Base-Catalyzed Rearrangement Reactions

Since the lithium salt rearranges faster than the potassium salt in this rearrangement from oxygen to carbon, it is likely that the lithium ion is coordinated to oxygen during the rearrangement. 

The last reaction shows that the benzyl group has a superior migration aptitude in the Stevens rearrangement. It is more reactive in nucleophilic 

With sodamide in liquid ammonia the Stevens reaction takes a different course.427 Alternative mechanisms are given for the first example shown below, one showing the proton removed from a methyl group, the other from a benzyl group. In the second example the product allows one of the corresponding mechanisms to be eliminated. 

Just as in its carbonium ion twin, the carbanion rearrangement takes place with retention of configuration of the migrating group. This proves at once that the reaction is intramolecular and that the displacement takes place at the front side.428 

A reaction related to the pure carbanion rearrangement, but one that eventually involves an external displacing agent as well, is the Favorskii reaction. It has been shown with the aid of isotopic tracers that the mechanism is 429 

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Addition reactions with tetracyanoethane have provided access to 2,5-diamino-3,4-dicyano-thiophene and -selenophene (58JA2775,81ZOR1958). Base catalyzed rearrangement gives the isomeric pyrrolethiol (Scheme 53). 

Curran s synthesis of ( )-A9(l2)-capnellene [( )-2] is detailed in Schemes 30 and 31. This synthesis commences with the preparation of racemic bicyclic vinyl lactone 147 from ( )-norbomenone [( )-145] by a well-known route.61 Thus, Baeyer-Villiger oxidation of (+)-145 provides unsaturated bicyclic lactone 146, a compound that can be converted to the isomeric fused bicyclic lactone 147 by acid-catalyzed rearrangement. Reaction of 147 with methylmagne-sium bromide/CuBr SMe2 in THF at -20 °C takes the desired course and affords unsaturated carboxylic acid 148 in nearly quantitative yield. Iodolactonization of 148 to 149, followed by base-induced elimination, then provides the methyl-substituted bicyclic vinyl lactone 150 as a single regioisomer in 66% overall yield from 147. 

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. 

There is kinetic evidence that the migration step in these base-catalyzed rearrangements is concerted with ionization. Thus, in cyclopentane derivatives, the rate of reaction depends on the nature of the trans substituent R, which implies that the migration is part of the rate-determining step.26 27 28 29... 

It has been pointed out (75TL213) that path A behavior might be more complex in some instances, and involve initial ylide formation followed by intramolecular proton abstraction (Scheme 202). The aryloxy quaternary salt (274), formed by reaction of pyridine 1-oxide with an arenediazonium salt, undergoes an interesting base-catalyzed rearrangement that is believed to take the course (path E) indicated (Scheme 203) (71JA3074). 

The chemical, physical, and thermal properties ana resistance to degradation of polysiloxanes is the result of the high energy (106 kcal/mol) and the relatively large amount of ionic character of the siloxane bond. The ionic character of the Si—O bond facilitates acid and base-catalyzed rearrangement and/or degradation reactions. Under inert conditions, highly purified polydiphenyl- and polydimethylsiloxanes are stable at 350 to 400 °C. 

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],... 

Base-catalyzed rearrangement of ethylene oxides is a topic that baa, until now, received only limited attention in the literature, chiefly because epoxides undergo simple nudeophilio attack rather than isomerisation with most bases. Strictly speaking, a base-catalysed epoxide isomerization is one in which the initial event is direct proton abstraction from the oxide ring. This may bo followed by redistribution of bonding electrons in any of several possible ways, to give ultimately one or more carbonyl compounds. For the general case the course i>f such a reaction may be depicted a in Eq. (480),... 

The ability of ethylene oxide to undergo rearrangement to acetaldehyde was mentioned (see section. L2.) in connexion with the thermal decomposition and photolysis of ethylene oxide, and also (see section m.l.C.) in connexion with catalytic ethylene oxidation at elevated temperatures. This characteristic property is discussed, again below with regard, to reactions of epoxides with Qrignard reagents (see section IV.4.F,). For the purposes of this section the subject of epoxide isomerization can be divided into two parts. The first, and most extensive, is concerned with thermal and acid-catalyzed ethylene oxide isomerisation the second involves base-catalyzed rearrangement. 

Base-catalyzed rearrangement of Af-(aryloxy)pyridinium salts (11) leads to 2-arylpyridines (12) via intramolecular attack on the ortho position of the aryloxy ring by the 2-pyridyl carbanion. When X was 3-CChMe, pyrido[3,2-J]coumarins (13 R = NO2, CN) were directly obtained in useful yields from this reaction. When in (11) X was 3-COMe and R was 4-NO2, 10-hydroxy- 10-methyl-6-nitropyrido[2,3-J]benzopyran (14) was obtained in 10.8% yield.89... 

Epimerization and the Enediol Rearrangement One of the most important aspects of sugar chemistry is the inability, in most cases, to use basic reagents because they cause unwanted side reactions. Two common base-catalyzed side reactions are epimerization and enediol rearrangement. 

Another base-catalyzed side reaction is the enediol rearrangement, which moves the carbonyl group up and down the chain, as shown in Mechanism 23-3. If the enolate ion (formed by removal of a proton on C2) reprotonates on the Cl oxygen, an enediol intermediate results. Removal of a proton from the C2 oxygen and reprotonation on Cl gives fructose, a ketose. 

In an alternative approach to molecules of this type, Dauben and Hart examined the base-catalyzed rearrangement of vinylogous 0-hydroxy ketones such as 305 and 308 (Scheme 48).321 Their conversion to 306 and 309 can be accounted for in terms of a vinylogous retro-aldol condensation followed by intramolecular 1,4 addition of an intermediate dienolate to the resulting enone moiety. Subsequent to this reaction, conjugation of the double bond away from the ring juncture followed by a transannular vinylogous aldol condensation produces the observed products. 

The 7-methoxy-l-acetyl and 1-benzoyl tetrahydrodiazepinones (43) have been prepared by reaction of 10 with an acid chloride and pyridine in methanol or from 32 by treatment with acidic methanol.30 These compounds (43) undergo base-catalyzed rearrangement to 5-methoxy-3-methyl-4-phenyl-2-pyrrolealdehyde.36 The formation of this pyrrole can be explained through an alternative cyclization of an intermediate of the type 37. 

N-Alkylation—Secondary Mannich bases can give the corresponding tertiary derivatives by treatment with particular alkylation agents, such as epoxides (affording P-ami-noalcohols) and acrylic derivatives. Tertiary Mannich bases, mostly, are submitted to N-alkylation in order to produce stable quaternary ammonium salts to be subsequently subjected to deamination (Sec. A.2). However, different quaternary ammonium byproducts can be readily given by the reaction. " For instance, a base-catalyzed rearrangement is afforded by allyl ammonium salts 365 (Fig. 144), obtained by N-alkylation of acetylenic Mannich bases with ally] halides. " In the presence of sodium hydride, the compounds 365 yield a wide range of 3-amino-5-hexen-l-yne derivatives 366. 

A number of alkylation reactions, Michael-type additions, and base-catalyzed rearrangements have been previously reported for Reissert compounds. These reactions appear to proceed through the conjugate... 

Relatedly, there is the temperature-dependent equilibration in TFIF of the sodium salt of 1-methylnonafulvene-l-oxide or acetyl-[9]-annulenide (equation 3). As the temperature is lowered to —52°C, equilibrium shifts to the right and the sodium cation in the contact ion pair acquires another solvent molecule to form a solvent-separated ion pair. At —52°C, the rate constant of the reaction from left to right is >330 s with AG > 43 kJ mol and AH = —29 kJmol. Analysis of the species involved in the base-catalyzed rearrangement of equation 3 would be most informative since data for 1 -methylheptafulvene-1-oxide may point to enhanced stability over the tropenide, with its antiaromatic 8-jT-electrons carbocyclic system. ... 

Methylenecyclopropane itself is best prepared by base catalyzed rearrangement of methylcyclopropene which is formed by an intramolecular reaction of the vinyl-carbene (Route a) generated from commercially available methallylchloride. The latter was reported to react with NaNH2 in anhydrous THF to give pure 1-methylcyclopropene in 40 % yield 107). Appropriate variations of the base and/or the reaction conditions allow to prepare methylenecyclopropane in one or two steps, respectively, and in 70% overall yield 1 5> (Eq. 51). Following the procedure given in Ref. 5), methylenecyclopropane can be obtained even on a kilogram scale. 

The classic example for conversion of allylphenols to propenylphenols is the base-catalyzed rearrangement of eugenol 117 to isoeugenol 118 (eqnation 52) °. Silyl protected phenolic tertiary cinnamyl alcohols 119 undergo a lithinm-ammonia indnced hydrogenol-ysis with concomitant double bond migration. This reaction serves as a nniqne approach to prenyl-snbstimted aromatic componnds 120 (equation 53)" " . ... 

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