Resonance-stabilized cations

Alkylation of pyrimidin-2(or 4)-amine on a ring-nitrogen gives an imine, e.g. (8), of quite high basic strength (pjSTa 10.7) because its cation, e.g. (13 R = Me), has typical and effective resonance stabilization indeed, methylation of pyrimidine-2,4-diamine gives a still stronger base (pjSTa> 13) due to an even more resonance-stabilized cation (14). 

Mass Spectra of Alkenes and Arenes. Resonance Stabilized Cations... 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

With the exception of 2,5-diaminopyrrole, which exists predominantly in the 2,5-bisiminopyrrolidine form, the C-aminopyrroles possess the structure of normal aromatic amines and this is generally reflected in their chemical properties. The C-aminopyrroles are, however, less basic than one might expect for an aromatic amine and it is evident that 2-aminopyrroles do not form the pyrrolylammonium ions, but are protonated at the 5-position, giving rise to the resonance-stabilized cations, e.g. (475) (68TL4605, 76S51). 

As early as in 1956, Braude et al.92 suggested that the selective oxidation of unsaturated alcohols with the quinone o-chloranil (82), can be explained by the intermediacy of a resonance-stabilized cation resulting from a hydride abstraction. Later, detailed mechanistic studies confirmed this hypothesis94c,95e in oxidations performed with the more common quinone DDQ. 

Amides are protonated on oxygen because a resonance-stabilized cation is produced. Protonation on nitrogen gives a localized cation. 

The process occurring here is reminiscent of the N.I.H. shift, which is well known to occur in iron hydroxylases such as cytochrome P-450 and mammalian PAH [1,167], For example, action of PAH on [4-3H]phenylalanine produces >90% [3-3H]tyrosine. Here, a presumed electrophilic iron-oxy species produces a carbonium ion intermediate from which a 1,2-shift occurs, giving a resonance stabilized cation rearomatization through loss of H+ (or 3H+) gives the observed product as a result of a heavy atom isotope effect. Thus, it appears that the N.I.H. shift mechanism for copper has been discovered for a chemical model system prior to its observation in proteins. 

Benzyl-type linkers are the most common anchoring groups for various kinds of functionality. Esters, amides, amines, alcohols, and thiols, in particular, can be immobilized by this linker family. This was demonstrated by Merrifield [2] and Wang [19] and is the starting point of modern linker development. Benzylic linkers are typically cleaved by strong acids (for example trifluoroacetic acid, TFA), which cause protonation and subsequent elimination. A nucleophilic scavenger usually quenches the resonance-stabilized cation thus formed. 

Net hydride transfer may also occur in a stepwise fashion without radical intermediates. There may be a-bond formation between donor and acceptor, particularly when resonance stabilized cationic acceptors react with a donor containing nucleophilic lone pairs on heteroatoms (9). Plausible fragmentations then lead to the products of transfer to the cation of hydride 0- to the heteroatom. 

The solvolysis product distribution and the kinetic data (solvent effect33) m = 0.583 — 0.505, substituent effect34) q = —2.98) were clearly consistent with a SNj ionization process involving the anchimeric assistance of the triple bond (kA) 35) leading to the resonance stabilized cation 63 with its positive charge delocalized through the adjacent triple bond into the aryl (or cyclopropyl) group 13). 

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

Fragmentation in the mass spectrometer gives resonance-stabilized cations whenever possible. The most common fragmentation of alkenes is cleavage of an allylic bond to give a resonance-stabilized allylic cation. 

Figure 12-20 shows how the radical cation of trans-hex-2-em undergoes allylic cleavage to give the resonance-stabilized cation responsible for the base peak at m/z 55. 

The most common fragmentation of amines is a cleavage to give a resonance-stabilized cation an iminium ion. This ion is simply a protonated version of an imine (Section 18-15). 

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