Elimination reactions base-catalyzed

VBase-Catalyzed Elimination Reactions. Base-catalyzed elimination reactions are more complex. In the type of reaction which appears to be the most common, the process is initiated by the attack of base at a /3-hydrogen atom (i.e., a hydrogen atom attached to a carbon atom which is adjacent to the one holding the functional group). A transitory car-banion (pp. 34 and 123) is thus formed (V), and displacement of the ion... 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

The mechanism for the following reaction in methanol is p.t., addition, then elimination. A base-catalyzed addition (sodium hydroxide) is followed by an acid-catalyzed elimination (hydrochloric acid). Write out the product and the mechanism with all its steps. Note the aldehyde has no acidic hydrogens to remove. Remember to choose between the three types of additions, Adg2, AdN2, or AdgS, then choose between the three types of eliminations, El, E2, or ElcB. 

Thiols. 5-Acetyl derivatives can be prepared by the reaction of acetic anhydride and a thiol in the presence of potassium bicarbonate. Several disadvantages to the 5-acetyl group in peptide synthesis include /3-elimination upon base-catalyzed hydrolysis. Also, sulfur to nitrogen acyl migration may be problematic. 

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. 

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. 

The mechanistic pattern established by study of hydration and alcohol addition reactions of ketones and aldehydes is followed in a number of other reactions of carbonyl compounds. Reactions at carbonyl centers usually involve a series of addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid-catalyzed or base-catalyzed. The rate and products of the reaction are determined by the reactivity of these tetrahedral intermediates. 

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

The treatment of enamines with acid halides which possess no a hydrogens results in the simple acylation of the enamine (7,12,62-67). If the acid halide possesses an a hydrogen, however, ketenes are produced in situ through base-catalyzed elimination of hydrogen chloride from the acid halide. The base catalyst for this reaction may be the enamine itself or some other base introduced into the reaction mixture such as triethylamine. However, if the ketene is produced in situ instead of externally, there still remains the possibility of a side reaction between the acid halide and the enamine other than the production of ketene (67,84). 

The mechanism of the Fiesselmann reaction between methylthioglycolate and a,P-acetylenic esters proceeds via consecutive base-catalyzed 1,4-conjugate addition reactions to form thioacetal Enolate formation, as a result of treatment with a stronger base, causes a Dieckmann condensation to occur providing ketone 8. Elimination of methylthioglycolate and tautomerization driven by aromaticity provides the 3-hydroxy thiophene dicarboxylate 9. 

Bischler-Napieralski reaction conditions can be attributed, again, to the destabilizing ability of the trifluoromethyl group to the cationic transition state of the acid catalyzed elimination. Formation of compound 29 was ultimately accomplished by base catalyzed methanol elimination-conditions conditions that are quite unusual for isoquinoline formation. 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

The knowledge of the valence tautomerization of benzene oxides to oxepins12 prompted several groups to synthesize oxepins by dehydrohalogenation of 7-oxabicyclo[4.1.0]heptane derivatives. Numerous examples have been described for the base-catalyzed elimination of hydrogen bromide from the 3,4-dibromo-7-oxabicyclo[4.1.0]heptane system. The reaction products are usually obtained as mixtures of oxepin 1 and benzene oxide 2. The 2,7-bis(hydroxy-methyl)oxepin 1 p obtained by this route can be converted to the 2,7-dicarbaldehyde with man-ganese(IV) oxide.23... 

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... 

Dibenz[c,e]azepine (30) is obtained in excellent yield as a stable crystalline solid by base-catalyzed elimination of methylsulfinic acid from 6-mesyl-6,7-dihydro-57/-dibenz[c,e]azepine (29. R = Ms).5 An analogous reaction is noted with 6-nitro-6,7-dihydro-5//-dibenz[c,e,]azepine (29, R = N02). 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

The surprising selectivity in the formation of 4 and 5 is apparently due to thermodynamic control (rapid equilibration via the 1,3-boratropic shift). Structures 4 and 5 are also the most reactive of those that are present at equilibrium, and consequently reactions with aldehydes are very selective. The homoallylic alcohol products are useful intermediates in stereoselective syntheses of trisubstituted butadienes via acid- or base-catalyzed Peterson eliminations. 

Base-catalyzed, Add-catalyzed and Thermal Eliminations of Trimethylsilanol. Peterson Reactions... 

Because of the large number of publications in this field, this chapter wiU be limited to giving an idea of developments in this area. Because the Peterson reaction can be viewed as a special case of base-catalyzed reaction, this reaction will be discussed in Section 10.2 after general base-catalyzed eliminations of trimethylsilanol... 

Vinyl ethers can also be generated by thermal elimination reactions. For example, base-catalyzed conjugate addition of allyl alcohols to phenyl vinyl sulfone generates 2-(phenylsulfinyl)ethyl ethers that can undergo elimination at 200° C.223 The sigmatropic... 

Reaction of allylic silanes with enantiomerically pure 1,3-dioxanes has been found to proceed with moderate enantioselectivity.104 The homoallylic alcohol can be liberated by oxidation followed by base-catalyzed (3-elimination. The alcohols obtained in this way are formed in 70 5% e.e. 

Silyl enol ethers and silyl ketene acetals also offer both enhanced reactivity and a favorable termination step. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCl4. This reaction provides a method for introducing tertiary alkyl groups a to a carbonyl, a transformation that cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

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