Tetraalkylammonium salts, elimination reactions

A highly active zinc powder can be generated in bulk by electrolysis of a tetraalkylammonium salt in dimethylformamide between a zinc anode and a platinum cathode [183]. This reagent has been used in stoichiometric amounts for Re-formatsky-type reactions where the tribromide 61 is used as a precursor for the isoprenyl group [184]. Zinc promoted 1,4-elimination of bromine to give the butadiene system is followed by a Reformatsky reaction of the remaining allylic bromine substituent. 

Let us return to the finding that k 2 cjs > k 2 tmm in the E2 eliminations from the isomeric tetraalkylammonium salts in Figure 4.19. The transition state geometries of these reactions are derived from the chair conformers shown in Figure 4.19. The stereostructure is fixed in each... 

Aprotic solvents such as acetonitrile [15,16] or dimethylformamide [17-20] considerably improved the stability of the radical anions but normally had little effect on the reactions of the more basic dianions [19-21]. The increased irreversibility of the dianion formation is probably due to the ability of dianions to abstract protons even from the solvent, or, by Hofmann elimination, from the tetraalkylammonium salts that are common supporting electrolytes in aprotic solvents [2],... 

The reaction of potassium carboxylates with alkyl halides without the use of a solvent in the presence of catalytic amounts of a tetraalkylammonium salt proceeds smoothly to give the corresponding esters in good to excellent yields, except for cyclohexyl bromide where elimination mainly takes place. Alkylation of o- and p-hydroxybenzoic acid selectively takes place at the CO2K group. ... 

The basic decomposition of tetraalkylammonium salts (the Hofmann degradation), has been reviewed extensively so.135) and will not be discussed here in detail. However, it should be noted that both displacement reactions and a-proton abstraction reactions may occur in addition to elimination reaction 30>. Ingold and Patel 75> report that the amount of substitution relative to elimination varies depending upon both the substituent on nitrogen and the base. 

The insertion reaction is stereospecific and syn. Moreover the /S-hydride elimination is also syn. For acyclic alkenes there is free rotation in the organopalladium intermediate so that the more stable /ra .v-alkene is formed. Electron-withdrawing groups in the alkene also increase the rate of the insertion reaction and give higher yields generally, but the reaction is limited to relatively sterically unhindered alkenes. In general, polar solvents such as DMF or acetonitrile are most commonly used. There are several common additives which aid in the reaction. These include lithium or tetraalkylammonium chlorides and bromide, silver salts, or cuprous iodide, but exactly how they function is unknown at present. 

The reactions of alkyl sulfonates with sodium, potassium or tetraalkylammonium carboxylates generally proceed with inversion of configuration. In the case of f-alkyl sulfonates, however, a varied amount of elimination products is invariably formed. Of all metal carboxylates, cesium salts (Scheme 43) have proved vastly superior with respect to reactivity and the suppression of undesirable side reactions. ... 

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