Elimination example mechanism

Primary and secondary aliphatic amines, morpholine and 2-methylaziridine and aniline and even the sterically hindered 2.2,6,6-tetramethylpiperidine readily react with 6-bromo-trithiadiazepine 7, in certain cases in the presence of /V./V-diisopropylethylamine, at room temperature by substitution of the bromine atom ammonia, for example, yields trithiadiazepin-6-amine 22 (R1 = R2 = H). There is compelling evidence that these reactions proceed by an elimination-addition mechanism via the heteroaryne, trithiadiazepyne 21.391... 

Several types of compound undergo elimination on heating, with no other reagent present. Reactions of this type are often run in the gas phase. The mechanisms are obviously different from those already discussed, since all those require a base (which may be the solvent) in one of the steps, and there is no base or solvent present in pyrolytic elimination. Two mechanisms have been found to operate. One involves a cyclic transition state, which may be four, five, or six membered. Examples of each size are... 

Although hydrolysis as well as other nucleophilic reactions of A-acylazoles (alcoholysis, aminolysis etc.) most likely follow the addition-elimination (AE) mechanism, there are indications that more complex mechanisms must be taken into account for hydrolysis under specific structural conditions. For example, for neutral hydrolysis of imidazolides with increasing steric shielding of the carbonyl group by one, two, and three... 

Carbanion mechanisms may give either syn or anti elimination. For example, Hunter and Shearing studied the butoxide-catalyzed elimination of methanol from 35 and 36. Since deuterium exchange with solvent is in close competition with elimination, the mechanism is probably (EjcB)b. The ratio ofsyn/anti... 

It should be noted that the problem of transient noise pulses can in principle be circumvented by use of suitable playback technology. For example, in the case of gramophone disks the use of a laser-based reader should eliminate any mechanical resonance effects and thus reduce the artifact to a large click which can be restored using the methods of previous sections. Of course, this does not help in the many cases where the original source medium has been discarded after transcription using standard equipment to another medium such as magnetic tape ... 

Detailed mechanisms of intramolecular rearrangement reactions have been difficult to determine. Classical rate measurements seldom lead to unambiguous mechanistic predictions. Generally only after extensive examination of concentration, solvent, and substituent effects on the reaction rate can a general mechanistic class be proposed for example, intra vs intermolecular or bond rupture of a bidentate chelate vs non-bond rupture twist pathways. Indeed, only two examples of slow complexes are known where detailed rate comparisons for geometrical and optical iso-merizations were made and used to eliminate several mechanisms however, a single most probable pathway was not demonstrated in either case.12,13 Only with DNMR can detailed environmental site interchanges be directly observed and with this in-... 

Further poor substrates for nucleophilic substitutions are amines, quaternary ammonium salts, amides, nitriles, and azides these will usually undergo substitution in particularly reactive substrates only or under harsh reaction conditions. Some rare examples of successful substitutions of these leaving groups are shown in Scheme 4.9. The last reaction is Scheme 4.9 presumably proceeds via an elimination-addition mechanism. 

This is an example of the first step of an addition-elimination reaction mechanism converting an ester (methyl acetate) to an amide (A - mcth y I acctam ide). For clarity, the anion was repositioned in the scheme. Arrow pushing is illustrated below ... 

The reaction presented in this problem is known as a Friedel-Crafts acylation. Technically, this example belongs to a class of reactions referred to as electrophilic aromatic substitutions. Furthermore, the actual mechanism associated with this reaction, utilizing Lewis acid reagents as catalysts, proceeds through initial formation of an electrophilic acyl cation followed by reaction with an aromatic ring acting as a nucleophile. This mechanism, shown below, reflects distinct parallels to standard addition-elimination reaction mechanisms warranting introduction at this time. 

Example 3.7. An elimination-addition mechanism—aryne intermediate. 

In certain cases in which the substrate carries an a hydrogen, there is strong evidenced " that at least some of the reaction takes place by an elimination-addition mechanism (ElcB, similar to the one shown on p. 1406), going through a sulfene intermediate, " for example, the reaction between methanesulfonyl chloride and aniline. 

We have seen that electron-withdrawing groups activate aryl halides toward nucleophilic substitution. In the absence of such activation, substitution can be made to take place, by use of very strong bases, for example. But when this is done, substitution does not take place by the mechanism we have just discussed (the so-called bimolecular mechanism), but by an entirely different mechanism the benzyne (or elimination-addition) mechanism. Let us f rst see what this mechanism is, and then examine some of the evidence for it. 

If a single rate-controlling step exists, Table 7 shows the apparent v calculated from Equation (45) when the step being considered is rate-limiting and the reaction velocities for the step are determined by tracers C and B. For the two examples in Table 7, v will be either unity or infinity. In Newsome s example, v is assumed to have a value of unity for both tracers C and B. For the value of unity for v, mechanism 1 in Table 7 can be eliminated while mechanism 2 is possible, and step 2 is the rate-controlling step. 

Several control experiments with radical initiators and inhibitors clearly exclude a radical chain mechanism (33). Also, various elimination-addition mechanisms are unlikely on the basis of appropriate control experiments (33). For example, Eq. (7) does not apply, since fluorenone fails to react with allyltrimethylsilane. 

Many production problems are solved by adjustments in process parameters, elimination of mechanical breakdown, and cleaning parts of equipment. But there are also many instances when production quality and rate can be restored by adjustments in formulation. Here are examples ... 

When large, multicellular organisms developed, vith specialized organs, some of these abilities became a handicap, and were lost. Let us continue with thiamine as an example. All mammals require thiamine in their food, in order to live. The ability to synthesize thiamine was lost by an early ancestral vertebrate, several hundred million or a billion years ago. This animal was ingesting plants, which provided an ample supply of thiamine, about 5 mg for each 2500 kcal of food energy. The sjmthetic mechanism was not needed, and it was a burden it cluttered up the cells, added to the body weight, used energy that could be better used for other purposes. When a series of mutations occurred that eliminated this mechanism, the mutant was favored over the wild type, which failed to meet the competition and died out. The victory of a mutant strain of a bacterium over the wild type in the competition for survival in the presence of an ample supply of the essential nutrient has been verified by direct experiments in the laboratory by Zamenhof and Eichhorn. ... 

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