Acidity, also elimination reactions

Ammonium ions have also been used as templates for creating pockets capable of stabilizing positively charged transition states. For example, antibodies raised against hapten 77 accelerate the dehydration of P-hydroxy ketone 75 to give enone 76. The ammonium group in 77 served as a surrogate for the protonated alcohol formed in the acid-catalyzed elimination reaction, and evidence obtained with... 

Disubstituted furans with different functionalities were synthesized through an acid-catalyzed elimination reaction of 2-alkylidenetetrahydrofurans, in turn, prepared via the cyclization of 1,3-bis-silyl enol ethers with l-chloro-2,2-dimethoxyethane with high regioselectivity <05EJOC2074>. 2-Substituted furans and bicyclic furans can also be... 

SET-sensitized irradiations of 39c, 39d, and 40b also afforded, in addition to the corresponding cyclopropanes, the nitriles 53a, 53b, and 54, respectively, resulting from ehmination of acetic acid. This elimination reaction cannot be considered a thermal process because it does not occur in the triplet-sensitized irradiation of oxime acetates. The mechanism shown in Scheme 13 for compound 39d is proposed to account for the formation of these nitriles. It involves the generation of a radical-cation centered on the CN double bond 55, followed by elimination of acetic acid to afford the radical-cation 56, which yields the nitrile 53b by back electron transfer (Scheme 13). 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

Other Syntheses. Acryhc acid and other unsaturated compounds can also be made by a number of classical elimination reactions. Acrylates have been obtained from the thermal dehydration of hydracryhc acid (3-hydroxypropanoic acid [503-66-2]) (84), from the dehydrohalogenation of 3-halopropionic acid derivatives (85), and from the reduction of dihalopropionates (2). These studies, together with the related characterization and chemical investigations, contributed significantly to the development of commercial organic chemistry. 

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 ... 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

Elimination reactions of fluorine compounds are not limited to the removal of simple molecules Frequently, large molecules or combination of smaller ones are formed as by-products, especially in pyrolytic reactions For example perhalo genated acid chlorides lose not only carbon monoxide but also chlorine fluoride [106, 107] (equations 74 and 75)... 

Thus, in contrast to benzothiepins, dibenzo compounds can be synthesized by direct acid-catalyzed elimination of water from hydroxy derivatives, or of amines from amino derivatives, at elevated temperatures due to their thermal stability. As in the case of benzothiepins, dibenzo derivatives can also be prepared by base-catalyzed elimination from the corresponding halo derivatives however, the yields are somewhat lower compared to the acid-catalyzed reactions. As a special case, an aziridine derivative was deaminated by palladium-catalyzed hydrogenation to afford the corresponding dibenzothiepin.69... 

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. 

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). 

In a reaction similar to the (>-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent [>-alkoxidc elimination to provide Rh-OCH3. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

These enzymes invariably involve a cofactor, pyridoxal phosphate (vitamin B6). In addition, pyridoxal phosphate is also required for most decarboxylations, racemizations, or elimination reactions in which an amino acid is a substrate. Pyridoxal phosphate is not involved in decarboxylations in which the substrate is not an amino acid. So if a question... 

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