ElcB elimination reactions

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. 

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. 

Ketene itself is usually made by high-temperature pyrolysis of acetone but some ketenes are easily made in solution. The very acidic proton on dichloroacetyl chloride can be removed even with a tertiary amine and loss of chloride ion then gives dichloroketene in an ElcB elimination reaction. 

Step 2 Base brings about an ElcB elimination reaction that has homocysteine as the leaving group. 

Carbanions are involved as intermediates in ElcB elimination reactions. 

Our aim is to find suitable systems where the individual reaction steps of Scheme I can be studied separately. In as much as some of these steps are also relevant to nucleophilic vinylic substitutions (1), ElcB elimination reactions (2) and proton transfer reactions involving both normal acids (3) and carbon acids (4) we hope to contribute to a better understanding of these reactions as well. 

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

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. 

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. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Secondary alkyl halides Sjvj2 substitution occurs if a weakly basic nucleophile is used in a polar aprotic solvent, E2 elimination predominates if a strong base is used, and ElcB elimination takes place if the leaving group is two carbons away from a carbonyl group. Secondary allylic and benzyiic alkyl halides can also undergo S l and El reactions if a weakly basic nucleophile is used in a pro tic solvent. 

In the ElcB reaction, C-H bond-breaking occurs first. A base abstracts a proton to give an anion, followed by loss of the leaving group from the adjacent carbon in a second step. The reaction is favored when the leaving group is two carbons removed from a carbonyl, which stabilizes the intermediate anion by resonance. Biological elimination reactions typically occur by this ElcB mechanism. 

ElcB reaction (Section 11.10) A unimolecular elimination reaction in which a proton is first removed to give a car-banion intermediate, which then expels the leaving group in a separate step. 

The fact that the rate law of hydrogen bromide elimination is first order with respect to the base may be interpreted by an E2 mechanism. The antiperiplanar position of the hydrogen and the bromine atoms in Ih also makes this mechanism very likely. Earlier the same mechanism was proposed for the elimination reaction of some tertiary a-halo ketones (ref. 19). Other mechanism, such as ElcB or El, seems to be very improbable considering the lack of any deuteration at C-2 or the lack of any rearrangement and the fact that the generation of a-keto cations requires acidic conditions (ref. 20). 

We have already seen examples of carbanions involved as intermediates, e.g. (40), in elimination reactions, i.e. those that proceed by the ElcB pathway (p. 251), for example ... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

More O Ferrall and Slae (1970) reported a secondary /3-deuterium KIE of 1.01 per deuterium ((kHfkD)0 = 1.03 at 25°C) for the rate-determining formation of the carbanion formed in the first step of the Elcb /3-elimination reaction of 9-fluorenylmethanol (reaction (35)). 

L is the hydrogen or deuterium atom that is not transferred in the elimination reaction and T is tritium that is present in tracer quantities. These substrates were chosen so that the reactions would have transition states ranging from very ElcB-like for [22], to central or intermediate for [23], to El-like for [24]. For practical reasons, the base/solvent system could not be kept constant as was originally intended. EtO /EtOH was used in the reaction with substrates [22] and [23] whereas Bu,0"/But0H was used with substrate [24]. Although the secondary tritium KIE (when L = H) for the reaction of [22] was... 

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