The Versatile Phenylthio Group. A Powerful Aid in Synthesis

Abstract

The replacement of the phenylthio group by lithium by the use of aromatic radical-anions has become one of the most powerful methods for the production of organolithium compounds. The major reasons for the power of this method are the ease of incorporation of the phenylthio group into molecules, the ability of divalent sulfur to stabilize any electronic arrangement on the atom to which it is attached, the relative ease of reductive cleavage of C-S bonds despite the stability of this type of bond under most other reaction conditions, and the fact that the rate determining step of organolithium production by this method is apparently the formation of a carbon radical. Since the order of stability of radicals in solution is the reverse of that of carbanions, reductive lithiation is capable of producing less stable organolithiums more rapidly and under milder conditions than more stable ones. Recent advances in synthetic and mechanistic chemistry based on carbanions generated by this method are presented. The dependence of the methods on the properties of divalent sulfur mentioned above and the generation and uses of heretofore virtually unavailable tertiary organolithiums are stressed. Topics covered include the production and synthetic or mechanistic uses of otherwise difficulty attainable 2-lithiotetrahydropyrans, allyllithiums, homoenolate and bis(homo)enolate anion equivalents, allylidenecyclopropyl ketones and homoallyl anions. The latter readily undergo a synthetically useful 1,2-vinyl rearrangement that is some cases leads to a ring contraction.