicals, hydro derivatives of l-silacyclopent-3-ene [106, 107], 1,1,3-trimethyl- and l,l,3,4-tetramethyl-l-silacyclopent-3-enes... 

It has been observed that the concentrated solution viscosity decreases upon addition of TMEDA to solutions of poly(isoprenyl)lithium 93). This would be consistent with the process shown in Eq. (17) or (20) and not with Eqs, (18) or (19). The decrease in viscosity would be consistent with interaction of TMEDA to form an unassociated complex (Eq. (20)), but this does not seem to be in accord /with the stoichiometry observed by calorimetry. It is noteworthy that the break observed by calorimetry at R = 0.5 is consistent with the stoichiometric dependence of spectral, kinetic and microstructure effects 90). Again this shows that these kinetic effects are related to the stoichiometry of formation of base-organolithium adduct, i.e. that they are ground-state solvation effects. 

Among unsolvated organolithium compounds only the alkyllithiums are soluble in noncoordinating solvents such as alkanes and arenes. Their states of aggregation depend on the structure close to lithium. Thus primary, tertiary and secondary alkyllithiums, all unsolvated, assemble into respectively hexamers, tetramers and equilibrium mixtures of hexamers and tetramers. Most organolithium compounds dissolve in and coordinate with donor compounds such as ethers and tertiary amines. The actual structures depend critically on the nature of the donor. Thus, diethyl ether solvates tend to be mainly cubic tetramers (with some dimers) while THF favors mixtures of monomers and dimers. Tertiary vicinal diamines such as TMEDA and 1,2-di-Af-piperidinoethane, DPE, favor bidentated coordinated dimers. Finally, in the presence of triamines such as pentamethyl-triethylenediamine PMDTA and l,4,7-trimethyl-l,4,7-triazacyclononane TMTAN, many organolithium compounds form tridentately complexed monomers. 

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. 

Synthetic Application of (Benzyllithium —TMED A and Benzyl-lithium—TED. All the synthetic application studies were run with an aged (benzyllithium)2-TMEDA toluene solution that had 0.8% of the total organolithium compounds present as the tolyllithium isomers (only meta present) or with benzyllithium-TED crystalline complex free of ring isomers. 

Our interest in the synthetic utility of N-chelated organolithium compounds was prompted by this early work. Since detailed experimental procedures were not generally available at that time, we initiated a study of the effects of time, temperature, stoichiometry, etc., on the reactions of BuLi-TMEDA with benzene. We found that optimum metalation of benzene occurs when the preformed BuLi-TEMEDA complex, prepared from equimolar amounts of the organolithium reagent and diamine, is... 

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