Bases 3-elimination reactions

Tertiary halides undergo a combination of SN1 and Ej reactions. If the reaction is kept cool, and the nucleophile is a relatively weak base, it is possible to get nucleophilic substitution. At high temperatures, or with strong bases, elimination reactions predominate. 

Sulfur occurs directly beneath oxygen in the periodic table. Therefore, sulfur compounds are weaker bases but better nucleophiles than the corresponding oxygen compounds. Sulfur compounds are excellent nucleophiles in SN2 reactions, and because they are relatively weak bases, elimination reactions are not usually a problem. Yields are good with primary and secondary substrates. For similar reasons, phosphorus compounds also give good yields when treated with primary and secondary substrates in Sn2 reactions. The following equations provide examples of the use of these nucleophiles ... 

Smith periodate degradation, partial acidic hydrolysis, reduction of uronosyl residues, acetolysis, as well as base elimination reactions and other degradation procedures that can be applied to uronosyl residues, and n.m.r. spectroscopy [261]. Increasing availability of purified enzymes with well-defined action patterns should assist future studies [106, 262], Structural features can also be recognised by monoclonal antibodies [268],... 

N,N,N, N -tetramethyl-l,8,-naph-thalenediamiDe M.P. 51 C. A remarkably strong mono-acidic base (pKg 12-3) which is almost completely non-nucleophilic and valuable for promoting organic elimination reactions (e.g. of alkyl halides to alkenes) without substitution. 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

When we discussed elimination reactions in Chapter 5 we learned that a Lewis base can react with an alkyl halide to form an alkene In the present chapter you will find that the same kinds of reactants can also undergo a different reaction one m which the Lewis base acts as a nucleophile to substitute for the halo gen substituent on carbon... 

We have seen that an alkyl halide and a Lewis base can react together m either a sub stitution or an elimination reaction... 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

Hydrolysis. Esters are cleaved (hydroly2ed) into an acid and an alcohol through the action of water. This hydrolysis is cataly2ed by acids or bases. The mechanistic aspects of ester hydrolysis have received considerable attention and have been reviewed (16). For most esters only two reaction pathways are important. Both mechanisms involve a tetrahedral intermediate and addition-elimination reactions i7i7... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Pyridine base eliminations of a-bromo ketones cannot be recommended for general use because of the side reactions already discussed. The semi-carbazone-pyruvic acid method should be employed if strict absence of isomerization is required in the dehydrobromination of 2- or 4-bromo-3-ke-tones. This procedure is not applicable for the preparation of -3-ketones,... 

The opening of epoxides with neighboring keto groups, accompanied by vinylogous jS-elimination, has already been mentioned (section V). This is an extension of the jS-elimination reactions which occur when a,p- or jS,y-epoxy ketones are opened with acid or base. a,jS-Epoxy ketones give a-substituted a,jS-unsaturated ketones [(162), for example], and j5,y-epoxy ketones give y-substituted a,jS-unsaturated ketones [(163), for example] ... 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

The E2 P-elimination reaction is a base-catalyzed concerted mechanism ... 

Note that the key to Maxam-Gilbert sequencing is to modify a base chemically so that it is removed from its sugar. Then piperidine excises the sugar from its 5 - and 3 -links in a /3-elimination reaction. The conditions of chemical cleavage described in Figures 12.4 and 12.5 are generally adjusted so that,... 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

A 1,8-naphthyridine, nalidixic acid (39), shows clinically useful antibacterial activity against Gram-negative bacteria as such, the drug is used in the treatment of infections of the urinary tract. Condensation of ethoxymethylenemalonate with 2-amino-6-methylpyridine (36) proceeds directly to the naphthyri-dine (38) the first step in this transformation probably involves an addition-elimination reaction to afford the intermediate, 37. W-Ethylation with ethyl iodide and base followed by saponification then affords nalidixic acid (39). 

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