Rearrangement - An Example

Organic chemists have conducted experiments to understand the formation of multiple products in acid-catalyzed reactions such as that of 3,3-dimethyl-2-butanol. Let s consider the acid-catalyzed dehydration reaction of 3,3-dimethyl-2-butanol and explore the possibility of rearrangement. 

The higher boiling points exhibited by alcohols with respect to comparable dkanes and ethers are primarily due to  

An organic chemist synthesized and isolated the above represented compound. If this compound undergoes oxymercuration-demercuration reaction, the major product is most likely  

Direct oxidation of which of the following can result in a carboxylic acid  

Choose the correct product formed as a result of the reduction of a ketone. 

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No molecule is completely rigid and fixed. Molecules vibrate, parts of a molecule may rotate internally, weak bonds break and re-fonn. Nuclear magnetic resonance spectroscopy (NMR) is particularly well suited to observe an important class of these motions and rearrangements. An example is tire restricted rotation about bonds, which can cause dramatic effects in the NMR spectrum (figure B2.4.1). 

In some cases, NMR studies in superacid media have permitted the observation of successive intermediates in a series of rearrangements. An example is the series of cations originating with the bridgehead ion F, generated by ionization of the corresponding chloride. Rearrangement eventually proceeds to the tertiary ion K. The bridgehead ion is... 

This is usually called the quasi-Favorskii rearrangement. An example is found in the... 

Another popular use of radiotracers in studying chemical reaction mechanisms is the study of molecular rearrangments. An example of this class of reactions that illustrates the use of radiotracers is the cyclization of w-phenoxyaceto-phenone (I) to 2-phenylbenzofuran (II). Two possible mechanisms are shown below ... 

Hoffmann and Pete [167,168] have discovered intramolecular ortho photocycloaddition of 3-alkenyloxyphenols. The efficiency of the reaction increases if it is carried out in the presence of added acid and the primary adducts undergo acid-catalyzed rearrangements. An example is given in Scheme 56. 

Finally, it is important to recognize that an SN1 reaction that forms an allylic carbo-cation often provides more than one site at which the nucleophile can bond. The nucleophile may bond to either of the carbons that bear the positive charges in the resonance structures. If the allylic cation is not symmetrical, this will result in the formation of two products one normal and one rearranged. An example of such an allylic rearrangement is... 

Sigmatropic rearrangements are also thermally allowed and are quite important in synthesis. When the six atoms involved are all carbons, the reaction is known as the Cope rearrangement. An example is shown in the following equation ... 

In weakly alkaline solutions aldoses and ketoses undergo rearrangements. An example is the Lobry de Bruyn -Alberda van Ekenstein transformation of... 

The mechanism discussed is in accord with all the facts when the halo ketone contains an a hydrogen on the other side of the carbonyl group. However, ketones that do not have a hydrogen there also rearrange to give the same type of product. This is usually called the quasi-Favorskii rearrangement. An example is found in the preparation of Demerol ... 

Isomerization may occur to compete with propagation, particularly in the 1-alkenes, other than isobutylene (the illustration used here). This is linked to the possible rearrangements of the propagating cation to more stable cations. Monomers such as styrene and vinyl ethers are not affected since they do not have structures available for rearrangement. An example... 

Acid Catalyzed Rearrangements. An example of a photoresist based on an acid catalyzed rearrangement is the diaryliodonium salt photoinduced cyclization of cis-1,4-polyisoprene shown in Equation 18. This facile cyclization which has been reported previously (11) by non-photochemical processes results in a polycyclic polymer whose physical properties and solubility characteristics are considerably different than the initial polymer. Exploitation of these differences in the exposed and unexposed regions of the polymer film permit their use as either positive or negative tone resists. 

The Oxa-Di-n-methane reaction and Related Processes. Interestingly, some P,y-unsaturated ketones do not undergo 1,3-acyl shifts or the oxa-di-Ti-methane rearrangement. An example of this is compound (61), which on irradiation undergoes only a Norrish Type II hydrogen abstraction. ... 

In addition to the ring opening/carbonylation pathways illustrated in Eqs. 2-5, photooxidation of metal carbynes can result in other types of ligand rearrangement. An example is the cyclization of butenyl carbyne complex 8 depicted in Eq. 6. Deuterium... 

Another related reaction which has found utility in natural product synthesis is the dianionic oxy-Cope rearrangement. An example of this is seen in the syntheses of ( )-palominol and ( )-dolabellatrienone by Corey. Diol 171 was deprotonated with excess base, resulting in the Cope rearrangement, which was followed immediately by an intramolecular aldol addition to give a 1 1 mixture of 172 and 173 in good yield. 

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