Carbamates heteroatom-stabilized

The stabilization by a heteroatom of the lithiated enolate can be realized by using carbamates and methylmethoxyethers (MEM) of trifluoroethanol. As shown in Figure 2.19, these enolates are powerful tools for the synthesis of difluorocompounds, especially to access difluorosugars. ... 

Deprotonation of ethers is another route to the a-alkoxy anions, but this pathway is often precluded by a kinetic barrier. Unless the a-carbon is benzylic [175], surmounting this barrier usually requires conditions that are not favorable to the survival of the anion [164]. Notable exceptions are the hindered aryl esters studied by Beak [176], Figure 3.13a, and the carbamates studied by Hoppe [177], shown in Figure 3.13b. In both cases, ec-butyllithium is required for deprotonation, and the carbonyls which direct the metalation by a complex-induced proximity effect [178] must be shielded from the base by large alkyl groups. Once formed, the organo-lithiums are chelated and stabilized by the heteroatom-induced dipole [179]. 

Other groups capable of stabilizing the anion generated by decarboxylation may be present in place of the ketone group. These include esters (so that the substrate is a malonate), nitriles, nitro groups, imines and sulfones. Even alkynes may be used, although the stereochemical outcome of the reaction indicates that, in this case at least, the nucleophile remains bound to palladium (Scheme 9.61). Decarboxylation of heteroatom derivatives, such as allyl carbamates, is included in Section 9.2.8. 

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