Elimination reactions direction

Quaternary ammonium salts as we have seen are useful m synthetic organic chem istry as phase transfer catalysts In another more direct application quaternary ammo mum hydroxides are used as substrates m an elimination reaction to form alkenes... 

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... 

Quaternaiy ammonium salts, as we have seen, aie useful in synthetic organic chemistry as phase-transfer catalysts. In another, more direct application, quaternaiy ammonium hydroxides aie used as substrates in an elimination reaction to fonn alkenes. 

A 1,8-naphthyridine, nalidixic acid (39), shows clinically useful antibacterial activity against Gram-negative bacteria as such, the drug is used in the treatment of infections of the urinary tract. Condensation of ethoxymethylenemalonate with 2-amino-6-methylpyridine (36) proceeds directly to the naphthyri-dine (38) the first step in this transformation probably involves an addition-elimination reaction to afford the intermediate, 37. W-Ethylation with ethyl iodide and base followed by saponification then affords nalidixic acid (39). 

This direct condensation of alcohols has limited use as a route to polymers because many alcohols undergo other reactions under the conditions required for condensation. Nature forges ether linkages in a more roundabout way than by eliminating water directly. The most important pol Tners containing ether linkages are two biochemical macromolecules, starch and cellulose. 

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that... 

The presence of PCHE in the HDN product (Table 1) indicates that at least part of the HDN reaction of PCHA proceeds through elimination of ammonia rather than by direct hydrogenolysis. Nevertheless, the direct product of the elimination reaction (allylcyclohexylamine) was not observed. This must be due to its strong adsorption and fast hydrogenation to PCHA. 

I -Propylaminopropene is thus on a logical route to dipropylamine, which can form directly by hydrogenation of its C=C bond. This condensation-elimination reaction may indeed involve the support [2] or at least the metal-support interface. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

This is clearly an elimination/addition mechanism [in contrast to the addition/elimination of SN2 (aromatic)] and formally parallels, in its genesis, the elimination reactions of simple alkyl halides that we shall consider subsequently (p. 246). Direct evidence in support of the aryne pathway is provided by the fact that the halides (98), (99) and (100),... 

Electrophilic metals or metal complexes, when incorporated into either the acyl or alcohol functions of the ester, might be expected to increase the rate of addition of amine. This might occur through direct carbonyl-0 or alcohol-0 coordination (21 or 22, Scheme 21) or by being positioned at a discrete distance from these (cf. 23 and 24). When the metal is attached to the alcohol function loss of this group might also be accelerated (in a stepwise addition-elimination reaction), but with acyl activation loss of alcohol might be expected to be retarded. 

Saunders10 and by Sims and coworkers11 have shown that the magnitude of the leaving-group heavy-atom isotope effect varies linearly with the extent of C—X bond rupture in the transition state for concerted elimination reactions and for nucleophilic substitution reactions, respectively. Since the magnitude of the isotope effect is directly related to the amount of C—X bond rupture in the transition state, these isotope effects provide detailed information about the structure of the transition state. 

In the first example of an elimination reaction, the strong acid, H2SO4, does not count as a reactant. It is not directly involved in the reaction. It is a catalyst a compound that speeds up a reaction but is not consumed by it. 

Following the determination of the geometry and the thermochemistry of transition states, the rate parameters for the two silane decomposition pathways can be obtained directly by the TST formulation presented earlier. These calculations have led to unimolecular rate constant expressions 10 exp(-91000/RT)s-S and 10 exp(-62000/l r)s" for Si-H bond scission and H2 elimination reactions, respectively. These results clearly... 

Important features favoring the reductive elimination reaction have been discussed based on theoretical and experimental studies [128-130]. The reductive elimination of an azohum salt from a palladium NHC alkyl complex (Fig. 13) proceeds under direct formation of Pd° in an exothermic process with a low activation barrier [128]. The coligands at the palladium atom play an important role. It has been shown that the Caikyi-Pd-CNHC angle becomes more acute during the reductive elimination to allow for an optimal orbital overlap of the groups to be... 

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