Tetraalkylammonium salts, elimination

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... 

Reacts with tetraalkylammonium salts to yield alkenes (Hofmann elimination Section 24.7). 

Finally, chemical or thermal-induced dealkylation (Hofmann elimination) from tetraalkylammonium salts can occur even in the absence of base as shown in Scheme 6.4. The formation of a palladium bis-imidazole complex from PdCl41 and [C4Ciim][BF4] has been reported to take place during the hydrodimerisation of butadiene1291 and an analogous product was obtained from a [PdBr4]2 imidazolium salt in the presence of the radical initiator 2, 2 -azabisisobutyronitrile.[30]... 

The tetraphenylarsonium cation is useful in coordination chemistry because of its large size. Tetraphenylarsonium cyanate dihydrate was prepared byNor-buryand Sinhain 1968. The preparation involved precipitation of the cyanate from an aqueous solution, but the method suffers from the disadvantage of low yields. Tetraphenylarsonium cyanide has been prepared from methanol solution with fairly good yields, but the product is isolated as the monohydrate. The ion-exchange method developed for the tetraalkylammonium salts can be readily applied to the preparation of anhydrous tetraphenylarsonium cyanate and cyanide, thus eliminating many of the problems initially involved in the synthesis of these componds. 

In certain solvents, such as dimethylformamide and dimethyl sulfoxide, water is a rather poor proton donor [347], and other impurities may be responsible for the protonation of the basic intermediates (radical anions, anions, and dianions). During preparative experiments the impurities may be reprotonated by water or, in case tetraalkylammonium salts (except tetramethylammonium salts) are used as supporting electrolyte, by attack on the cations (Hofmann elimination). Treatment of the medium with active alumina may lower the concentration of such protonating impurities [38,260]. 

The most commonly used quaternary ammonium salts are tetrabutylammonium perchlorate (TBAP), tetrafluoroborate (TBAT), the halides (TBACl, TBAB, and TBAI), and the corresponding tetraethylammonium salts, such as the perchlorate (TEAP), but also the tetramethyl- or tetrapropylammonium salts have been employed the former cannot undergo a base-promoted Hofmann elimination. However, evidence has been found for the formation of trimethylammonium methylide [460]. In nonpolar solvents it may be necessary to employ tetrahexyl- or tetraoctylammonium salts. The tetraalkylammonium ions are soluble in many nonaqueous media, and they may be extracted from an aqueous solution by means of chloroform or methylene chloride [461,462], and tetraalkylammonium salts may thus be prepared by ion extraction [462]. Tetrakis(decyl)ammonium tetra-phenylborate is soluble even in hexane [442,443]. 

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. 

It is known that tetraalkylammonium salts can undergo dealkylation processes that are either thermally or chemieally indueed. The dealkylation of the imidazolium nucleus ( Hoffman elimination ) was also observed in the catalytic hydrodimerization of butadiene by Pd(ii) compounds immobilized in ILs (Scheme 13). ... 

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. 

Alkyl carbanions are potent bases, so they are protonated by almost any source of hydrogen ions, especially adventitious water in the solvent or supporting electrolyte. In the absence of water and other added proton donors, tetraalkylammonium cations can serve as proton donors toward alkyl carbanions deprotonation of tetraalkylammonium ions leads, via the Hofmann elimination, to the corresponding trialkylamine and olefin. When tetramethylammonium salts are used as supporting electrolytes, there is evidence that proton transfer gives rise to trimethylammonium methyUde [34,35]. 

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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