Esters, reaction with elimination

The subsequent step is the slowest in the sequence and appears to involve attack of a base (water) at the alpha hydrogen of the chromate ester concurrent with elimination of the HCr03° group. There is an obvious analogy between this step and an E2 reaction (Section 8-8A) ... 

Cellulose xanthate is another modified cellulose obtained from alkali cellulose RceiiOH NaOH + CSj RceiiO-C(S)SNa + HjO Xanthates undergo a pyrolytic reaction with elimination, similar to other esters ... 

Each reaction produces another ester group and the resultant polymer is a polyester resin. This reaction with elimination of a small molecule is termed a condensation reaction, hence this type of polymerization has also been termed condensation polymerization. 

Phosphoms halides are subject to reactions with active hydrogen compounds and result in the elimination of hydrogen halide. They are convenient reagents in the synthesis of many esters, amides, and related compounds. However, because the involved hydrogen halide frequendy catalyzes side reactions, it is usually necessary to employ a hydrogen halide scavenger to remove the by-product. 

Enols and alkoxides give chelates with elimination of alcohol. For example, in the reaction of the enol form of acetylacetone [123-54-6] all four alkoxide groups attached to zirconium can be replaced, but only two of the four attached to titanium (Fig. 3). Acetoacetic esters react similarly. 

Alcohols react with boric acid with elimination of water to form borate esters, B(OR)3. A wide variety of borate salts and complexes have been prepared by the reaction of boric acid and inorganic bases, amines, and heavy-metal cations or oxyanions (44,45). Fusion with metal oxides yields... 

The submitters have shown that these reactions proceed by dehydro-chlorination of the acid chloride to the ketene, which is then trapped by reaction with the phosphorane. The resulting betaine decomposes to the allenic ester via an oxaphosphetane. In contrast, the reaction of acid chlorides with 2 equivalents of phosphoranes involves initial acylation of the phosphorane followed by proton elimination from the phosphonium salt. ... 

An example of cleavage ol the sulfur-oxygen bond in trifluoromethane-sulfonic ester has been reported Tnfluororaethyl triflate reacts with neutral or anionic nucleophiles with elimination of carbonyl difluoride and formation of trifluoromethanesulfonyl fluoride [57] (equation 32) The mechanism of this reaction involves elimination of fluoride ion, which is a chain carrier in the substitution of fluorine for the trifluoromethoxy group... 

The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8). 

A treatment of 2-butyltelluroaniline with an equimolar amount of bromoacetic acid results in spontaneous cyclization of the formed telluronium salt 31 to give 1-butylbenzotellurazinonium bromide 30. That the alkylation occurs at the tellurium and not at the nitrogen atom of 2-butyltelluroaniline has been proved by the isolation of the methyl ester of 31 in 60% yield when the amine was coupled with methyl bromoacetate under the same reaction conditions. Elimination of butyl bromide from 30 readily occurs on heating of its DMF solution leading to 2//-l,4-benzotellurazin-3(4//)-one 32 in 90% yield. 

The reaction with a carbonyl substrate 3 is similar to a Grignard reaction. Hydrolytic workup then yields the /3-hydroxy ester 4. Sometimes product 4 easily eliminates water to yield directly an o ,/3-unsaturated ester. 

Exchange reactions with esters are significant only if the resulting ester can be easily eliminated. 

Since 1,4-butanediol (BD) undergoes dehydration side reaction in the presence of acid resulting in THF formation, the hydroxy-ester interchange reaction is the preferred method for the preparation of PBT. The first stage of reaction is carried out at 150-200°C and consists of a hydroxy-ester interchange between DMT and excess butanediol with elimination of methanol. In the second stage, temperature is raised to 250°C and BD excess is eliminated under vacuum. Tetraisopropoxy-and tetrabutoxytitanium are efficient catalysts for bodi stages (Scheme 2.20). 

The same elimination strategy was used for the synthesis of the natural product (i )-(-)-dysidazirine 15 as is shown in Scheme 10 [23]. The requisite aziri-dine ester was prepared by treatment of sulfimine 19 with the lithium enolate of methyl bromoacetate. This reaction is a Darzens-type condensation leading to czs-M-sulfinylaziridine ester 20. The elimination of sulfenate was accomplished in the same manner as mentioned above (see Scheme 9). The natural product 15 (see Fig. 1) was obtained in 42% yield. Attempts to prepare azirinomycin 14 in a similar fashion all failed [23]. 

In an analogous reaction the catechol titanate 2136 is converted by TMSOTf 20, via 2137, to give the bis-(trimethylsilyl)ester 2138, which eliminates HMDSO 7 to regenerate 2136 [65] (Scheme 13.18). The intermediate compound 2137 apparently serves as catalyst for the reaction of l-0-trimethylsilyl-2,3,5-tri-0-benzyl-D-arabino-furanose with O-silylated alcohols to afford mainly the l-/ -arabinofuranosides. 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

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