Heteroatom-stabilized organolithium

Some heteroatom-substituted or chelate-stabilized organolithium compounds, on the other hand, can be sufficiently stable toward racemization to enable their use in stereoselective reactions with electrophiles [223, 225, 271, 531, 543, 552-554] (Scheme 5.75). This increased configurational stability of a-heteroatom-substituted carbanions might be due to the stronger pyramidalization of such carbanions [261,555] and fixation of the metal by chelate formation. 

Reports on organolithium compounds with internal heteroatom stabilization and their reactions with alkylating agents7 12 -14 are informative for stereoselective protonations and deuter-ations. 

The addition of carbon nucleophiles to the carbene carbon atom generates a new C-C bond. Organolithium reagents add to alkoxycarbene complexes to give alkyl metalates which undergo alkoxide elimination upon addition of acid or exposure to silica gel and thus may serve as precursors for non-heteroatom-stabilized diarylcarbene complexes (Scheme 1). [14b]... 

Stabilized organolithium compounds, such as lithium enolate intermediates [5], a-organolithium compounds bearing electron-withdrawing groups (RSO, RSOj, CN, NOj, etc.) and heteroatoms such as sulfur [6], selenium and phosphorus as well as functionalized aryllithium compounds will not be consider in depth in this chapter due to the limited length of this review. 

In cases where an alkyl has no (5 hydrogens (or no accessible ]8 hydrogen), an important alternative process, a elimination, can occur. M. L. H. Green40 has proposed the process shown in Scheme 3 to explain the formation of the ylide complex shown. Another interesting example (Scheme 4) is due to Shaw.6 It is not known whether Schrock s62 remarkable chemistry (equation 45), which led to the first examples of carbenes not stabilized by heteroatoms, also goes via a elimination or, perhaps more likely, by deprotonation of an alkyl at the a position by an organolithium reagent or other base. 

The ability of sulfur to stabilize an adjacent anion will be discussed in Chapter 46, and it means that sulfur heterocycles are much easier to deprotonate than THF. The most important of these contains two sulfur atoms dithiane. Deprotonation of dithiane occurs in between the two heteroatoms, and you can see some chemistry that arises from this on p. 1234. For tire moment, we will just show you series of reactions that illustrate nicely both dithiane chemistry and the ring opening of oxygen heterocycles in the presence of BF3, This substituted derivative of dithiane is deprotonated by BuLi in the same way to give a nucleophilic organolithium that will... 

Arylsilver compounds ArAg, where the aryl group carries one or two substituents containing a heteroatom, have been obtained by transmetallation reactions of silver bromide with the corresponding organolithium compounds 2-[(dimethylamino)methyl]phenyl silver (17)70, (2,6-dimethoxyphenyl)silver (18)70 71 and (2,4,6-trimethoxyphenyl)silver (19)70. The compounds show an enhanced thermal stability with decomposition temperatures well above 150 °C. 

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