Carbocations diazonium salts

Intramolecular insertion has been reported. The positively charged carbon of the carbocation (23), generated from the diazonium salt of the triptycene compound (22), reacted with the CH3 group in close proximity with it. ... 

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

As seen in Figure 13-24, secondary amines react directly with acidic sodium nitrite to form a nitrosamine. (These compounds are very, very toxic.) Primary amines react under similar conditions to form unstable diazonium salts (see Figure 13-25). Diazonium salts readily lose the very stable N2 to form reactive carbocations that are useful in a number of synthetic pathways. Figure 13-26 shows the resonance stabilization of a diazonium ion. 

Diazonium salts can be regarded as combinations of carbocations R with N2 and, because of the considerable stability of nitrogen in the form of N2, we would expect diazonium salts to decompose readily with evolution of nitrogen and formation of carbocations. This expectation is realized, and diazonium salts normally decompose in this manner in water solution. The aliphatic diazonium ions decompose so rapidly that their presence can only be inferred... 

Primary amines react with nitrous acid to yield a diazonium salt, which is highly unstable and degradates into a carbocation that is capable of reaction with any nucleophile in solution. Therefore, reacting primary amines with nitrous acid leads to a mixture of alcohol, alkenes, and alkyl halides. 

Alkyl diazonium salts can lose nitrogen to give 1°, 2°, or 3° carbocations, depending on the nature of the alkyl group. 

If R is an alkyl group, this diazonium salt is very unstable and immediately loses nitrogen gas to give a planar carbocation, which normally reacts with a nucleophile in an S l process (Chapter 17) or loses a proton in an El process (Chapter 19). It may, for example, react with water to give an alcohol. 

If R is an aryl group, the carbocation is much less stable (for the reasons we discussed earlier— chiefly that the empty orbital is an sp2 rather than a p orbital) and that makes the loss of nitrogen slower. If the diazotization is done at lowish temperatures (just above 0 °C, classically at 5 °C), the diazonium salt is stable and can be reacted with various nucleophiles. 

You met diazonium salts in Chapter 23. Arene diazonium saits are stabie compounds, but alkyl diazonium saits, whlph are formed by protonation of diazo compounds, are not. They decompose rapidiyto carbocations—this was how the carboxyiic acid got methylated at the beginning of... 

A carbocation may lose a proton to form an alkene. For example, 1-propyl carbocation generated from diazonium salt may lose a proton (H+) to form an alkene (propene). Alternatively, 1-propyl carbocation may rearrange to more stable secondary carbocation, which may also lose a proton to give propene (Scheme 2.1). 

Phenols are formed when a diazonium salt is heated in boiling water. Nitrogen is evolved and the aryl cation reacts rapidly with water (Scheme 8.15). Since this is a nucleophilic displacement, it is preferable to use acidic conditions to ensure that no phenoxide ions are present, since this could react with unchanged diazonium salt. Sulfuric acid rather than hydrochloric acid is preferred for the diazotization to avoid trapping the highly reactive carbocation with chloride ion. 

Alkyl diazonium salts are generally not useful compounds. They readily decompose below room temperature to form carbocations with loss of N2, a very good leaving group. These car-bocations usually form a complex mixture of substitution, elimination, and rearrangement products. 

This reaction is another example of electrophilic aromatic substitution, with the diazonium salt acting as the electrophile. Like all electrophilic substitutions (Section 18.2), the mechanism has two steps addition of the electrophile (the diazonium ion) to form a resonance-stabilized carbocation, followed by deprotonation, as shown in Mechanism 25.4. 

Alkyl diazonium salts are unstable even at low temperature. They decompose to form carbocations, which go on to form products of substitution, elimination, and (sometimes) rearrangement. Keeping this in mind, draw a stepwise mechanism that forms all of the following products. 

Ionization after an initial reaction that converts one functional group into a leaving group, as in protonation of an alcohol to give an oxonium ion or conversion of a primary amine to a diazonium salt, both of which ionize to the corresponding carbocation ... 

No matter how produced, RNj are usually too unstable to be isolable, reacting presumably by the S l or 8 2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations they should give the same ratio of substimtion to ehmination to rearrangements, and so on, as carbocations generated in other 8 1 reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc, and so on, depending on how the diazonium ion is generated) is the counterion.One class of aliphatic diazonium salts of which several... 

Diazoketones are relatively easy to prepare (see 16-89). When treated with acid, they add a proton to give a-keto diazonium salts, which are hydrolyzed to the alcohols by the S l or Sn2 mechanism.Relatively good yields of a-hydroxy ketones can be prepared in this way, since the diazonium ion is somewhat stabilized by the presence of the carbonyl group, which discourages N2 from leaving because that would result in an unstable a-carbonyl carbocation. 

Nitrosation (Section 22.16) Nitro-sation of amines occurs when sodium nitrite is added to a solution containing an amine and an acid. Primary amines y e d alkyl diazonium salts. Alkyl diazonium salts are very unstable and yield carbocation-derived products. Aryl diazonium salts are exceedingly useful synthetic intermediates. Their reactions are described in Table 22.7. 

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