Solvation TMEDA-solvated dimer

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

The structure of mixed aggregates involving ester enolates is also of major interest to macromolecular chemists, since ionic additives are often introduced in the polymerization medium. The more stable arrangement between lithium 2-methoxyethoxide and MIB lithium enolate was thus calculated (at the DFT level) to be a 5 1 hexagonal complex with similar O—Li lateral coordinations212. The same team has recently extended this study to complexes formed between the same enolate in THF and a-ligands such as TMEDA, DME, 12-crown-4 and cryptand-2,1,1213. Only in the case of the latter ligand could a separate ion pair [(MIB-Li-MIB),2 THF]-, Li(2,l,l)+ be found as stable, still at the DFT level, as the THF solvated dimer [(MIB-Li)2,4 THF]. 

Three examples of nitrile-stabilized enolates have been described by Boche et al. Two of these structures incorporate the anion of phenylacetonitrile. Hie TMEDA-solvated dimer (178) crystallizes out of benzene solution however, the mixed nitrile anion LDA-(TMEDA)2 complex (179) is obtained when excess LDA is present. This latter complex has often been mistaken as a geminal d anion since it frequently gives products that appear to arise from a dianion. The crystal structure of the anion l-cyano-2,2-dimethylcyclopropyllithium (180) consists of an infinite polymer (181) that is solvated by THF. Interestingly, there are C— Li contacts in this structure and the carbanionic carbon remains tetrahedral. 

N-Phenylpyrrole (194) is monolithiated at the 2-position of the heterocyclic ring. This monolithium compound crystallizes as the TMEDA-solvated dimer (195). This structure agrees well with the Li- H 2D heteronuclear Oveihauser NMR spectroscopy (2D-HOESY). The structure serves to predict correctly that the second lithiation to a dianion occurs at the ortho position of the phenyl ring located closest to the lithium in the monoanion. 

Several ester enolates have also been examined by X-ray crystallography. The enolates of /-butyl propionate and /-butyl 3-methylpropionate were obtained as TMEDA solvates of enolate dimers. The enolate of methyl 3,3-dimethylbutanoate was obtained as a THF-solvated tetramer. 

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 . ... 

Ai,Ai -Diphenylbenzamidine undergoes metaUation in toluene, the crystalline precipitate (232) is probably polymeric and contains solvating toluene (average 0.7 mol per Li atom). The lithium imidinate solid produced in the presence of HMPA is dimeric (233) whereas the solids obtained in the presence of bidentate and tridentate ligands (e.g. TMEDA and PMDTA) are monomeric (234 and 235). The products were characterized by H NMR spectra and XRD crystallography, except for 232, for which no crystals suitable for XRD could be prepared . 

Recent NMR study suggested that PhLi exists as a mixture of tetramer and dimer in diethyl ether and that the addition of a stoichiometric amount of a coordinating solvent such as THF, dioxolane, DME or TMEDA induces complete conversion of the mixture to dimeric solvates. In a THF solution PhLi exists as a mixture of dimer and monomer. It was noted that the addition of 12-crown or HMPA increases the reactivity and decreases the selectivity of PhLi in THF. 

A kinetic study of the addition of n-Bul.i to a chiral aliphatic aldimine has explored the roles of TMEDA ( /V,/V,lV, lV -tetramcihylc(.hylcncdiaminc) and solvents (toluene, diethyl ether) on relative rates and diastereoselectivity41 Evidence for four mechanisms was obtained, including monomeric and dimeric n-BuLi cases, and a cooperative TMEDA-Et20 pathway. Hence the roles of chelation, aggregation, and cooperative solvation all need to be considered when solvents and additives are varied in such reactions. 

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. 

In the presence of the corresponding pyrrolidine diamine, the chiral lithium pyrrolidide amide yields dimeric chelates composed of a lithium pyrrolidide amide dimer solvated by a pyrrolidine diamine, (Li-6)2 6, as shown by NMR spectroscopy39. The lithium amide gives two 6Li NMR signals in a 1 1 ratio. The addition of TMEDA to Li-6 results in a similar complex where TMEDA coordinates to the lithium pyrrolidide amide dimer, (Li-6)2 TMEDA. 

NMR studies have shown that, upon addition of one equivalent of THF to a Et20 solution of the lithium amide, the Et20 ligand is replaced by THF in the dimer. Further addition of THF or TMEDA at low temperature was reported to yield monomers solvated by THF, Li-8 nTHF, where n = 1 or 2 (Figure 1). 

The structures of many compounds solvated by thf, dme, tmen, etc., are also known.41 Thus [LiBu (OEt2)]2 is a dimer similar to (3-II), [LiBu"(tmeda)] has a zigzag chain, while [LiMe(thf)]4 has a structure similar to that in Fig. 3-4 but with THF bound to each Li atom. 

In contrast, diisopropyl ether-solvated 2-lithio-3-bromobenzofuran dimer 4 contains two crystallographically identical Li-C-O cycles, a difference ascribed to monodentate ether versus bidentate TMEDA in 3. Again the anionic carbon lies ca. 0.3 A closer to the other lithium LijA than to Lij. Bond distance t/(Li-0) = 1.954(6) A is much smaller than that for unsubstituted 3, while d(C-0) = 1.470(4) A is almost 0.1 A longer than typical for benzofuran C-O bonds. 

A dimer (58) of a-lithiated 2,6-dimethylpyridine crystallizes with TMEDA solvation. This dimer is completely unlike the polymeric benzyllithium (53) in that no T) -intramolecular bonding is observed. The centrd core of the dimer (58) consists of an eight-membeied ring formed from two intermolecular chelated Li atoms and nearly ideal perpendicular conformations of the a-CHaLi groups. Dimer (58) is a relatively rare example of a lithiated ir-system where Li exhibits only one carbon contact. 

---