Dimers ester enolates

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. 

Oxidation of arylmethyl ketoximes by phenyliodoso diacetate in glacial acetic acid was second order overall, first order each in substrate and oxidant.145 Iodine allowed the oxidative dimerization of glycine ester enolates with low to moderate diastereoselec-tivity that is consistent with kinetic control.146 Although malonic acid is not oxidized by iodate under acidic conditions, oxidation proceeds in the presence of catalytic ruthenium(III). A mechanism is put forward to account for the observed orders of reaction.147 The rate of periodate oxidation of m-toluidine in acetone-water increases with ionic strength.148... 

An ion-pair derived from the substrate and solid NaOH forms a cation-assisted dimeric hydrophobic complex with catalyst 39c, and the deprotonated substrate occupies the apical coordination site of one of the Cu(II) ions of the complexes. Alkylation proceeds preferentially on the re-face of the enolate to produce amino acid derivatives with high enantioselectivity. However, amino ester enolates derived from amino acids other than glycine and alanine with R1 side chains are likely to hinder the re-face of enolate, resulting in a diminishing reaction rate and enantioselectivity (Table 7.5). The salen-Cu(II) complex helps to transfer the ion-pair in organic solvents, and at the same time fixes the orientation of the coordinated carbanion in the transition state which, on alkylation, releases the catalyst to continue the cycle. 

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]. 

A fine early account of the relevant information on this topic is featured in Seebach s 1988 review200. Enolates seem to crystallize only as even aggregates (from dimers to octamers). Actually, it has been known for a while that ester enolates are stable crystalline solids220, but it took many efforts to obtain and handle crystals of a quality sufficient to achieve X-ray crystallography. Indeed, they tend to decompose into ketene and alkoxide, even in the crystalline state221. 

We complete this section with a study of the effect of TMEDA on the polymerization of methyl methacrylate lithium enolate in THF262. It was concluded that TMEDA hardly affects the kinetics of the polymerization and therefore the monomer-dimer equilibrium. From these figures, TMEDA does not seem to be a better ligand for lithium ester enolates than THF, in line with previous observations by Collum on other organolithium compounds263. 

Furthermore, by assuming that the methoxy unit maintains its integrity through the reaction, the formaldehyde fragment mentioned earlier must proceed from the oxymethylene unit of I. At this stage it is timely to focus one s attention on product III. It is a dimeric form of dimethyl ketene. This ketene must come from the ester enolate of I (see Problem 21). Therefore, the simultaneous generation of dimethyl ketene VII and formaldehyde is conceivable. 

Few ester enolate crystal structures have been described. The lack of structural information is no doubt due to the fact that the ester enolates undergo a-elimination reactions at or below room temperature. A good discussion of the temperatures at which lithium ester enolates undergo this elimination is presented in the same paper with the crystal structures of the lithium enolates derived from r-butyl propionate (163), r-butyl isobutyrate (164) and methyl 3,3-dimethylbutanoate (165). It is significant Aat two of the lithium ester enolates derived from (163) and (165) are both obtained with alkene geometry such that the alkyl group is trans to the enolate oxygen. It is also noteworthy that the two TMEDA-solvated enolates from (163) and (164) are dimeric, while the THF-solvated enolate from (165) exists as a tetramer. 

The least highly substituted amide enolate whose structure is known is the lithium enolate of N -di-methylpropionamide (170). This enolate is obtained as a dimer solvated by TriMEDA, i.e. (171). The alkene geometry in (171) is opposite that found in the ester enolates from (163) and (165). Thus in the... 

Another example of the nucleophilic chloro ligand substitution involves the reaction of the zirconium dichloride complex 468 with lithium ester enolates (Scheme 112). Its reaction with 2 equiv. of stable lithium ester enolates such as lithium tert-butyl isobutyrate in TF1F produces the bis(ester enolate) complex 499 as a crystalline solid.344 The same reaction but with the unstable lithium methyl isobutyrate leads to the isolation of the decomposition product, the bis(methoxide) complex 500, which exists as a dimer in the solid state. Treatment of the bis(ester enolate)... 

The reaction of the methyl zirconocene ester enolate Cp2ZrMe[OC(OBut)=CMe2] 862 with 1 equiv. of B(G6F5)3 in THF at 0°C leads to the selective formation of the cationic enolate complex 870668 (Scheme 219). The cation of this ion pair decomposes rapidly at 20 °C in THF with concomitant elimination of 1 equiv. of isobutene to form the cationic zirconocene carboxylate species 871. The same reaction in CD2C12 leads to the direct, rapid formation of the dimeric /x-isobutyrato-zirconocene dicationic species 872, which also gives carboxylate complex 871 upon dissolution in THF. However, when the ester enolate 862 is treated with [Ph3C][B(C6F5)4], a 15 85 mixture of the dicationic... 

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