Reactions base-catalyzed displacement

Base-catalyzed displacement reactions, such as base-catalyzed halogenation, hydrogen exchange, and condensation reactions, belong to a separate class of reactions. The reactive species, in this case, is the phenolate ion rather than unionized phenol, and the reactive positions are limited to those ortho and para to the hydroxyl group. In a guaiacyl propane unit, no reaction occurs at positions 2 and 6, and displacement at position 5 will generally proceed more rapidly than the displacement of the side chain. 

Conversion of sugar phenylhydrazones into olefinic azo-sugars on treatment with acetic anhydride and pyridine was shown by Wolfrom and co-workers (28) (The acetylated forms of the acyclic phenylhydrazones of D-glucose, D-mannose, and D-galactose readily lose the elements of acetic acid to yield 1-phenylazo-frans-l-hexenetetrol tetraacetate when treated with warm aqueous ethanol (28, 30). It is assumed that atmospheric oxygen partakes in this elimination reaction.) This is a special case of base catalyzed -elimination reactions of the type proposed by Isbell in 1943 ( 31), involving consecutive electron displacement (which actu-... 

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... 

In Mannich reactions performed in basic media, the active aminomethylating species is less well defined. It may be a methylenediamine (9), a hydroxymethylamine (8), or an alkoxymethylamine (ROCH2NR2 ) (in alcoholic solvents). Aminomethylation in base-catalyzed Mannich reactions, as shown for cyclohexanone, is believed to occur by an 5n2 mechanism in which the enolate (13) displaces either NR2", 0H , or OR from one of the intermediates above (Scheme 2). 

In all the examples of basic catalysis considered above, the characteristic effect of the basic catalyst is the increase in concentration of the basic group involved in the reaction. This is accomplished by the familiar displacement of one base by another for example, the displacement of the ester anion from ethyl acetate by the ethoxide ion catalyst in the Claisen reaction. In some reactions, however, displacement apparently does not result, but the combination of the basic catalyst with the reacting substance merely localizes the electron excess on one atom, which can then act as a base. An example is the shifting of the electron pair of the C—H linkage toward the hydrogen in the base-catalyzed Cannizzaro reaction, thus enabling a second molecule of aldehyde to remove a hydride ion. 

Organometallic compounds or carbanions undergo a number of reactions in which the carbanion or carbanion-like moiety of the organometallic compound acts as a nucleophilic displacing agent. Examples are the formation of hydrocarbons from alkyl halides, alkyl halides from halogens, and ketones from acid chlorides or esters. The latter two reactions are closely related to the base-catalyzed condensations and are perhaps additions as well as displacement reactions. Related addition reactions are the carbonation of organometallic compounds and the addition to ketones or aldehydes. 

The carbonium ion rearrangement is essentially an internal displacement reaction in which a carbonium ion becomes bonded to some other portion of the same molecule. Certain base-catalyzed rearrangements have been discussed in the section on carbonium ion rearrangements because they have the distinguishing characteristic of being internal electrophilic displacements by an atom bearing at least a partial positive charge. 

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

Figure C shows an extreme case of the dependence of a substitution reaction rate on the nature of the incoming group. This happens to be the hydrolysis of the trisacetylacetonate complex of silicon (IV), cationic species, which Kirchner studied first—the rate of racemization or rate of dissociation. We studied the base-catalyzed rate of dissociation and showed that a large number of anions and nucleophilic groups, in general, would catalyze in the dissociation process. We found that the reaction rates were actually for a second-order process, so these units are liters per mole per second. But the reaction rate did vary over an enormous range—in this case, about a factor of 109—and this is typical of the sort of variation in rates of reaction (that you can get) for processes that seem to be Sn2 bimolecular displacement processes.

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

In addition to participating in acid-base catalysis, some amino acid side chains may enter into covalent bond formation with substrate molecules, a phenomenon that is often referred to as covalent catalysis.174 When basic groups participate this may be called nucleophilic catalysis. Covalent catalysis occurs frequently with enzymes catalyzing nucleophilic displacement reactions and examples will be considered in Chapter 12. They include the formation of an acyl-enzyme intermediate by chymotrypsin (Fig. 12-11). Several of the coenzymes discussed in Chapters 14 and 15 also participate in covalent catalysis. These coenzymes combine with substrates to form reactive intermediate compounds whose structures allow them to be converted rapidly to the final products. 

Kinetic isotope effect for lysozyme. A secondary kinetic isotope effect is expected because a molecule with H in the number 1 position can be converted to the corresponding oxocarbenium ion somewhat more easily than the molecule with 2H in the same position (Eq. 12-13). For example, in the nonenzymatic acid-catalyzed hydrolysis of a methyl- glucoside, a reaction also believed to proceed through a carbocation intermediate,41 75 the ratio h / h, is 1.14 for the a anorner and 1.09 for the (3 anorner.53 In the base-catalyzed hydrolysis of the same compound, which is believed to occur by a double-displacement reaction involving participation of the neighboring OH group on C-2, the ratio /c1h / /c2h is 1.03. The corresponding ratio measured... 

Chloroacetic Acid (ClCHiCOOHf. [CAS 79-11-8 J. Chloroacelic acid can be synthesized by the radical chlorination of acetic acid, treatment of trichloroethylene with concentrated H S04. oxidation of 1.2-dichloro ethane or chloroaceialdehyde. amine displacement from glycine, or chlorination of ketene. It behaves as a very strong monobasic acid and is used as a strong acid catalyst for diverse reactions. The Cl function can be displaced in base-catalyzed reactions. For example, it condenses with alkoxides to yield alkoxyacetic acids CICH COOH... 

Cyanohydrins can be formed by the acid- or base-catalyzed reaction of hydrogen cyanide with an aldehyde or ketone, the displacement of bisulfite ion by cyanide ion on the bisulfite addition compounds of aldehydes and... 

Polymerization and Spinning Solvent. Dimethyl sulfoxide is used as a solvent for the polymerization of acrylonitrile and other vinyl monomers, and as a reaction solvent for other polymeri7ations. Tt is also used as a solvent for displacement reactions, solvent for base-catalyzed reaclions, extraction solvent, solvent for electrolytic reactions, cellulose solvent, pesticide solvent, and clean-up solvent,... 

The mechanism of the Parikh-Doering oxidation involves formation of O-di-methylsulfoxonium sulfate from the reaction of DMSO with S03. Displacement on the sulfur by the alcohol gives the alkoxysulfonium salt intermediate, which undergoes base-catalyzed elimination to afford the dimethyl sulfide and the aldehyde. This reaction must be carried out so the hydrogen sulfate of the alcohol is not formed first, otherwise no oxidation would occur. 1143 44 ... 

The finding of a small positive value for p was something of a surprise, bui together with the idea that a catalyst complex must somehow be formed, we can draw some conclusions. The complex could be some form of protonation of the alcohol, but it is difficult to envision that species carrying out a substitution. A more likely possibility is that the mechanism more closely resembles Schowen s work with buffered carboxylic acid solutions. In this scheme, we propose that the carboxylic acid first protonates either the alcohol or the alkoxysilane. The resulting carboxylate then forms a complex with the alcohol as postulated by both Schowen and Boe for the base catalyzed reaction. This is then followed by a concerted displacement reaction as shown below ... 

Mechanistic features of SNAr displacement reactions involving amino nucleophiles have been the object of many investigations, a major point of interest being the occurrence of base-catalyzed paths. Strongly activated aryl halides react readily with ammonia and with primary and secondary amines to give the corresponding arylamines. Thus, for example, 2,4-dinitrofluorobenzene is used to tag the amino end of a peptide or protein chain. 

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