Carbocations nitrenium ions

At least two types of nitrenium ions, the nitrogen analogs of carbocations, can exist as intermediates, though much less work has been done in this area than on carbocations. In one type (46), the nitrogen is bonded to two atoms and in the other... 

Not only is the leaving group important, but if the reaction has SnI character, the stability of the positive charge left behind is also important. In the examples above, a primary carbocation is difficult to form and therefore the reaction of busulfan with glutathione would likely be a SN2-type reaction, whereas the nitrenium ion and carbocation formed from N -acetylaminofluorene and safrole, respectively, are relatively stable and likely to be Sn 1 -type reactions. 

Table 2 Yukawa-Tsuno parameters for the reaction of carbocations and nitrenium ions with H20. ...

Mechanism A proposes initial attack by N-7 of d-G on the nitrenium ion to generate the N-7 adduct 110, followed by an intramolecular rearrangement to form the cationic C-8 intermediate 111. The evidence for the mechanism includes the precedent of other electrophiles, including carbocations, that usually react at N-7 of d-G," and the reported isolation of 113 (Scheme 50) and its reduction with NaBH4 into 114." Novak and co-workers were not able to isolate an adduct similar to 113 from the reaction of 76n and 76o... 

The nitrenium ion (120) is isoelectronic with the carbocation. Until the middle 1960s it was unknown, but at that time Gassman began an intensive investigation... 

Reactive intermediates " are believed to be transient intermediates in the majority of reactions. The main types of reactive intermediates of interest to organic chemists are carbocations, carbanions, radicals, radical ions, carbenes, nitrenes, arynes, nitrenium ions and diradicals. 

In an alternate mechanism, water leaves the protonated starting material prior to involvement of the nucleophile (the aromatic ring). This gives a nitrenium ion, which acts as the electrophile. Cyclization leads to the same intermediate carbocation, 4-81, as before. 

Hydride shift, 84 in carbenes, 115 in carbocations, 107 in nitrenes, 118 in nitrenium ions, 120... 

Nitrenium ions may occur during bioactivation of aromatic amines and amides, which are usuaUy A-oxidized into A-hydroxylated derivatives. By sulfation or esteriUca-tion foUowed by elimination of the newly formed leaving group, the latter may be transformed into highly reactive nitrenium ions. In the case of aromatic nitrenium ions they are in equiUbrium with their tautomeric aromatic carbocations, which react with ceUular nucleophiUc macromolecules (nucleic acids, etc.). 

Evidence that the a-cyano carbocations did display some nitrenium ion character was obtained by Olah and coworkers " . A study of the ion (105) generated from benzophe-none cyanhydrin indicated from chemical shift data that the ion included a substantial contribution from the nitrenium ion form (106). 

Carbocations occupy a unique place in the world of electron-deficient cations. Electron-deficient nitrogen and oxygen compounds (nitrenium and oxenium ions, respectively) are extremely unstable and are rarely or never seen. Electron-deficient boron and aluminum compounds are quite common, but these species are not cations. Silenium ions (R3Si+) are extremely unstable kinetically still, it is useful to think of the R3Si+ group as a large, kinetically more stable H+. 

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