Elimination, ElcB

It was found that 1,2,4-triazine 4-oxides 55 are active enough to react with cyanamide under basic conditions according to the deoxygenative mechanism to form 5-cyanamino-l,2,4-triazines 73 (00TZV1128). This reaction seems to be facilitated by the easy aromatization of cr -adducts by the Elcb elimination of water. 

I Primary alkyl halides S 2 substitution occurs if a good nucleophile is used, 2 elimination occurs if a strong base is used, and ElcB elimination occurs if the leaving group is two carbons away from a carbonyl group. 

In Europe, interest has centered particularly on polyhydroxybutyrate, which can be made into films for packaging as well as into molded items. The polymer degrades within 4 weeks in landfills, both by ester hydrolysis and by an ElcB elimination reaction of the oxygen atom p to the carbonyl group. The use of polyhydroxybutyrate is limited at present by its cost—about four times that of polypropylene. 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter what the geometry of the original compound, the more stable situation is the one where the larger of the D-E pair is opposite the smaller of the A-B pair and the corresponding alkene should form. If the carbocation is not completely free, then to that extent, E2 type products are formed. Similar considerations apply in ElcB eliminations. ... 

Why does this pathway occur instead of the Robinson annulation when the seemingly trivial change of increasing the concentration of NaOH is made Good question. It is not clear. It seems likely that the Robinson annulation does occur first (because quick quenching helps to increase the quantity of Robinson product), but the Elcb elimination at the end of the annulation mechanism is reversible in the presence of NaOH as base. It seems likely, then, that if NaOEt were used as base instead, only the Robinson product would be observed regardless of the quantity of catalyst. 

Fluorines present in the /i-position (/f-fluorination) can also influence the outcome of elimination reactions of fluorocarbons as earbanion intermediates are involved in the ElcB elimination mechanism.83 The regio- and stereoselectivity of the elimination step is governed by CH acidities,84 which, as has been discussed, arc influenced by /f-fluorination. 

An attempt to study resolved (( )-18) as a probe for the detailed mechanism of the Adn—E vinylic substitution reaction has been complicated by intervention of a competing reaction route this is believed to involve a competing (ElcB elimination-addition, for which antiperiplanar orientation of H and Cl is not a requirement.7 a-Deuterated (ca 50%) E- and Z-substitution products (which do not themselves exchange deuterium) are obtained on reaction with MeS in 9 1 CD3CN-D2O but no incorporation of deuterium in unreacted ((/r)-18) occurs and neither does isomerism to ((Z)-18) precede elimination. 

The impossibility of a conjugated alkene in 15 as well as 13, makes this an exceptional case in aldols of aldehydes. Without the branch at the a-carbon, the product is more usually a conjugated enal.6 So the linear isomer 17 of 16 with the same base gives the enal 18 in good yield.7 The true product of the dimerisation is the anion 19. This is in equilibrium with the enolate that allows an ElcB elimination of water 20 to give the enal 18. 

We need a formaldehyde equivalent that is less electrophilic than formaldehyde itself and will therefore add only once to enol(ate)s. The solution is the Mannich reaction.7 Formaldehyde is combined with a secondary amine to give an iminium salt that adds 47 to the enol of the aldehyde or ketone in slightly acidic conditions to give the amino ketone (or Mannich base ) 48. If the product of the aldol reaction 50 is wanted, alkylation on nitrogen provides a good leaving group and ElcB elimination does the trick. 

The first step is a Ferrier mercuration9 reaction on the enol ether double bond, which initiates ring-opening of the pyranoside to form 13 (Scheme 11.5). An intramolecular aldol addition reaction then ensues to give 14. After 0-mesylation, 15 undergoes an Elcb elimination reaction via enolate 16. 

Fig. 4.4. Energy profile of the C=C-forming step of the four mechanisms according to which the /3-eliminations of Figure 4.3 can take place in principle as a function of the chemical nature of the substituent Het and the reaction conditions. The conceivable starting materials for this step are, depending on the mechanism, the four species depicted on the left, where k is for E2 elimination,2> is for /3-elimination via a cyclic transition state,3> is for El elimination, and4> is for Elcb elimination.

Unimolecular Elcb Eliminations Energy Profile and Rate Law... 

Fig. 4.39. A bimolecular Elcb elimination and its energy profile K refers to the constant of the acid/base equilibrium.

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