Dimers lithium enolate aggregates

Jackman and Szeverenyi were the first to systematically correlate the quadrupoiar interaction with molecular structure, i.e. with the aggregation of lithium enolates ". They noted that a tetrameric aggregate had a smaller QSC than a dimeric aggregate, ca 135 kHz compared to an estimate of 230 kHz for the dimer. The QSC of the tetramer was shown to be of the same magnitude in three different ethereal solvents. 

After the proton transfer completion, the enolates tend to merge in (1 1) mixed aggregates with the excess lithium amide34. These species have so far been the object of relatively little attention48. Then, as the enolization proceeds to completion, the aggregated enolates form at the expense of the mixed dimers. Another aspect to be considered is the interaction between the lithium enolate and the amine released after protonation of the amide49. This phenomenon will be discussed in the section dedicated to the enantios-elective reactions of enolates. 

Lithium ester enolates are extremely important in polymer chemistry as initiators and active centers of the anionic polymerization of acrylic and methacrylic monomers in polar solvents. Thus, HF-SCF studies, comparable to those mentioned above, were undertaken on monomeric methyl isobutyrate (MIB) enolate210,211. The overall conclusions on the aggregation and solvation trends are exactly the same, the bent rj3-0,C mode being preferred over the rj1-O planar one by ca 3.3 kcalmol-1. While the dimeric MIB enolate solvated by four molecules of THF was found to be the enthalpically most stable aggregate, the prismatic S6 unsolvated MIB hexamer was computed as the preferred structure in non-polar solvents (Scheme 55)212. In the latter case, the supplementary oxygen of the ester acting as a side-chain ligand for the lithium seems to explain this remarkable stability. 

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]. 

SCHEME 57. Various types of solid-state aggregation of /3-ketocarbonyl compounds (A) unsolvated dimer [(CH3C(OLi)CH-COOEt]2]222 (B) unsolvated hexameric [(CH3C(OLi)CHCOOBu-t]6223 (C) tetrasolvated dimer of 1,3-cyclohexanedione lithium enolate exhibiting both O—Li and O—H coordinations224... 

SCHEME 60. Solid-state aggregation of (thio)ester lithium enolates (A) chelated dimer [(c-Pr= C(OLi)SBu-f)2, 2 TMEDA]236 (B) chelated hexamer of racemic (NH2CH(Me)CH2CH=C(OMe) OLi]6240... 

SCHEME 61. Solid-state aggregation of amide lithium enolates (A) tetrasolvated dimer [(C-C7H12C(OLi)NMe2)2, 4 THF]241 (B) intramolecular chelated [(PhC(OLi)=NPr- )6, 2 THF]243... 

The solvent effect has long been recognized as an important factor in that it affects the lithium-oxygen bond polarization but also the electrophilic reagent380,398. The effect on aggregation was evaluated by measurement and comparison of the reactivities of monomeric, dimeric and tetrameric forms of LiPhIBP and LiPhAT or LiPhIBP in various ethers252. In the less polar solvent methyl-tert-butyl ether, lithium enolates are tetrameric and do not react with benzyl bromide. On the contrary, with added HMPA the dissociation of the tetrameric LiPhIBP is accompanied by solvation of each monomer by 1 -2... 

Pratt and Streitwieser performed ab initio (HF/6-31G and HF/6-311-FG ) calculations to examine the formation of mixed dimer and trimer aggregates between the lithium enolate of acetaldehyde (lithium vinyloxide, LiOVi) and lithium chloride, lithium bromide and lithium amides. Gas-phase calculations showed that in the absence of solvation effects, the mixed trimer (LiOVi)2 LiX (20) was the most favored species. 

The pinacolone lithium enolate condensation product with pivaldehyde (147) has been characterized as the tetrameric aggregate (148).However, an attempted condensation reaction of pinacolone with itself as shown in Scheme 8 led to crystallization of a product derived from subsequent dehydration and reenolization, i.e. (149). This dienolate (149) was characterized as the dimer (150) solvated by ditnethyl-propyleneurea (DMPU). ... 

Two lithium enolates (174) and (175) derived from the vinylogous urethanes (176) and (177) have been crystallized and subjected to X-ray diffraction analysis.Although the individual enolate units combine to form different aggregates, they are very nearly identical in conformation, i.e. s-trans around the 2,3-bond however, both the aggregation state and the diastereoselectivity of the enolates differ. The enolate (175) is obtained from benzene solution as a tetramer and (174) is obtained from THF solution as a dimer. The origin of the diastereoselectivity shown by these enolates is subtle. 

Any discussion of enolate geometry must include the structure of the enolate. It is well known that metal enolates exist as dimers a or other aggregates in ether solvents S (see Section 9.2.C. for a discussion of aggregate formation with LDA).28d Jackman and Szeverenyi suggested that the lithium enolate of isobutyro-phenone exists as a tetramer (31) in THF solution but exists as a dimer (32) in DME. o Such aggregates were proposed by House et ah,3 who found that ketone enolates of groups 1 (lA), 2 (llA), and 3 (lllA) metals... 

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