Rearrangement of Carbonium Ion Intermediates

The mechanism involves carbonium ion formation and substituent migration, assisted by electron release from the remaining hydroxyl group  

Another method for carrying out the same net rearrangement involves synthesis of a glycol monosulfonate ester. These compounds rearrange under the influence of base. This method can be used to change the nature of rearrangement from that 

Aminomethylcarbinols yield ketones when treated with nitrous acid. This reaction has been used synthetically to form ring-expanded cyclic ketones, a procedure known as the Tijfeneau-Demjanov reaction, The diazotization procedure generates the same type of jS-hydroxycarbonium ion that is formed in the 

Ring Expansion of Cyclic Ketones with Diazo Compounds 

More recently, it has been found that trimethylsilyl cyanide reacts with ketones to give trimethylsilyl ethers of cyanohydrins. These compounds can be directly reduced to the aminomethylcarbinol by lithium aluminum hydride. Another method for 

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The hydrolysis of epoxides is pH dependent and can occur through acid, neutral, or base-promoted processes. Because the acid and neutral processes dominate over environmentally significant pH ranges, the base-catalyzed process can often be ignored. The reaction products resulting from epoxide hydrolysis are diols, and to a lesser extent, carbonyl products, which result from the rearrangement of carbonium ion intermediates. 

On the basis of the evidence discussed, the rearrangement of arene oxides to phenols clearly involves dienone intermediates. However, from kinetic results it is evident that several carbonium ion intermediates must also be involved. The evidence for involvement of carbonium ion intermediates is ... 

The reaction of a-ketodiazonium ions is of interest because there is considerable evidence that loss of nitrogen can occur by an 8 2 mechanism (p. 337-347). If this is generally true, the possibility arises of a comparison between the reactions of diazonium ions and those of alkyl halides and tosylates under conditions that do not lead to the formation of carbonium ion intermediates. In the discussion of the molecularity of the rate-determining step, the reaction of ketodiazonium ions was supposed to proceed with simple substitution by an external nucleophile. Product analyses, on the reactions of diazoketones with acids and the deamination of aminoketones, show, however, that extensive rearrangement and molecular fragmentation can occur in suitable alkyl structures. The simplest of these reactions have the following stoichiometric form (Baumgarten and Anderson, 1961) ... 

A further advantage is that the reaction appears to be free from rearrangements involving carbonium ion intermediates.41 Thus norbomene (9) is converted in close to quantitative yield into exo-norborneol (10) of > 99.8% purity. 

Using platinum oxide as catalyst, aliphatic ketones in HF are reduced by hydrogen to hydrocarbons. Rearrangements typical of carbonium ion intermediates are observed. "... 

Because of the involvement of carbonium ion intermediates, rearrangement is a possibility. Reaction of t-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged product. The rearranged acetate may also be... 

Skeletal rearrangements have been observed in hydrogen halide additions when hydrogen or carbon migration leading to a more stable carbonium ion can occur. These rearrangements are indicative of carbonium ion intermediates. 

The nature and stereospecificities observed in the rearrangement of (68b) to (69a) and (69a) to (70a) suggests that these rearrangements involve two discrete carbonium ion intermediates A and B (see Chart II). 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

Although HCo(CO)4 is a strong acid in aqueous solution and is capable of protonating even weak bases like dimethylformamide, there is no evidence that it protonates olefins in hydrocarbon solvents to form carbonium ion intermediates which might then rearrange by conventional 1,2-hydride shifts followed by proton elimination ... 

The use of N-trifluoroacetyl in place of the piotonated N-methyl function in these oxidative cyclization reactions has been explored. Generally these substrates lead to products of the O-methylflavinanthine type. In one instance, the delocalised carbonium ion intermediate 34 was found to undergo a competitive rearrangement when lack of a nucleophile in solution led to a slow demethylation step [137],... 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

The formation of these products may be rationalized in terms of a mechanism involving protonation by sulfuric acid at the terminal carbon of an isopropenyl group to produce an a-carbonium ion intermediate, followed by rearrangement of vicinal silyl group from silicon to this electron-deficient carbon. 

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