Ambident cations

The formation of solvent-incorporated products 8 suggested that triarylallenyl cations 7 are formed by photolysis of the corresponding chloroallenes 6 (equation 2). However, the products 8 obtained by photolysis were attributed to attack by nucleophiles at /-positions of the allenyl cation. Although allenyl cations are ambident cations and can produce allenyl or propargyl derivatives by attack at the a- or /-position, respectively, only /-attack was observed in this photolysis of triarylchloroallenes. This result is parallel to that observed by Schiavelli and coworkers9 in the solvolysis of these systems (vide infra) which therefore supports the formation of allenyl cations in the photolysis. 

However, the character of the reaction of 3-nitro-substituted TV-oxide (169) with the C,C triple bond is changed 371. Apparently, the first step affords the normal cycloadduct A, which is successively rearranged into aziridine B. Elimination of the nitronate anion from the latter compound gives rise to ambident cations C and D, which, after quenching with methanol, form dihydrooxazines (170) or (171) depending on the nature of the substituent R. 

We have recently developed a novel method for the generation of alkylideneallyl cations from alkylidenecyclopropanone acetals (8, 9). This method provides a nice opportunity to examine the selectivity of reactions of the ambident cation with various nucleophiles including siloxyalkenes (10) and furans (11). The reaction of the cation with the carbon nucleophiles gives [4 + 3] and [3 + 2] cycloaddition products as well as simple nucleophilic addition products. These results are summarized in this chapter. 

In nitration, where we have an ambident cation, the important frontier orbital will be the LUMO of N021, and this is a similar orbital to the HOMO of N02, except that the cation is linear. The nitronium ion, N02+, always reacts on nitrogen, both with soft nucleophiles like benzene and with hard ones like water. 

According to Bredereck et al.,17,18 the reaction of carboxylic acid amides with dialkyl sulfates proceeds via the intermediate ambident cation such as (5). In the case of caprolactam, when excess dialkyl sulfate is taken, the A,(9-dialkyl derivative (6) is formed, which with base forms 3 (Scheme 1). 

In principle, ambident cations may be defined in the same way, but the solvent dependence of their dual reactivity with nucleophiles has not yet been investigated thoroughly [368]. 

In analogy with ambident anions, mesomeric ambident cations do exist, but the solvent influence on their dual reactivity with nucleophiles has not been thoroughly investigated see reference [368] for a review. 

Homing and Muchowski have recently presented a modification of the general procedures described above. In an attempt to replace selectively the acyl halogen in 48 with azide ion, these authors examined the reactions of dimethylformamide-acyl halide complexes (49) with nucleophiles (equation 35). The site of attack of a nucleophile on the ambident cation (49) is markedly influenced by the nature of the solvent and the temperature. Control reactions with aniline as the nucleophile enabled the optimum conditions for attack at the acyl carbon atom to be elucidated. With azide ion under these conditions, acyl azides were obtained in 60-100% yield (based on... 

Acyloxonium ions are ambident cations that is to say, they can react with anions in two ways. The cis pathway (21 22) leads to... 

The D-ido acetoxonium salt (62) exhibits both of the reactions characteristic of an ambident cation (see Section 1,3). With ethanol-pyridine, 62 reacts by the cis pathway to give the orthoester 65. It is noteworthy that, in pyridine solution, no equilibration takes place between the u-ido salt (62) and the o-gluco salt (59). After hydrolysis of solutions of these salts and subsequent further acetylation, the pure pentaacetates of D-idose or D-glucose, respectively, are recovered. It may be supposed that, in pyridine solution, a salt such as 62 is converted at once into the non-isolable pyridinium orthoester (67), whereby each further acetoxonium rearrangement is prevented. In pyridine-ethanol, the salt 62 evidently reacts initially to give the intermediate 67, which reacts further with ethanol to give the orthoester 65. 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Since the cyanide anion is an ambident nucleophile, isonitriles R—NC may be obtained as by-products. The reaction pathway to either nitrile or isonitrile can be controlled by proper choice of the counter cation for the cyanide anion. 

The proposed intramolecular solvation is not the only feature differentiating between the polystyryl and polymethyl methacrylate salts. The former are classified as the salts of carbanions, whereas the latter are ambident salts having the character of allylic enolates with the cation interacting with the partially negatively charged carbon and oxygen atoms. The degree to which the one or the other interaction is favored is affected by the size of the cation. 

The compound 70 has also been reported showing the ambident character (both C- and N-coordination) of the cyano-stabilized ylide as ligand. The authors have also transposed their work concerning the keto-bis-ylide and palladium, with the synthesis of the C-bonded complex 71 or the new cycloplatinated or-thometallated compound 72. The latter by various treatments allows one to obtain other ylidic cationic complexes of platinum such as 73. A C,C,C-terdentate coordination of the keto bis-ylide, already observed with the palladium is also obtained from the reaction of 73 with gold derivatives. 

The intermediacy of a carbocation or complex-equivalent is attractive, if one considers that the nucleophilic ambident cyanide ion may be accomodated on secondary or tertiary cationic sites. Where exceptions (e.g., 125,126,134-136 cf. Sect. 4.3) exist, the cationic intermediate resides on a primary allylic carbon. The following skeletal types are examples of some biogenetic schemes offered in conjunction with the structural determination of isocyanoterpenoids ... 

The photoelectrochemical reduction of the N = N double bond of the diaryl azo dye methyl orange can be similarly sensitized by colloidal titanium dioxide les, isoj The reaction was sensitive to pH and the identity of the organic redox reaction could be shifted by conducting the photoreaction in the presence of surfactants. Cationic surfactants increased the efficiency of oxidative cleavage by inhibiting charge recombination. Polyvinyl alcohol instead favored reduction. The ambident photoactivity of methyl orange thus makes it an attractive probe for activity of irradiated semiconductor suspensions. 

There is another correlation that seems to have validity in many situations, at least where kinetic control is dominant namely, the.freer (less associated) the ambident anion is from its cation, the more likely is the electrophile to attack the atom of the anion with the highest negative charge. Thus O-alkylation of the sodium enolate of 2-propanone is favored in aprotic solvents that are good at solvating cations [such as (CH3)2SO, Section 8-7F],... 

Bicyclo[2.2.1]heptane (norbomane) and bicyclo[2.2.2]octane, when treated with nitronium tetrafluoroborate in nitrile-free nitroethane, unexpectedly gave no nitro products. Instead, only bicyclo[2.2.1]heptane-2-one and bicyclo[2.2.2]octan-l-ol were isolated, respectively.500 Observation of bicyclo[2.2.1]heptane-2-yl nitrite as an intermediate and additional information led to the suggestion of the mechanism depicted in Scheme 5.48. In the transformation of norbomane the first intermediates are the 2-norbornyl cation 126 formed by hydride abstraction and nonclassical cation 127 formed through insertion of N02+ into the secondary C—H bond. In the case of bicyclo [2.2.2]octane, the oxidation of bridgehead tertiary C—H bond takes place and no further transformation can occur under the reaction conditions. Again these electrophilic oxygenation reactions testify to the ambident character of the nitronium ion. 

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