Lithium alkoxides, reactivity

The spontaneous rearrangement of allyl p-toluenesulphenates to allyl sulphoxides was independently recorded by Mislow and coworkers and Braverman and Stabinsky. Mislow and colleagues201 have demonstrated that simple allyl alcohols such as 149, on conversion to the corresponding lithium alkoxides followed by treatment with arenesulphenyl chlorides, may be smoothly transformed at room temperature via the sulphenate esters into allylic sulphoxides 150 (equation 83). Braverman and Stabinsky202 have found that when the more reactive trichloromethanesulphenyl chloride is treated with allyl alcohol and pyridine in ether at — 70°, it affords trichloromethyl allyl sulphoxide and not allyl trichloromethanesulphenate as reported by Sosnovski203 (equation 84). 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

The reactivities of the hydroxyl groups of D-glucal and D-galactal are in the order 0-4 > 0-3 > 0-6 for methylation and benzylation conducted in Ar,Ar-dimethy 1 form am ide via the sodium or lithium alkoxides or electrochemically.86 These observations are corroborated by semiempirical calculations87 and are in interesting contrast to the selectivities just given. A possible explanation is that while the last-reported data pertain to reactions of oxyions, the others may refer to those of the undissociated alcohols. 

C-to-O silyl-group transfer without alkene formation is relatively uncommon. However, Oshima, Utimoto, and co-workers reported that metalation of dichloromethylsilane 96 and addition to aldehydes provided fi-alkoxysilanes (e.g., 97) that imderwent 1,3-rearrangement and alkylation to 98. Notably, the addition of HMPA promoted the 1,3-Brook rearrangement, presumably by increasing the reactivity of the lithium alkoxide intermediate. As the authors pointed out, this sequence demonstrated 96 as a methylene chloride dianion equivalent. ... 

The polyaminophosphazene base t-BuP4 was used in combination with alkyllithium as initiator for the anionic polymerization of EO (Scheme 15(a)). The space inside the molecule is sufficient to host the compact lithium cation and the base works as a cryptand for Li ions with the polar amino and imino groups located inside the globular molecule and the outer shell formed by alkyl substituents. The equilibrium between complexed lithium alkoxide ion pairs and reactive free anions is thus shifted allowing polymerization. 

Alkyltrifluorosilanes and disubstituted difluorosilanes are themselves quite reactive with nucleophiles such as lithium amide bases [102, 103 104], alkyl-lithium reagents [1051, Gngnard reagents [105], or alkoxides [105] (equations 82 and 83)... 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

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