Lithium complexes amides

An investigation of lithium diisopropyl amide (LDA) by solid state NMR led to the observation of dramatic differences between the spectra of the solid polymer and the complex crystallized from THF. Li as well as "C and "N MAS spectra showed large sideband patterns in the former case and only a few sidebands in the latter. For both materials X-ray data are available and establish a helix structure for the polymeric material, which is insoluble in hydrocarbon or ethereal solvents, and a dimer structure of the THF complex (25, 26, Scheme 4). The obvious difference between both structures, apart from the solvent coordination in the THF complex, is the magnitude of the structural N-Li-N angle, which is close to 180° in the first case and close to 90° in the second (176° and 107°, respectively). Thus, a large difference for the electric field gradient around the Li cation is expected for the different bonding situations. 

The crystal structure of the lithium complex [Pr 2N(Ph)COLi(p-tmp)(p-Bu )ZnBu shows that the amide solvates the lithium ion through oxygen with zincation of the aryl occurring via interaction of the ortho aryl hydrogen Bu or tmp group." The crystal... 

A few years later Arnold and co-workers also reported the synthesis of lithium complexes of the neutral and anionic salts of a tridentate amino bis-carbene ligand (Scheme 2).13 Treatment of the cationic amino bis-imida-zolium salt with three equivalents of //-butyl lithium affords the lithium amino bis-carbene chloride complex (5). Deprotonation with four equivalents of n-butyl lithium affords the lithium amide salt (6). Although the complexes were not characterised in the solid state, characteristic shifts in the multinuclear NMR spectra and elemental analysis are consistent with the lithium complexes being formed. NMR spectra of 5 suggest formation of a cluster of lithium chloride ions with lithium-NHC bonds (13C NMR NCN 203.9 ppm) and NH-chloride bonding interactions. Following further deprotonation to form 6 the complex also retains lithium chloride and exhibits a similar C2 chemical shift (13C NMR NCN 203.4 ppm). 

The utility of carbonylation of lithium amides for the synthesis of complex molecules has been also demonstrated. V.V.V V -Tetrasubstituted ureas 5 were obtained in good yields by reaction of lithium alkyl amides in THF solution with carbon monoxide under mild conditions (0°C, 1013 mbar), followed by treatment with oxygen prior to work-up... 

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. 

As most organometallic compounds, lithium enolates are highly polar entities susceptible to combine in various types of (eventually solvated) aggregates that undergo dynamic equilibria in solution. This phenomenon explains why enolate solutions are difficult to describe by the classical spectroscopic, physicochemical or theoretical methods, a difficulty enhanced by the sensitivity of these equilibria to many physicochemical factors such as the concentration, the temperature or the presence of complexing additives (lithium halides, amides, amines, HMPA,. ..). The problems due to dynamics are avoided in the solid state where many clusters of lithium enolates, alone or co-crystallized with exogenous partners, have been identified by X-ray crystallography. 

Under similar conditions but with different oxidants, esters are hy-droxylated in the a positions with respect to the ester groups. Ethyl phen-ylacetate in tetrahydrofuran treated with lithium diisopropyl amide at -78 °C followed by oxidation with molybdenum oxide complex gives ethyl mandelate in 58% isolated yield [531]. Another way to obtain a-hydrox-ylated (or methoxylated) esters is by oxidation with iodobenzene diacetate in the presence of potassium hydroxide (or sodium methoxide) (equation 466) [794]. 

Three other groups reported alternative methods for the synthesis of 3-alkenyl-substituted P-lactams. Durst [71] prepared these compounds via a Peterson olefination reaction. Thus, for example, treatment of 3-trimethylsilyl-azetidinone 116 with lithium diisopropyl amide followed by addition of pro-pionaldehyde gave a mixture of 117 and 118 in 66% yield. Tanaka [72] converted allylic stannanes 119 to bromides 120 which were smoothly cyclized to P-lactams 121 in good yield. Ley [73] treated 7i-allyltricarbonyliron complexes with benzylamine in the presence of Lewis acids to afford the corresponding lactam complexes. Oxidation with cerric ammonium nitrate served to liberate the desired P-lactams (Scheme 14). 

Zarges W, Marsch M, Harms K, Boche G (1989) X-ray structure analysis of alpha-lithiophenylacetonitrile. Lithium diisopropyl amide. 2 Tetramethylethylenediamine. A quasi-dianion complex. Angew Chem Int Ed 28 1392-1394. doi 10.1002/anie.l98913921... 

The similarity between the cryptands and the first of these molecules is obvious. Compound 7 7 is a urethane equivalent of [2.2.2]-cryptand. The synthesis of 7 7 was accomplished using a diacyl halide and l,10-diaza-18-crown-6 (shown in Eq. 8.13). Since amidic nitrogen inverts less rapidly than a tertiary amine nitrogen, Vogtle and his coworkers who prepared 7 7, analyzed the proton and carbon magnetic resonance spectra to discern differences in conformational preferences. Compound 7 7 was found to form a lithium perchlorate complex. 

Heterocyclic structures analogous to the intermediate complex result from azinium derivatives and amines, hydroxide or alkoxides, or Grignard reagents from quinazoline and orgahometallics, cyanide, bisulfite, etc. from various heterocycles with amide ion, metal hydrides,or lithium alkyls from A-acylazinium compounds and cyanide ion (Reissert compounds) many other examples are known. Factors favorable to nucleophilic addition rather than substitution reactions have been discussed by Albert, who has studied examples of easy covalent hydration of heterocycles. 

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