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methanethiolate. This led to an efficient route to highly substituted a,(3-unsaturated ketones under mild conditions with regiospecific sequential introduction of three alkyl substituents, as shown in the accompanying scheme.
a) Alkyl halide - ÊÎÍ - TOMAC (cat.) in DMF. b) Alkyl halide - NaH in DMF. c) Silica gel. d)
CuCl2 in Me0H-H20.
220.127.116.11 Reductive lithiation of phenyl sulfides a
So far we have considered sulfur-containing carbanions obtained by *’
metallation with appropriate bases of a to sulfur atom(s) acidic C-H bonds. For synthetic purposes this method has been by far the most used.
Among the variety of other routes to such carbanions  we shall consider two methods which seem to be general and promising: the reductive lithiation of phenyl thioethers and the thiophilic addition of organometallics to thiocarbonyl compounds (vide infra Section 4.1.2).
The cleavage of alkyl phenyl sulfides by naphthalenelithium or a lithium „
dispersion in THF to afford alkyllithium reagents has been studied initially |
by Screttas and Micha-Screttas [309,310] as part of their hydrolithiation of *
a-olefins process. They prepared primary, secondary and tertiary alkyllithium reagents in fair to good yields, as shown by carbonation.
R-ObCHa + PhSH
R-CHjCHa SPh + 2 Li -------> R-CH2CH2 Li + PhSU
The use of reductive lithiation to prepare a carbanion from a dithioacetal is due to Cohen :
His group [312,313] has done important work on the reductive cleavage of the C-S bonds of phenyl thioethers. These workers have shown the potential of two radical anions: lithium p,p'-di-t-butylbiphenylide (LDBB) and lithium l-(dimethylamino)naphthalenide (LDMAN) as reducing species.
The strongly reducing agent LDBB is preferred when no separation problem of the aromatic by-products is to be expected during the work-up, whereas LDMAN is the reagent of choice for an easy separation of the dimethylaminonaphthalene, soluble in dilute acidic aqueous solution, from the other reaction products. Using these lithium arenides the authors established the reductive lithiation of phenylthioethers as one of the most powerful methods to prepare organolithium compounds. Moreover, the tertiary organolithiums are conveniently prepared by that route. The proposed mechanism explains the observed order of the rates of formation of organolithiums,
and more generally that less stable organolithiums are formed more rapidly and under milder conditions than more stable ones [312, 313]. The rate-determining step is the formation of a radical, and the rate of the overall process follows the relative stability of that radical intermediate and not that of the anion, which is the reverse as far as the order of substitution is concerned.
tertiary > secondary > primary
I C 4
A wide variety of organolithium reagents have been prepared by this reductive metallation, a number of which take advantage of the complementarity of the method to the more classical deprotonation. A few examples are given here [312,313].
a-Lithioethers: R‘CH(Li)OR2 (derived from readily available
Preparation of the following compound is described in Organolithium Compounds :
The synthesis of cycloalkenyllithiums from cycloalkanones is particularly attractive . The required cycloalkenyl sulfides were obtained in good yields according to .
Montmorillonite, KSF (CH-J,' PhMe, Ä (ÑÍã).
Lithium p,p'-di-tert-butylbiphenylide (LDBB)
To an oven-dried three-necked round-bottomed flask, equipped with a glass stirring bar, rubber septum and argon inlet, was added THF (7 ml) and DBB (0.85 g, 3.2mmol). Lithium ribbon was prepared by scraping the dark oxide coating off the surface while it was immersed in mineral oil. The metal was dipped in pentane, in order to remove the oil, and then weighed (24 mg, 3.4 mmol) in a tared beaker containing mineral oil. The metal was cut into small shiny pieces while immersed in mineral oil. The small lithium pieces were dipped again in pentane and quickly added to the THF/DBB mixture while the flask was rapidly being purged with argon. The mixture was cooled to 0°C and vigorously stirred for 4-5 h. The dark green-blue colour of the radical anion appeared within 10 min, and formation was complete in 4-5 h. This procedure produced a 0.45 ì solution of the radical anion.