Elimination reactions reversibility

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

Is the electrophilic addition of hydrogen chloride to 2-methyl-propene the reverse of the El or the E2 elimination reaction of fe/t-butyl chloride ... 

It has been noticed that the reverse reaction of Eq. (5) is a particular type of the Hofmann elimination reaction (26) via either an E2 or an ElcB mechanism. An E2 mechanism seems to be more obvious for this reaction than an ElcB mechanism, however. 

An elimination reaction is, in a sense, the reverse of an addition reaction. It involves the elimination of two groups from adjacent carbon atoms, converting a saturated molecule into one that is unsaturated. An example is the dehydration of ethanol, which occurs when it is heated with sulfuric acid ... 

Benzothiepins 13 and their 2,3-dihydro precursors 12 can be oxidized by two equivalents of 3-chloroperoxybenzoic acid to afford the sulfones 15 and 14, respectively, in moderate to good yields.2,9 83 Sulfones 15 can be prepared using two routes, the reverse order (oxidation, followed by elimination) also being possible (see Section 2.1.2.1. for a description of the elimination reactions). The preferred route must be decided for individual cases. 

Isomerization and elimination reactions of alkyls and aryls Isomerizations of mono-alkyls and aryls have been widely studied [107] many ds-Pt(PR3)2ArCl undergo rapid isomerization in the presence of free phosphine, a reaction inhibited by Cl- with a mechanism believed to involve a 3-coordinate Pt(PR3)2Ar+ intermediate that is then attacked by Cl-. The cis- and trans-isomers of Pt(PEt3)2(Ph)Cl undergo reversible isomerization when irradiated at the wavelength of charge-transfer transitions (254 and 280 nm). 

Early investigations have indicated that sulfinyl radicals apparently do not add, at least in the usual way, to olefmic double bonds24. However, some recent results by lino and Matsuda25 obtained by studying the thermal decomposition of benzhydryl p-tolyl and benzhydryl methyl sulfoxides in the presence of cis-/2-deuteriostyrene lead one to believe that sulfinyl radicals add reversibly to CH2 =CHPh. The molar ratio of trans to cis /3-deuteriostyrene that they observed at nearly 50% conversion was explained by addition-elimination reaction of sulfinyl radicals. 

Many elimination reactions in which C=0 bonds are formed were considered in Chapter 16, along with their more important reverse reactions. Also see 12-38 and 12-39. 

C) Reactions involving ligands in two adjacent (i.e., cis) coordination positions. They include certain insertion reactions (e.g., the insertion of cyanide into the Co—C bond) and the reverse elimination reactions. 

Reactions Where the DNA Base Serves as a Leaving Group Thiolysis, Hydrolysis, Elimination, and Reversible DNA Alkylation... 

Elimination reactions are, of course, essentially the reversal of addition reactions the most common type is the loss of hydrogen and another atom or group from adjacent carbon atoms to yield alkenes (p.246) ... 

The nitro-aldol approach is impractical for the synthesis of 2,2-disubstituted 1-nitroalkenes due to the reversibility of the reaction when ketones are employed as substrates. Addition-elimination reactions are used for the preparation of such nitroalkenes (see Chapter 4). 

The transmetalation of trimethylsilylphosphanes with germanium and tin halides is a useful way to prepare compounds with P—Ge and P—Sn bonds by simple chlorosilane elimination. The reverse reaction, i. e. formation of P—Si bonds by chlorosilane cleavage of germyl- and stannylphosphanes has not yet been reported. Recently, we observed that hexachlorodisilane "transsilylates di-r-butyl(trimethyl-silyl)phosphane 1 much faster than tetrachlorosilane to give trichlorosilylphosphane 2 ... 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

Since 8-elimination reactions are often rapid and reversible, it is surprising that no examples of metallacyclobutane to metallacyclopentane ring expansion reactions according to equation (6) have been found. 

In order to study the mechanism of reverse Cope elimination reactions in the condensation of pentenal and hexenal with iV-methylhydroxylamine. it seemed reasonable to synthesize unsaturated nitrones (226). 

In an elimination reaction, atoms are removed from a molecule to form a double bond. This type of reaction is the reverse of an addition reaction. One reactant usually breaks up to give two products. The organic product typically has fewer atoms bonded to carbon atoms than the organic reactant did. 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

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