Lithiation heteroatom facilitated

Heteroatom-Facilitated Lithiations Heinz W. Gschwend and Herman R. Rodriguez... 

Heteroatom-facilitated lithiations , Gschwend, H. W. and Rodriguez, H. R., Org. React., 1979, 26, 1 Directed metallation of 7r-deficient azaaromatics strategies of functionalisation of pyridines, quinolines, and diazines , Queguiner, G., Marsais, F., Snieckus, V., and Epsztajn, J., Adv. Heterocycl. Chem., 1991, 52, 187 Metallation and metal-assisted bond formation in 7r-electron deficient heterocycles , Undheim, K. and Benneche, T., Heterocycles, 1990, 30, 1155 Directed ortho metallation. Tertiary amide and O-carbamate directors in synthetic strategies for polysubstituted aromatics , Snieckus, V., Chem. Rev., 1990, 90, 879. 

Ortho Lithiations. Heteroatom-containing substituents on aromatic rings facilitate metalation by organolithium reagents... 

H.G. Gschwend and H.R. Rodriguez, Heteroatom-facilitated lithiation, in "Organic Reactions,"... 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Protons attached to sp carbons are more acidic than protons attached to nonallylic sp carbons. Also, the inductive effect of a heteroatom further increases the acidity of an adjacent sp C-H bond, facilitating a-lithiation. The relative activating effect of heteroatoms is sulfur > oxygen > nitrogen. Thus, treatment of 2-ethoxy-l-(phenylthio)ethylene with t-BuLi results in exclusive lithiation at the phenylthio substituted carbon. ... 

Proton removal adjacent to a heteroatom is further facilitated if the lithium can be internally coordinated to proximate electron donors, such as the carbonyl oxygen, permitting the formation of dipole-stabilized carbanions. Thus lithiations of various amides, thioamides, imides, esters, Boc derivatives of cyclic amines (pyrrolidines, piperidines, and hexahydroaze-pines), thioesters, )V,iV-dialkylthiocarbamates, and various formamidine derivatives are achieved conveniently using s-BuLi (eqs i3 i7) 32,43b.44.47a,48a Subsequent addition of electrophiles followed by hydrolytic removal of the activating carbonyl, carbamoyl, or formamidine moiety provides a valuable synthetic route to a variety of a-substituted amines, alcohols, and thiols. Successful alkylation of the dipole-stabilized car-banions may require the conversion of the initial lithio carban-ions into their organocuprate derivatives, e.g. by the addition of n-PrC=CCu. ... 

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