Oxygen-lithium bond

When the counterion is varied from lithium to sodium to potassium, the proportion of inversion increases. The relatively covalent lithium-oxygen bond favours a retentive mechanism. However, as the metal-oxygen bond becomes more ionic, the components may function more independently, allowing attack of RO on the back face of the silicon tetrahedron while electrophilic assistance by M+ of the leaving group aids inversion of configuration. 

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... 

It is known that the anionic polymerization of dimethyl trimethylene carbonate (DTC) in toluene with lithium cation as a counterion proceeds slower than that with potassium one due to the more covalent character of the lithium-oxygen bond compared with the potassium-oxygen bond this leads to a lower nucleophilicity of the lithium alkoxide and also the low tendency for complexation with PEG favors lithium as... 

Mechanistically, the nucleophilic ring-opening of the P-lactam by the lithium alkoxide of the baccatin requires the carbonyl of the p-lactam be aligned with the lithium-oxygen bond in the transition state. It is shown that when the lithium is located outside of the concave face of the baccatin, due to its solvation by THF, the P-lactam is situated underneath the baccatin. This alignment gives rise to the four possible orientations depicted in Figure 3(a)-(d). 

A second example exploits the fact that the mixed hydride reagent is capable of hydrogenolysis of certain carbon-oxygen bonds. Thus, treatment of cyclohexanone ketal (Chapter 7, Section IX) with lithium aluminum hydride-aluminum chloride results in the rupture of a C-O bond to give the oxyethanol derivative. 

Benzyl-nitrogen, sulfur, or -oxygen bonds are somewhat susceptible to hydrogenolysis, either during catalytic hydrogenation (30, 31), or upon treatment with L-selectride (32), or even lithium aluminium hydride (33). 

Walker RJ, Hanson GN, Papike JJ, O Neil JR, Laul JC (1986) Internal evolution of the Tin Mountain pegmatite, Black Hills, South Dakota. Am Min 71 440-459 Wenger M and Armbruster T (1991) Crystal chemistry of lithium oxygen coordination and bonding. Eur J Mineral 3 387-399... 

Alone formed by the reaction of lithium aluminum hydride and aluminum chloride in ether cleaves exclusively the carbon-oxygen bond in cyclic monothioketals derived from ketones and mercaptoethanol, and on refluxing in 100% excess for 2 hours produces -hydroxyethyl sulfides (yields 66-91%) on prolonged heating with the reagent these -hydroxyethyl sulfides are further reduced to the corresponding ethyl sulfides (thioethers) (yields 28-81%) [936]. 

A completely different approach to lithium homoenolate synthons uses a carbon-oxygen bond cleavage. Lithiation of acrolein diethyl acetal 180 with lithium and a catalytic amount of DTBB (2.5%) in the presence of different carbonyl compounds in THF at 0°C gave, after final hydrolysis, the corresponding y-products 181 in different diastereomeric ratios (Z/ 3/1 to 20/1) (Scheme 63) . 

Dioxanes can be opened by an arene-catalyzed lithiation only if the carbon-oxygen bond to be cleaved occupies an allylic or benzylic position. This is the case of the vinyl-dioxane 416, which reacted with lithium and a catalytic amount of DTBB in THE at 0 °C, and the allylic intermediate 417 generated reacted at the y -position with tridecyl iodide to yield the compound 418, used in the synthesis of plasmenyl-type lipids (Scheme 117) . 

When 2,2-diphenyl-l,3-dioxolane (410, R = Ph) was lithiated with lithium and a catalytic amount of naphthalene (4%) in THF at —40°C (see Section VI.F.l) and then reacted with an aldehyde as electrophile, intermediates 437 were generated. The further lithiation of these compounds at the same temperature cleaved the second benzylic carbon-oxygen bond giving new organolithium intermediates 438, and a second electrophile could be introduced to give 439, after hydrolysis. In these products, two different electrophilic fragments have been incorporated, so the starting material behaves as the 1,1-diphenylmethane dianion synthon (Scheme 122) °. 

Equation 4 indicates a rapid, irreversible formation of hydroxypentacyano-cobaltate(III) and is analogous to the cleavage of hydroperoxides by lithium aluminum hydride (17) involving oxygen-oxygen bond scission. Equation 5... 

As unidentate ligands, carboxylates are expected to (i) lose the equivalence of the two carbon- oxygen bonds found in the anion and (ii) have one metal-oxygen distance considerably shorter than the next shortest M—O contact. Lithium acetate dihydrate exemplifies this14 with C—O distances of 133 and 122 pm and Li—O distances of 227 and 257 pm. Most examples of unidentate carboxylate complexes have this classical configuration of M(0—C) and C=0 respectively so certainly the presence of features (i) and (ii) unambiguously determine this mode of coordination. 

The reductive lithiation of cyclic benzofused ethers, for example, 510, with 4,4 -di-/< rt-butylbiphenyl (DTBB) and lithium gives intermediate organolithiums, for example, 511 and 512, that can be quenched with a variety of electrophiles to give general products 513 and 514 (Scheme 92). The process is not synthetically useful for 4H-chromene as carbon-oxygen bond cleavage occurs in both directions <2002TL4907>. 

Treatment of substituted phthalans 1172 with lithium metal in the presence of catalytic quantities of naphthalene leads to reductive cleavage of the arylmethyl carbon-oxygen bond to form a stable dilithium compound 1173, which upon trapping with carbon dioxide furnishes isochroman-3-ones 1174 (Scheme 289) <1996JOC4913>. 

Aldol reactions of boron enolates are frequently more diastereoselective than aldol reactions of, for example, lithium or aluminium enolates. This is partly ascribed to the relatively short boron-oxygen bond length (B-O = 1.36-1.47 A, Li-0 = 1.92-2.00 A, Al-0 = 1.92 A) which exacerbates the unfavourable 1,3-diaxial interactions that occur between the boron substituent... 

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