Hoffman eliminations

Hoechst slurry process Hoeppler viscometer Hoffman degradation Hoffman eliminations Hofmann degradation... 

Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

Displacement of a tertiary amine from a quaternary (eq. lb) iavolves the attack of a nucleophile on the a-carbon of a quaternary and usually competes with the Hoffman elimination (173). The counterion greatiy iafluences the course of this reaction. For example, the reaction of propyltrimethylammonium ion with hydroxide ion yields 19% methanol and 81% propylene, whereas the reaction with phenoxide ion yields 65% methoxybenzene and 15% propylene (174). 

Here is where we get back to mechanisms. Whether we are talking about Zaitsev vs. Hoffman elimination reactions or about Markovnikov vs. anti-Markovnikov addition reactions, the explanation of the regiochemistry for every reaction is contained within the mechanism. If we completely understand the mechanism, then we will understand why the regiochemistry had to be the way it turned out. By understanding the mechanism, we eliminate the need to memorize the regiochemistry for every reaction. With every reaction you encounter, you should consider the regiochemistry of the reaction and look at the mechanism for an explanation of the regiochemistry. 

The exact nature of the occluded organic cation is more difficult to determine. The increased intensity of the silanol band as this type of TEA is removed strongly supports the presence of the SiO"-TEA species, which has been claimed to be present in as-synthesized ZSM-5 (10). On the other hand, the presence of the broad band centered around 3450 cm" (see figure la) could correspond to OH " acting as counterions of TEA cations. The removal of this organic material would take place through a Hoffman elimination (9) ... 

Hoffman eliminations -ofquaternary ammonium compounds [QUATERNARY AMMONIUM COMPOUNDS] (Vol 20)... 

Chemical Properties. Reactions of quaternaries can be categorized into three types Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la). 

Displacement of a tertiary amine from a quaternary (eq. ]b) involves the attack of a nucleophile on the o -carbon of a quaternary and usually competes with the Hoffman elimination. 

The onium carbanion formed under phase-transfer conditions is unstable depending on the anion source, and in the absence of an electrophilic reaction partner, degradation of the accumulated onium carbanion in the organic phase may be observed. This is known to proceed via Hoffman elimination, nucleophilic substitution and/or Stevens rearrangement (Scheme 1.4) [4f,6,7]. The direct decomposition of onium salt, as influenced by the strong inorganic base at the interface, may be also operative. 

Tetraalkylammonium fluoride (R4N 1 F ) is well known as being highly receptive to protic compounds such as hydrogen halides and water, affording non-stoichiometric hydrogen-bonded adducts, R N F (HY) , in non-polar solvents. This property reasonably accounts for the hygroscopic nature of ammonium fluorides. However, under strictly anhydrous conditions, intramolecular interactions are predominant and result in self-destruction of the tetraalkylammonium cation via Hoffman elimination to furnish tetraalkylammonium bifluoride, trialkylamine, and olefin (Scheme 9.12) [22]. Consequently, the resulting tetraalkylammonium bifluoride,... 

Anionic polymerization of (meth)acrylates with hindered ester functions can most likely be conducted at room temperature and above to remove the heat of polymerization with low boiling solvents. Polymerization of the important methyl and ethyl (meth)acrylate members of the family, however, are still plagued by chain termination at higher temperatures. The phosphorus based counterions have a stability advantage over tetraalkylammonium counterions which undergo Hoffman elimination. 

A number of selective transformations (Fig. 10) have been described which include the selective allylation on alcohols in the presence of amides [47], the Lewis acid catalyzed cleavage of benzyl alcohol esters with secondary amines to afford tertiary amides [48], the synthesis of ketones from Weinreb-type amides [49], and the synthesis of tertiary amines by a Michael addition/alkylation/Hoffman elimination sequence [50],... 

The hydroxymethyl resin-supported regenerated Michael (REM) linker 1.38 (96), has been used to support secondary amines during elaboration followed by release as tertiary amines by classical Hoffman elimination [DIEA/(diisopropyl ethylamine)-DMF at rt over 18 h] of the resin-bound quaternary ammonium salt. 

Bandermann and coworkers studied the MMA polymerization initiated by 37 . Whenever the initiator is contaminated by traces of diethyl 2-ethylmalonate, an inhibition period is observed, which was accounted for by a transfer reaction of the propagating enolate to diethyl 2-ethylmalonate. The polymerization cannot start until this contaminant is completely consumed. Consistently, no inhibition is observed when the initiator is highly pure. Finally, a Hoffman elimination in the propagating ammonium enolate (38) could occur with formation of dead chains (39) tri-n-butylamine and 1-butene (equation 43) . 

Dithiolane 1,3-dioxide 363, prepared in 84% yield by oxidation of 1,3-dithiolane 362 with MCPBA <1998JOC3481, 1998J(P1)2771>, was smoothly converted to the dimethylamino derivative 364 in 100% yield. Hoffman elimination using methyl iodide and Hilnig s base at room temperature gave 365 in 86% yield (Scheme 46). 

Figure 17-20 Hoffman elimination and hydrolysis reactions of atracurium.

An important property of an ionic liquid in terms of its applications in biomass processing is its thermal stability, which is usually measured by thermogravimetric analysis. Nucleophilicity of the anion has been found to have an effect on the thermal stability of many ionic liquids due the role of the anion in the degradation of the cation [76], Quaternary ammonium, imidazolium, phosphonium and pyrrolidinium cations degrade at lower temperatures in the presence of highly nucleophilic anions such as halides due to Hoffman elimination and reverse Menschutkin reactions [76-79],... 

A highly regioselective and traceless solid-phase route to N(l)-N(7)-disubstituted purines 73 has been developed. The reaction involves coupling of 6-chloropurine 72 to the REM resin (Michael addition), oxidation, N(l)-alkylation, quatemization and product release through Hoffman elimination <05JCC734>. 

This process intercepted the vinyl ether intermediate from the existing bulk route in 24% overall yield and is highlighted by a crystallization driven diastereoselective condensation, an intramolecular cyclization that controlled the selectivity of the glycosidation, and by a selective Hoffman elimination. It also addressed the main objectives BSCA was removed from the process, the cryogenic step that utilized lithium tri-5ec-butylborohydride had been circumvented, and a dimethyl titanium carbene olefination was not required. However, this was not viewed as the ultimate manufacturing route. A limitation was that the sequence was lengthy and it incorporated a large number of operations. 

Under neutral conditions the C2-H bond of the imidazolium nucleus can add oxidatively to electron-rich Ni(0) or Pd(0) complexes to generate stable carbene-metal-hydride compounds [48]. Dealkylation of the imidazolium nucleus ( Hoffman elimination ) has been observed in the catalytic hydrodimerization of butadiene by Pd(II) compounds immobilized in ILs (Scheme 3.5-11) [49]. 

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