Elimination of alcohols

This Reaction should be carefully distinguished from the Claisen Conden-tation, which is the condensation of an ester, under the influence of sodium ethoxide, with (i) another ester, (ii) a ketone, or (iii) a nitrile, with the elimination of alcohol. For details of this condensation, see Ethyl Acetoacetate, p. 264. 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

Oxidative Carbonylation of Ethylene—Elimination of Alcohol from p-Alkoxypropionates. Spectacular progress in the 1970s led to the rapid development of organotransition-metal chemistry, particularly to catalyze olefin reactions (93,94). A number of patents have been issued (28,95—97) for the oxidative carbonylation of ethylene to provide acryUc acid and esters. The procedure is based on the palladium catalyzed carbonylation of ethylene in the Hquid phase at temperatures of 50—200°C. Esters are formed when alcohols are included. Anhydrous conditions are desirable to minimize the formation of by-products including acetaldehyde and carbon dioxide (see Acetaldehyde). 

The elimination of alcohol from P-alkoxypropionates can also be carried out by passing the alkyl P-alkoxypropionate at 200—400°C over metal phosphates, sihcates, metal oxide catalysts (99), or base-treated zeoHtes (98). In addition to the route via oxidative carbonylation of ethylene, alkyl P-alkoxypropionates can be prepared by reaction of dialkoxy methane and ketene (100). 

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. 

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

This information coupled with the proposed mechanism of the Conrad-Limpach reaction, reasonably lead to the below proposed mechanisms. Conjugate addition of aniline and elimination of alcohol provides the P-anilinoacrylate 14, which upon heating to 180-320 °C gives species, like 34a,b, which undergo 6n-electrocyclization to 35 or 36, respectively. Loss of ethanol from 36 gives 35 and tautomerization provides 4-... 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

On the other hand, the action of acid chlorides on ethyl acetoacetate in pyridine leads to the formation of O-acyl-derivatives, whilst the O-alkyl-derivatives can only be obtained indirectly from the acetals (p. 140) by elimination of alcohol (Claisen). 

The elimination of an alcohol from a neutral 2-alkoxy-1,2-dihydro-1//-imidazole leads to the formation of NHCs [Eq. (18)]. Upon heating, the elimination of alcohol forms the NHC, which in the case of imidazolin-2-ylidenes dimerizes to... 

The best-known exception to exponential kinetics is the elimination of alcohol (ethanol), which obeys a linear time course (zero-order kinetics), at least at blood concentrations > 0.02 %. It does so because the rate-limiting enzyme, alcohol dehydrogenase, achieves half-saturation at very low substrate concentrations, i.e at about 80 mg/L (0.008 %). Thus, reaction velocity reaches a plateau at blood ethanol concentrations of about 0.02 %, and the amount of drug eliminated per unit of time remains constant at concentrations above this level. 

Though the rates of metabolism and excretion of many drugs are measured in terms of half-lives, the elimination of alcohol primarily by liver enzymes occurs as a constant amount per time regardless of its concentration. On average, the BAG decreases 0.015% per hour. Thus, it would take about five hours before someone s BAG reached zero if he or she consumed enough alcohol to have reached a BAG of 0.075%. Also, if one consumed... 

On the contrary, while working in the synthesis of the alkaloid ru-brolone, Bogerand Zhu (91TL7643) have found 0-alkyl a,/3-unsaturated oximes 98 to participate as effective 4ir components of an intramolecular Diels-Alder reaction with an electron-deficient dienophile. Thus, 98 was prepared from butane-1,4-diol and heated in triisopropylbenzene to furnish 2-pyrindine derivatives 99 by virtue of in situ elimination of alcohol (Scheme 25). 

The mechanisms that underlie ethanol s teratogenic effects are unknown. Ethanol rapidly crosses the placenta and reaches concentrations in the fetus that are similar to those in maternal blood. The fetal liver has little or no alcohol dehydrogenase activity, so the fetus must rely on maternal and placental enzymes for elimination of alcohol. 

When the reaction proceeds by this pathway, 24 and similar intermediates are not involved and the mechanism is exactly (by the principle of microscopic reversibility) the reverse of El elimination of alcohols (7-1).149 It is likely that the mechanism involves both pathways. 

Elimination of water introduces a C=C bond between two carbons that previously were saturated (as in the enolase reaction see Fig. 6-23). Similar reactions can result in the elimination of alcohols and amines. 

Finally, upon treatment with acid, both diene and allyl ether complexes of [Os(NH3)5]2+ yield Os(IV) i73-allyl species, the latter reaction proceeding by the elimination of alcohol (180). 

An important class of reactions of alkoxides and aryloxides involves the substitution of an alkoxide or aryloxide by protonation and elimination of alcohol or phenol (equation 56). 

There are also a vast number of reactions known between alkoxides and non-trivial alcoholic reagents such as glycol, catechols, alkanolamines and hydroxylamines. The fundamental reaction involves elimination of alcohol, but with the reaction products complicated by the nature of chemistry of the other substituents.1... 

A considerable amount of work has been done on the hydroformylation of alkyl acrylates. The formation of y-oxobutyrates has previously been reported (15). Iwanaga (70) studied the effect of solvent variation and found that the rate of hydroformylation was in the order alcohols > acetone > toluene. Pyridine and some of its homologs also increased the rate of reaction. At higher temperatures and pressures, lactones were formed (32, 35, 135), presumably by reduction of the Oxo ester to the hydroxy ester followed by ring closure with elimination of alcohol. 

The shortest Ge—H bond yet found in an organogermane occurs in the recently reported complex hydride [(CH3)3Si]2CH 2(OC2H5)GeH (118). The shortness of the Ge—H bond (1.46 A) must, at least in part, arise from the hybridization demands of the electronegative ethoxy group. Distortions in the tetrahedral environment about the germanium (Table I) arise from the steric requirements of the bis(trimethyl-silylmethyl) moieties, while the authors attribute to the same source the stability of this compound with respect to the reductive elimination of alcohol normally observed (127). 

In the synthesis of specially substituted methylene diphosphines, made from secondary phosphines and carbonyl derivatives (7), a carbenium ion adjacent to trivalent phosphorus as the transition state has been discussed. The transfer of this reaction principle to primary phosphines and suitable carbonyl compound revealed a further pathway to derivatives of dicoordinated phosphorus (8). Aromatic phosphines react with carboxylic acid amide acetals under elimination of alcohol giving dialkylamino-alkylidene phosphines (Scheme 5). A modification of the synthesis... 

The elimination of alkyl halides is done under basic conditions, the elimination of alcohols is done under acid conditions. Under basic conditions, an E2 elimination would require the loss of a hydroxide ion as a leaving group. Since the hydroxide ion is a strong base, it is not a good leaving group and so the elimination of alcohols under basic conditions is difficult to achieve. 

The activation of propargylic ethers also provides the generation of ruthenium allenylidene species with elimination of alcohols (Eq. 13). This reaction has been used in the catalytic transformation of benzyl propargyl ethers into 1,3-dienes via dealkoxylation, addition of benzyl alcohol to the a-carbon atom of the allenylidene intermediate and hydrogen-transfer reactions according to Scheme 21 [89]. 

The elimination of alcohols 13A to give cinnamic acids trans-13B in aqueous sulfuric acid has been studied, If optically active 13A is used and the reaction stopped at 10% completion, the starting material is found to be completely racemized. What can you deduce about the mechanism of the elimination step ... 

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