Aryl cations, as intermediates

Important additional evidence for aryl cations as intermediates comes from primary nitrogen and secondary deuterium isotope effects, investigated by Loudon et al. (1973) and by Swain et al. (1975 b, 1975 c). The kinetic isotope effect kH/ki5 measured in the dediazoniation of C6H515N = N in 1% aqueous H2S04 at 25 °C is 1.038, close to the calculated value (1.040-1.045) expected for complete C-N bond cleavage in the transition state. It should be mentioned, however, that a partial or almost complete cleavage of the C — N bond, and therefore a nitrogen isotope effect, is also to be expected for an ANDN-like mechanism, but not for an AN + DN mechanism. 

Among the evidence for the SnI mechanism19 with aryl cations as intermediates,20 is the following 21... 

Photochemistry offers a convenient access to the formation of aryl—alkyl bonds under mild reaction conditions. This can be accomplished either via fragmentation of an aromatic derivative to produce a trappable aryl radical or aryl cation as intermediate, or via the activation of an aliphatic component that then reacts with the arene derivative. 

The experimental work of the groups of Swain and Zollinger on the dediazoniation mechanism of arenediazonium ions, which started in 1975, provided good evidence for the existence of aryl cations as steady state intermediates (see Sec. 8.3). These results also initiated theoretical work on aryl cations, in part combined with further calculations on the structure and reactivity of arenediazonium ions. Publications that contain data on arenediazonium ions and aryl cations will therefore be discussed in the chapter on dediazoniation reactions (Sec. 8.4). In the rest of this section we will concentrate on investigations that are concerned with the geometries and electron densities of diazonium ions but not, or only marginally, with energetics of the dediazoniation reaction. 

However, measurements of substituent effects supported the hypothesis that the aryl cation is a key intermediate in dediazoniations, provided that they were interpreted in an appropriate way (Zollinger, 1973a Ehrenson et al., 1973 Swain et al., 1975 a). We will first consider the activation energy and then discuss the influence of substituents, as well as additional data concerning the aryl cation as a metastable intermediate (kinetic isotope effects, influence of water acitivity in hydroxy-de-di-azoniations). Finally, the cases of dediazoniation in which the rate of reaction is first-order with regard to the concentration of the nucleophile will be critically evaluated. 

In Section 8.3 the mechanism of heterolytic dediazoniation of arenediazonium ions was discussed, and it was shown that the hypothesis of Crossley et al. (1940) that the aryl cation is the characteristic metastable intermediate in those reactions was not consistent with some experimental facts found in 1952 by Lewis and Hinds. Nevertheless, these facts did not have significant influence on the scientific community, which continued to accept the original and apparently convincing hypothesis of the rate-limiting formation of an aryl cation as an intermediate as correct . The incom-patabilities of various mechanistic hypotheses with experimental facts were, however, discussed in some detail only two decades later (Zollinger, 1973 a). Another year passed before I performed a crucial experiment that refuted a number of hypotheses (Bergstrom et al., 1974, 1976). ... 

I will now discuss the development of the elucidation of the dediazoniation mechanism in terms of Kuhn s cycle normal science -> crisis -> revolution -> normal science . After Crossley et al. postulated the aryl cation as key intermediate of dediazoniation in 1940 and the strong support given to that hypothesis in Hammett s book Physical Organic Chemistry (1940), work in the area of dediazoniation... 

NJ by hydrogen may also occur (5.42). and this implies an aryl radical as intermediate. In many systems it is possible that both cationic and radical mechanisms operate. 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

In the field of (hetero)aromatic photochemistry substitution reactions are also quite useful. The two most useful classes are the SrnI reaction [16] and SnI reaction [17], involving respectively the aromatic radical anion and the aryl cation as the key intermediates. In the former case, (generally photoinduced) electron transfer generates the radical anion of an aryl halide. With less strongly bonded derivatives (usually iodides) the intermediate cleaves to an aryl radical that gives the new product via a chain process (see Scheme 2.6). 

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

Simple mechanistic considerations easily explain why heterolytic dissociation of the C — N bond in a diazonium ion is likely to occur, as a nitrogen molecule is already preformed in a diazonium ion. On the other hand, homolytic dissociation of the C —N bond is very unlikely from an energetic point of view. In heterolysis N2, a very stable product, is formed in addition to the aryl cation (8.1), which is a metastable intermediate, whereas in homolysis two metastable primary products, the aryl radical (8.2) and the dinitrogen radical cation (8.3) would be formed. This event is unlikely indeed, and as discussed in Section 8.6, homolytic dediazoniation does not proceed by simple homolysis of a diazonium ion. 

Addition of hexafluorophosphate salts reduces the dediazoniation rate of 4-me-thylbenzenediazonium tetrafluoroborate in TFE/H20 (1 1) (Maskill and McCrud-den, 1992). However, as the concentration of these salts (0.12 — 0.23 M) does not affect the rate, it is evident that these salts are intercepting one of the intermediates, i.e., either the ion-molecule pair or the aryl cation. 

An interesting observation (Becker and Israel, 1979) is that the photochemical de-diazoniation gives the same product ratio as the thermal reaction in a given solvent. Therefore both types of reaction probably proceed via the same intermediate, i. e., the aryl cation (see Sec. 10.13). 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

Vinyl cations have been postulated as intermediates in the aqueous acetone solvolysis of 257 (214). Unimolecular kinetics were observed, and the sole products of solvolysis were the corresponding amides, 258. A rho value of about p = —1.2 was observed for the effect of substituents X and p = —.7 for substituents Y in 257. The relatively small value of rho for the aryl substituents... 

The wide utility of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are several general mechanisms by which substitution can occur. One involves unimolecular thermal decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly unselective.86 Either the solvent or an anion can act as the nucleophile. 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

In summary, the copper ion transfers an electron from the unsaturated substrate to the diazo-nium cation, and the newly formed diazonium radical quickly loses nitrogen. The aryl radical formed attacks the ethylenic bond within the active complexes that originated from aryldiazo-nium tetrachlorocuprate(II)-olefin or initial arydiazonium salt-catalyst-olefln associates and yields >C(Ar)-C < radical. The latter was detected by the spin-trap ESR spectroscopy. The formation of both the cation-radical [>C=C<] and radical >C(Ar)-C < as intermediates indicates that the reaction involves two catalytic cycles. In the other case, radical >C(Ar)-C < will not be formed, being consumed in the following reaction ... 

Cleavage of a C—C bond gives a distonic radical cation as an intermediate, while concerted cleavage of two C—C bonds yields the corresponding ArO and ArO in cycloreversion of aryl-substituted cyclobutane Therefore, the cycloreversion mechanism is related to dimerization of ArO where tt- and a-dimers are detected during PR of ArO such as... 

Diols such as 87 are converted in excellent yields [89] into acetoxy chlorides (88) by treatment with trimethyl orthoacetate and trimethylsilyl chloride [90] or into acetoxybromides (89) with trimethyl orthoacetate and acetyl bromide [91]. These reactions proceed through nucleophilic attack on an intermediate l,3-dioxolan-2-ylium cation [91] with inversion of configuration. In the presence of an aryl substituent as in 87, displacement occurs exclusively at the benzylic position. With aliphatic diols such as 90, the halide is introduced mainly at the less hindered position and acetoxybromides 91 and 92 are formed in a ratio of 7 1. Treatment of the acetoxy halides 88 or 89 under mildly alkaline conditions affords epox.de 93 in 84-87% yield while the mixture of 91 and 92 is converted to epoxide 94 in 94% yield. Because both... 

Cyclobutyl, substituted cyclobutyl, and related carbocations were reviewed.23 A study of the photophysical properties and photochemistry of 1-adamantyl aryl ethers in methanol solvent showed that, although the majority of the ethers underwent photolysis by homolytic pathways, irradiation of the 4-cyanophenol ether resulted, in part, in the methyl ether, implicating an ionic mechanism with the 1-adamantyl cation as an intermediate.24 Labelling experiments demonstrated that the fragmentation of 7-norbomyloxychlorocarbene to 7-norbornyl chloride proceeds with both retention and inversion.25 While an ion pair [Cl CO R+] is a possible intermediate, computational studies suggest that the fragmentations proceed via transition states that lead to either retention or inversion. 

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